Dave Richards AA7EE

November 17, 2013

A Tuned Loop Antenna For The AM Broadcast Band

As a follow-up to the previous post, in which I discovered that the Sony SRF-59, though cheap to purchase, offered surprisingly good performance due to a rather creative and interesting receiver architecture. I did some reading up on external antennas to help pull in weak stations.  Among the Ultralight DX’ing crowd (those who DX the AMBC band with small, cheap receivers) FSL antennas are a source of great interest – they offer good gain and directivity in a small and portable package.  However, I had almost all the materials on hand to build a simple tuned loop and as, typically, I don’t pursue these things in too much depth, figured this would be the way to go.

First off, let’s get to grips with the rather complex schematic of this thing. The SRF-59 doesn’t have an antenna jack, so the external antenna will need to be coupled to the receiver inductively, which just makes the circuit diagram even simpler (at this point, it couldn’t really be any simpler) -

There are many different ways to construct a loop of this type. Big ones give more gain with deeper nulls, but space is at a premium for me and as this was an initial experiment, I decided to go for something modest in size.  You can use a cardboard box, plastic crate, or any number of things on which to wind the turns, but I opted to construct a frame specifically for the purpose.  Hardwood is nice, but I don’t have any woodworking tools. A trip to Michael’s craft store yielded a display of balsa and basswood in pre-cut and finished sizes. Balsa is very easy to cut, but is also very soft, and wouldn’t be very hard wearing in duty as a portable loop antenna.  Basswood is a little harder, but can still be cut with a sharp craft knife, so I decided to try a frame made form basswood. I bought 2 pieces of basswood pre-cut to 3/16″ x 3″ x 24″ and a length of 1/2″ square rod to strengthen the frame. At this stage, I have cut 2 slots in each of the 2 main pieces -

I slotted the 2 pieces together and glued 2 pieces of the square section to them with epoxy, to act as strengthening pieces. The square section was held in place with small clamps while the glue was setting. Here’s the finished result -

I wanted to have a rough idea how many turns would be needed, so found an online calculator for exactly this purpose.  I had a nice air-spaced variable capacitor that had been donated by a friend (thanks Jason!) With both gangs in parallel, it has a capacitance swing of 16 – 705pF.  This frame has sides equal to about 16.5″ in length and using the calculator, I figured that 10 turns, with 0.25″ spacing, should tune the AM BC band. Before winding the lopp, I mounted the variable capacitor -

I split a length of narrow-gauge zip cord in two for the loop. Halfway through winding it, Sprat The QRP Cat bit clean through the wire while my back was turned, so I had to solder a new length on in order to continue winding. She also chewed a small part of the frame while I wasn’t looking. It’s a good thing I love that little kitty!

Here’s the finished loop -

The space between the windings is 1/4″, with a wider 1/2″ gap in the middle. This is in case I later decide to use a rod or piece of square section wood as a supporting mast – it can fit through that larger gap -

Another view of the completed loop -

Of course I was keen to try it out, so I switched the SRF-59 on, placed it close to the loop, tuned to a weak station, then tried tuning the loop and moving the receiver around for optimum coupling. Nothing I tried seemed to work and although I could tune the loop to resonate at the frequency I was listening on, it wasn’t enhancing the received signal at all. In fact, reception was better without it. This was all rather dispiriting and I was about ready to throw the towel in and think about adding a few parts to convert the loop to a novel crystal set receiver when, after taking some shots of it outside on my balcony (the 2 pictures above with the concrete on the floor, and the one below), I decided to set up the radio and try it there. It worked! (All the previous tests had been made in my apartment indoors).

For good inductive coupling between the loop and receiver, you want to orient the loop so that both it’s turns, and the turns on the ferrite rod of the receiver, are in the same plane.  The rod in the SRF-59 runs across the top of the case, so this is how it is oriented (you can also place it inside the loop) -

In the above picture, the loop will receive maximum signal from stations to the left and right of the picture (broadside to the winding) – and it does!  My test was only brief, conducted in the daytime, with signals that were of moderate strength. They were of such a strength that there was some noise and static when receiving them with just the radio. On placing the radio next to the loop and tuning it to resonance, all static and noise disappeared, yielding a more pleasant signal to listen to.  To make operation easier,  when orienting the loop for maximum signal, I rested the receiver on one of the diagonal arms in the frame. If the loop were on a stand, one of the arms would be horizontal.

My loop seems to tune well above the top end of the BC band, but doesn’t cover the bit from 530 to about 600KHz.  A fixed capacitor across the variable should bring the tuning range down a bit.  I’ll fiddle around with it in the next few days. I may also make a recording if the spirit moves me :-) EDIT – I did. See below.

I already had the wire and variable capacitor, so this loop cost me $8.58 in wood from the craft store. The SRF-59 receiver cost me $6.50 inc shipping from eBay, so my complete AM BC band DXing set up set me back a whopping $15.08. I like the kind of fun that can be had for such a small outlay :-)

This afternoon, I went out onto my balcony and made a short recording of KZSF in San Jose.

The recording starts with the SRF-59 receiver without the loop, then I place the receiver inside the loop, which has been pre-tuned to resonance and oriented in the direction for maximum signal. I remove the receiver, and then place it back in the loop for comparison. KZSF is not a DX station from my location in Oakland. It is a 5KW station in San Jose – just 40 miles away. It is entirely possible that I could have found a nearby position from which to get a better signal on the receiver without the loop, but this recording was made to show how a loop such as this can provide a meaningful and useful boost to a marginal signal.

November 11, 2013

AM Broadcast Band Dxing – With A $3.50 Radio!

After finishing the VK3YE Micro 40 DSB Transceiver, I did fool around with crystal radios a little, but didn’t pursue those experiments very far. Perhaps they will continue at some point. However, thinking about crystal radio sets did keep me on the subject of Medium Wave AM Broadcast Band listening for long enough to find out about the hobby of Ultralight DXing, which is the hobby of listening for distant stations (usually on the MW AM BC band) using modest portable receivers.  Some enthusiasts cite a receiver price of $100 and less as a cut-off point, and that seems like a reasonable definition.

It’s a neat hobby, and there is a lot to be heard for the dedicated listener. The fact that it can be done with a modest set-up only adds to the appeal.  In 2007, Gary De Bock N7EKX discovered that a little Walkman radio from Sony, the SRF-59, had very good AM performance, and cost under $20 new. Others acquired their own SRF-59′s and also found that considering that it’s just a small, cheap receiver with an analog tuning dial, it has surprising sensitivity and selectivity. Unfortunately, in order to achieve the best performance, an alignment is recommended, as many of them came out of the factory with less than optimum performance. Earlier models, such as the clear-cased prison issue SRF-39FP, had much better factory alignment as well as a higher quality tuning capacitor, but they cost more.  If you’re willing to pop open the case yourself and perform 2 fairly straightforward adjustments, you can have a sensitive, selective and very portable receiver for the 530-1700KHz broadcast band.

How does a cheap receiver like this manage to provide sensitivity as well as selectivity, with excellent image rejection and almost no birdies? Well, take a look at the one I scored on eBay for $3.50 plus $3 shipping, and I’ll tell you -

The Sony SRF-59 uses a low 55KHz IF on the AM band for good selectivity, combined with a local oscillator quadrature mixing scheme that cancels out images – and it operates from a single AA cell with long battery life too!

This receiver uses a proprietary Sony chip – the CXA1129N. They have not released any data on this chip but after it had been on the market for a while, the basic architecture was figured out. This radio uses a low IF of just 55KHz on the AM band. Yes – that’s not a typo – the IF is 55KHz, which gives great selectivity. Think about those other cheapie portables you have that cannot receive a weak station on a channel adjacent to a local powerhouse. The selectivity on this receiver really helps with those kinds of situations. The problem with such a low IF is, of course, images, which would only be 110KHz apart.  Sony get around this by using a quadrature mixing scheme that splits the LO signal into 2, and phase shifts one of the signals, before mixing them back together. This cancels out the images that would otherwise be a serious problem in this design. What a great idea to implement a scheme like this in such a cheap little receiver! It runs off a single AA cell too – reportedly, the main chip will operate down to 0.95V.  On reading about this, I had to have one, and when I found the above used one for just $6.50 inc shipping on eBay, it was a no-brainer.  It came with the Sony earbuds pictured above though when supplied as new, it comes with a set of light headphones.

Out of the package, it sounded pretty good but I had the nagging feeling it wasn’t receiving as well as it could. Gary De Bock, who has performed many alignments on these units for DX’ers, reported that a significant number of them benefited from adjustment. Although the frequency calibration wasn’t too far off on most, nearly all of them needed some tweaking to the 2 tracking adjustments. Mine, it turned out, did too.

I won’t describe the alignment process in detail, as there is all sorts of info about it documented by more knowledgeable people than me. This post by Gary in the Ultralight DX Group on Yahoo Groups, describes it in detail. Also, this page shows how to disassemble and reassemble the receiver and has some good info too.  Both links open in new browser windows. Here is Gary De Bock’s first review of the SRF-59, published in late 2007.

When I first popped the case off, according to instructions I had read, the board is glued to the back part of the case, so the front part is supposed to separate first. It didn’t happen that way for me – my back part came off first. This image also shows the trimcap that is adjusted for maximum signal at about 1400KHz.  If you don’t have a signal generator (I don’t) you can use a weak off-air signal -

This view shows both parts of the case separated from the board -

The view from the other side -

The other adjustment that needs to be made is shown in the next image. The smaller coil is secured to the ferrite rod with wax. The wax is scraped away (I used a small jeweler’s screwdriver) so that it can slide up and down the rod. Then, with the radio either listening to a signal from the sig gen at 600KHz, or a weak off-air signal at or near 600KHz, the smaller coil is slid up and down the rod until the point of maximum signal is found. Gary recommends to use a small piece of tape or woodworking glue to secure the coil in it’s new position; I smeared the wax that I had previously scraped off back onto the coil and warmed it very briefly with a match to melt it again. In this photo, I had already made this adjustment (my coil needed to be moved closer to the main coil for maximum signal) -

A closer view -

You can, if you wish, adjust the frequency dial calibration too. This process is described in the links I have provided, but it was relatively close in the unit I had. There is a limit to how accurate such a basic dial can be anyway, and it is not too hard to figure out where you are if you use powerful local stations as markers. The fact that US stations are spaced at standard 10KHz intervals helps a lot as well.

I have only spent a couple of evenings listening at home so far. The electrical QRM is quite severe in my place at night. It clears up significantly when I walk out into the street, but standing in the middle of my street at night is not the most comfortable position for a long listening session! So far, I have heard stations up and down the west coast, from Mexican “border blasters” on the Mexican side of the border, San Diego, Los Angeles, Las Vegas and up into Oregon, from my home in the SF Bay Area, as well as stations in the central California valley. This is all straightforward stuff – to be hearing stations up to 500 miles distant, but I’m really looking forward to hearing my first Trans-Pacific (TP) DX. There are quite a few powerful broadcasters in Asia that can be heard here on the west coast, as well as inland, when conditions are good.

Anyway, instead of waiting until I had logged some serious DX, I wanted to share my excitement at this neat little receiver. It has reminded me of the pleasure I used to get from simple radios as a teenager. In fact, I even took it to bed last night and went under the covers with it and a flashlight (to see the dial)! The last time I did this with a radio was as a youngster :-)

The appeal of the Sony SRF-59 for me is similar to the appeal that some sports cars hold for driving enthusiasts. In the same way that basic suspension and a lack of luxury features in sports cars like the early British Triumphs made the driver feel closer to the road, there is not much in the Sony SRF-59 to get between you and the AM band. Having said that, it performs better than it’s counterparts from a few decades ago. I love the fact that a newcomer to DXing could, if he/she kept an eye out for a good used deal, get started with this radio, and a small notebook for a logbook, for less than $10. Excellent! I have already had lots of fun for my $6.50, end expect to have much, much more. Stick this in your bag or shirt pocket the next time you go for a walk or hike (or camping), and you’re guaranteed lots of listening fun.

PS – I bought this radio for the AM performance in such a small, cheap radio (and the novelty of the technology used in such a package). It sounds nice on FM but the reason to own this receiver, IMO, is for it’s AM band.

PPS – This little receiver has quite a dedicated set of followers.  Some people have hooked the board up to air-spaced variable capacitors and vernier drives, with larger cases, knobs, input/output jacks etc.  Others have modded it for different bands. With such a cheap radio, there’s not much to lose if you mess up your mod.

PPPS – Some have commented on how the tuning is too fiddly with the small thumbwheel. I haven’t found this to be a problem – I engage my thumbnail with the teeth of the thumbwheel and find it easy to make small adjustments. If you have very short nails, this might not work for you. I saw a mod in which the rectangular slot for the thumbwheel was widened, exposing a greater width of the thumbwheel.

October 19, 2013

The VK3YE Micro 40 DSB Transceiver

I have attempted to build 2 DSB transceivers now with limited success – a Manhattan version of G3WPO and G4JST’s DSB80 – here and here  (the original kit version which Richard F5VJDD sent me for reclamation, worked fine) and the ZL2BMI rig, here and here. Both of them worked FB up to and including the TX driver stages but as soon as I added the PA, I had constant feedback/oscillation, even when not modulating the TX.  In retrospect, I think a simple partition to separate the driver and final from the earlier stages of the TX would have done the trick in both cases (or even building the driver and PA on the other side of a double-sided board.)

The kit version of the DSB80 that Richard F5VJD very generously sent me was a fantastic piece of nostalgia (I owned one as a young man) and a very satisfying project, but I still wanted to be able to build at least one DSB transceiver from scratch and have it be fully operational.

Enter Joel KB6QVI from stage left. Joel is an avid homebrewer of QRP rigs – both from kits (he’s currently working on a BitX using the original board from India, which he is putting on 40M) and from scratch, Manhattan style. Joel is a fan and big user of the MePADS and MeSQUARES from QRPMe (as am I) and has constructed several QRP rigs using them. Joel and I communicate on Twitter, on which he was singing the praises of the VK3YE Micro 40 that he built. I think he was trying to get me interested in building something again, and his enthusiasm couldn’t help but pique my interest. I’ve made a number of jokes in the past aimed squarely at that trusty favorite of many a QRP homebrewer – the LM386. I usually end up using it with a 10uF cap between pins 1 and 8, which gives lots of gain but also quite a lot of noise.  Joel told me several times about the configuration of the LM386 AF amp that Peter uses in this little rig which still gives enough gain to easily drive a speaker, but has much lower noise than the typical high gain configurations of this chip. Then one of my other non-ham projects came to a temporary pause and I got to looking at this enclosure which I originally made for the second beta run of NT7S’ CC-series transceivers. That beta run ended up using a much smaller board and a smaller custom case, so this blue enclosure has been sitting on the shelf for the last 2 years, just waiting for something to be built in it -

The blue enclosure that was originally made for the second beta run of the Etherkit CC-Series transceivers. The 2 pushbuttons on the front were intended for the CC-Series beta. Only one of them would remain for the Micro 40 DSB rig.

Joel got me to thinking that a little DSB rig in this case sure would be neat, so I rummaged in the parts drawers and fitted the controls and connectors I’d be using if I were to build the Micro 40. I kept telling myself that, as I was trying hard not to commit myself at this point :-) Note the little electret condenser mic insert in the middle. I thought an internal mic would make it easier to use, especially if out in the field. Also note that even though the enclosure is 2 years old, the coat of lacquer I applied has kept the copper looking pretty good -

The trouble is, on seeing a neat little case like this with a few controls and connectors installed, it’s hard not get enthusiastic about actually building it. Notice the small hole drilled above the right-hand pot for the locator lug. I used to break these little spigots off until an incident with my Fort Tuthill 80, in which the volume pot came loose and twisted round. I don’t know exactly what happened, but one of the potentiometer terminals contacted something else, causing a blue LED that was being used as a voltage regulator to blow. From that point on, I started using the locator lugs to help keep the pots in the same position -

At this point of course, I was committed, and set about building what I hoped would be the first DSB rig I’d build from scratch that would actually work.   I have made a few changes to VK3YE’s schematic, and will describe them here.  I hope you don’t mind that instead of using the conventional symbols for the 2 chips, I have represented them as rectangular blocks. It makes it a bit harder to figure out what’s going on with the circuit, but easier to visualize the physical layout when building -

I do have one problem with this rig. In fact, it is the only issue I have with my version, and that is a loud feedback howl from the speaker on going from TX to RX. I am thinking that Peter’s method of directly keying the mic amp with the PTT button would switch the mic amp off a fraction of a second before the relay kicked in and switched the LM386 RX AF amp on, thereby avoiding the feedback perhaps? This loud howl, which you can hear in one of the recordings linked to at the end of this post, was the only thing I wanted to cure. Everything else about the rig is great.  Note – see point 10) at the end of this list.

2) I changed the value of the cap that couples the output of the mic amp to pin 1 of the NE602 from 1uF to 0.1uF (100n). On-air reports indicated that my audio was a bit bassy. Admittedly, I was using a microphone that was designed for recording and broadcasting applications, and was way overkill for this use, but I figured it wouldn’t hurt to gently roll off some of the lower frequencies in the TX, regardless of what mic was used. It did help, but I’m now thinking that the value of that 1uF cap in the base lead of the mic amp could stand to be reduced also. Feel free to experiment :-)

3) In Peter’s version, the cap that couples the collector of the BD139 final to the output network is a 47nF.  I didn’t have any of those. I could have put two 100nF caps in series but figured that a single 100nF would work just as well.

4) Peter bypasses the wiper of his tuning pot to ground with a 47nF cap.  I used 100nF.  No biggie. Perhaps I should have used a 10nF instead……..

5) Peter bypasses pin 5 of the NE602 to ground with a 47nF cap. I used 100nF.  He couples pin 5 of his NE602 to the top of the AF gain pot track with a 220nF cap, while I used 100nF. My substitutions are based on what I have in my parts box, rather than any meaningful analysis of the circuit :-)

6) For tuning, Peter uses 2 banks of diodes, each consisting of four 1N4002′s with a switch to achieve the frequency coverage in his rig. With the switch in circuit, both banks of diodes are used, and with the switch out of circuit, just the one bank of 4 diodes are connected between the resonator and ground. He also has a 10uH inductor in series with the ceramic resonator. My 7.2MHz resonator was obtained from hamshop.cz and seems to have very desirable properties. With no series inductor, and just one 1N4004 diode (I didn’t have any 1N4002′s so I used what I had), I achieved coverage of 7207 – 7335KHz.  Placing a 3.3pF cap across the diode (shown as Cx in the schematic) changed the coverage to 7183 – 7295 – almost all of the phone portion of the US 40M band. What luck! Both Jason NT7S and Joel KB6QVI did tests with 7.2MHz resonators from hamshop.cz and achieved very similar coverage. They don’t always have these resonators so my advice would be buy a small stash of them when you see them in stock. These things are like gold! Not all ceramic resonators are created equal – others have different amounts of coverage.

The key advice with ceramic resonators in rigs like this is to experiment in order to get the coverage you want. However, if you are in the US, with the band going up to 7.3MHz, and you have one of those resonators from hamshop.cz, this circuit should give you excellent coverage. Other resonators will most likely give very different results, and you may need to experiment with different diodes, different numbers of diodes in parallel, and perhaps a series inductor (which I believe has the effect of extending the bottom end of the frequency swing.)

7) Pin 1 of Peter’s LM386 is connected to ground via a 47uF cap and a 33 ohm resistor. I didn’t have a 47uF, but I did have a 33uF.  Given the wide tolerances of electrolytics, it probably doesn’t matter much but I substituted a 33uF cap and a 47 ohm resistor. There is an interesting article in SPRAT 116 on page 4 that talks about the use of the feedback resistor and capacitor between pins 1 and 5, as well as the use of an RLC network between pin 1 and ground to create a high gain amp that has a peak at 500Hz for CW reception. With a resistor as low as 3.3 ohms, gains of 74dB and even higher were achieved. This configuration doesn’t use an inductor, or such a low value resistor, but still has plenty of gain without resort to the the more common method of connecting a 10uF cap between pins 1 and 8 – a method that has (in my opinion) done a great deal to give the 386 it’s reputation for high hiss. It does have a lot of hiss when used this way, so don’t do that – use this circuit instead. It is far more pleasing to listen to!

8) I added a 1N4148 diode from pin 8 of the LM386 to ground as detailed in SPRAT 155 page 26. This is designed to help with squeal on going from RX to TX. It did seem to help a bit, but my bigger issue was the squeal in going from TX to RX. Feel free to leave it out, or put it in. Whatever you’d like to do!

9) I really liked the receiver and was surprised at how good it sounded, considering the simplicity. However, at certain times of day, I did experience a small amount of low level breakthrough from AM broadcast stations in the 550-1700KHz band. Joel KB6QVI didn’t have this with his Micro 40 but then, he lives in a less built-up area, about 12 miles outside Medford, Oregon.  I am in the city of Oakland, in the San Francisco Bay Area, and close to many AM broadcasters. This breakthrough didn’t actually stop me from copying any ham stations but it was there and as such, was mighty annoying. Then I noticed that while the problem occurred when I connected the Micro 40 directly to my outside antenna, it disappeared when my ATU was inline. A quick look at the schematic of the ATU revealed that it was a high pass filter (as many ATU’s are). Aha – problem solved!  I installed a simple high pass filter permanently in the receive antenna lead and the breakthrough completely disappeared. The receiver now sounds great.

If you don’t live close to many powerful AM broadcasters, or you are planning to use this rig only out in the field, in the boonies, then you could most likely leave the AM BC band filter out. However, if there is any uncertainty about the circumstances under which you’ll be using it, why not install it? It’s just a couple of toroids and 5 caps (unless you have 2,000pF caps in which case it’s only 3 caps, as you won’t have to double up on the 1,000pF caps).

10) A word about the bypass cap on the TX +ve supply line – the one marked Cy. In Peter’s version, this cap is 220uF. His mic amp is permanently connected to the +ve supply and switched off by a 100nF cap in the emitter lead, which is shorted out by the PTT button on TX. To achieve this, his PTT button keys the -ve side of the TX/RX relay. I understand now why he did this but in my “wisdom” I decided to permanently connect the mic amp +ve supply line to the TX driver final supply line and key them together. A side effect of doing it this way is that when the PTT is released, the remaining charge in the 220uF bypass cap on the TX supply line keeps the mic amp energized for about a second, causing a loud squeal in the speaker. I found that decreasing the value of Cy to 10uF gave a much shorter squeal that I could live with. I am hoping that this lower value of capacitance will still bypass any audio on the TX DC supply line.

As is usually the case with such projects, I built the AF amp first. Touching the input of the amp chip (in this case an LM386) to hear a loud buzzing sound always provides good positive feedback (pun intended :-) ). In the following 2 pictures, the AF gain pot hasn’t been hooked up yet. The curved red lead is a temporary power connection -

The MePADS and MeSQUARES from Rex at QRPMe have become a firm favorite of mine. Every Manhattan project I build uses them. I just realized that I can buy SMT chips from now on if I like, as the sheets of MePADS contain pads for mounting SMT devices too.

The next stage was the point at which things started to get interesting. This is the VXO using a 2N3904 and a 7.2MHz ceramic resonator. Thru-hole resonators for frequencies such as 3.58 and 3.68 are easily available, but ones for 7.2Mhz are a little harder to come by.  When I discovered that http://www.hamshop.cz stocked them, I ordered 3 and gave away one, leaving me with just 2. Now I’m realizing that I should have ordered more, because on firing up the VXO, I found that the coverage with just one 1N4004 diode used as a tuning diode and no series inductor, was 7220 – 7335KHz. Of course, 115KHz of swing is quite a lot but what surprised me more was the fact that this 7.2MHz resonator was happily being pulled so high above it’s nominal frequency. A 3.3pF capacitor placed in parallel with the tuning diode brought the tuning down to 7169 – 7297KHz, which I consider very satisfactory, encompassing as it does the majority of the phone portion of the US 40M band. I like that the upper limit is 7297 as this means I won’t inadvertently transmit out of band. What a cracking little resonator! The resonator is the blue thing just below and to the left of the tuning pot (the top pot) in the photo below -

Fantastico!

Then, things started to get really good, because I built the VXO buffer (an MPF102) and installed the NE602. At this point, I could connect an antenna to determine whether I would be able to hear signals. The first thing I usually do at this point is to turn the power on my K2 right down to 01.W and give a few short bursts of carrier. Even without an antenna attached, the little DC receiver picked it up with no problem and I knew we were in business. The antenna input coil is on the lower left of this next picture. You’ll notice that I have also built the 2N3904 mic amp. The blue wire was a temporary connector to the BNC at the rear of the case, so I could plug in the antenna for listening. If you look closely at the AF gain pot, you’ll see that I soldered a short grounding wire from the body of the pot to the chassis.  Without this lead, you may get hum whenever your hand comes close to the pot.  This doesn’t happen if the pot is used the other way round i.e. if the track is wired between the input of the chip and ground -

This is always the point at which building transceivers gets tricky for me, as I spend so much time listening to the receiver, I lose momentum. I was even beginning to wish that I had set out to build just a receiver.

Here is a synopsis of what had been built up to this point. I had removed the PTT pushbutton to make soldering in that area easier -

You know how when you move house or apartment, you reach a point where you feel as if you’re very nearly done? That’s usually the point at which you are only halfway through (or even less.) All I had to do to turn this rig into a full transceiver, was add driver and PA stages, and I was in business. It wasn’t quite that simple, as I also had to cut and fit a partition, and wire up the transmit/receive switching. Here’s the first view of what I thought at the time was the completed rig. If you’re sharp-eyed, you’ll notice that the electret mic has been replaced by a phono socket.  This was because I kept getting a motorboating sound on TX which was coming from the mic amp. Peter VK3YE said that I should either try a dynamic mic, or try lowering the gain of the mic amp if I wanted to use it with an electret mic.  I decided to take the easier route, and replaced the internal electret mic with a mic socket. That way, I could experiment with different dynamic mics to find the best one. Also, the 2N3053 driver is fitted with a heatsink, wheras the BD139 final is not. KB6QVO said that his driver got warm, while his final ran cool.  For this reason, he used a heatsink on his drvier, but allowed the final to go au natural.  I simply copied him ( it was easier than doing my own research!) -

Well, this little rig works well. See the video at the end of this post to see and hear it in action. As mentioned before, the only issue I was having with the receiver was low level breakthrough from local AM broadcast stations. It was the only downside to what was otherwise a neat little receiver. I won’t retell the story related in point 9) near the beginning of this post, but the simple high pass filter I installed to attenuate signals in the AM BC band did the trick. Here’s a view of the completed transceiver with the high pass filter installed in the receive antenna line. The 2 toroids wound with green wire and the 5 blue caps in the upper left-hand side of the picture are the receive-only high pass filter -

I cut two small triangular pieces out of the bottom of the partition on both sides to allow wires to pass through. One cutout was a little bigger, as it had to allow more wires through. In the following picture, you can just see one of the triangular cutouts (I cut the pieces out with a flush wire cutter – perhaps not the best idea, but the cutter seemed to be undamaged) -

I suppose that at this point extra images just seem gratuitous, but perhaps one of them will contain an extra detail revealed by a slightly different camera angle that will help a hopeful builder somewhere -

This one might be useful in determining what goes where -

Here’s a view of my VK3YE Micro 40 from the back. The hole on the right is unused.  I will cover it up with a piece of electrical tape on the inside -

And here is what this little DSB beauty looks like with it’s cover on, and viewed from the front -

You might wonder about the practicality of covering a tuning range of over 100KHz with a 1-turn pot.  In fact, you are covering this range with just 300 degrees of rotation, which is not much.  The tuning is a bit touchy but I was surprised to find that I got used to it.  If you want, you could use a 10-turn pot for tuning, or a 1-turn pot fitted with a turns counter. The value is not critical – I often use 10K pots for tuning. One advantage to a 1-turn pot for tuning in simple rigs is that you can see roughly where you are in the band with a quick glance. Also, it is great for quickly scanning the band for activity.  A second 1-turn pot for fine tuning would make it easy to exactly tune stations, while still keeping the cost lower than a 10-turn pot or a turns counter. I’m thinking a 100K pot for rough tuning and a 5K for the fine adjustments.

I plug a little MFJ-281 ClearTone speaker into the phone jack and it sounds great, with easily enough volume for comfortable listening. Current consumption is about 30mA on receive with no signals, peaking up to 100 – 125mA on very loud signals. I didn’t measure the current consumption on TX.

Here’s a recording of me in QSO with KE7NCO 180 miles away. At this point, the 220uF capacitor was still in use bypassing the +ve supply line to the TX, causing the very noticeable feedback when switching from TX to RX -

On changing that bypass cap from 220uF to 10uF, the feedback reduced considerably. Here I am in QSO with N7UVH. He is my greatest DX to date, being 736 miles away – not bad for 800mW of DSB (equivalent to 400mW of SSB) -

Here’s a video demonstration of the receiver (boy, I really need a new video camera. This one is 10 years old and limited in resolution!) -

This was me checking into the daily Noontime Net on 7268.5KHz with Jim W6FHZ, who is 180 miles from me -

On the scale of dollars spent for fun and satisfaction had, this little rig is high up there on the list. I built mine with components I had on hand but even if you had to purchase all the parts, I calculated it would cost you around $23 (not including shipping from the various different suppliers.) This is one fun little rig – and it wouldn’t be hard to whip up a simple matching network for an end-fed halfwave antenna, and take a small battery with you for some portable fun.

As an aside, I went to Pacificon last weekend and had the pleasure of meeting Steve the Goathiker, WG0AT. Here he is at the Buddipole booth holding the packet that contains his entire portable station – a KD1JV MTR with a small key and end-fed half-wave antenna. Fantastic!

Note on Ceramic Resonators – sourcing suitable resonators for projects like this can be tricky. The supply of hamshop.cz 7.2MHz parts seems to have dried up. Mouser have some 3 terminal 7.2MHz ones that, even with the internal capacitors out of circuit, wouldn’t resonate much higher than 7.15MHZ (myself, NT7S and KB6QVI all got the same results). More recently, Patrick W9PDS found some 7.3728 MHZ resonators from Mouser that seem to fit the bill. Joel KB6QVI just reported that with a 27pF cap across this part, and using a polyvaricon for tuning, he is getting a freq coverage of 7.175 – 7.303MHZ. It sounds like this would work in the Micro 40! You’ll need to experiment a bit with parts values to get the coverage you want, but suitable ceramic resonators for 40M are getting as rare as hens teeth, so you might want pick up a few while you can. You can find these ones (while supplies last) here.

September 22, 2013

Building and Installing the K60XV 60M Adapter and Transverter Interface Option For The K2

When I first built the CW-only 10W basic K2 about 2 years ago, I was fairly certain that the basic version was all I would need.  Indeed, at the time, it was. I had made a commitment to operate QRP CW exclusively and was having no trouble sticking to that. So although the basic K2 was a fairly good chunk of change, I was able to justify it. Thing is, that it just begs to be added to. There was plenty of empty space left in the case and although some options, such as the 100W internal PA, promised to relieve me of a good portion of my ham radio budget, there were others that required a lot less (oooh – 160M receive and a separately-switched receive antenna for $40, ooh – SSB for $130, ooh – a nice AF filter for $90, ooh – well, you get the idea.)  So it was that in short order, I ended up with the K160RX option, the 20W internal ATU, and the KSB2 option.  In that post, I did mention that the K60XV 60M adapter and transverter interface option would most likely be the next to be added, and that is how it panned out a few days ago.

Living just 50 miles away from Elecraft is great. I called and spoke with Madeleine in the morning, and the next day this arrived via US Priority Mail (First Class Mail would have cost just 2 bucks and very possibly would have gotten it here in a day also, or 2 days max). The small envelope to the right was an extra headphone jack (just in case.)  Whenever I order from Elecraft, I include a few of the more commonly needed extra parts. Heavily used headphone jacks on the K2 tend to wear out over time – especially if physical stress is placed on them, such as that from a bulky adapter. This probably won’t happen to mine but it will be good to have if, some years down the road, I need a new jack and the current part is no longer available -

Jingles, a new addition to the family (who is blind, but you’d never know it) was trying to ascertain what a K60XV is and what it means for her -

She then figured it out and cast her vote -

There aren’t many parts, and the board doesn’t take long to assemble.  Modification of the main RF board inside the K2 in readiness for the installation probably takes as long, but I’ll get to that a bit later. Here’s the K60XV board after assembly -

I suppose it’s hard to imagine how I can make such a meal out of a fairly simple project by taking so many pictures, but I sure do like taking pictures -

There were a small number of inconsistencies and points I felt could have been made a bit clearer in the assembly manual. I’m going to send Elecraft an e-mail with my suggestions for corrections in the next few days. I won’t detail them here, as it may well confuse if they have been corrected by the time you read this. I will mention the more salient ones in the text of this post though.

There was a diagram showing which side of the board the multi-pin connectors P1 and P2 should be soldered. I found the diagram a bit confusing, so figured it out by looking at the board and the space it was going to fit into in the K2. This photo should help though. look at how wonderfully thick that high-quality board is – and just get a gander at those large plated-through holes. Beautiful!

After finishing the board, the main RF board of the K2 has to be modified to accept the new option.  A jumper has to be removed, and a small number of parts have to be removed and new parts substituted – the exact details of which depend on which revision of the main board you have. Good quality solder-wick is a boon here, and helps to suck up all the solder from those plated-through holes. These boards are well-made, so will not be damaged, provided you have a good iron, good solder-wick, and don’t completely fry the thing :-)  The other main modification is the addition of a length of RG-174 coax to the main board as shown here -

The assembly manual recommends putting a short length of heat-shrink tubing over one end of the co-ax as follows (to prevent the braid from inadvertently making contact with the board). The screws that secure the PA transistors to the heatsink are prevented from falling out with small strips of electrical tape applied to the top side of the board. One of them is visible here -

I thought that it would be a good idea to use heat-shrink tubing on the other end of the cable too, so I did just that.  I had some tubing that was a little narrower in diameter than that supplied with the kit, yet it still fitted over the co-ax, so I used that instead -

A view from the top.  There are 2 sets of holes for the transverter input/output sockets. The user can either install BNC’s in the top cover, or RCA phono sockets in the lower heatsink plate. I decided to go with the latter, and you can just see the 2 phono sockets poking out of the back in this shot. The K60XV board is at the back of the K2, to the left of the K160RX board. The large plated-through holes are so you can still easily adjust the 40/60M, 80M and 30M bandpass filters without having to remove the K60XV board -

One more shot, showing the 3 options I now have installed in the main case (20W internal ATU in the top cover, but that is not visible here, of course) -

On finishing the installation, and switching the rig on, 60M was coming through just fine.  Readjustment of the VCO inductor, L30, was required to keep the VCO voltage within an acceptable range for all bands. This is fully covered in the K60XV and K2 manuals.  I completed the alignment process and was soon hearing much band noise on the 60M amateur band (no activity heard until the next evening) and plenty of AM broadcast stations on both the 60M and 49m broadcast bands.  Funnily enough, the first signal I heard was Radio Havana, Cuba, promoting a film screening that was happening just a few miles away in San Francisco!  I have since heard a few ragchew QSO’s on 60M USB as well as W5GHZ calling CQ on CW, though he didn’t hear me calling him. There was one slight problem with the testing process of the transverter interface part of the option. When in transvert mode, the K2 can develop a low-level signal (1mW or below) to send to the transverter. Firstly, I noticed that when set to an output power of 1mW (at the transverter output phono jack), the K2 was only generating 0.2mW. A few Google searches revealed something that was also in the assembly manual, had I taken the time to read it thoroughly. When using the internal 20W ATU, it has to be taken out of auto mode in order to develop the full 1mW. You can do that either from the menu, or directly from the front panel by pushing the “Display” and “Ant 1/2″ buttons simultaneously. Problem solved? Not quite, as the K2 was now putting out about 50mW – more, but still not enough.

At this point, it was 2:30 am and time for bed. I went to sleep, and woke up the next morning concerned that I had made some kind of boo-boo with the board assembly and/or installation. However, another Google search revealed yet another solution that, had I not been so dog-tired the night before, I would have seen in the assembly manual.  For anyone with a K2 that has the internal 20W ATU, there is a 47 ohm resistor at the input of the op-amp on the ATU control board that can load down the transvert interface to the point where it won’t develop the full 1mW output power. The recommendation is to swap that 47 ohm resistor for a 470 ohm (supplied with the K60XV kit). I did so and – bingo! – the K2 was now putting out 1mW into the transverter output when in transvert mode. I love it when things work :-)

This would be a good time to talk just a little about using the K2 to receive out of the ham bands. Being optimized for the ham bands, with bandpass filters centered on those portions of the spectrum, sensitivity does fall off as you tune away from them. Then as you continue tuning, at some point, the VCO loses lock and you can’t tune any further. However, within these limitations, you can cover most of the SWBC bands with the K2, albeit at reduced sensitivity for some. If you’re a casual SWL only, the reduced sensitivity isn’t as important an issue at it might seem. Each K2 will vary in terms of it’s out of band coverage and sensitivity outside the bands for which it was designed, but this report from Neil WA7SSA will give you an idea of what you can expect.

“But the K2 isn’t set up for AM”, I hear a few people say, “it only receives CW and SSB.”  I have actually seen this argument made in a few online forums and of course, the K2 receives AM quite well, as long you take care to accurately zero beat the carrier. Doing this is easy. Let’s say yours is set up for a CW offset of 500Hz. You select either LSB or USB. I’ll use LSB for this example. Tune away from the carrier until you reach zero-beat with the spotting tone. Let’s say that zero beat occurs at 9580.52KHz.  Subtract 500Hz form this figure and that is where you need to tune the receiver. In this example, you would retune to 9580.02KHz.  Easy! If you were using USB, you’d add 500Hz. Use whichever sideband provides nicer sounding audio. Of course, the width of the crystal filters limits how good an AM broadcast station can sound on the K2, but you get used to the slightly restricted audio. Sensitivity on the 49M BC band is a little low but you can still listen to the stronger regulars on that band (Arnie Coro fans take note!)

Here is a short clip of Radio Habana, Cuba on 6000KHz in the 49M band recorded from the headphone socket of my K2 using the 7-pole crystal filter in the KSB2 option. This filter has a -3dB b/w of about 2.3KHz – less than is ideal for AM SW broadcast reception. This should give you an idea of what to expect when listening to SWBC stations on the K2 -

Funny how that back panel continues to fill up with connectors…….

And if it’s not too much of an imposition, please allow me just one picture of the new addition to the family. This is Jingles. She is 7 years old, blind, and completely adorable. Unfortunately, just like my other 2, she has shown no interest so far in learning the code but she has valiantly (and successfully) taken on the task of leaving little tell-tale pieces of fur on my various homebrew projects as a reminder of her presence :-)

June 24, 2013

The Etherkit CRX1 – A Handy Little VXO-controlled 40M Receiver Kit

Filed under: Ham Radio,QRP — AA7EE @ 5:14 pm
Tags: , , ,

Just a quick post to mention that Jason NT7S has developed what looks like a neat little 40M receiver kit, the CRX1.  It is VXO controlled, covers about 7030 – 7034KHz, and comes with  muting, transmit/receive switching and user-enabled sidetone, as well as a port for connecting an external VFO.  It sounds like a great little receiver for combining with home-brew transmitters and with the external VFO port, there is room for further development. It is all SMT, but with larger-sized SMT components and a board that is not very densely packed, making it a great first project for an experienced builder who wants to get his/her feet wet with SMT.

Here are the specifications (copied and pasted from Jason’s site) -

Frequency Range: Approximately 7.030 to 7.034 MHz (at +13.7 VDC power supply)
IF Bandwidth: Approximately 400 Hz
Current Consumption: 25 mA (at +13.7 VDC power supply)
Power supply: +9 VDC to +14 VDC
MDS: -123 dBm
3rd Order IMD DR: 84 dB
IF Rejection: 74 dB
Image Rejection: 67 dB
PCB dimensions: 70 mm x 100 mm
Antenna Connector: BNC
DC Power Connector: 2.1 mm barrel jack
Phone Jack: 3.5 mm stereo
Key Jack: 3.5 mm stereo
Muting, sidetone (user enabled), T/R switch, external VFO port included

More info here on Jason’s blog.

The CRX1 is not available as a “proper” kit yet but instead of selecting beta testers, as he has done in the past, Jason is selling 8 beta kits on his website for the sum of just $29. Because it comes with minimal documentation, it is only recommended for experienced kit builders. I have built 3 of Jason’s beta kits before and can testify that if you are good at soldering and know how to follow simple instructions, you’ll be fine. The beta documentation probably won’t give you a lot of hand-holding, but if you’ve done this sort of stuff before, you won’t have any problems.

If you’re in the mood for building something and have $29, this sounds like a good idea to me.

NOTE – I just noticed that the Etherkit store is now out of stock of the CRX1 beta. Hopefully we won’t have to wait too long for the production version of the kit.

May 24, 2013

Taking Stock, New Desert Ratt 2 Recording, and A New Tut80 Run

When I started this blog almost 4 years ago, I was getting a (very small) handful of page views every day and had no idea that anyone would find it at all interesting or useful. In fact, I don’t think anyone did at first. Then I started building a few things and found that some people enjoyed looking at the pictures of my builds and in some cases, were encouraged to try building things themselves.

I used to think that in order to have a blog, website, or other kind of internet presence, you needed to be really, really good at something or it wasn’t worth putting your stuff out there, but I was missing the point. I think that the point is sharing. I don’t need to be one of the best at something, because everyone does things differently. If I do my best at something, and share the way that I did it, that information could well be useful to someone else who was trying to figure out how to do the same thing. Maybe my approach will present an interesting alternative to someone who was thinking of a different approach.

My last post on the NA5N Desert Ratt 2 Regen is quite a good example of this. I certainly didn’t design it so wasn’t offering anything radically new, but for anyone wanting to build one, there aren’t very many examples on the internet, with pictures, of DR2′s. Of the ones that exist, there aren’t a lot of detailed pictures, with discussion of construction details, all in one place. Perhaps someone was interested in building it, but was wanting to know the winding information for toroids (which doesn’t seem to be available online), or was wondering whether the tuning would be too fine, what kind of reduction drive to use etc. This is why I like to include this kind of information in my posts, in case it can help someone.

Since I sold my FT-817 2 years ago with a desire to rely more on homebrew gear, things have gone quite well. Admittedly the main rig at AA7EE has been a K2, which is not exactly home-brew, but it still felt good to prove to myself that I could assemble a kit of such complexity.

Apologies for the following 2 lackluster photos (it has to do with my inability in using flash to light indoor scenes, among other things) but here are the main bits of gear I have built in the last 2 or 3 years. These are the ones that worked; I left out the partially completed projects (which includes 2 DSB rigs that have working receivers but not fully working transmitters) .

On the top shelf, from left to right, is the 40M DC receiver using a Hi-Per-Mite filter, and an OHR WM-2 QRP Wattmer.

On the middle level is my K2, Fort Tuthill 80 (see news about a further release of Tut80 kits below), and the NA5N Desert Ratt 2 Regen.

On the bottom level you can see the Norcal 2N2/40, the first beta of the CC-20 and the first beta of the CC1 (it’s successor), both sitting on top of the G3WPO DSB80, and the N1BYT WBR Regen Receiver for 40M.

On the desk in front of that lot is a little 2 transistor TX on 7030 based on the Pixie 2 design. I have used it successfully with the WBR for a 100% home-brew on-air experience! -

The reason I arranged all these projects at my operating position and took their picture together is because I wanted to review my progress so far.  My interests are shifting, and it looks like ham radio will be taking a backseat to other pursuits for the next few months. This was a good way of putting a bookend on this period before I begin another one. This color shot shows why I usually drag my projects into outside light in order to photograph them.  I really need to work on my flash lighting skills (note the blown-out red channel on the freq displays – a bit of HDR work with Photoshop could have helped this, but sometimes I just want get on with things and post them!)

In other news, the videos I posted of the Desert Ratt 2 were intended to give a general sense of what this neat little regen is like to tune around the bands.  It doesn’t really give a good sense of what the audio from the receiver sounds like though, as I was using an MFJ-281 ClearTone speaker, which has a restricted audio response. On top of that, I was using the internal microphone of an old compact camera (Canon A80) to record it. To remedy this I made a recording the other night from the speaker jack of the DR2 directly into the line input of a little flash recorder (the Marantz PMD620, if you’re interested) and posted it to Soundcloud.  This will give you a much better idea of the quality of the audio from this receiver. Unfortunately, band conditions weren’t too good, so I wasn’t able to find any consistently strong signals with little in the way of QSB, but this recording of Radio Habana, Cuba isn’t too shabby.  It has been edited down, and the edit points are marked by cowbell sounds. When the signal gets strong, you can hear the wide frequency response and good fidelity of the Desert Ratt 2 -

And finally, I’ve had the pleasure of an e-mail chat with John K5JS, of the Arizona ScQRPions, and he informs me that they will be producing a final run of the Dan Tayloe designed Fort Tuthill 80 Direct Conversion CW QRP TX/RX. They already have the boards and many of the parts, so it sounds as if they just need to order some more parts and have a kitting party. This is no mean feat, as kitting is an awful lot of work. I don’t know when this will be happening, but it is definitely in the works. As you no doubt know, QRP Kits are selling versions of this rig for 15M and 160M, but I think it would be fun to buy another of the Tut80 kits when they come out and mod it for 40M. Has anyone done this? Could they post details of their mods to the Tut80 Yahoo Group if they have, perhaps?

In the meantime, the weather has been getting nicer and combined with the fact that the bands haven’t been in great shape, it’s as if mother nature is coaxing me to get out more. I plan to do just that. My bicycle has a new chain, and the weather is perfect for bike rides.  There’s also a new camera calling my name, which will require new photo software, and the inevitable upgrade of my operating system (I’m still on XP), as well as much time spent outside taking lots of pictures. I’ve been looking at my rather old photo portfolio and realizing that there is much work to be done, and much fun to be had.

Much work. Much fun,  I love it when the two go together :-)

May 11, 2013

The NA5N Desert Ratt 2 Regen

EDIT If you’re thinking of building the Desert Ratt 2, although the pictures in this post are numerous and quite large, I do recommend reading all the text too, as I have included what I thought were relevant details on the construction as part of my narrative. Also make sure to read the comments and replies.  Previous blog-posts have taught me that readers often ask pertinent questions, so you may be able to glean a little more information from them too.  In fact, just before I wrote this, Paul NA5N made a comment which includes a usefiul piece of information about the 2 x 1,000pF (0.001uF) capacitors in the regen stage.

I’ve been wanting to build NA5N’s Desert Ratt regen ever since I first found his very attractively drawn schematic for it online. I then found the updated version, called the Desert Ratt 2, and a very good description of how the circuit works – all of these documents available on Paul’s website. What more could an avid regen builder want? Not much, it turned out. Late last year, when N2CX and N2APB dedicated an episode of Chat With The Designers to the Desert Ratt (and to the subject of regens in general), I just had to listen and of course, it fueled my interest in building the DR2 even more. The whiteboard for this particular episode of CWTD is here, and the podcast audio is here.

The WBR was a successful regen for me and while it worked well on SSB/CW, it didn’t seem to quite have the gain with AM stations. This makes sense, as a regenerative detector has to be set below the point of oscillation for AM reception, at which point it has less gain than when it is oscillating (which is where you set it for SSB/CW reception.)  Even so, I had read that bipolar transistors tend to work better as regen stages for AM, as they have higher gain when not oscillating. The search was on for such a receiver, and this was one of the key deciding factors in building the DR2 for me. In fact, Paul has mentioned (I forget where I saw it, as I have done so much reading on this receiver) that the Desert Ratt doesn’t do so well with SSB/CW as it does with AM. My experience with it backs up this assertion, thought it’s a pretty neat receiver for AM.

In particular, I wanted a receiver for covering the 49M SW BC band as although my Elecraft K2 covers a few of the BC bands, 49M is not one of them. There were a few things I found interesting about the design. The use of a phase splitter transistor to convert the single-ended output of the detector to a balanced output in order to drive the LM386 in differential mode was novel. Paul talks about how much RF is flying around inside regen receivers, and how the common-mode rejection of the 386 when used in differential mode can be advantageous in such an environment. I was also intrigued by the detector consisting of 2 germanium diodes – I think I was just looking for an excuse to build something with Germanium diodes again to remind me of my crystal-set building days as a kid :-)

If you look at the schematic of the DR2,  you’ll see that one of the changes in the design from the original DR is that instead of a variable capacitor, it uses 1N4004 diodes as varicaps. I have a bit of a “thing” for nice air-spaced variable capacitors, and I had in mind a nice Millen 50pF capacitor that I picked up on eBay for a very fair price last year. Combined with a 6:1 reduction drive, it made a good combination with a very useable tuning rate for tuning in AM stations.

Anyway, I’m getting ahead of myself here. I did make a few changes to the original schematic for my version, so allow me to introduce my rather wobbly circuit diagram -

The differences between my schematic and Paul’s are as follows -

- I added an RF attenuation pot at the antenna input. After building the DR2, I found that using a relatively short piece of wire indoors as an antenna was causing a lot of common-mode hum.  On top of that, I wanted to be able to increase the signal level into the receiver with the use of my regular outside antenna (A 40M dipole fed with 300 ohm balanced feeder.)  Using the attenuation pot allowed me to use the large outdoor antenna without overloading the receiver.  Use of my outdoor antenna created enough separation between the receiver and antenna that the hum problem almost entirely disappeared.

- Earlier versions of the Desert Ratt included instructions for winding the coil on a plastic 35mm film canister and on an IC shipping tube. The DR2 schematic doesn’t include such instructions, but I wanted to use a toroid, so I experimented a bit and came up with a scheme that seems to work OK.  I used a T68-6 former and the turns info is on my schematic above – a T50-7 would take up a little less space. More about this later.

- I had a few 2-position center-off switches that I wanted to use, so I used one of these for a bandswitch instead of the SPST switch in NA5N’s DR2 schematic. I had originally thought that using the 50pF tuning capacitor with no padding would make the upper limit of frequency coverage too high, resulting in too large a frequency swing in one band, but there must have been more stray circuit capacitance than I had anticipated, as the coverage with no extra padding was about 7.3 – 13MHz. This band became the center position.

- I was attempting to power the DR2 from my shack power supply, which is about 45AH of sealed lead acid batteries with a float charger constantly connected.  This also powers my K2, and the DR2 was picking up processor noise from the K2, as well as a low-frequency “burbly” kind of noise of undetermined origin. The problem went away when I powered the receiver from a separate SLA. but I decided to add extra filtering to the power line anyway.  I found that a 1mH choke as well as a 1,000uF electrolytic almost (but not quite) got rid of the unwanted interference on the power line.  For good measure, I added a 0.01uF RF decoupling capacitor across the power line at the input connection.

- I added an AF preamp stage directly after the diode detector to ensure enough power to easily drive a speaker - even with weak signals.

- The inputs to the LM386 are the opposite way around from the way indicated in NA5N’s DR2 schematic.  With the inputs connected as shown in Paul’s diagram, the LM386 emitted a loud screeching sound.  Swapping the inputs cured this. I was not the only person who had this problem, as I discovered from this post in the GQRP Yahoo Group (you need to be a member of the group to read the post).

-  I left pin 7 unconnected. I don’t understand the way that NA5N has it connected to the junction of the series resistor and capacitor connected between pin 5 and ground in his diagram.  Most circuits that use pin 7 call for a decoupling capacitor direct from pin 7 to ground (usually about 10uF).  This helps reduce large signal distortion, though Paul does say that in this application, it may not do a great deal to help and is therefore optional.  I elected to leave it unconnected.

Now for some pictures.  I didn’t want to spend a lot of time constructing an enclosure, so decided to make a simple PCB L-shaped chassis and build the circuit directly onto that.  With the variable capacitor mounting bracket, it still ended up taking quite a while to construct though. All my projects begin like this, with the main components and control being laid out on the front panel, while deciding on the basic layout -

I’ll spare you the words at this point and apologize for all the pictures that are about to come. If you’re living in a remote area and are still relying on dial-up, then I feel a bit sheepish about the sheer number of images to follow!  I’ve talked before about constructing enclosures from PCB material, so won’t repeat that information here. As well as constructing the chassis from PCB material, I also made a mounting bracket for the variable capacitor and a tuning pointer to attach to the reduction drive with 2 small screws – all from double-sided copper-clad laminate.

I applied several thin coats of lacquer from an aerosol spray.  It was sprayed from a distance, resulting in a light, and stippled coating, which you can see in these pictures. I’d rather apply too light a coat than risk overdoing it. The downside of this is that oxidation will being to affect the appearance of the copper fairly soon. Oh well. The capacitor mounting bracket received a thicker coat. You can see the smoother, shinier finish.

I got the 6:1 reduction drive from Midnight Science. A number of others sell them, and one place that springs to mind is Mainline Electronics in the UK. They are the suppliers for Jackson Bros components (I think they have the rights to manufacture and sell the parts).  They sell on eBay using the name anonalouise.

The enclosure looked a little bit different by the time the DR2 was finished, as the hole for the nylon toroid mounting hardware hadn’t been drilled in the base at this point.

Look at that gorgeous variable capacitor!

A close-up view of the Millen 21050 50pF air-spaced variable capacitor and mounting bracket. This component is silver-plated (the vanes are probably brass), and has double bearings and a ceramic base. It is a very nice variable capacitor, and had never been soldered to before being used in this project. It is at least 35 years old – most likely older!

Boy, was I glad to finish the chassis so that I could start work on wiring it all up.  I decided to build the AF amp first and work backwards, my thinking being that the AF amp would be relatively straightforward. The act of touching the input with a metal screwdriver and hearing a hearty buzz in the loudspeaker would give a welcome psychological boost! If I started by building from the antenna end, I’d have to wait until the entire receiver was built before getting any clue as to whether it was working.

Here’s the chassis with the LM386 amp, the 2N3904 phase splitter, and the 2N3904 preamp built. As has been the case with all my projects since I started using then, I used W1REX’s wonderful MePADs and MeSQUAREs to build the circuit -

Here’s a close-up. The 2N3904 preamp is just below the 6:1 reduction drive, and the 2N3904 phase splitter is to the left of the LM386.  The 100uF capacitor that decouples the supply line to the LM386 straddles it. I read that it is best to ground it to pin 4 instead of to some other point on the chassis to avoid instability, hence the reason for this placement. The other electrolytic that is straddling the chip is the 10uF capacitor between pins 1 and 8 that sets it to the maximum gain of 46dB. The black shielded cable connecting the AF gain pot to the circuit on the PCB is lavalier mic cable.  It has 2 conductors, each of them in it’s own shield, which is ideal for wiring up potentiometers. It is fairly thin and very flexible. I use it in all my home-brew projects. I bought it from a local pro-audio store which recently closed down, so will now need to find another supplier.

In this view, you can clearly see the extra DC supply line filtering that I added, consisting of a 1mH choke in series with, and a 1,000uF electrolytic across, the DC supply. After seeing these pictures, I noticed that there wasn’t very much solder on the joint connecting the choke to the power jack, so I re-flowed the joint and melted a bit more solder onto it.

The power indicator LED’s main function is as a voltage regulator. NA5N marked the various voltages on his schematic for the DR2, and I chose an LED with a forward voltage drop to match those voltages as close as I could.  A green LED in a variety pack I got from Radio Shack had a forward voltage drop of 2.1V, which seemed about right.  The 1N4148 had a forward drop of about 0.65V.

The next stages to be built were the detector and impedance converter/buffer stages.  The description of the DR2 on NA5N’s site gives more info on these stages (as it does for the whole receiver). I couldn’t be sure these stages were working, but bringing my finger close to the diodes resulted in a pleasing cacophony of stations in the headphones – and at a louder level than in doing the same to subsequent stages, so I figured there was some detection/amplification going on :-)

I didn’t know how many turns I was going to use on the toroid, but using the calculator on W8DIZ’ site and an online resonant frequency calculator, I figured that 36 turns on a T68-6 should be a good starting point for the whole winding from pin 3 to pin 6. In Paul’s version, with the coils wound “traditional style”, the tickler winding was about 1/3 of the whole winding.  Coupling between windings is tighter with a toroid than a “regular” coil, so I reduced the number of turns on the tickler. I found that regeneration was occuring at only about 25% rotation of the regen pot, so further reduced the number of turns. Using the turns shown on my schematic at the beginning of this post,  the regen stage moved into oscillation at anywhere between 40 and 50% rotation on the pot, so I left it at that. For the same reason of tight coupling, I used fewer turns on the antenna winding too and because I am using an outdoor antenna, could probably have used even fewer turns.

The toroid was fixed to the PCB with nylon nuts, bolts and washers that I got from my local Ace hardware store.

Here are some pictures of my Desert Ratt 2 with the circuit finished -

The red wires running along the back of the front panel are the regulated 2.1V and 2.75V lines.  I would have run them on the main board but ran out of room due to lack of planning, so went vertical.  Incidentally, although I refer to the 2 regulated lines as 2.1V and 2.75V,  the exact voltages aren’t important.  That’s just what they turned out to be in my case.

The RF amp and regen stages can benefit from transistors with high hfe. I got a cheap Harbor Freight DMM that measures hfe from an eBay vendor for under $6 including shipping.  hfe varies depending on the collector current, but I was doing this mainly for comparative purposes rather than absolute values, so the fact that I didn’t know what value of collector current was used to measure hfe in this cheap meter didn’t matter. It just so happened that my 2N2222A’s tended to have higher hfe than my 2N3904′s, so I ended up using a 2N2222A that measured in at hfe = 203 for the RF amp, and a 2N2222A with hfe = 223 for the regen stage.  The other stages don’t require high-gain transistors. NA5N talks about it in this post on QRP-L from 1999. Bear in mind that he was talking about the original version of the Desert Ratt in this post (just so you don’t get confused when he identifies the various transistors).

I did promise that I’d give a bit more detail on the toroid. Mine was wound on a T68-6 former. The main winding was 30 turns tapped at 27 turns from the top (3 turns from the bottom). The antenna coupling winding was 5 turns.  All turns are wound in the same direction. I used 26 gauge wire, but the precise gauge isn’t important. 26 gauge was narrow enough to easily fit all the turns on the former, yet stout enough to lend some stability to the oscillator, as the toroid isn’t sitting close to the board, and the leads are relatively long. When putting taps on coils, I used to not cut the wire i.e. I would simply make a loop in the wire, twist it, tin the twisted part and keep on winding.  Now I find it is easier to treat them as 2 separate windings connected together. If you can get heat-strippable wire, please do – it makes winding toroids so much easier and more pleasurable.  I wound the first winding of 27 turns, stripped and tinned the end, then stripped and tinned the end of another piece of wire, twisted and soldered them together, and carried on winding the last 3 turns in the same direction (this is important).  The separate antenna winding of 5 turns is also wound in the same direction.  I’m afraid I didn’t write down (or if I did, I have since lost it) the lengths of wire used. I did notice that the turns calculator on W8DIZ’ site (linked earlier in this post) was quoting lengths that are too short for the T68-6 former.  All you have to do is wind one turn around your former, measure that length, multiply it by the number of turns you’re going to wind, add an extra inch or two for the leads and, as we say in England, Bob’s yer Uncle and Fanny’s yer Aunt (meaning – you’re home free!)  When winding toroids, I often find that the first 1 or 2 turns aren’t quite as tight as the rest so when I’ve finished winding, I will unwind one turn from the beginning of the coil, then wind an extra one at the end, to keep the total number of turns the same.  Sometimes I will repeat that exercise a few more times until all the turns are nice and tight.  For this reason, I use enough wire to leave several extra inches at each end.

The next picture shows an anti-hiss filter that wasn’t in the earlier pictures, which I tried and ended up removing due to a low-frequency oscillation it was causing at the higher volume settings.   It was a series 0.01uF capacitor and 4.7K resistor connected from pin 1 of the LM386 to pin 5.   From what I have read, too low a value of resistor or too high a value of capacitor can cause the oscillation. I have seen other anti-hiss filters that used a 0.01uF cap and a 10K resistor, so it is very possible those values would have cured my problem. However, I was near the end of the project and itching to move on, so I just removed it. You can also see the 0.1uF capacitors on the inputs of the IC that have been swapped over to stop the uncontrolled oscillation, and are now crossing each other.  You may not have to cross these caps if you plan your layout accordingly -

Other than the problem with the loud screeching that was solved by swapping over the inputs to the LM386 (my schematic reflects the way the inputs were finally connected), the only other problem I had was with what appeared to be a defect in the 0.001uF (1,000pF) capacitor that leads from the tap on the coil to the emitter of the regen transistor.  I wasn’t getting any regeneration at all but on replacing this capacitor, the circuit broke into a nice loud hiss when advancing the regen pot.

I do have one ongoing issue that I hope someone can shine a light on for me, and that is a loud crackling sound when adjusting the tuning capacitor. At first, I thought a dirty rotor connection was the problem, but it only happens when extra padding capacitance is switched in by the band-switch   With no extra capacitance switched in, the tuning is smooth, but on the lower frequency bands, the receiver crackles when being tuned.  I need to try bypassing the band-switch and soldering the padding capacitors into circuit in case the switch is the problem. I’ll report back when I’ve done further work on this.

Incidentally, the main tuning range on mine covers approximately 7250 – 13000KHz.  Switching in a 47pF capacitor changes the range to 5825 – 8050KHz. I’m a bit limited with my receiver and test equipment here, so haven’t yet been able to determine the coverage of the lowest frequency band.

When first listening to the DR2, I had no idea what frequency I was listening to – only that I was probably somewhere between 5 and 12 MHz. I had no antenna connected (and at this point, hadn’t even built the RF amp stage) but started hearing CW. Lo and behold, it was Hank W6SX 180 miles away from me in Mammoth Lakes, CA. His CW signal was coming through well and in fact, this was the only time I have received CW in a satisfactory fashion on the Desert Ratt. There was no antenna – he was being picked up directly by the toroid.  Any concerns I might have had about the sensitivity of this receiver would have been immediately allayed.

I know the main question that is probably on your mind is – how does it sound, and what is it like to use? How does it “handle”? There are some videos of my Desert Ratt 2 in action at the end of this post. Apologies for the poor video quality, but my only video camera is 10 years old (and has a faulty CCD sensor). You’ve probably read articles about regens that describe the many and subtle adjustments that need to be made when tuning a regen in order to coax maximum performance from it. If you haven’t operated a regen before or if it’s been a while, it does take some time to get the hang of getting the best out of it. As you get further away from the setting of the regen pot where it breaks out into oscillation you lose selectivity and gain, so you need to try and keep the control set just under the point of oscillation. Loud stations can overload the detector, resulting in audio distortion, so it’s worth keeping an eye on the RF attenuation pot too. Also, if the attenuation pot is set too high (too little attenuation), you may get breakthrough from stations on other frequencies. There’s quite a bit going on to keep under control, but if you manage to keep all controls adjusted well, you can coax some pretty decent performance out of the set. I think this is why regens appeal to some people – we are incurable knob-twiddlers!

Stability is easily good enough for AM reception and with a logging scale fitted to the front panel, I don’t think it would be hard to find specific frequencies, as the majority of SW BC stations stick to 5KHz channels. In my casual listening so far, I have heard The Voice Of (North) Korea on 9435 and 11710KHz, Radio Habana, Cuba on the 49M band, Radio Australia on the 31M band, coastal station KLB (South Korea) on 8636KHz, the BBC World Service (forget which band or frequency), China Radio International on 9790KHz, WTWW on 5830KHz, and a number of other evangelical Christian stations (sorry, I tune them out and don’t pay them much attention.)

To sum up, you can definitely have a lot of fun and engagement with the bands on this set.  Being a regen, it is not the easiest receiver to operate, but you shouldn’t let that put you off. The best analogy I can think of is to reference the way that although an older British sports car may not have the finesse and performance of a newer sports model, it’s a lot of fun, and it’s lack of suspension gives you an exhilarating feel for the road that the more expensive cars cannot.

The Desert Ratt 2. A logging scale fixed to the front panel would make frequencies in the SWBC bands easy to find. I must do this sometime :-)

Please note that in the following videos, an MFJ-281 ClearTone speaker was used. My understanding is that this speaker has a slight resonant peak at around 700Hz (helpful for CW) and a relatively restricted overall bandwidth that is good for communications applications. This probably means that it’s not optimum for getting the maximum fidelity from an AM SW broadcaster (not that those stations have a lot of fidelity, but they tend to have a bit more than your average SSB transmission). On top of that, the audio was captured with the built-in mic in my old Canon A80 compact. Please don’t judge the quality of the Desert Ratt 2 audio from these clips. It’s better than this! I’m working on a few audio only recordings that will better demonstrate what the DR2 sounds like, and will put them up in the next blog-post (hopefully within a week or so).

April 17, 2013

My Ongoing Pre-Occupation With High Quality Air-Spaced Variable Capacitors

I’ve blogged before about air-spaced variable capacitors.  I’ve always liked ‘em, but I think my understanding of what makes a good one is maturing a little more. I was the winning bidder on a really nice-looking specimen on eBay a few days ago.  Ever since placing the winning bid, I had been excitedly looking at the pictures of it posted by the seller. It looked great. How exciting when it arrived in the mail yesterday and I got a chance to see it “in person”, as it were!  I got it for $11.50 and I think I scored -

It is NOS (New Old Stock) meaning that while it is old, it has never been used. Surprisingly, there seems to be quite a few of these high-quality NOS caps still floating around. Here are the specs for this series of variable capacitors from Hammarlund -

The cap that I scored has nickel-plated brass vanes. Brass is good, as it expands and contracts with changes  in temperature less than aluminum does (the other main material from which variable capacitor rotors and stators are made.) Also good are the bearings on each end of the rotor shaft. I can’t see them, but I assume the bearings are hidden away. It gets better. This capacitor has wide-spaced plates, meaning less change in capacitance with temperature changes than a part with closer spacing. Oh – and this is all firmly mounted on a ceramic base. Ceramic is a great insulator and I’m thinking that this must also be good for the physical stability of the component with regards to changes in temperature.

I just noticed something. As you rotate the shaft clockwise, the capacitance increases. It’s normally the other way around.  An end-stop prevents the shaft from rotating more than 180°, or this wouldn’t be an issue.  This must have been intended for use with a drive mechanism that translated the rotation of the tuning knob into rotation of the capacitor shaft in the other direction. I hope that the length of shaft protruding from the other end is enough for me to connect to, otherwise it might end up on the shelf for a few more decades!

One thing you may not appreciate from these photos is the feeling of solidity. This is a beautifully engineered part. See how the shaft is off-center? This makes for a non-linear relationship between the rotation of the shaft and the change in capacitance.  The change in capacitance occurs in such a way as to make the higher frequencies a little less cramped together, which is what happens with a capacitor where the relationship is strictly linear.

I mean, really – do variable capacitors get much better than this?  I don’t have definite plans yet for this little beauty but if my current interest in regens continues, I can see it paired up with the Jackson Brothers Dual Ratio Ball Drive and Dial I just ordered from the UK and used as the main tuning cap in a general coverage regen receiver – all built on a generously-sized aluminum chassis with front panel. (EDIT – unless I am able to connect the ball drive to the rear end of the shaft, this is not going to happen. Fingers crossed.)

Scroll back up to the top of the page and look at this fabulously engineered piece of American history sitting on top of it’s original box. That’s what it feels like to me – a piece of American history, and I got it for a few bucks. I will feel terribly privileged to be able to incorporate it into my own project at some point, though I’m going to hang onto that box.

Incidentally, while riding around Oakland, I noticed that this commercial space is up for lease.  It would be a good place for a ham-oriented business don’t you think? EDIT – It is now March 2014 and I recently noticed that this space has been turned into a coffee bar – the type that looks like it is part chemistry lab, with much glassware used in the brewing of the coffee.  Aah well – better than being left empty!

March 29, 2013

Good Ops, Ben’s Best Bent Wire, and Some New Home-Made QSL Cards

Nearly every evening on 40M, I hear Bill Crane W9ZN for an hour or two coming in from Chicago. He’s a good op. I’m not sure what his top speed is, but I often hear him conversing easily with others at around 25wpm.  He always matches the speed of whoever he is talking to, which I think is one mark of a good op. I remember the first time I QSO’ed with KA7PUN a couple of years ago.  We were conversing easily at what was my comfortable speed back then (which was probably around 16-17 wpm).  I thought that was his regular comfortable speed until I heard him in QSO a few days later with another station sending much faster. I realized that he had matched my speed and felt very grateful to him for making me feel comfortable in that QSO.

Anyway, back to Bill. I first noticed him on the band for a style of sending that incorporates a variation on the “Ben’s Best Bent Wire” routine that commercial operators used to use in order to loosen up their wrists before a shift. At the time, I wasn’t familiar with this type of routine and only knew that Bill had a style that made him stand out on the band for me.  Here’s what I’m talking about.  This is Bill as recorded last night -

I imagine that a few decades ago, this kind of routine was more prevalent on the bands, but W9ZN is the only station I have heard doing it.  Some people would probably prefer to perform their warm-up routines off the air, but it sure is a good way for Bill to be instantly identifiable. A little online research seems to indicate that he was a Chicago radio personality in the 60′s and 70′s, going by the name of Bill “Butterball” Crane. I’d sure like to QSO with him, but he never hears my puny 5W sigs.  He’s running QRO, and a regular presence in the segment from 7031 – 7034 most evenings.

I’ve also been busying myself with making some new QSL cards, firstly for QSO’s I make with the CC1 beta.  I was inspired by NT7S’ CC1 beta card, and wanted one for myself. I’m lucky to have Photoshop (CS2) and to have finally figured out the importance of layers and how to use them.  The initial version of the CC1 beta card that I came up with looked good on the screen, but due to the fact that I didn’t have a profile for the printer at my local Fedex Kinko’s (they probably don’t have one), the card printed out a lot darker than it looked on my monitor, and some of the text ended up being buried in the background.  I did eventually come up with 2 versions, both of which look OK when printed. One, in my opinion, looks better in print than the other, but I’m waiting to hear back from NT7S as to whether he agrees before I print up a few of one of these two.  These are not scans of the printed cards, but jpeg renditions of the original Photoshop files. Bear in mind when you’re looking at these, that the printer in my local Fedex kinko’s prints files darker than they look on-screen, so if you’re thinking these images look a bit light, that is why -

I finally seem to be getting the hang of using Photoshop to do these kinds of layouts so, bolstered by the success of these cards, decided to make another one. It took me a while to scan the G-QRP Club logo and change it from black on a white background to white on a transparent background, but now I know how to do it, it’s a piece of cake -

Of all these cards, my favorite is my basic 2 color one.  The following image, unlike the previous ones, is not a jpeg generated from the original Photoshop file, but a scan of the final printed card.  I did this because the color of the card stock does a lot to make the card look good.  It’s called “Sawgrass” and unfortunately, my local Fedex Kinkos won’t be restocking it once their current stock is gone -

It’s simple, effective, and prints out well on a variety of printers – no complex graphics that need to be rendered in accurate tones. On top of that, if I need to make a lot and am feeling a bit skint, it doesn’t look too bad in monochrome either.

March 25, 2013

The First CC1 to CC1 QSO – and a QSL as a Memento

Filed under: Amateur Radio,Ham Radio,QRP — AA7EE @ 7:17 pm
Tags: , , ,

I made my first ever QSO with the CC1 beta a week or so ago. It was with Jason NT7S (the designer of the CC1 and proprietor of Etherkit) and on top of that, it was the first ever CC1 to CC1 QSO. Very exciting!  I was hoping to have been the first ever QSO Jason had with his CC1. That honor actually went to WA0JLY, but I did get to be Jason’s 2nd QSO.

He was recording video of the QSO, which is up on his website if you want to take a look, though I am rather embarassed by my sending.  For some reason, I hadn’t plugged the paddle into the jack on the CC1 properly, and when I came back to Jason, the paddle went nuts and wasn’t sending what I wanted it to at all. I finally discovered the error, plugged it firmly into the jack and continued with the QSO. Jason for his part, (due also to nervousness at our historic QSO I’m guessing, just like me,) wished me 71 at the end of our brief exchange.  I like that! As I pointed out to him, 71 is like 72, but even better.  From now on, whenever I QSO with NT7S, I am going to sign off by wishing him 71. Perhaps that could become the default sign-off for any CC1 to CC1 QSO’s in the future? He also told me that my mess-up in sending due to not plugging the paddle in properly is one of those things that help create a narrative to remember these occasions with. Well, I guess so :-)

I don’t normally collect QSL cards, but some are special. This one from Jason is one of those in my collection that have great meaning.  In the early days of radio a QSL, instead of being seen as merely the final courtesy of a QSO, was the much-desired proof that a hard-worked for contact had taken place.  The early hobbyist would labor hard building his entire station, and spending many hours adjusting and tweaking in order to make contacts with other amateur stations. QSL’s were highly-prized pieces of proof that validated the work of the experimenter. I got some of that feeling on receiving this card from Jason -

This, in my opinion, is a QSL in the best time-honored tradition of amateur radio.  I’m running off to Fedex Kinko’s this morning to do a test-print of the custom QSL I’ve designed for my CC1 beta and hope to be spotting myself on QRPSpots later this week once I repair the final that I fried. I’m still not completely sure what I did, but it most likely had something to do with a stray clipped component lead or metal screwdriver :-)

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