Dave Richards AA7EE

September 21, 2016

Greater Harmonic Suppression and a Narrower RX Filter for the SST20

My scratch-build of the Wilderness Radio SST20 has been a great success. I’m not a very active operator but I’ve had 18 QSO’s on it so far, running it into a horizontal loaded dipole (a Buddipole) at 25 feet. The furthest was Hawaii at about 2350 miles distant, followed closely by Midland, PA at about 2200 miles away, as the crow flies. I previously thought the output power was 2.25W, but it looks as if it’s closer to 1.5W. I’ll explain why in this post.

Ever since completing it, I had been having uncertainties about the low-pass filter on the output. I understand that spurious emission requirements had used to be a little more lax for QRP transmitters, specifying that for transmitters under 5W, spurious emissions needed to be greater than 30dB below the level of the fundamental emission. This is no longer the case, as the requirement is the same for all HF transmitters in the amateur service, and is found in 97.307 –

“(d) For transmitters installed after January 1, 2003, the mean power of any spurious emission from a station transmitter or external RF power amplifier transmitting on a frequency below 30 MHz must be at least 43 dB below the mean power of the fundamental emission.”

Although I am not set up to make spectrum measurements on transmitters, a very rough test using the S-meter of my K2 indicated that my SST was emitting a 2nd harmonic that, at best, was only a few S-points below that of the main signal, It looked very much as if I wasn’t even fulfilling the older requirement of a minimum of 30dB suppression of spurious emissions. Digging around on the internet a bit, I found this paper on the GQRP site, giving practical values for low-pass filters for 50 ohm systems for all the HF amateur bands that I thought should satisfy current requirements. The suggested LPF for 20M was this –

As well as greatly improving harmonic suppression over the original SST filter, the use of this LPF in the receive path also improves image rejection as the image, with this receiver, is on the high side of the VXO.

 

Fitting it into the same space that the previous 2-stage LPF had occupied was a tough call, but I managed it. I did have to break one of my own rules of decorum, and allow one of the toroids to be slanted slightly, so that the layout looked less uniform (Oh, the horror!) but I was happy that it fitted into the space at all. The yellow T37-6 toroids of the new LPF are easily visible in this close-up view –

The first thing I noticed was that despite careful peaking of C28 (at the output of the TX mixer), the output power as measured on my OHR WM-2 wattmeter was now only 1.5W. Given that the 28MHz harmonic was previously very possibly only 2 or 3 S-units down on the main emission, I’m thinking that a significant portion of the 2.25W I was measuring at the output before was 2nd harmonic energy (and perhaps some unwanted TX mixer products too). The good news was that listening to the 28MHz 2nd harmonic on my K2 now revealed it to not even register S1, when the fundamental at 14Mhz was S9 +40dB! That is an excellent result, and one that falls well within FCC requirements. I observed a very similar result with the VXO signal at 18MHz. My K2 doesn’t cover the unwanted TX mixer product frequency of 21.932MHz (VXO freq + 3.932) – at least not with full sensitivity, and I don’t have a general coverage receiver with an S-meter unfortunately, so I can’t check the level of that unwanted emission.

The levels of the harmonics emitted from the antenna jack now easily comply with FCC Part 97. Due to the previous harmonic level I measured, I’m thinking that a significant level of 2nd harmonic is being delivered to the input of the PA and being amplified, before then being attenuated by the LPF. It would be preferable for that harmonic to be filtered out before the PA stage, and I may take a look at doing this in the future. In his SST40, JN3DMJ added an extra stage to the bandpass filter after the TX mixer to increase the level of spurious attenuation. You can see it here, under the heading “Upgrading of the filters”. With a better BPF in place, it may be possible to get an honest 2 – 2.5W from the final, with all of that power consisting of 14MHz energy.

As if to confirm to me that all was well, the little rig gave me a brief daytime QSO with KD3CA in Midland, PA – 2200 miles away. Not bad for 1.5W into a detuned dipole with an SWR of nearly 6:1!

The other thing that I took a look at was the crystal filter on the receiver. I had used the values of C6, C7, C8 and C9 suggested by Rich K7SZ on QRP-L for a wider response than even the values given in the manual for a wider filter. The stock values give a particularly narrow filter, and not all users will want that. I changed the values to those given in the manual as a mod for greater width. Does the way I’m explaining it make sense? I was going from extra wide to moderately wide, so to speak. Here are AF response curves taken by measuring the AF response of the entire rig at the headphone jack. The program used was Spectrogram.

Using K7SZ’ “extra wide” values of C6, C9 = 47pF, and C7, C8 = 120pF (the red vertical marker is at 400Hz – the sidetone I use) –

and using the “stock mod values” from the manual, for “regular wide response” – C6,C9 = 68pF and C7,C8 = 180pF (the red line represents an aggregate of all the peak values taken over a 16 second period, while the blue line is a response in one instant in time) –

Not sure if I’ll decide to go narrower with the filter, or leave it as it is. I need to experience a few more contest weekends before making any further decisions 🙂

On an entirely different tack, I have been using a new camera for the one photo in this post, and all the photos in the previous post about my SST build. It’s much lighter, and more compact than the camera I used for the photos in all the other posts on this blog. I’m still trying to decide whether the image quality is up to par for me. It’s a different lens, with a slightly wider focal length, a different sensor, and I’m using different software to process the images. I am not quite yet used to using this particular lens in order to “see” my subject the way I want, and so there are a lot of factors in deciding whether it’s going to cut the mustard for use in this blog. However, it’s a great and relatively inconspicuous camera for carrying around with me. Occasionally, I like to do what one might call street and candid photography, and it excels at that. This is one of the things I do when I’m not slaving over a hot soldering iron –

But, for the most part, I prefer taking photos of radios. They are very relaxed and compliant subjects, and don’t give me a hard time when I point a camera at them, unlike some members of the general public (gee, I wonder why they would do that).

I’ll get back on topic and talk about radio in the next post, I promise. In the meantime, as I am getting the “neither here nor there” figure of 1.5W of power out of my SST20, I turned the drive down to 1W, to make it a nice, even figure. I am looking forward to sending “PWR 1W” during QSO’s!

September 17, 2016

A Single Lever Paddle From QRP Guys

When building the SST for 20M, my plan was to kit it out with a paddle, battery, and an easily deployable antenna, and head for the hills. That’s still the plan. I don’t operate portable very often, preferring the comfort of the operating position in my small apartment, where I can make as many drinks and snacks as I want, and do it all with the company of my 3 kitties. Bliss! However, having just made a small, lightweight CW rig, I have to take it out in the field at least once in order to prove it’s mettle.

Currently, the only paddle I have is a Bencher, which is a bit too heavy and cumbersome to carry in my backpack for a portable set-up. There are some really neat portable paddles on the market, but I didn’t want to spend much, so settled on the idea of making one from PCB material, inspired by Wayne NB6M’s paddle, and KI6SN’s version, which was based on Wayne’s design. Two things happened to stop that idea in it’s tracks though. The first was that, nearing the end of building my SST, I was beginning to feel a bit lazy. Occasionally, when wading my way through a scratch-built project, I ponder how nice it would be to build a kit and give my brain a rest. At around the same time, I came across the website for The QRP Guys and realized I’d hit paydirt. They have a selection of small and low-priced kits for the QRP’er, including some small paddles made from PC board for very affordable prices. Perfect! The QRP Guys are Chuck Adams K7QO, Doug Hendricks KI6DS, Ken LoCasale WA4MNT, John Steven K5JS, and Dan Tayloe N7VE. Holy moly – that is some serious QRP starpower. I think we’d all be well advised to keep an eye on what these guys are up to.

QRP Guys ship out once a week on Wednesdays. With any small ham business such as this, where the owner/operators have many other things going on, setting expectations is an excellent idea. I decided on a single-lever paddle, and ordered it over the weekend. Later in the week, a small bag of parts arrived in the mail –

QRP Guys provide a scale for you to gauge how easy or difficult each of their kits is to build. On a scale of 1 to 5, with 5 being the most difficult, this paddle kit is rated as a 4. They do mention that some kits may also be rated as requiring what they term “expanded skills” – meaning, I assume, more difficult than 5. The rating of 4 for this kit makes sense. The PCB paddle parts need to be positioned fairly accurately. The way to do it is with a light tack solder in one point, re-adjusting until the exact positioning is reached, at which point you can commit with fully soldered joints. There are quite a few small screws, washers, and other small parts, so care, and a container to put all the small parts in are good ideas.

Here’s the final paddle. What a neat-looking little assembly –

A view of the underside –

This paddle is intended to be fixed to a panel, such as the side of a portable transceiver. I wanted it to be on a base, so decided to fabricate one from single-sided PCB material –

The cable is a cord from an old set of earbuds that came to an early end in the washing machine. It has a small molded 3.5mm stereo jack on one end, which is perfect for the task. It is held to the paddle base with a loop of twisted wire that threads through 2 holes in the base. At the point where it is secured, the cable was covered with 2 layers of shrink tubing. Luckily, the flexible wires in the earbud cord were insulated with heat-strippable enamel, so all that was necessary to remove the insulation was a generous gob of solder on the tip of the iron, and a few seconds, for the enamel to burn off –

You can’t see them, but there are 4 stick-on vinyl bumpers on the underside, purchased from the local Ace hardware store – the same type I used on the SST –

The copper is not lacquered, so if I take the same photo in a few months, it won’t look quite as shiny –

For size comparison, here’s the paddle with the SST20 and a pack of playing cards –

Despite the little stick-on feet under the base, I’ve found that the most comfortable way to send with this paddle is to hold it in my hand. This will work well for portable ops, when a suitable surface on which to place it might not be available. The lever is made from springy stainless steel. Doug Hendricks reminded me of an old tip for finding suitable flexible metal strips for making your own paddle, if you wish to do so. Just visit your local auto parts store and purchase a feeler gauge – the tool that is used for measuring spark plug gaps. It contains multiple flexible metal strips, of varying thicknesses (and degrees of springiness), so you can pick the exact one to suit your preference.

I experienced a small learning curve with this paddle. Firstly, I had never used a single lever paddle and secondly, I wasn’t used to the springiness of the lever, as most keys and paddles use stiff metal for the pivoting part. It doesn’t take long to get used to though. If you’re looking for a cheap and rugged paddle, this is a good value for the money. QRP Guys have both single lever and iambic paddles, with and without a base.

September 9, 2016

A Scratch-Build of N6KR and Wilderness Radio’s SST for 20M

Note – all links in this post open in a new browser window. It would be a good idea to clear your cache from time to time, to make sure your browser loads the latest version of this post. As an example, I just found an error on the schematic, and uploaded a newer, correct diagram.

I’ve been wanting to build an SST for a few years now. It’s a plucky little rig, with a lot of character. Designed by Wayne N6KR in the late 90’s, appearing as a full article in QRPp, and a kit sold by Wilderness Radio, it ignited the imaginations of a whole generation of builders for it’s combination of simplicity, performance, and willingness to accept modifications cheerfully. The review from Adventure Radio Society was quite positive. It is a VXO-based QRP CW transceiver, with a simple superhet receiver (SST = Simple Superhet Transceiver), and a TX that puts out up to 3W, depending on your choice of transistor in the final (it can be dialed down for battery-powered outings). It has very fast, clean QSK – so fast, in fact, that it feels as if I can hear the band all the time I’m sending (W6JL would approve). You actually listen to your own signal as you’re sending – there is no separately generated sidetone. The sidetone level does vary with the volume control, as opposed to being a fixed volume, but I only find this to be an issue when I have the AF gain all the way up, in which case I either quickly adjust the volume knob, or partially pull the earbuds away from my ears while sending. By the way, the sidetone on this little rig sounds really nice. It’s a feature which helps to make operating the SST an enjoyable experience.

QRP’ers loved their SST’s. There was a lively discussion about the minimalist rig on QRP-L, with builders reporting back on the frequency coverage and performance of their builds, with details of mods they were trying. The kit came with a light gauge unfinished aluminum enclosure. The raw-finish aluminum was a blank slate which invited many different creative solutions to the age-old question of how to show off your project. Some folk endowed theirs with professional-looking paint jobs, while others used dymo labels, or simply scrawled right onto the panel with a Sharpie® for that authentic home-brew look. All approaches worked admirably well. I saw one SST that had been painted with a US flag, and looked great. Some of them were taken out on the trail many times, and showed many knocks and scratches on the case which, of course, just made ’em look better still.

original

SST Kit Version – Image from Wilderness Radio

original1

SST Kit Version – Image from Wilderness Radio

JN3-bong;y modified his SST with 2 varacters for extra frequency coverage. He also added a speaker, and a spot button. Click on this photo to go to his site, and read more about his SST.

Koichi JN3DMJ modified his kit version of the SST40 with 2 varacters for extra frequency coverage. He also added a speaker, a spot button, and expanded the LPF for better filtering of the transmitted signal. Click on this photo to go to his site, and read more about his SST. He says that this SST is his favorite rig. He has made over 1300 QSO’s on it so far. Thank you for the permission to reproduce this photo and link to your site Koichi.

Recently, I decided it was time to build my own SST, only to find that I had missed the boat on a kit, as Wilderness Radio had discontinued it at some point in the recent past. I called QRP Bob on the phone, hoping that there would perhaps still be a board and/or enclosure available, but I was out of luck. In fact, it meant that I was in luck, as I would have to scratch-build one, and that’s a good thing.

There is plenty of documentation for this rig online. The initial write-up was in the Spring 1997 edition of QRPp, available from Chuck K7QO here. Note that the preceding link is a PDF of all 4 volumes from that year. The entire QRPp archive is also on Chuck’s site and accessible from the main page. Ken WA4MNT has a copy of the manual for the Wilderness SST kit on his site. Other essential documentation is the mods and information collected from QRP-L messages from 1997-2004, from Ken Larsen AL7FS, here as an HTML file, here as a text file, or here as a .doc file.

Mine differed just slightly from the original. Here’s my hand-drawn version of the schematic, reproduced here with kind permission of N6KR. Don’t rely just on the following schematic, as my drawing is a bit goofy. Best to refer to the original circuit diagram in the manual, and use mine to see how it differs –

 

Note – I do not recommend using the LPF comprised by C34, C35, C36, L2 and L3. Although it just met FCC specs at the time of release, it is very unlikely to provide enough filtering for this rig to meet current FCC rules regarding suppression of spurious emissions. For more satisfactory filtering, see my post here.

Component designations (e.g. C27, R11 etc) are the same as in the schematic in the Wilderness Radio manual. There are 3 parts in the above schematic that do not have such designations – they were added by me (the series capacitor and resistor from pin 5 of U3 to ground, and the 1N5817 diode in series with the 10-16V DC supply line).

Differences between the stock SST and mine are –

1)  Inclusion of a series  diode in the supply line for polarity protection. I did consider using a P-channel MOSFET so as to avoid the voltage drop, but decided to go with a Schottky barrier diode. Some diodes of this type have a big enough reverse leakage current such that they are not effective in this role, but not so the 1N5817. It’s forward voltage drop is about 0.34V in transmit. I still don’t like losing that much, but it’s an improvement on the higher drop of a regular silicon diode.

2) The use of trimcaps in the RX and TX oscillators in order to place the received signal in the center of the passband, and to put the TX signal on the same frequency as the RX signal. I’m still about 20Hz off due, I think, to touchy trimcaps, but it’s close enough – until I get the urge to tweak and adjust again 🙂

3) The use of 68pF capacitors for C19 and C20 instead of the 100pF values specified. With 100pF feedback caps, my VXO wouldn’t oscillate, so I swapped them for 68pF ones, and it sprang into life (thanks to LA3PNA for the help on that). It may well also have oscillated with 82pF caps, and that is an option if you want your frequency coverage to be a little lower.

4) An MRF237 was used for the final instead of a 2N3553. This substitution was suggested in the manual for higher output power, as the MRF237 has higher gain. If you want a cheaper and more modern alternative to the 2N3553, I’m thinking that a BD139 should work as a direct replacement. Joel KB6QVI just told me that W8DIZ has the 2SC5706 at 10 for $4, and I’m wondering how it would work in this application.

5) A Zobel network was added to the output of the LM386. Mine was unstable at high volume settings. A series capacitor and resistor from pin 5 to ground is very commonly used in these circuits, and the inclusion of these 2 parts tamed my instability immediately.

6) Alternate values of the capacitors in the crystal filter were used to widen out the response. The original values were reported to be giving a particularly narrow bandwidth of around 200 – 300Hz at the -6dB points. I wanted something a bit wider. There were several suggestions in the QRP-L archived discussions. K7SZ tried widening his SST20 out with the help of these suggestions, but it still wasn’t wide enough for him. He suggested the use of 47pF for C6 and C9, and 120pF for C7 and C8, which is what I used. Thanks Rich. A few builders went further, and implemented the ABX (adjustable bandwidth crystal filter) mod that was used in the Wilderness Radio version of the NorCal Sierra. EDIT – K7SZ notes in his ARRL book “Low Power Communication” that his mod brought the center frequency of the filter down too low for him on his SST30, so he ended up going back to the stock values. I have found that using the values of C7/C8 = 120pF and C6/C9 =  47pF that Rich suggested on QRP-L for his SST20, I set the sidetone at 400Hz ( a new thing of mine – I’m experimenting with lower than normal sidetone pitch), and the center of the filter passband was still about 20Hz lower, which I guess is pretty close. If you like higher sidetones though, you may be better off with one of the 2 sets of stock values in the SST manual. SECOND EDIT – I plan to tighten the response of the filter, by using the alternate values quoted in the manual. After using the SST for a while, I think my version is a little wide.

Although the SST didn’t come with a keyer, many users added their own, the Wilderness KC1 keyer and frequency readout being popular. At the time of writing, this is still available – the only kit that Wilderness still supplies. I decided that I wanted to build a keyer onto the same board as an integral part of my SST. I didn’t need much in the way of special features, my only 2 requirements of a keyer for this rig being that it will operate in iambic mode B, and that it has a speed pot – a feature I think of as essential. Changing speed on the fly during a QSO is tricky if the option is accessible only via menus, but a piece of cake if all you have to do is reach out and twist a knob. Perhaps I didn’t look hard enough, but the only freeware I found didn’t support a speed pot. I remembered how well the N0XAS Super PicoKeyer that Dar W9HZC had given me had worked, and a light went on in my head. Dale sells spare chips for his PicoKeyer Plus at $6 each. I purchased 4, used one in this rig, and saved the others for future projects. The manual on Dale’s site will give you the info on all the features of this keyer, and how to access them. The surrounding circuitry is simple (the genius is in the coding), so it was easy to incorporate into the SST –

The on-board keyer used a replacement PicoKeyer-Plus chip from Dale N0XAS

The on-board keyer used a replacement PicoKeyer-Plus chip from Dale N0XAS

The keyer uses a piezo transducer to announce the responses to command inputs made via the CMD pushbutton and the paddle. It would be possible to feed this audio into the AF amp of the SST so that it can be heard in the earphones, but I elected to fit a small piezo transducer on the edge of the board. I had intended to punch a small hole in the side of the case to make it easy to hear, but this appeared not to be necessary. Note that in the schematic, I have called it a piezo “buzzer”. It is actually a transducer, but allow me to explain. There are piezo buzzers available to which you apply a voltage, and the unit rewards you with a loud piercing tone, generated by an internal oscillator. Some of the units available are called piezo buzzers, but they don’t contain the audio oscillator – just a transducer, which is often sharply resonant at a specific audio frequency, to enhance the volume of the emitted tone. I bought a 5-pack of so-called “piezo buzzers” on eBay. They looked too small to contain an internal oscillator and I was correct – they consisted of just the transducer, which was exactly what was needed.

This build looked great when it started (as they all do 🙂 ). A nice, clean board, with nothing but potential. As projects progress, I tend to become more anxious that in a single unconsidered moment during a late night soldering session, the iron will slip, the odor of burning plastic will waft into my nostrils, and all the hard work will be undone in a careless fraction of a second. In truth, there aren’t that many errors that can’t be corrected, but this early shot of the board was the best it ever looked! As with all my projects, all the Manhattan pads were MePADS (for IC’s) and MeSQUARES (for everything else) from Rex at QRPMe

If you compare the above photo to later pictures, you’ll notice that I ended up changing the layout of the front panel controls.

In the next picture, the AF amp and the VXO have been constructed, as well as the 8V regulated supply line. Temporary DC and headphone jacks were also connected, so the circuit can be plugged in to see if it works. The shielded cable that connects to the tuning pot was installed, but left longer than needed to allow for the final install in the enclosure. Not too much of the circuit had been built at this point, but it was already possible to test the voltages at the input and output of the regulator, as well as ensuring that the LM386 made a nice, loud noise when the input terminals were touched with a screwdriver (a highly controlled and accurate test 🙂 ) The VXO was tuned in on a nearby receiver and tested for frequency coverage. With the 20M version, the VXO is in the 18MHz range, and by subtracting the IF of 3.932MHz from the highest and lowest frequencies it oscillates at, you can estimate the final coverage of the SST, and make adjustments at this stage if you wish. The discussions on QRP-L (which are linked earlier) contain a lot of info on tailoring the coverage, so I won’t repeat it all here, but your options involve using different varactors, connecting a second crystal in parallel with the VXO crystal, using different values of rubbering inductor, and adjusting the value of R5. I won’t explain how these all affect the frequency coverage and stability, as this is discussed at length in the QRP-L archive and also, to a certain extent, in the manual. There is plenty of homework reading to be done if you are thinking of building this rig!

Here’s another view, with the VXO in the foreground –

Suddenly, the product detector and BFO burst onto the scene. In the next shot, the 3.932MHz BFO crystal is the one closest to the camera, with the trimcap for centering the passband just behind it. I used 60pF trimmers, as that is what I had the greatest quantity of. Something a little smaller might have made the adjustment less touchy though. I’ll leave you to experiment, if you want to. Things are getting pretty exciting at this point, because when you a touch a wire or metal screwdriver screwdriver to pin 1 of the product detector IC U2, you hear honest-to-goodness atmospheric noise – a distinctly different sound from what you hear when touching the input of the AF amp IC. It’s instructive, not to mention really cool, to hear this progression in the sounds you hear, as you touch the inputs of stages closer and closer to the antenna, as the build progresses. If you have a signal generator, you can inject that into the circuit, and look at the output on a ‘scope. Don’t despair if you don’t have a full stable of test gear though – it’s important not to underestimate the power of touching and listening. Once you’ve done it a few times, you get used to knowing what sorts of things you should be hearing. See the curved red power wire that supplies 8V regulated to the BFO/product detector? You’ll notice in later photos that it was replaced with a different-shaped wire. It’s rarely possible to get everything right the first time you construct something, so one-off builds like this tend to morph somewhat as they progress. It’s OK to change things as you go along –

The next stage to construct was the crystal filter. Do you notice how, on the board for the kit version of the SST, the crystals for the filter were lined up with the short edges parallel to each other, so that the filter takes up a significant length of one side of the board? You usually see filters with the long edges of the crystals lined up parallel to each other. I don’t know why Wayne did it this way, but it did occur to me that with this physical configuration, the input and output of the filter are further apart than they would be with the more conventional placement pattern. Perhaps this was an attempt to decrease the possibility of filter blow-by? It seemed like a good idea, so I replicated it in my Manhattan copy. The crystals in the filter are not yet grounded in this next shot –

As far as matching the 3 crystals for the filter, I placed them into an oscillator circuit, and measured the frequency of oscillation. My cheap Chinese stand-alone frequency counter only had a resolution of 100Hz, but then I remembered that my K2 had a built-in counter with a 10Hz resolution, so I used that. I needed 5 x 3.932MHz crystals total – 3 for the filter, and the other 2 for the oscillators in the TX mixer and the BFO/product detector, so I picked the 5 that were closest in frequency. Out of that group of 5, I took the 3 closest and used them for the filter, while the other 2 were used for the local oscillators (but not the VXO, which required an 18MHz crystal).

To verify that the receiver is working, you’ll need to also build the antenna LPF, consisting of L1, L2, and associated parts. Without it, you won’t be able to peak the antenna input trimcap C1. Notice that if you touch the input of the crystal filter, the noise from the phones sounds much more restricted than when you touch the output of the filter. In fact, you can work your way back through the filter, with the rushing atmospheric noise becoming more and more restricted-sounding as you touch each stage of the filter with your metal screwdriver. These quick checks help to confirm that your project is pretty much on track. Adding the receive mixer means that the receiver is complete. After peaking C1 for maximum band noise, you should be able to receive off-air signals. Congratulations! If you substituted a trimcap for C10, you can also adjust it to place the received signal in the center of the passband, an adjustment that will depend on what pitch of sidetone you like to listen to.

My receiver didn’t work particularly well at first – I was getting very low audio out of it. One or two posts in the QRP-L archive made the same observation. I was beginning to talk myself into believing that the design was deficient in the audio department, and resolving to substitute a different audio chain, when I discovered that the coax which delivered the output of the VXO to the input of the RX mixer wasn’t properly soldered at the output of the VXO, resulting in low drive to the RX mixer. Re-soldering the joint solved the problem, and I can happily report that the audio output is more than adequate to drive a quality set of earbuds or a pair of reasonably sensitive headphones. If you attempt to drive a speaker, you will find that the level is only adequate for monitoring whether a frequency has activity or not, in a quiet room. That’s fine, as this was designed as a trail-friendly rig, with low current consumption in mind, and it certainly achieves that. VK3HN mentioned to me the idea of adding a lower noise AF chain designed to drive a speaker, and retaining the original AF output stage, feeding the inputs of both in parallel. The advantage of this would be that you’d retain the AGC action provided by the LED.

See the VXO in this next shot, with it’s 18MHz crystal? It has a total of just 10 parts, including the tuning potentiometer. I know that it represents old, well-established technology, but I feel that it still has it’s place in ham home-brew. Only 10 parts, and yet it has great stability and signal purity too. As long as you can deal with the fairly limited frequency range, a VXO is still a great choice as the frequency control in a simple rig –

This is always the point, when building a transceiver, where I slow down and spend some time playing with the receiver. I was dead chuffed, as we Brits say, that I had successfully built a little superhet receiver with a narrow crystal filter, that was sensitive, and sounded good.

But at some point, the momentum needs to be capitalized on before it is all gone, and so the build proceeded, with the addition of the transmit mixer. I also added the keying line (the green wires around the edge of the board) so that I could key the TX to see if it worked. If it did, then all that would be required would be to amplify the output of the TX mixer with the driver/buffer, and the PA. We were really getting close at this point! You’ll notice that there is a “channel” of space separating the crystal filter from the rest of the circuit. I did this for two reasons – firstly, as I thought it couldn’t hurt to physically separate the filter a little, to help prevent filter blow-by. Secondly, if there was excessive blow-by, it would give me enough space to erect a screen made from PCB material. C39, the 470uF AGC capacitor, is not present in this shot, nor in the later overhead notated view. I was planning on mounting it off the board, on the inside front panel, but eventually decided to mount it on the board. It ended up occupying the space between the AF amp and the edge of the board –

If you look carefully at these pictures, you may notice one or two components changing position slightly. As the build progresses, I will occasionally move a part or two in order to refine and improve the layout. I’m not going to point out which parts this applies to, as I don’t show these photos with the intention that you follow the layout closely. I started out by following the layout of the stages on the board from the SST manual but as the build progressed, realized I’d be able to move the position of the driver/buffer, thereby freeing up space for the on-board keyer, in one of the back corners of the board. Here’s another view of the board in the same state as in the above photo, with the receiver fully built, as well as the TX mixer. If you have a scope, you can measure the output of the VXO, which should be between 200 and 500mV RMS (that’s 0.565 – 1.414V peak – peak). You can also adjust C28 to peak the signal that will drive the buffer –

This was really the point at which I felt that I was home free. The TX/RX switching was working well, and the rig was putting out a small signal on the operating frequency in the 20M band. All that was left was to amplify it – and even if that didn’t work, I still had a cool little receiver and let’s face it – receivers are cooler than transmitters 🙂

The next shot shows the rig fully built, with the exception of the keyer, with the board temporarily mounted in an enclosure. I ended up changing the layout of the front panel controls, which necessitated the use of another enclosure. In both cases, I used the LMB Heeger 143 enclosure in plain aluminum finish (they also have it in black and grey). I used an MRF237 instead of the 2N3553 in the PA, in order to provide a bit more output, and you can see that transistor, wearing it’s heatsink. To the right of the PA transistor is the orange top of R12, the drive control, and to the right of that is the LT1252 driver/buffer stage. The RF input to the LT1252 is carried by a single wire underneath the board. There are only 2 wires under the board – the one just mentioned, and a length of RG174 coax connecting the output of the VXO to the input of the RX mixer –

A view of the completed board, with the N0XAS keyer in the rear left-hand corner (which is actually the rear right-hand corner, if you are looking at the board from the front panel end). I mounted the keyer chip in a machined socket. I run as many of the control cables as possible underneath, and drill holes in the board for them to enter. I think it looks neater that way –

This next view shows the layout. I got carried away and labeled a few too many parts. The side of the board that faces the front panel is the left edge. As with the previous overhead anotated view, C39, the AGC capacitor, is not shown here, though it did end up being mounted on the board. Remember those SMD SA602’s I was giving away for the price of postage a while back (courtesy of KV7L)? I hadn’t used any myself, until now. 3 of them are in this little rig –

Time to get this thing in a case. The LMB Heeger 143 is an ongoing favorite of mine. It measures 4″ x 4″ x 2″ high, and has 2 small lugs at the front and back of the cover that engage with the front and back panels to prevent them from being pushed in. This feature adds rigidity and stoutness. One of the things I don’t like about most clamshell cases is that the front and back panels can be flexed; not so with this model. It comes in grey and a sort of black wrinkle finish, if you don’t want the raw aluminum. All the pots are Alpha brand. The 3 small ones were $1.29 each from Tayda. The larger tuning pot is also an Alpha part, but is very slightly smoother in action, and I wanted to optimize the experience of tuning this rig. It is Alpha part # RV16AF-10-20R1-B10K-LA (I got it from Mouser).  There is a small dummy load plugged into the back in this next shot, because I was having fun using the rig as a code practice oscillator 🙂

This SST is quite a triumph for me, as it is the most complex project I have built so far with Rex’s MeSQUARES and MePADS.

The piezo transducer for the keyer was fixed to the edge of the board with a small spot of hot glue (on the high temperature setting of the hot glue gun, as it flows better) –

The keyer CMD pushbutton was a 22 cent cheapie from Tayda. These types are available with plastic and metal shafts. The ones with the metal shafts have a slightly smoother, more positive action. Get those ones. 4 vinyl bumpers from the local hardware store keep the SST from slip-sliding on my desk –

This enclosure is higher than the kit version, at 2″ high. I wasn’t initially planning on such a high case but the advantages are that it supports a larger tuning knob and, as you can see from the next shot, there is room for an internal battery pack, speaker, ATU or other add-on –

Here’s a view of the SST-20 upside-down and from the rear. From left to right – RF gain (rarely used), Antenna, DC power, and paddle –

I have a couple of spare covers for this enclosure, from projects that didn’t go as planned, and am thinking that it would be possible to have different covers with different accessories built in. For instance, one cover could have a speaker and extra AF amplifier, for operation at home. Another cover could contain a battery pack, for portable ops –

The red and yellow knobs look a bit garish, and I’m still getting used to them, but the thinking is that yellow = audio (AF gain and headphones), while red = keyer (speed pot and CMD button). It was also a way of using the cheap knobs I got from Tayda for 49¢ each 🙂

So how does it perform? Well, in 2 words – very well. I don’t operate a lot, but I do spend a lot of time listening. I’ve had 6 QSO’s so far with a horizontal loaded dipole (a Buddipole) at 25 feet above ground at my home QTH. 3 of them were with stations in Colorado, about 900 miles distant, one with KE5AKL who was doing a SOTA activation in NM, also 900 miles from me, and one with a mobile station in Hooks, Texas, who was running 25W. He was 1600 miles away as the crow flies. The other was with a local station. The receiver is as sensitive as you’d need a receiver to be, and there’s a good amount of opposite sideband suppression. I haven’t measured it, but you only hear the opposite side of the signal weakly when tuning through a very strong station. The RF gain only needs to be backed down when in the presence of very strong stations, as the use of an SA602 in the front end can cause it to crumble under such circumstances. I haven’t needed to use it yet, and from what I’ve read, it doesn’t need to be used very often – hence the reason it is on the back panel. My frequency coverage, with an MV209 varacter, is approximately 14055 – 14064KHz, a swing of 9KHz, which is about as much as you’d want when tuning with a 1-turn pot. Many users mounted a switch on the front panel to switch in another varacter (usually an MVAM108, which was also supplied with the kit) to extend the coverage downwards. With the MRF237 in the final, my WM-2 wattmeter indicates an output power of about 2.25W with 11.61V at the input to the rig (11.27V after the polarity protection diode). When supplied with 13.8V from a PSU, the power output was about 2.8W. I didn’t measure the current consumption on transmit, but on receive it is between 26 and 27mA. This is low, but somewhat higher than the 15-16mA quoted in the manual. The keyer consumes <1mA, so that isn’t the reason for the difference.

The AGC LED is a rather unique feature. I’ll let you read up about it in the manual but for the addition of a few extra parts, it will save your ears from the worst ravages of sudden loud signals – and the LED is fun to watch too 🙂 Most red LED’s have a forward voltage drop of about 1.7 – 1.8V. If you want to raise the AGC threshold, look for a red LED with a higher Vf – some of them go as high as 2.2V.

Although all the parts for this little rig are still available, a few of them are a bit harder to find than others. I purchased the LT1252 from Digi-Key – they have them in both through-hole and SMD versions. Chuck K7QO tipped me off to a supplier on eBay who was selling them in 10 packs. I couldn’t resist purchasing a pack. Thanks Chuck 🙂 W8DIZ has the MPN3700 PIN diodes, though see the next paragraph for a worthy substitute.  There are several different choices for the PA transistor. 2N3553’s and MRF237’s were available on eBay when I was looking. Try to buy legitimate parts from a reputable supplier (my gut feelings seem to serve me well in this regard). I’m thinking that a BD139 would work in this position too. All the crystals are still available from Digi-Key. The part numbers are the same as in the SST manual, with the exception of X1-X5 for the 30M version. The manual quotes the Digi-Key part # as X007-ND. It is, in fact, CTX007-ND. Perhaps it changed. It has, after all, been 19 years since the kit was introduced!

Even though this design is now quite old, I think it is still very relevant. An experienced home-brewer can build this into a fairly small case, and take it on the trail with a simple tuner and, say, an EFHW, for a compact and effective portable set-up. All of the parts are still available, though it would be great to see a partial redesign, utilizing more modern and widely available parts. I’m thinking of a redesign of the buffer/driver and PA stages. BS170’s are cheap, and 3 of them in parallel, in class E, could provide close to the full QRP gallon. The original SST had room in the case for a 9V lithium battery, and could be dialed down to lower output powers to help battery life. Nowadays, newer battery technologies make more power available in a light and small package, so running 4 or 5W while portable with a small rig like this is practical. Kenjia JH1PJL used an NPN transistor in his driver, instead of the LT1252 IC. He also used a 1N4004 instead of the MPN3700 PIN diode (you can see pictures of his SST scratch-build here). In fact, all the diodes in the 1N4001 – 1N4007 series have the relatively slow recovery time of 30µS, giving them PIN characteristics. Any of them should work fine in place of the MPN3700. If a 1N4000-series diode is good enough for RF switching in the Elecraft K2 (the 1N4007), then it’s good enough for us!

Here’s a brief video of it in action, with a surprise appearance by Jingles the blind kitty. I fed her just before starting the video, and forgot that her routine after eating is to jump up on the desk to relax and digest her meal for a few minutes. I’ll work on producing a slightly better video, though videos are not my strong point. Apologies for the slightly crackly audio. It’s a combination of operator error and a camera that was designed primarily for stills, and not video –

The yellow knob was making me uncomfortable. It has since been replaced with a black one, and I am feeling much calmer now 🙂

10 minutes later, and the red knob has now been changed for a solid and dependable black knob also. I finally feel that I know where I am in the world again 🙂

For the near future, the next tasks are to –
a) add an extra stage to the LPF between the antenna and the rig for greater harmonic suppression  and
b) tighten up the crystal filter a bit. I have decided that it’s just a little too wide 🙂

Both the above were done here.

 

Note – as of early Oct 2016, I received this very informative message from Walt K3ASW –

I have an actual SST20, with several mods.

If you’d like more VXO range, try adding a small value NP0 or C0G from the crystal-RFC junction to ground. Mine has a 3 pF and I get about 14042-14064. However, the VXO voltage to the two mixers (RX, TX) drops by about 1/4 to 1/3 over the lower 2-3 kHz. Also, I have a 1u8 RFC in series with the 5u6 below the crystal.

Currently, it has the K8IQY filter mod, but I’m likely to change that per my crystal measurements and modelling (via W7ZOI’s GPLA).

I added a JFET amplifier (MPF-102) between the crystal filter and product detector ‘602. This reduced the RX noise level quite a bit and I don’t have to run the AF gain as high. This simple circuit is from NA5N’s little handbook he published some years ago. If one does this mod, a dual JFET keying switch needs to be added between the 602
‘product detect and LM386; otherwise, the sidetone is way too loud. (My first version of the JFET switch circuit didn’t work, so I’ll have to try again. It is a tight fit! – on the   Wilderness Radio PWB  version.)

I’d also noticed the output of the TX mixer needs another filter section. I’m trying to figure out how to shoehorn it in on the existing PWB. (You won’t have that problem with your HB layout.)

I’ve worked quite a bit of DX on mine over the years. When I lived in a condo (before 2000), the antenna was a `98 foot horizontal loop in the attic above the fourth floor and I regularly worked into EU from here in MD.

Hope you like yours; it’s a neat little rig.

73 Walt K3ASW

– and a few days later. Walt sent these extra SST tips –

A couple other simple mods: Add a 0.033 or 0.039uF between the product detector pins 4 and 5; this reduces some of the high frequency noise. Also, a 10 or 15 uF from the phone jack + to ground will reduce the noise a bit more (22uF is too much – its starts attenuating the desired signal). The latter is a RC low pass filter for audio; the R is the AF gain control.
73 Walt K3ASW
Thanks for the helpful tips Walt!

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