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.

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