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 loop, 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.

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