I came across the NA6O RFI DF Loop Antenna which helps to solve both the low-frequency hunting problem and the size problem in one swoop.  This is a receive-only antenna using a tuned magnetic loop with an inner Faraday coupling loop.  Magnetic loop antennas, as I’ve learned, have a very deep, very narrow null in their reception pattern.   When you’re edge-on to the source, it’s at its greatest intensity, and when you’re flat across it (ie, looking through the loop), you’re in the extremely narrow null.  As you might guess, a very deep, very narrow null is perfect for direction finding.  You just turn the antenna until you hear basically nothing, and then you’re looking through the loop either at the source, or at something 180 degrees away from the source.  As always, you need more than one bearing to get a location.  It’s a little bit more complicated than using a Yagi, because 0 degrees and 180 degrees are equal in intensity, but the process of triangulation is very similar- you just need to be mindful that your source may lie anywhere in the direction your loop is facing, both in front of you and behind you.

I don’t have a very well-stocked junk box, so I sourced some stacked Polyvaricon variable capacitors on Amazon (no financial interest) for the tuning.  I did have a 220pF capacitor in my junk box, and I soldered one leg across the two stators, and the other leg to the rotor.  This gave me an effective range of about 280 – 820 pF.  By the chart in the NA6O doc, this should provide a tuning range of about 3 MHz to about 6 MHz, and that’s exactly what it does.  I also bought some cheap RG174 coax for the coupling loop, and used some AWG 16 primary wire that I already had in my junk box for the magnetic loop.

The form for the magnetic loop is a 12-inch bamboo embroidery hoop from Michaels.  Thought not strictly required, I found it made it easier to keep the magnetic loop neat and well aligned.

Other parts came from Home Depot: a 1-gang stainless steel outlet box with 1/2-inch knockouts and a blank steel cover, a bag of plastic cable push-through plugs, and a bag of aluminum knock-out plugs.   I used the push-through plugs as bushings for the mag loop wires and coax to enter the outlet box.  In one knock-out plug, I drilled a 3/8-inch hole with a stepper bit to hold the BNC connector.  This I plugged into the bottom knock-out.

I had quite a lot of trouble working with the RG174 coax getting it soldered into a nice N4SPP Faraday loop (type 3 from his docs).  The next time I work on the coupling loop (if I need to again) I’ll use the same RG316 used by NA6O.  I found that it was easier to solder the far end to the exposed shield on the near end of the loop by stripping back about half an inch of the outer jacket, also stripping almost half an inch of the inner dielectric, exposing the center conductor for almost the entire length, cutting back the braid to about a quarter inch long, and the first soldering the braid to the center conductor.  Then I soldered just the center conductor to the exposed shield on the near end of the loop.  This required much less heat and much less time to solder than trying to solder braid-to-braid, and works just as well.

I drilled out holes in the cover plate to accommodate the Polyvaricon and its screws, hooked everything up, attached it to the SWR meter and got … nothing.  After some troubleshooting, I found that I’d missed a connection between the resonant loop, Polyvaricon, and the chassis.  Though the document describes connecting the stator to the chassis, I found that in the case of the Polyvaricon I had, I needed to connect the rotor pin to the chassis; the rotor pin is the one that connects to the knob with my Polyvaricon, and otherwise any time the rotor got close enough to the cover plate, I’d lose all sensitivity from the build.  So pay attention to how the connections work on your tuning capacitor, you may need to pick either the stator or the rotor depending on the configuration.

(Speaking of which, there’s no pinout included with the Polyvaricons I got on Amazon, so if you use these, fold out all the pins, then find the side with 5 pins, one of which is in the center halfway up.  With the knob side on the top, the two pins on the top row are your stators, and the pin in the center is the rotor.  You can snip all the other pins.)

I was never able to get anything remotely close to the SWR described in the original document, but I strongly suspect this comes down to two things: coupling loop size, and the super cheap coax that I bought – it may not even be properly 50 ohms.  I do, however, get a nice sharp dip in SWR where the antenna is resonant, and it’s more than good enough for DF work.  Some day when I run out of other things to do, it may be fun to play with the coupling loop size to try to get a better match.

I also sourced a used, inexpensive, battery-operated shortwave radio, a Tecsun PL-330, and a pigtail to convert between BNC and the 3.5mm TS jack the Tecsun uses for an external antenna.  As an unexpected bonus, the Tecsun PL-330 also gives you a dBu reading of the received signal, which is really handy when direction finding.  You don’t need to rely solely on your ears.   I found that it still seems to use the built-in antenna when an external antenna is connected, which can confuse things when hunting, so I removed the screw holding the telescoping whip and removed the whip antenna entirely.

In the first field test of the antenna, I was able to quickly discern one of my “usual suspect” poles, and when turning the antenna null in the direction of the pole, the null was deep enough that I could hear WWV clearly on 5 MHz.  Being only a degree or two off was enough to bring the powerline buzz back.  I also found that because the intensity with some of these poles is so high, going up in frequency some to 5 MHz (rather than my monitor frequency of 3.540 MHz) helped avoid overloading the shortwave receiver — at 3.5 MHz, the signal is so strong that it even gets picked up with the null facing the source, maybe due to some diffraction, maybe just the internal wiring of the radio still receiving such a strong signal regardless of the antenna.

In any case, if you have the time, this is a great device to have in your arsenal for direction-finding powerline RFI.  I think the total cost of the project, excluding some tools I bought myself to make a few things easier, came up well under $50.