70cm Magnetic loop

Because UHF loop antennas have a substantially better efficiency as the rubber duck that usually comes with your handheld (or FT-817), I decided to make one for my Standard C-528 HT. I still had this HT laying around in the shack. After 15 years of service, it had some faults that I recently repaired, and now that it is working properly again, this was the ideal candidate to do loop experiments with.

Usually, you will find that the efficiency of a rubber duck is only around 10% (that's right, you will loose 90% of your precious power that is not being radiated). A UHF loop however could have an efficiency of around 95%.

The loop I ended up making is shown on the left. I used 6mm soft copper pipe to construct it. The loop inside diameter is 55mm and I used something round laying around the shack of that diameter to bend the pipe around. The connector used is a BNC-male chassis type. 2 Little screws hold 2 solder terminals salvaged of cinch connectors that were used to solder the copper pipe to. With that out of the way, I used my hacksaw and opened up the top of the loop - a 5mm gap in my case, big enough for the variable capacitor which I soldered across the gap. The capacitor is 0.5-5pF.

The loop is fed with a little 10mm diameter loop, constructed out of silver plated copper wire. One end soldered to the BNC inner pin, the other end to ground (Asymmetric inductive coupling).

Usually, adjustment is as easy as tuning the capacitor for maximum noise. In my case, using the Standard C-528 with just FM, that proved not that easy. A real noise maximum could not be established, so I had to adjust the loop another way. You could tune the radio into a know repeater frequency ie. and tune for maximum signal strength. I am lucky to have a Rohde & Schwarz service monitor in the shack, so I used that. With a little antenna on the output of the generator, I generated a signal on the desired frequency with just enough level so the HT a few inches away could pick it up. Then, using a non-metallic adjustment tool, I adjusted the loop for maximum signal.
This picture shows the adjustment setup, and also gives you a nice idea of the size of the loop sitting on top of the HT. It takes a little tweaking to get the loop adjusted as loops are very sensitive and suffer from hand effect. Move you hand away and the loop detunes a bit. Once you get the hang of it, it is easy though.

Test results
Now for the real test - how does the loop compare to the rubber duck ? I am impressed. I tuned into a 70cm repeater about 20 miles away from where I live, the HT just on the bench as shown in the picture. With the loop on, I could copy the repeater very comfortably above the noise, the S-meter actually showed 2 bars as well. With the HT in exactly the same position, I replaced the loop with the rubber duck that came with the HT. No signal, just noise !! As another test, I replaced the rubber duck with an MFJ-1812 - a dual band telescopic whip for handhelds. With this antenna, I could copy the repeater, but just above the noise floor and no S-meter reading.

2006-2007 - PA3CNJ