The Hoop Loop  antenna is remotely tuned by an inductor and capacitor. The inductor makes up part of a radio-frequency transformer. With Si4734-based radios, from 150-1710 kHz, the tuning capacitor is inside the radio. Above 1710 kHz, the radio switches to the whip antenna and the capacitor is set at a low, fixed value. The term "medium frequency" applies to signals up to 3000 kHz. There isn't much between 1710 and 3000 kHz that the chip can demodulate, as it lacks a BFO. But there are a few stations of interest. Three in particular caught my attention, as they are 50-kW broadcast stations in Australia's Northern Territory. They're on 2310 (Alice Springs), 2325 (Tennant Creek) and 2485 kHz (Katherine).

This week I've been experimenting with the Hoop Loop connected to a communications receiver, using a discrete, manually adjusted tuning capacitor. Last night I thought, 'Why not try this on the PL-380?' I'd removed the whip antenna from my PL-380, but it was easy to connect the tuned circuit in its place. I used a small additional secondary winding on the transformer. I was able to copy WWV on 2.5 MHz very well, so I went to bed with high hopes.

The Australian stations  are night-time only, and sunset in the NT is about 2 hours ahead of my sunrise right now. I was up at 5 a.m. local time, 0900Z. About 1000Z I started noticing a hint of signals on 2310 and 2325 kHz, and a noisy local buzz on 2485 kHz, so I gave up on that frequency. Tennant Creek on 2325 kHz never got loud enough to hear audio, but Alice Springs (10,300 miles from me) did, about 1020Z, 20 minutes before my sunrise. The audio wasn't strong, but I could hear a man speaking, and what sounded like five "chip" beeps every 5 minutes, with some variation at 1045Z and 1100Z, about 20 minutes after my sunrise. Shortly after 1100Z, the signal faded into the noise.

The tuned part of the transformer is 30 turns of #28 on an FT50-61 core, resonated with a 150-pF (max) variable capacitor (a 365-pF variable capacitor would work, but the smaller capacitor has less capacitance variation per degree of shaft rotation, making it easier to peak a signal). The Hoop Loop is matched through 7 turns bifilar-wound, and there's a 2-turn winding going to the whip-antenna terminal and ground inside the PL-380. I chose two turns to couple to the radio because it felt right, and it seems to be. Band noise increases when I connect the Hoop Loop, and can be peaked with the tuning capacitor. Now I only have to wait for the right propagation some morning!

The Hoop Loop is a very versatile antenna. Above 3 MHz, this type of loop becomes omnidirectional, but still can be quieter than a random-length wire. Up to 2.5 MHz it shows definite directivity and nulling. I was able to turn it to minimize lightning crashes from the west, even though Alice Springs is due west of me (bearing 268 degrees). I've now used the Hoop Loop to copy trans-oceanic signals on the PL-380 from 153 to 2310 kHz, simply by changing coils, and adding a variable capacitor for frequencies above 1710 kHz. As noted, with a discrete tuning capacitor it can be used with any receiver. (The Hoop Loop is a natural for the PL-360 too, as the matching transformers can plug right into the top of the radio. ) I'll be adding information to the Hoop Loop page over the next few days, but if you'd like more details, drop me an email.

73,
 
Jim, KR1S
http://kr1s. kearman.com/ 
http://qrp.kearman. com/