In Part 1 we discussed construction of the antenna. This section will discuss computed vs. actual on-air performance
I calculated the performance of the loop using the AA5TB spreadsheet available at Steve's website:
The loop's performance is exactly what I was looking for in a compact package. The following graph shows the computed efficiency and bandwidth of the antenna:
The following graph highlights the antenna's Q and capacitor parameters (assuming 100W transmit power):
Also notice the yellow line in the second graph above, which is the voltage through the coax stub in our antenna. Loops are capable of tremendous voltages. If coax stubs are to be used to tune the loop, ensure the coax is something suited to high voltages. RG-8X is not going to cut it here, folks! RG213/U or RG8/U would be my minimum recommendation. Even with good coax stubs, I would still avoid more than about 50W using high duty-cycle digital modes or CW due to voltage through the stub. SAFETY NOTE: Due to the high voltages running through this antenna, mounting it at ground level can pose a hazard for severe RF burns and RF field exposure. Precautions should be taken to ensure personnel and pets are kept well away from the antenna during any operations.
Summarizing the performance graphs, I expected the antenna to operate well as an NVIS antenna on 40-60-80m bands, and also enjoy moderate performance for DX on some of the higher bands. Anecdotally, upon connecting the antenna to a transceiver I was pleased to find that receive performance was excellent. The loop was noticeably quieter than my 43' vertical in an urban setting. SNR's were excellent and pulling in signals, even outside the transmit bandwidth of the antenna was very easy. On 40m during the daytime, the antenna has been a tremendous NVIS performer. I have made contacts with ops in Prescott, east Mesa, southern Utah, Kingman...all previously unreachable with my 43' vertical. The antenna seems fairly omni-directional as an NVIS antenna...in the far-field I didn't notice any major gaps in coverage due to sharp antenna lobes. I did attempt to work the Flying Pigs Club Run For The Bacon Monthly QRP Contest on 40m last night to see how weak signal performance stacked up. Due to the low mounting height, for DX work the house to the east of the antenna gets in the way. I did make a California contact (approximately 300 miles away) on 40m running only 3W. This was NVIS propagation at its best, and helped validate the theory that path losses over NVIS easily accomodate QRP operation.
I also attempted some local NVIS work with Virgil on 40m with mixed results. Late one afternoon I tried Virgil on 40m while the sun was still up. His QTH is approximately 30 miles from mine as the crow flies; however, there are two 700 foot mountains in direct line of sight path between us. Virgil was able to copy me via LSB when I ran 50W or so. He was able to copy CW when I was running 5W. We tried an EasyPal picture running 75W (B Mode/2.2/QAM4/RS2), but it wasn't quite enough to get the signal to lock in. Overall, I'd call it mixed results, although once again I was impressed with the efficiency of CW for QRP NVIS ops.
This afternoon I hooked up my 17m stub and figured I'd try out the antenna for some DX. Although I expected the loop to be efficient on 17m, I also realized that the TX pattern would not resemble that of a small loop because the loop is too large in circumference to be considered an SMTL on 17m. Over a casual 2 hour operating period I made SSB contact with S58N in Slovenia (6000+ miles contact), and CW contact with HA9RT, ZP6CW, HR5/F2JD, JQ2IQW, and XE2AI. On 17m the antenna seemed directional in the plane of the loop, but I was unable to assess any side lobes from transmissions. Coupled with my fancy new Flex3000 radio (thanks to W6SDM!) the loop seems to be a more than adequate performer for higher band DX, as well as local ops.
After initial testing, this loop has met and exceeded every expectation I had for a small antenna that would comply with HOA restrictions. I still have work to do on the antenna. My vision for the finished product would be a loop with a cleaned up gamma match, as well as a stepper motor driven vacuum variable capacitor to replace the coax stubs, and a TV rotor on the base to allow me to tune the loop and spin it from inside the house. As such, I learned an enormous amount about antenna construction through this project and have already started a list of "things I would do better next time." Again, I want to emphasize the need for appropriate antenna safety for any who may consider ground-mounting theirs, as I did. Voltages can exceed 5kV running as little as 100 watts. Please keep people and pets away from this antenna while in use! Overall, I think the small magnetic transmitting loop is an excellent answer to those who live in HOA-governed communities, and I have been impressed with its RX performance, small footprint, and low cost. It has proven itself to do everything I expected and a little more. I can't wait to put the finishing touches on this one and start on my mobile HF loop!