Introduction:
I have been doing some tinkering with loop aerials lately. The present example is approximately 12 ft of #12 solid wire on a square frame of white PVC pipe. Some experimentation a receiver noise bridge enabled a coupling loop to be configured so that a 50 ohm impedance is presented to the coax. A ceramic bodied 15-150 pF trimmer at the top of the loop permits tuning from ~6 MHz to ~15 MHz and is the only purchase made specifically for the project. For the latest experiments, the loop was suspended from a tree, borrowing the feeder for my long-wire. This disposition is somewhat close to the house, and not especially "out in the clear", but does allow the loop tuning to be trimmed, which is quite demanding on account of the sharp peak, and the need to use a long trimmer tool so as to minimise one's influence on the loop due to stray capacitance. The loop was tuned in situ, on transmit, using a small portable receiver to detect peak output.
Experiment 1: Receive
In this endeavour, I compared the loop against the "random wire dipole" that has become the de facto standard for 22m HiFER reception. Firstly, I noticed a higher (atmospheric) noise floor against the dipole. Not much man made "hash" that I could identify. Attention turned to the cluster of signals around 13,555.+++ kHz (though I did try the top and bottom of the bands as always). The dipole showed some activity from both SIW stations, and IIRC WM. See the
http://lwca.org/sitepage/part15/index.htm website for details. Switching to the loop brought traces that were obviously a continuation, but would be hard to have logged without the previous observations.
Intermission:
I decided to experiment with feeding PVC's signal to the loop, to compare the received signal at the receive dipole with the numbers I see from PVC's usual (separate) dipole. I'd played before (indoors) and seen S9 +30 dB, now, curiously, the signal was swinging (hint) from barely S9 to S9 +20 dB. Then I realised the swing had a fairly consistent period/time constant. I visited the loop and saw the issue immediately. With a little stabilisation (another piece of string) I arrested the twisting motion. Also, I arranged the loop so that in theory, it's pattern should match the direction of the dipole. Result: S9 +30 dB and steady, very similar to the dipole.
Return to receive:
The re-oriented loop showed improved sensitivity to the HiFERs that could also be seen with the dipole. It would be instructive to monitor both simultaneously, as an A/B comparison is at the mercy of the short timescale variations in propagation. However, the loop's performance is very encouraging, especially considering its compact size. It might be expected that better results could be obtained with improved siting. The dipole against which the comparison took place has it's apex in a tree at ~20 ft and the ends > 6 ft above ground. By contrast the loop center was perhaps 6 ft above ground, but further down the hill, and potentially affected by other nearby structures - quite a disadvantaged site in my opinion.
This work provides anecdotal evidence to support the enthusiasm for tuned loops that I have seen from a significant number of credible internet sources. Attention to a few details appears to show that these aerials are suitable for aspects of HiFER work. One aspect that has not been addressed is the desire/requirement to adjust tuning remotely for easy optimisation, and convenient use on a wider range of frequencies. This is under consideration at present.
PVC is transmitting from the loop this week, so reports from 13,558.400 would be very welcome.
Regards,
Ed
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