HFU HF Underground

Loggings => 22 Meter Band Beacons => Topic started by: Ed H on May 17, 2017, 1846 UTC

Title: PVC loop aerial experiments.
Post by: Ed H on May 17, 2017, 1846 UTC
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 (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.

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.


Title: Re: PVC loop aerial experiments.
Post by: tesla on May 20, 2017, 1320 UTC
I listened a few times, total time about  1 hour this week...no joy.
I only heard FRC a couple times so condx out this way have been poor.
Title: Re: PVC loop aerial experiments.
Post by: g0ifi on May 22, 2017, 1026 UTC
I use this homebrew one indoors on a first floor flat and get very good reception from LF to HF. As good as you could expect anyway. I'm sure it'd be fantastic outside. https://www.google.co.uk/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&cad=rja&uact=8&ved=0ahUKEwia74m-pIPUAhVJ8RQKHaPuAnAQjRwIBw&url=http%3A%2F%2Fwww.pa1m.nl%2Fpa1m%2Fsimple-active-receive-loop%2F&psig=AFQjCNELDgO3T_ix-UGlvE39-ZoLggBkrw&ust=1495535083606626
Title: Re: PVC loop aerial experiments.
Post by: tesla on May 25, 2017, 1216 UTC
I listened from 2200 - 2230 utc May 25...I thought I heard it but could not
be sure it wasn't imagination...so no joy as of yet. At least I tried...
FRC was again very weak with only a few fad ups at that time...
Title: Re: PVC loop aerial experiments.
Post by: Ed H on May 26, 2017, 1812 UTC
Thanks for keeping an ear on it! I may need to retune the loop. PVC is back on the dipole until I can attend to matters.

I do want to spend more time using it for listening as well. It is excellent, especially considering it's size!
Title: Re: PVC loop aerial experiments.
Post by: tesla on May 29, 2017, 1500 UTC
I'll be listening off an on today for PVC since FRC has been making the trip...
Title: Re: PVC loop aerial experiments.
Post by: ThElectriCat on May 29, 2017, 2157 UTC
 I have been listening to the area right around the PVC center frequency with my HP8594E, and any ability to hear it has been obscured by a loud signal centered on 13560.15 Kilocycles, it is pulse width modulated with pulses occurring every 23 ms or so, and it has very wide ( the better part of 1 Mc) sidebands, have any other listeners encountered this signal while trying to receive PVC? or is it just a local noise source?

I have a 27 meter long wire antenna, and live in humboldt county, CA
Title: Re: PVC loop aerial experiments.
Post by: Ed H on May 31, 2017, 2117 UTC
I have persistent signals in the 13,560 to 13,562 range, that I think comes along with the cable TV lines or other utilities. Wandering locally with a portable radio suggests a large number of things using the 22 m  ISM band, radiating intentionally or unintentionally over short ranges, usually within a quarter mile.

PVC is back on it's rooftop 1/4 wave dipole for the time being. I will retune the loop and reconnect again soon. Updates in this thread.