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I
wasn’t particularly active on radio but it used to be a pleasure to switch on
and have a rag chew on 160M 80M & 40M (also on 5MHz when I received my
licence NoV) with other UK operators and do a little bit of DX on 20M &
17M, both SSB & PSK31 with the occasional contact on 10M & 6M during
sporadic E season. But
then sometime during 2007 there was an increase in general noise levels on
80M & 160M resulting in interference up to S9 across some parts of the
lower bands most of the time. I purchased an MFJ-1025 noise canceller, strung
out another length of wire down the garden and managed to “phase” out most of
the noise. During
2008 the noise got worse with a S9 level across all of 160M & 80M all of
the time. So I tried various types of “Quiet” antenna with the coaxial
bazooka type dipole on 80M being about the best, but the noise level was
still high and I was unable to hear anyone under an S6 and even signals at S9
were uncomfortable to listen to for prolonged periods. After
a holiday at a remote farmhouse where I used my portable radio for two weeks
in the Elan Valley in Central Wales, I returned home to realise that the
noise levels were worse than I remembered and I had obviously got use to it.
The noise free environment of Central Wales had spoilt me and I found myself
losing interest in HF operation from home to the point where I had thought of
selling my HF transceivers. But
during my holiday in Wales I remember having a QSO with a station that was
using a G5RV type antenna on transmit, but on receive a coaxial loop that was
just over a metre in diameter. They also had high noise levels and my signal
could hardly be heard on the G5RV but was perfectly audible on the loop. I
had seen and read about receive only loops in the past but never considered
them, as only one antenna for transmit and receive had been sufficient until
now. I read how successful others had been using them to receive on 160M
& 80M and the general opinion was that these loops have an excellent
ability to reject noise. I
decided to set about building my first loop antenna using the design in the
drawing below as a last attempt to keep using HF from home.
I
used RG6 Satfoam Satellite TV coax, each side of the “loop” is approx 750mm
and a 30mm section of the braid is removed opposite the feed point, the inner
remains connected. Three 100pf
capacitors were soldered in parallel and then a piece of 50 Ohm RG58 was
soldered to one side of the loop and capacitor. The
supporting frame is made from 25mm PVC conduit with a 8mm wooden diameter
dowel as the spreader.
When
I first connected it to my FT817 it was sitting on the ground and I could
null out noise from the neighbours property and hear stations clearly on 80M. I
left the loop sitting in the frame on the lawn for about a week and connected
it to the auxiliary port on my MFJ-1025 noise canceller. I turned the gain of
the auxiliary port up to maximum and the main antenna gain to minimum. This
enabled me to transmit through my 80M Bazooka antenna but receive on the
loop. The difference in reception was certainly noticeable,
while signals from stations were 1 – 3 S points down, the noise level was on
average S1 instead of S6 - 9 so stations that were S6 or less were now in the
clear on the loop. After a year or so
of listening to 80M with S9 noise it was a now a pleasure to come into the
shack switch on the radio and hear stations with little or no noise at all.
Even with the loop not much above ground level I was even able to hear
stations from as far away as North America on 80M very easily. I
realised that using HF was now going to be a pleasure once more, so I set
about researching my next project a loop that I could transmit on. |
