Home : Loop Antenna : HF APRS
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.
I also wasn’t happy with the receiver on
my Yaesu FT897, I had always found Yaesu receivers to be noisy at the best of
times and asked myself why I bought it as the IC-706MK2G I had previously was
much better and the IC-7000 I use in the car is far superior, but as it is
said, you live and learn.
So earlier this year (2009) I purchased an
Icom IC-7200 as I had always liked Icom for their quiet receivers. The IC-7200
was a big improvement over the FT897 but the general noise levels have now
crept up to 40M.
After using the IC-7200 at a remote
farmhouse for two weeks in the Elan Valley in Central Wales while on holiday, 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 IC-7200.
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 an 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 amazing.
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 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.
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