80 Meter antenna considerations for UK Contests
When I was first licensed I was anxious to get on
as many bands as I could, as quickly as possible. Then, as is still
often the case today, most operators turned to the G5RV antenna. To
be fair it is an excellent starting antenna and can be made to “work”
on most bands. By “Work” I mean it can be tuned with an ATU to
show 50 ohms to the radio but that does not necessarily mean it is
works well.
As with most multi-band antenna, the G5RV is a
compromise. You get to work lots of bands but it is also a noisy
antenna that requires a robust ATU to work. Sometimes the tuner built
into many modern rigs will not cope very with it on all bands.
Noise is the perennial enemy of the amateur radio
operator and, as I have already mentioned, the G5RV has a reputation
of being a noisy antenna. This is for two reasons;
It is a non-resonant antenna, especially on
80 meters, and
It has an element of vertical polarisation in
it.
Non-resonant antenna do not discriminate between
the signal you do want and the noise that you don't want. As such
they will quite happily receive that weak amateur signal in the 80
meter band that you are looking for but it will also pick up lots of
interference from other bands and domestic QRN without favour. A
resonant antenna however acts, not quite like a band pass filter, but
it does positively discriminate towards the signals that it is tuned
to receive. The benefits here are obvious and when you compare the
difference between the signal that you do want and the noise level
when using a G5RV and a resonant dipole you are likely to find an
improvement of about 2 “S” points when on the later.
Vertical antenna are renowned for working DX but
in the modern domestic environment they are also plagued by QRN from
just about every electrical source in the home. I recently used a
resonant half wave vertical antenna during the Club Field Day and got
some excellent results from it while in the electrically quite
environment of the field in Surfleet but, at home, it is virtually
unusable due to the S7 noise level it hears from surrounding
properties. The G5RV antenna has an element of vertical polarisation
in it especially on 80 meters when part of the ribbon feeder is used
as the antenna. Horizontally polarised dipoles are less prone to
domestic QRN than vertical antennas.
When working search and pounce in a Contest it is
also beneficial to have an antenna that has a reasonable usable
bandwidth so that the tuner does not have too much work to do when
searching the whole band for contacts. Unfortunately the best antenna
for bandwidth is the Doublet fed with open wire feeder, followed by
the full size dipole fed with coax cable. On 80 meters both of these
are quite a formidable length. Also there is the fact that for a
dipole to work most efficiently it must be at least half a wavelength
above ground level and horizontal. Not many of us can manage that
with our pocket handkerchief gardens!
Fortunately we can improve matters a little. I
have been working on making some 40 meter traps so that I can make an
80/40 meter trap dipole. This gives me the advantage of having an
antenna that is resonant on two bands and also, the traps serve to
shorten the overall length of the antenna to somewhere close to that
of a full size G5RV. There is a down side as the bandwidth is reduced
but if you cut and tune your antenna to the relevant part of the
band, the ATU in a modern transceiver should be able to cope with it.
I use an LDG AT100Pro2 Autotuner and that manages very well indeed.
Before the Contest I “train” the ATU by tuning every 15 k/c's or
so and the settings are stored in its memory for rapid recall later.
For the inter-G contacts that we require during UK
contesting we do not really need a good DX antenna with a low angle
of radiation. In fact what we require is an antenna with a radiation
pattern that is virtually straight up and down and here lack of
height is a distinct advantage. A half wave trap dipole with a centre
support of only 10 meters and the ends sloping down to about 2 meters
has the type of radiation pattern that we are looking for. Not so
good for DX but excellent for more local work.
but instead of making them up on the commonly used
drainpipe former, I just loosely wrap the coax length into a coil as
per the idea by W6NBC in an article in QST magazine available here;
http://w6nbc.com/articles/2011-02QSTtraps.pdf
This has the advantage of easier to make and waterproof and slightly
improved bandwidth.
When tuning the length of the antenna do so to the
CW end of the band which is about 3 feet longer than the phone
section. You then simply fold back the ends and cable tie them
closely to the rest of the wire until it is resonant in the phone
section. When the phone Contest is over you then simply un-clip the
ends and you are back on the CW part of the band again.
The resulting antenna that I have fitted into my
bungalow garden works very well for UK AFC, Sprint and CC contesting
I hope that this information encourages some of you to improve your
antenna system for 80 meters without breaking the Bank or moving to a
house with a bigger garden. I also hope that it encourages you to
help your Club by adding a few points during Contests.
73 de Jim G0HGH
SDARS
FISTS and AGCW
