by Peter Parker VK3YE - first appeared in Amateur Radio, April 1999
Though antennas are things of great beauty
to radio enthusiasts, people who are not interested in radio often think
otherwise. However antennas are necessary for all types of on-air amateur
activity.
Many amateurs live in dwellings where, for
various reasons, it is important to keep a low profile. This may mean running
only moderate power to reduce interference risks, refraining from erecting tall
masts stacked with beams, and finding inconspicuous ways to bring antenna
feedlines inside. Too many amateurs when faced with these difficulties either
go off the air entirely or transmit only from their vehicle.
However, most of the problems mentioned
above can be resolved. It is possible to enjoy amateur radio without
neighbours or landlords knowing. And, given the current high sunspot activity
and good HF radio conditions, now is the ideal time to establish your own
low-profile amateur radio station.
This month we provide some ideas for
low-visibility outdoor HF antenna systems and suggest ways that they can be
concealed.
The author has used thin insulated wire
concealed in a timber fence. The
antenna consists of an end-fed wire 40 metres long as described in Novice
Notes June 1998. Most of the wire is threaded through the slats of a wooden
fence approximately 1.6 metres tall. Thin enamelled copper wire was used. This
antenna has been used on bands between 1.8 and 21 MHz.
The antenna's main advantages is its
multiband capability. It is also unlikely to be noticed by neighbours, spouses
or landlords.
Because of its low height, the antenna has
poor low-angle radiation. This makes the antenna quite effective for local
operation (up to 1000 km) but poor for longer distances. Nevertheless, the
author has worked Japan on 15 metres CW with two watts on this antenna, so DX
is still possible if conditions are good.
Where it is not possible to erect
stand-alone masts, trees are good ways of concealing and supporting antennas
and masts.
Some people will tell you that an antenna in
the clear is better than one surrounded by foliage. This may be true, but
either antenna is better than none at all! The use of a tree can provide height
that is impossible by other means.
The end-fed wire antenna shown in the
picture (not presented here) is about 12 metres long. It extends to the top of
the tree and is almost perfectly vertical. Because the tree is not tall enough
to support the full length, the top part of the wire is bent towards itself and
forms an acute angle at the top of the tree. A small pea-sized lead sinker is
attached to the free end to keep the antenna wire taut. Choose green or black
covered insulated wire to reduce visibility.
Though this antenna is shorter than the
fence-mounted wire described above, its height makes it more effective on bands
above 7 MHz than the longer antenna.
Trees can also be used to conceal antennas
other than end-fed wires. For example, a fixed-position two element quad with
wire elements for six or ten metres can provide an effective gain antenna that
does not attract attention. Alternatively, a single-element quad loop fed with
open wire feedlines can cover several bands if you have an antenna coupling
unit.
Balcony rails can appear attractive as
ground systems because of their sometimes considerable length. However, their
use is fraught with dangers.
The author's only attempt to use one was
greeted by a barrage of carrier signals heard while tuning across eighty
metres. These carriers were harmonics of local AM broadcast stations. The
harmonics originate not from the station transmitters themselves, but from bad
connections in the balcony rail, which act like diodes and cause harmonics to
be generated.
Using such a rail as part of an earth system
would be unwise - it would almost certainly generate TV interference (even
though your transmitter is clean and you have a low pass filter) and blow the
cover off your 'covert' transmitting activities.
If you do use a balcony rail (or other metal
structure) as an earth, make sure connections are good before proceeding. If
this is not possible, find an alternative earth for your antenna system.
The standard handbooks stress the importance
of having a station earth with short and stout connections to the equipment.
However, this is almost impossible to arrange for amateur shacks that are
several storeys up. Alternatives include the use of the plumbing system (if
there are copper pipes available) and radials a quarter wavelength long on the
bands of interest.
The author tried a (rather poor) attempt at
a station earth. It consists of an earth stake made of 12mm copper water pipe.
It is only 600 mm long - an earlier attempt at driving a longer stake into the
ground was not successful as it struck a rock and one did not wish to draw
attention by continuing to hammer the stake any deeper. Use stainless steel
clamp to attach the wire to the copper pipe. Alternatively, if you have access
to a large soldering iron or butane-powered torch, solder the connection
instead.
Ideally one would use a thick conductor such
as coaxial cable braid (leave the outer jacket on) for the lead from the earth
to the station. If appearances are a problem, other types of wire could be
used. In the author's installation, green and yellow insulated electrical earth
wire was used to make the earth system appear as part of the home electrical
system.
The simple ground stake as described above
will not be sufficient for good performance with some types of antennas. In
such cases, connecting radials to the earth stake will dramatically improve
performance. A small number of elevated radials is better than a larger number
of buried radials. However, elevated radials are unsightly, and the
experimenter may have to be satisfied with running a few radials along the
surface of the ground.
A thin wire running along the ground can be
almost invisible. This is especially the case if care is taken to choose the
colour of the insulation to match the colour of the ground.
A single 10 metre long radial was run from
the earth stake described above as an experiment on 40 metres. Improvements in
the strength of the transmitted signal ranged from nothing to 3 to 4 s-points
in some directions.
If the radial is run down the side of a
building or laneway it can go unnoticed. Some types of coaxial cable look like
plastic irrigation tubing, so it may be possible to run the radial beside a flower
bed without it attracting attention.
One major difficulty is getting the feedline
in. Gaps under doors can be small, and windows (especially those fitted with
flyscreens) do not always offer a solution. Some people get around the problem
by drilling small holes near the corners of doors or windows. These can be
filled in if you move out.
Open wire feeder is usually easier to get
inside than coaxial cable, particularly if you do not have metal-framed
windows. The picture shows home-made open wire feedline passed through a front
window. Security is not compromised as the window can still be locked with the
feedline in place. The use of white wire can sometimes improved the visual appeal
of installations.
Open wire feedline is less lossy than
coaxial cable. It also allows multiband operation with simple dipole and
loop-type antennas if you have an antenna coupling unit. The attempt to do the
same with coaxial cable would lead to quite high losses as a result of the
extreme impedance mismatches that would occur.
The above comments, have, I hope, given
several ideas for the amateur wishing to establish a low-profile amateur
station. With recent improvements to propagation conditions, there will never
be a better time to set up a station than right now!
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This page was produced by Peter Parker VK3YE parkerp@NOSPAMalphalink.com.au. Material may be copied for personal or non-profit use only.