by Peter Parker VK3YE - first appeared in Amateur
Radio, October 1997
Introduction
The trend towards smaller lot sizes and
inner-city living has made it harder for many amateurs to erect antennas,
particularly for the HF bands. This, along with the fear of causing
interference, has driven many to confine their operating to the VHF and UHF
bands and/or when away from home. This article shows that it is possible to
operate successfully from a confined space. I will concentrate on antennas for
two, ten and eighty metres, though it should be possible to apply the ideas
given to other bands.
Antennas for two metres
The helical antennas supplied with handheld
transceivers often perform poorly around the house. A better antenna is usually
needed. This can range from a simple ground plane or J-pole to a beam and
rotator. A difficulty often faced is finding a way of routing the feedline
inside without compromising security, particularly if your home is rented.
Indoor antennas can be quite effective if
you are near a repeater. Provided it can be placed near a window facing the
direction of interest, its performance should be acceptable. Indoor antennas
can take many forms. Some experimenters have successfully used self-adhesive copper
tape to build quad loops. In this case, the tape is simply stuck onto a window
pane or a piece of cardboard. Little space is required; a loop for two metres
is about 50 cm square, while one for 70 cm is less than 18 cm square. Depending
on how the loop is fed, it may radiate either horizontal or vertical
polarisation signals. More information appears in Reference One.
Another option is some sort of vertical
antenna. These go under various names, such as "J-poles", "Slim
Jim" and the like. Normally made out of metal tubing for outdoor use, the
keen experimenter should be able to use materials such as PVC tubing, coaxial
cable and 300 ohm TV ribbon to make an indoor version. Performance a little
over that provided by a half wave dipole should be obtainable.
Though a half-wave dipole offers less gain
than more elaborate antennas mentioned above, they are easier to build. Because
vertical polarisation is most common, a simple dipole can be hung vertically
behind a curtain or in a similar inconspicuous position. Or, when outside, hang
it from a tree branch for better coverage on VHF. Simple vertical antennas are
particularly useful when omnidirectional coverage of a local area is desired,
for example during club nets or local contests. Construction details of a
dipole for two metres made from coaxial cable appear in Novice Plus.
Antennas for ten metres
The existence of the 27 MHz CB band has been
a real boon for the antenna experimenter active on 28 MHz. Many CB antennas can
be modified to ten metres with very little work being required. For flat
dwellers, a yagi or quad is normally out of the question, though the
possibility of installing a VK2ABQ miniature beam antenna for a few decibels of
gain should not be discounted.
Those with sizeable balconies or a backyard
could try a horizontal dipole. The space required is about 5 metres. If fed
with open wire line, the dipole should also work on 21 MHz with the addition of
an antenna coupling unit.
Another option is a modified fibreglass CB
whip. This is effective for both local and overseas contacts and occupies very
little space. Longer whips give the best performance; a 1.8 metre whip is
suggested. Good height and a clear outlook are desirable. A ground system is
important. This can either be a metal roof, gutter, railing or one or two 2.5
metre long radials. Once installed, the whip is trimmed (using a hacksaw) to
make it resonant on 28 MHz. To avoid over-cutting, saw off small pieces at a
time (no more than 1 cm) and check the standing wave ratio (SWR) at the antenna
after each cut. If the antenna is too long, you will find that its SWR is
lowest at 28.1 MHz and gradually rises towards 28.6 MHz. Continue trimming the
antenna until the SWR is lowest around 28.4 MHz. It will rise either side of
this frequency but should be acceptably low over the whole Novice section of 10
metres. When you've finished, you will probably have sawn 8-10 centimetres off
the antenna.
Antennas for eighty metres
This is a challenging band for the amateur
with little space. Though a compact antenna is unlikely to yield regular DX
contacts, it should be possible in almost every case to enjoy fairly regular
QSOs up to about 1000 km when band conditions are quiet. There is always a
trade off between bandwidth and efficiency with small antennas. Always aim for
efficiency; it is better to be heard on one frequency than to be heard on none.
The use of 300 to 600 ohm open wire feedline
(instead of 50 ohm coaxial cable) can allow a dipole cut for one band to
operate on several higher frequency bands with the help of an antenna coupling
unit. Of greater interest to us, however, is the behaviour of such dipoles
below their normal resonant frequency. If your operating frequency is not much
less than an antenna's design frequency (eg transmitting on 3.6 MHz using a
tuned feeder dipole resonant at 5 MHz), such an antenna can be quite effective.
Tuned feeder dipoles much shorter than this do work but are inefficient. I
would suggest a dipole with a total length of at least 25 metres as a sensible
minimum for efficient operation on 80 metres.
Some operators use end fed wires. An
effective counterpoise is important, particularly if the wire is a quarter
wavelength (20 metres) long or less. Some people use the gutters on their house
for this. However, there is a risk that poor electrical contact between lengths
of guttering could act as crude rectifiers and cause interference-producing
harmonics to be radiated. Half wavelength-long end fed wires exhibit high
feedpoint impedances and are less dependent on an effective earth for correct
operation.
Vertical antennas are another possibility.
Again an extensive ground system is needed for good efficiency. This greatly
reduces their attractiveness to amateurs living in flats where access to any
ground, let alone a good one, is difficult. People with backyards too small for
a dipole may have sufficient space for a trapped vertical. Several 80 metre
operators known to the author have had good results with the commercially-made
verticals manufactured by Andy Coman.
A rotatable dipole can be formed from two
mobile whips. Such antennas have directivity and do not need extensive
grounding systems. A description of such an antenna appeared in Amateur
Radio last year. Bandwidth will be narrow, but experimentation with
remotely controlled relay switching schemes, to allow a choice of operating
frequencies, may prove fruitful.
A magnetic loop is perhaps the smallest
practical antenna for 80 metres and the only option for some. It consists of a
circle or square of metal tubing brought to resonance on the operating
frequency by a variable capacitor. A single loop can cover several bands. The
efficiency is lower than for larger antennas, but no ground system is needed
and the antenna does not have to be very high off the ground. Loop sizes as
small as 1.5 metres square are practical on eighty metres, though larger loops
will be more efficient.
An effective magnetic loop antenna will have
a thick, low-resistance element and good connections. The bandwidth will be
narrow at 3.5 MHz; 10 kHz is typical. A narrow bandwidth (or "high
Q") indicates that the antenna is efficient and resistive losses are low.
A design that has worked well for the author features elsewhere in Gateway.
Conclusion
This article has provided a few ideas for
those who may have thought that they had too little space to erect an antenna.
Provided that care is exercised in the construction and adjustment of the
antennas described here, all should yield acceptable results.
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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.