Tuesday, April 9, 2019

Antennas for confined spaces (Part 1) By Peter Parker VK3YE

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.

Picture of restricted space antenna

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 a 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 increasing sunspot activity and good HF radio conditions, now is the ideal time to establish your own low-profile amateur radio station.
I'll firstly give some points to think about when considering limited space antennas. Later there's some suggestions for the various HF and VHF bands.

Conceal, disguise, temporary or make small?

Concealment means place the antenna where it won't be seen, eg inside a roof or hidden in a tree.

Disguise may involve making the antenna look like something else, eg a flagpole or clothesline.

Another trick is to make your antenna temporary, so it's only up when you're on air, or at night. Collapsible squid poles are a real boon; you can go from 1 metre to 9 metres in a few seconds. Another possibility is a tilt-over mast, especially good if you have a long narrow yard. If you don't have the horizontal space, consider a partially retractable mast, made with timber, pulleys and a retractable centre portion.

 antennas may be so small not to be noticed, or even used indoors. Very small antennas can work well but note that out of (1) good performance, (2) small size, and (3) wide bandwidth, you can only have two of these in the one antenna, never three (despite claims sometimes made). In practice you should aim for good performance and small size while providing some tuning mechanism to allow the resonant frequency to be adjusted.

Aesthetics and design

Many antenna articles and handbooks are concerned with RF performance more than aesthetics. Authors may live on acreages where aesthetics or the need to acommodate close neighbours is less important.

An antenna that is effective in the open may perform much poorer when installed in a less obtrusive location just off the ground. These considerations may govern your choice of antenna. For instance, a well-built magnetic loop may work better than a very compromised trapped dipole or radial-less trapped vertical.

There's also trade-offs within the same family of antennas, such as horizontal dipoles. Consider the popular multi or fan dipole. It may have 3 or 4 wires coming off the feedpoint. It's fed with coax so has the convenience of not needing an antenna coupler. I put one up and once the interaction was sorted it worked OK.

But to the non-amateur all those wires looked like a spiderweb in the sky. So I took it down, replacing it with a tuned feeder dipole with just one wire. And, for good measure, giving it a higher feedpoint. The smaller number of wires made it less obtrusive, and dare I say it, more elegant. Similar comments apply to trapped dipoles, with the added complication that the weight of traps require thicker wire.

The same principles apply with beams. A smaller beam on a higher mast may be less obtrusive than a larger beam on a lower mast. And, provided feedline loss is low, performance way well be better.


Most landlords seldom visit their rental property, instead leaving it to an estate agent's property manager to do periodic inspections. My experience is that inspecting agents never look at the roof. Instead they look at general presentation and upkeep. That's basically that the front garden, kitchen, bathroom, carpets and walls are being kept clean and tidy. The average property manager supervises 100 or more properties and will not worry about or even notice discreet antennas unless other aspects of your tenancy, for instance rent payment and tidiness, are objectionable.

The squid pole

This features in many of my portable videos but is equally suitable for short term use at home. Key benefits include light weight, low cost (approx $50) and the ability to collapse when not in use. A squid pole will support light wire antennas but is not so good for heavy feedlines. This makes them most suitable for the end-fed type if antennas discussed later.

Antenna in fence

The author has used thin insulated wire concealed in a timber fence. The antenna consists of an end-fed wire 40 metres long. 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.

This is not what I would call a good antenna. Nevertheless it should still allow 80 and 40 metre contacts up to about 1000km with occasional DX on higher HF bands.

Antennas in the roof

If you're a top floor dweller, have a tiled roof and can get inside it, you may have hit the antenna jackpot. A nest of coax dipoles can work well in the attic, and, if you're in a 2 or 3 storey building, reasonable height can be achieved. In some cases you don't even need to drill holes to get the feedline inside; some flats and units have an exhaust hole above the stove down which feedlines can be dropped.

Interference and noise pick-up are potential problems. Nevertheless I've had better results with in-roof antennas than lower antennas strung through fence palings (described above).

Wire antennas through trees

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.
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

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. Having said that, balcony rails can be effective supports for vertical whip antennas, as used on the higher HF bands, VHF and UHF.

Ground stake

The standard handbooks stress the importance of having a station earth with short and stout connections to the equipment. However, this is 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.

Ground radial

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.

Getting feedline in

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.

Part 2, with antenna suggestions for various bands, will appear tomorrow.

PS: The items below may assist your experiments.  They are affiliate links meaning that I receive a small commission (at no extra cost to you) if you decide to purchase.


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