VK3YE amateur radio pages

Return to VK3YE antenna projects

Antennas for confined spaces

Picture of restricted space antenna Picture of restricted space antenna

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 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.

Compact 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.

Landlords

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 antenna 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.

Ideas for compact antennas by band

80 metres

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 500km 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.

First we'll talk about dipoles.

Of interest to those with limited space, is how well they operate below their resonant frequency. As a rough rule of thumb, you can cut about a quarter off a dipole's length without severely compromising performance. In other words an 80 metre dipole (40 metres of wire) will work with just 30 metres (5 metres cut off each end). If you droop each end down by 5 metres then you'll be able to fit it into a 20 metre span. Like with a full-sized dipole, it is acceptable for the mast supporting the feeder to be higher than the masts or trees supporting the ends; this is known as an inverted-vee.

Of course such a shortened dipole will have a feedpoint impedance wildly off 50 - 70 ohms and present some reactance as well. So you won't be able to use coax cable with it. Instead you need 300 - 600 ohm open wire feedline and an antenna coupling unit. As well as allowing the abovementioned short antenna (often known as a G5RV) to operate on 80 metres, you'll get other bands as well. It really is a lot of antenna for the money, provided you have a yard long enough to accommodate a 20 metre wire span.

Use much shorter than 30 metres of wire and the antenna's efficiency drops enourmously and it might be time to think about other methods.

One possibility is a 40 metre dipole (20 metres of wire) and end loading coils for 80 metres. This will operate on both bands but the bandwidth on 80 metres will be narrow. Nevertheless I've had a good results on that band and can recommend it. The one I built used just the longest element of the VK5AH 4 Bander.

The dipole can be shrunk further and even made rotatable by mounting two mobile whips end to end. Such antennas have directivity and do not need extensive grounding systems. Bandwidth will be extremely narrow, but experimentation with remotely controlled relay switching schemes, to allow a choice of operating frequencies, may prove fruitful.

Some operators use end fed wires. That may suit your situation if your shack is nearer then end than the centre of where your wire will go. Disadvantages include 'RF in the shack' and increased interference pick-up. Hence this arrangement is most suited to low power or temporary operation. If the wire length is exactly 20 metres you may be able to get away without using an antenna coupler. Otherwise you will need one.

An effective counterpoise is also 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, especially for people with backyards too small for a dipole or G5RV. They're especially attractive if you have a large metal roof, eg a shed or carport, and the vertical can be raised to be mounted on or against it. A full sized quarter wave vertical on 80 metres is truly a monster (20 metres high) but traps can be used to greatly shorten it. Several 80 metre operators known to the author have had good results with the commercially-made verticals manufactured by Andy Coman.

Trapped verticals can disappoint as much as delight; they don't offer the sure-fire predictability of a dipole. Traps risk loss. The extensive radials needed for best efficiency are either hard work (if buried) or an eyesore (if raised). And, most importantly, the vertical's radiation pattern, though good for very short (groundwave) and very long distances (low angle, towards the horizon), is often deficient in the 50 - 500km range that is so critical to the bulk of contacts on 80 metres.

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 over a 2 or 3 : 1 frequency range. Thir efficiency is somewhat 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 2 metres square are practical on eighty metres, though larger loops will be more efficient.

Picture of magnetic loop

A good loop will be made of thick copper tubing and have solid connections. The bandwidth will be very narrow; maybe 10 kHz on 80 metres. Such narrow bandwidth, though it makes the antenna a pain to adjust when changing frequency, indicates the antenna is low loss, has high Q and is performing efficiently.

I'm a big fan of magnetic loops. They've always worked when built. Effort and expense in building them well pays off. Their directionality is especially good on receive, allowing local interference to be nulled out. Thousands of amateurs living in flats or units have access to HF, thanks to the magnetic loop.

40 metres

Comments and recommendations for 80 metres also apply for 40 metres. 40 metres offers propagation characteristics well suited to low power and small antennas.

However size requirements are relaxed and it is vastly easier to build an efficient, yet compact antenna for the band. For example, it will be easier to fit a 40 metre dipole in the roof space or build an efficient magnetic loop for the band (even 1 - 1.5 metre loops work suprisingly well on 7 MHz).

No matter how small your accommodation is, time and money spent in building a good compact antenna for 40 metres will not be wasted.

30 - 12 metres

As wavelengths get shorter the variety of possibilities increases. If you don't have the yard length to install a full sized dipole it may be possible to fit in one with its ends bent up to form a square. Several dipoles on the one crosspiece can provide multiband capability. Look up 'cobweb antenna' for designs. Alternatively flagpoles can be a quarter wavelength on some of these frequencies so be a workable antenna if loaded against buried radials. An antenna coupler at the bottom (preferably automatic or switched) could allow operation on several bands with the one element.

10 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 such as ground planes or verticals can be modified to ten metres with very little work being required.

If building from scratch, a wire ground plane, vertical dipole or delta loop are worth considering. A good thing about 10 metres is that their dimensions are starting to get small enough to be concealed in a tree.

If you have less room consider a modified fibreglass CB mobile whip mounted on a gutter, roof or balcony. This is effective for both local and overseas contacts and occupies very little space. Longer whips give the best performance; a 1.5 metre whip or longer is suggested. Good height and a clear outlook are desirable. The ground system 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 most active SSB section of 10 metres. When you've finished, you will probably have sawn 8-10 centimetres off the antenna.

A horizontal dipole is another possibility. This could either be nested with dipoles for lower bands (noting the risk of interaction) or supported on its own poles or trees five metres apart. Unfortunately extra wires or more supports can be ugly; a consideration for amateurs wishing to keep their station low-profile. Even though people think that open wire feedline is ugly, it's actually

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.

A yagi or quad is normally out of the question for unit dwellers. However if you have a small courtyard, the possibility of installing a VK2ABQ miniature beam antenna for a few decibels of gain should not be discounted.

Two metres and 70 centimetres

The helical antennas supplied with handheld transceivers often perform poorly around the house.

A good step up is a simple vertical dipole that can be hung behind a curtain or in a similar inconspicuous position. They are best near a window facing the direction of interest.

Or, when outside, hang it from a tree branch for better coverage on VHF. This should be sufficient to provide access to local repeaters and nearby simplex contacts. Simple vertical antennas are particularly useful when omnidirectional coverage of a local area is desired, for example during club nets or local contests.

For permanent use some sort of outdoor vertical antenna mounted on the balcony or roof is desirable. A small 1/4 or 5/8 ground plane can be fitted on top of a TV antenna (move the antenna down the mast) or on the toilet's exhaust pipe.

The main challenge will probably be to get the feedline into the house. RG58 is better than RG-213 (or thicker) in this regard, but is lossier, particularly on 70 centimetres. This is why feedline runs need to be kept short, with thought given to equipment and antenna placement.

What if gain is needed? It's a toss-up between a tall collinear vertical (for FM work) or a yagi. Both have their mounting problems. A tall vertical may be unwieldy for supports (eg TV antenna 'hockey sticks') designed for lower antennas. Whereas a vertical yagi requires a non-metallic mast (or at least its top section) to avoid interference to its radiation pattern.

Beams also introduce the problem of rotating. It may be possible to do this by hand if the support mast extends to the ground. Or, if you can't be bothered with a rotator, a 2 element vertical yagi could be a good compromise, especially if you're near the edge of a city and most stations are repeaters are in a favoured direction. This is because a 2 element yagi has a broad front lobe and a substantial rear response.

At least two antennas are needed if you wish to use both FM and SSB on VHF / UHF. FM vertical and SSB horizontal. Luckily horizontal beams for 2m and 70 cm look exactly like TV antennas so should attract fewer objections.

Conclusion

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!

Earlier versions of this article appeared in Amateur Radio October 1997 and April 1999 with updates since.

 

 

Disclosure: I receive a small commission from items purchased through links on this site.
Items were chosen for likely usefulness and a satisfaction rating of 4/5 or better.

 

Books by VK3YE

 

All material on this site
(c) Peter Parker VK3YE 1997 - 2017.

Material may not be reproduced
without permission.