Wednesday, April 10, 2019
Antennas for confined spaces (Part 2)
Yesterday I gave some tips on accommodating amateur antennas in confined spaces. Today I'll give some ideas for compact antennas by band.
PS: I have written five books on various facets of amateur radio. They are available in electronic or paperback form. Find out more here.
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. Their 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.
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.
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.
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.
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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