Antennas from scratch part 1 – by Phil Parkman G3MGQ

“if you can’t hear ’em, you can’t work ’em”

This is the first of an occasional series on antennas; what to choose and how they perform. The following is based on what I’ve found for myself in books* and in discussion with my RCE students.  I’m not an expert, so feel free to let me have your views and contributions on what worked for you.

Which design to use
You’ve just got your amateur radio licence and have been looking at the mags gently salivating at the descriptions of the rigs on sale, the cost of which are mostly way beyond your budget! Then you remember your instructor’s words about power isn’t everything. It doesn’t matter whether you’ve 10W, 50W or 400W output, “if you can’t hear ’em, you can’t work ’em”. A good aerial is the most important element (excuse the pun) in making that challenging contact. A top of the range rig is next to useless unless you’ve at least a half decent aerial system. A study of the Club’s ARRL Antenna Handbook (available on loan to members from Phil, G3MGQ) seems to indicate you need vast real estate, like Uwing’s South Fork ranch, with at least two 100ft towers so that you can string up a dipole or rhombic for 160m & 80m and an array of Yagis for the higher HF bands. OK that might be the ideal but the reality is most amateurs get out quite well with a far more modest setup.

So where do we start? The first thing is to decide is what you want to do with your radio. And you need to be realistic about your ambitions. You’re unlikely to be a top DX station working from your first floor flat with no garden and clauses in the lease that prohibit you putting anything outside. But you can work quite readily on the HF bands and do not have to be confined to VHF just because a dipole is only then small enough to pin up inside. Safety and appearance are important issues – you don’t want your neighbours or your family opposing your proposed development. The space available is always a significant factor, but there are ways around this! And finding a way to suit your operating requirements is part of the fun of being a radio amateur.

The Internet is rich with all sorts of possibilities, so there’s sure to be something that’ll do the job – not necessarily ideal but good enough to make the effort you made to get a licence worthwhile. Wire antennas are a popular choice for homebrew aerials, not least on grounds of flexibility, availability & cost. There even wire beams although most beams will use rigid elements of copper or aluminium tube or rod. Wires themselves are not structures, so do not need planning consent but their supporting structures, if permanent, may unless you’re making use of an existing structure for which planning consent has already been given or deemed; e.g. use of the chimney, existing TV aerial or a tree.

Another determining factor is the location of the shack, and the constraints that may give to the feeder and risk of EMC.

First you need to decide which bands to use by considering the propagation conditions at the times you’ll be operating for the sort of operating you want to do. For local working on HF, a high elevation main lobe is required to give a Near Vertical Incidence Sky-wave if the station you’re working is close but beyond your ground-wave, or the stations are beyond ‘line-of-sight’ so VHF isn’t an alternative. A dipole at under half a wavelength high will have predominantly NVIS radiation but a straight 80m dipole is longer than the garden most of us have. However there are ways aerials can be shortened without reducing their performance significantly.

In the present low in the sunspot cycle, (which seems to be going on for ever, apparently forgetting it’s supposed to be on an 11 year cycle), DX propagation on the higher HF bands is usually too poor to bother about. That means you might like to start with the lower HF bands, perhaps looking at 40m, 30m, 20m & possibly 17m & 15m. For DX you need a low radiating angle, preferably with the main lobe below 20 degrees elevation. For the lower HF bands, it is generally impractical to get a dipole high enough to get a significant proportion of the radiation at the low angles required for DX, so a vertical antenna might be a better option and is also omni-directional.

However, for good efficiency a ground mounted quarter wave vertical requires a very good earth; e.g. an extensive earth mat or system of radials, preferably buried just below the surface. (The more the better – a commercial broadcast station will have 120 or more radials!) A modestly elevated vertical, even 6ft above the ground, can be an effective DX antenna when fitted with as few as 4 quarter-wave  radials, which don’t touch the ground. Those elevated radials are good conductors and, being resonant, take the majority of the current instead of the poorly conducting earth beneath. The downside is that the aerial is relatively narrow band. A logical extension is the vertical dipole, which then does not need radials or an earth, but again the height required is impractical on the lower HF bands for the average amateur.

It is possible; the GAP Voyager, which covers the four amateur bands 160m to 20m without needing an ATU, uses a capacity hat to reduce its height to 45ft high but needs supporting by 3 guy wires at 2 levels, one set of which are 57ft long counterpoises but at a price ($525 +P&P from the USA!). The dipole & vertical are essentially single band aerials (but also resonant at the odd harmonics of the fundamental, so a 40m aerial will also work quite well on 15m albeit with a higher feed impedance). Bending the top of the vertical over into an inverted “L” gives a vertical radiating section with a horizontal radiating tail whilst still accommodating a full resonant length. As with a dipole, to fit the aerial in, up to an eighth of the top can be bent down or sideways without appreciable loss of signal as the current at that end is minimal.

(but the RF voltages are high so the end needs to be kept well clear of people!) The off-centre fed dipole, like the Carolina Windom, will also have vertical radiation from its feeder as the currents are unequal, unless a choke balun is fitted to prevent this. If tuned feeders are used on a dipole, then called a doublet, in theory there should be no radiation from the feeders, as the feed currents are equal, so its radiation pattern is like a dipole & horizontally polarised. The doublet with a top span of at least a quarter of a wavelength at the lowest frequency, is suitable for multi-band operation using an ATU to transform the impedance of the antenna presented by the feeder to match the 50 Ohm output of your transceiver.

Then there are magnetic loops which are small, typically about a tenth of a wavelength circumference, which are directional and tend to pickup less noise from the house wiring & appliances but are very high Q and therefore narrow band, necessitating constant retuning over a band by a capacitor subject to several kV of RF even for only 10W input. There are loops that are a full wavelength long of various shapes; deltas, quads, rhombics, some having several elements and being directional. And, of course, there are the Yagi antennas like the common domestic TV aerial.

We seem spoilt for choice, or more probably confused with the profusion of possibilities. In practice, the practicalities of what you need to achieve your goal soon reduce the options. So, in the following parts in this series, I’ll look in more detail at some of those possibilities to determine their potential and their limitations. Meanwhile, don’t despair. There will be at least one design that can do the job you want with perhaps only a small compromise on performance.

My references include:-

The ARRL Antenna Handbook

HF Antennas for All Locations, Les Moxon, G6XN

Practical Wire Antennas, 2nd Ed. By Ian Poole, G3YWX
(originally by John Heys, G3BDQ)

G-QRP Club Antenna Handbook
(extracts from SPRAT, the G-QRP Club quarterly journal)

And many on-line sources.

73 de Phil, G3MGQ from Vital Spark February 2010.

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