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42 antennas for swls42 antennas for SWLs 1/3/04 11:55 am Page 42 If we forget all the theory and come down to hard practice, there are five things aboutan antenna that we can alter. Its length, height, direction, where we feed it and whatsort of wire we use. Graham Brown looks at how we can use this to our advantage.
thereof are ‘bad’ is based on diagrams assuming a perfect earth. Our radiate or receive best at right-angles to itself (Fig. 1a). If we have an antenna that U is a half-wave long at 3.5MHz (around Direction
39.6m) and we try to use it on 28MHz, we will see that To assess direction, you need to use a Great Circle Map based on the the favoured direction has changed; it now seems to UK (if you don’t have one of these, take a look at the Radio Active favour directions more towards the ends. So, if we put Bookshelf pages). The Great Circle projection makes a hash of up a half-wave on 3.5MHz and use it on all bands we shapes, but for our purposes it has the outstanding advantages that it will have one wavelength on 7MHz, two wavelengths gives true direction and distance. With a half-wave that has been put on 14MHz and four wavelengths on 28MHz. The up with its ends at the north and south, the main lobes will thus be preferred direction swings round progressively from east and west, plus or minus about 45 degrees. By and large this right angles at the lowest frequency to favouring the covers Europe, Asia, N. America and S. America. Australia and New ends at the highest (Figs. 1 & 2). That, at least, is what Zealand may, in fact, be heard over either the long or the short path depending on time of day. That leaves out Africa, but by changingbands or whatever we may be able to fill in the gaps. Not that there ismuch amateur activity from mainland Africa outside of ZS.
In practice, we live in a world of non-radio wires, cables and buildings;so our half-wave may have its two major lobes pushed to one side. We Feed Point
say it ‘squints’ (Fig. 1b). Worse, we cannot say that a squint affecting
Do you centre-feed or end-feed? Usually, the house is at one end of one band will have a predictable effect on another band. ‘suck it and the plot; alternatively your listening post may be in the garden shed at see’ is the motto, then - particularly if we are antenna-farming in a the other. Either way, you are stuck with end-feed. As SWLs, we can loft. So let’s look at the things we can alter and see how we can make end-feed happily enough, with some sort of tuning unit to turn whatever the wire just happens to be into what our receiver prefers,namely 50Ω. Indeed some SWLs disdain the use of ATUs, though I Height
There’s not a lot to say about the height of an antenna, save that the If we consider a half-wave dipole slung, say, between a couple of higher the better! This is simply because for long range we want our masts, then clearly the coaxial feeder hanging down from the middle antenna to favour signals from low angles. That’s not to say that a low tends to drag the middle part (which does most of the work of antenna won’t hear signals from faraway places - it usually will, but receiving) down towards the ground. We can only reduce the sag of they will be weaker and the more local signals (probably from Europe) such a coaxial-fed dipole by over-tightening the halyards, or cutting off will be louder. The argument that a quarter-wave and odd multiples the weight of the cable! Draw a Triangle of Forces and you will immediately see why, Fig. 3. Halyards that tight are an
invitation for something to break and dump all back on the ground. Usually that means dropping the mast to re-tie a
new halyard as well. Clearly, end-feeding has a lot going
for it, in practice, Fig. 4.
Obviously, the wire of a half-wave dipole supporting the
weight of umpteen metres of coaxial cable needs to be
pretty strong. On the other side of the coin, I have used
28swg. enamelled copper wire to make an end-fed Top
Band half-wave that lasted well and was almost invisible. It relied on the insulation properties of the nylon string - quite good enough, but the weight of a ceramic ‘egg’would have had it down in hours, while a coaxial cable’sweight would have made sure it broke while hoisting it. So- wire strength to suit the arrangement. I’m not too keen on pvc insulated wire, but if you’ve got some - use it! However, if you must make joints, solderthem and if they’re outside, paint them as well, otherwisethey may create noise later due to electrolytic corrosion, orworse, cause the apparent antenna length to change. Thinwire may be a bit lossy, but it’s not as lossy as no antennaat all! How about the half-wave centre-fed dipole on other bands? Fed with coaxial cable, no way! (You can, as a bodge, use a 7MHz dipole on 21MHz, though). Thus adipole used as a multiband antenna is fed with a twin feeder instead of coaxial cable. Twin feeder, though, 42 antennas for SWLs 1/3/04 11:55 am Page 43 Further Reading
At Radio Active we have a good range of suitable antenna books forthose who would like to learn more or try a little experimentation. Call0870 224 7830 for more details on any of the titles.
All About VHF Amateur Radio (AAVHF)
This title provides a readable and clearly illustrated introduction to
VHF amateur radio operation. Although American in origin, the
sections on propagation, antennas, modes and equipment are trulyuniversal and very helpful, including a section on wiring up thoseawkward plugs and sockets. £8.95 HF Antennas for all Locations (HFAFAL)
The author goes into great technical detail on how antennas work,
waves and field, feeders, propagation, designs and ideas. It also carriescomprehensive chapters dealing with making the antenna work,antenna construction and erection, as well as what kind of antenna helps solve this perplexing problem for the individual. Full of lots ofpractical advice and projects. £19.99 25 Simple Tropical and MW Bands Aerials (BP145)
This concise book describes how to build 25 simple and inexpensive antennas for operation on the medium wave broadcast band (550 -1600kHz) and on the 60, 75, 90 and 120m tropical bands. There are brings its own problems - the stuff breaks at the dipole feed point also designs for the 49m band. Other information included in the because the wind gives it such a thrashing; and it doesn’t survive all book gives the reader help with choosing which bands to try and what that long before ultra-violet ‘sees off’ the insulation. So, for twin feeder, make some proper open-wire line and/or take great care to besure that a gale can’t break off the connections up aloft. (Only 25 Simple Indoor and Window Aerials (BP136)
yesterday an SWL friend rang me to bewail that the gale had not only Many people find it difficult to use outdoor antennas, perhaps they snapped off his balanced feeder at the antenna but - worse - had live in flats or don’t have a garden. This doesn’t mean that you have wrapped it around the neighbour’s TV antenna! At the bottom, use a to forgo short wave listening as there are plenty of indoor antennas balanced ATU connection, and make the ATU also transform to 50Ω that can give good results. With a little effort and experimentation all unbalanced (coaxial) to offer the receiver maximum signal.
kinds of results are possible. This book shows how to build 25 indoorand window antennas that have given proven performance. £1.75 Lower Frequencies Yet!
If we have an end-fed half-wave for, say, 7MHz, we can make it match
More Out of Thin Air (MOOTA)
on 3.5MHz and even Top Band (1.8MHz) with our ATU. It will tend A compendium of antenna information and designs including the Slim to favour the same directions as on 7MHz, but for best results you Jim Vertical Antenna for 144MHz. ‘2BCX 16-element 144MHz must ‘go for broke’ in the way of earthing. For a centre-fed with beam, VHF/UHF Folded Collinear Antenna Array, a five-element coaxial or twin feeder we can go lower by joining each leg of the beam for 70MHz, a simple antenna tuning unit and an RF bridge.
feeder before they reach the ATU, put the joint to the ATU antenna There are articles on solar flares - outside broadcasts from the Sun terminal, and feed the result against earth as though it were a single wire. You may find it receives quite well all round the compass but ithas become somewhat unpredictable. 25 Simple Amateur Band Aerials (BP125)
Don’t forget a little theory: the difference between a dipole and an This book describes how to build 25 amateur band antennas what are isotropic radiator which is infinitely small, is a mere two-and-a-bit simple and inexpensive to construct and perform well. The designs decibels. Practically then we are talking, first, last and all the times start with the simple dipole and then proceed to beam, triangle and about reducing losses. If the antenna impedance is a couple of ohms, onto a mini-rhombic made from TV masts and wire. £1.95 and the earth impedance 100Ω, it doesn’t take a genius to work outthat most of the incoming signal will appear across the earth Wire Antenna Classics (WANTC)
impedance rather than the receiver input! Published by the ARRL and therefore American in origin, this bookcontains proven antenna designs from the pages of QST and other Summary
ARRL publications. Topics covered include: Dipoles, Loops, Wire A half-wave at the favoured band should be oriented for preference to Beams, Receiving Antennas, Collinear and Verically Polarised North-South, so that it fires East and West. However, in practice it will ‘squint’ a bit depending on your surroundings; higher in frequency, themain lobes will tend to slew round towards the ends of your wire. For Backyard Antennas (YANTS)
lower frequencies strapping the feeders produces useful and interesting In this book Peter Dodd explains how, by using a variety of simple if somewhat unpredictable results. With an end-fed antenna, work on techniques, it is possible to achieve very high performance from a the earthing side will be found useful.
compact antenna. He also explains how to make an antenna efficienton several bands at once. Covering end-fed, centre-fed, rotary beams, Abbreviations
loops, tuning units, VHF/UHF antennas and mast construction.
Receiving Antenna Handbook (RXANHB)
Your receiver is only as good as your Antenna! Joe Carr’s Receiving Antenna Handbook is a complete guide to high performance receiving antennas for longwave all the way up to the upper end of the shortwave spectrum. Topics discussed include: basic theory, signal propagation, construting a tuned antenna for below 30MHz and much
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