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372 THE SHORT WAVE MAGAZ1NE September, 1983

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right at one end of the plot, then one wants a basically end-fed arrangement, and about all you can do is to work hard on the earth and shove up a long-wire with its preferred directions the way you want to aim your RF. If you have a house at the centre of the plot, then the whole rangę of centre-fed devices comes to you, starting from the centre-fed Zepp arrangement plus an ATU; and the variant of this known as the G5RV either fuli or half-sized. Or you can go the whole hog and ask for a 120-foot tower with a four-element beam for each band up there - but get the planning permission through before you put it up, and spend at least as much on making the neighbours like it!

If there are no outdoor aerials allowed at all, then you have to apply Iow cunning, plus the knowledge that any bit of metal will radiate, albeit unpredictably, if you can match it to the transmitter - and there is always a suitable ATU for the chap who is prepared to work on the problem. As witness, ZE3JO and is Artificial Leg Aerial, hanging from the shack roof and fed, it is recorded, by a piece of wire from the ATU wrapped round the big toe of the peg leg (Mai doesn’t load the one he’s wearing, of course!). Another chap we know used to use the bedsprings as a counterpoise against an end-fed wire. Other possibilities include an indoor dipole for one-band operation (for example, a Top Band half-wave dipole bent into the loft), and a mobile whip on a hinged mount which can be poked out of a window as a vertical. In the end it’s a matter of using your noddles!

Travelling-Wave Aerials

Up to now, our elements have been open-circuit at the ends, so there has been nowhere for the current to go but back; and we know that on a transmission linę that means standing waves- remem ber? Right, so what we’ve been doing in fact is to try and generate standing waves on the aerial while minimising them on the feeder. Now we come to the types where the end of the element is terminated in a resistor; for examplc the Rhombic shown in Fig. 3. Here we are looking at an aerial many wavelengths long on each leg, fed with balanced feeder, and giving a quite fascinatingly good gain and polar diagram. If you transpose the feeder and the terminating resistor, the polar diagram reverses; but if you just disconnect the resistor and feed it, the missing backward lobes on the polar diagram reappear at fuli strength so that the aerial has a big lobe down the linę of the aerial and another one straight back. What happens is this: if the resistor is fitted the RF floats down the aerial to the far end, creating the polar diagram we expect, and then the resistor absorbs all the power left so nonę can be reflected. If the resistor isn’t there, then the reflected RF sets off back to the stan, and generates its own polar diagram but facing the opposite way. Notice that by terminating the Rhombic, despite its enormous gain, you arcn’t using morę than about half the transmitter output power!

Another aerial of this generał type, which is much beloved of Top Band addicts, is the Beverage. Here we are again talking about a long, long wire (several wavelengths long) pointed in the direction from which we want to hear the signals, and supported at intervals of, for Top Band, no morę than twenty feet, and preferably ten feet, for between one and three wavelengths distance - five hundred to 1500 feet long roughly. The terminating resistance will be of the order of 200-400 ohms and could be a non-inductive variable resistor of carbon, or better, cermet type. To set it up, look for a signal from the backward direction, and tune the resistor for least signal. The Beverage is normally reckoned to be at its upper limit at 2 MHz, but there are recorded cases of them being

“. . . one of my other interests is Egypiology. . . . ”

successful at 3.5 or 7 MHz. And, of course, it is a receiving aerial, working best over poor ground, but reąuiring a good earth at the recciver end. You will find the signal levels are down a bit, but the QRM level and the static is down even morę!

Conclusions

We have talked about aerials and hopefully whetted your interest. If so, go to some other texts: the RSGB Radio Communication Handbook, the ARRL Radio Amateur's Handbook, the ARRL Antenna Book, and then come back to G6XN’s HF Antennas for All Locations, published by RSGB, and read him-he will make you think and question all the assumptions you have been making. Then question all of his, too . . . and out of it all will come understanding in your own terms, which is what matters.

Next time, we will have another change of tack. Till then, cheerio and good DX!

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