I don’t know how popular the delta match is among radio amateurs. I first came across this method of feeding a dipole during a recent holiday in Spain. Climbing to the top of a hill to take a look at a lighthouse, I noticed a horizontal dipole. The thing had massive insulators at each end and the wire itself was pretty thick. I reckon that the reason they used the delta match was because of the sheer weight of the aerial and the tension of the wire. Why cut the wire in half and use a centre insulator? It’s far easier to use one length of wire and feed the centre with the delta match. By the way, my rough estimation was that the dipole was cut for the 4mHz shipping band.
The diagram below shows a half wave dipole cut in the centre where the feed impedance is around 72 ohms. For single band use, this may be fed with 50 or 75 ohm coax. This is the age-old classic dipole which works extremely well.
Now take a look at the second diagram (below). This is the same half wave dipole, but with one major difference. The length of wire is still one half wavelength long at the operating frequency, but the centre isn’t cut. Now for the frightening bit. The impedance of the antenna is the resistance between any two points equidistant from the centre. So, a transmission line having characterstic impedance of, say, 600 ohms, may be used by locating two points on the antenna either side of the centre where the feed point impedance is equal to the transmission line impedance.
The diagram below shows 75 ohm twin feeder fanning out at the aerial end to transform the impedance to 600 ohms. You don’t have to use 75 ohm feeder. The aerial may be fed directly with 600 ohm, 300 ohm, or whatever. At a point along the fanning out section, between 75 and 600 ohms, you’ll find 300 ohms. This is a great way of matching a balanced line to an aerial. I’m trying to find the formulae for working out the length of the fanning out section and the distance between the lines. There must be a mathematical table available somewhere.
