A balun is a trasnformer with several 'windings'. How those windings are connected can affect the balanced/unbalanced thingy. And just like any trnasformer, the ratio of the number of 'turns' between primary and secondary can affect the impedance, change it x2, x3, x4, etc. The idea of that impedance transformation is to match the impedance of the feed line to the input impedance of the antenna. That's just like a one-way street. If you get it backwards, you got a really bad mismatch and high SWR. There really isn't any need for other than a 1:1 impedance transformer with the typical 1/2 wave dipole, since the 'classic' input impedance of a flat top dipole is something around 72 ohms give or take a little. A 2:1 balun (transformer) would make the feed line see something on the order of (2 x 72 = 144 ohms), or the other way around, about 36 ohms. About a 3:1 SWR in the first case, closer to a 2:1 in the second. A 1:1 balun would give something like 1.5:1 SWR. that 1:1 balun just gets closer to what's wanted. If you use that dipole for multiple bands, the impedance changes get really wild, and not really predictable. So unless there's a particular need for that impedance transformation ability, don't bother. One of those particular 'needs' is with a multi-element antenna such as a beam. When you add elements in fron/behind the driven element they lower the typical input impedance. The more elements, the lower it goes. Usually, you end up with an input impedance close to about 25 ohms. Then a 1:2 (backwards 2:1) balun certainly can be useful. (Remember that 'one-way' street thingy, don't get it backwards.) About the only time you even need to think about that is with a 'directly' fed beam. If that beam uses a 'match' of some kind, gamma, hair-pin, whatever, ~it~ takes care of that impedance matching/transformation. So a 1:1 balun would/could be useful.
Using a beam is also one of those times when it's nice to be able to predict the shape of the radiation patter, where the signal is pointed, right? So it also means that using a balun to keep that balanced/unbalanced 'slewing' of the radiation pattern to a minimum is a good idea too.
On the lower HF bands, under 20 meters, sort of, a dipole is seldom ever at a height that is going to allow any directionality. It's typically a sort of weird shaped, omnidirectional, 'lumpy', radiation pattern. 'Smoothing' those 'lumps' out isn't going to change the radiation pattern much, so why bother? That's why I say baluns aren't of much use on the HF bands. See?
- 'Doc
After seeing this post... posted, and all the spelling etc, mistakes I started to correct it. But there's so many, @#$$ with it, just read around the mistakes.
