What determines which Balun you need? I have seen 1:1, 4:1, and 12:1 Balun's mentioned in a number of articles. Is there advantage to one over the other? Is there a formula to figure it out? Could someone please help?
Very basically, you need to know the input impedance of the antenna. Divide that input impedance by the feed line impedance and you have the ratio for the balun required. Since antenna input impedances can be almost anything, the most common way of determining which 'size' balun to use is to select the closest one to the impedance ratio between antenna/feed line. If it's close it should make things usable. It'll probably never be exactly 'right'.
- 'Doc
Huh? If you are using a typical T network tuner into balanced line the feedline impedance is not being matched. You are matching the antenna system inpedance. The actual feedline surge impedance only equals the antenna system impedance when the SWR is 1:1. Since most balanced wire fed dipoles are built to take advantage of low feedline loss at medium to high SWR levels, the feedline impedance will not be matched in almost all cases.
The "tuner" doesn't do anything to the feedline impedance; all it does is provide an impedance very close to 50 +/-j0 for the transmitter. If the "tuner" is sitting in the shack next to the transmitter, everything downstream from the "tuner" remains just as it was before.
I don't see anything at all about balanced feed lines in the original post. Or about tuners for that matter. So, if a (balanced to unbalanced) 'balun' is required then I would assume that coax is the feed line being used. Wouldn't you??
- 'Doc
I can't make that assumption because of so many people believing that winding a few turns of coax close to the feedpoint constitutes a "balun", when all it is, is a choke for common-mode current. It's still coaxial cable, and therefore UNbalanced, at the end that connects to the antenna.Pretty safe to assume unbalanced on one end and balanced on the other.
Usually someone asking for optimum ratios has been cruising the WWW and seeing the QRM on the subject.