[Elecraft] Baluns - give me a kit!

[k6se@juno.com]

Jerry, K6III wrote:

"... but wouldn't it be better [using a balun] to at least get close to
the 50-ohm impedance that the K1/K2 want to see?"
==========
The primary purpose of a balun is to prevent common-mode currents on the
feedline which are due to either the mismatch between an unbalanced
feedline (coax) and a ballanced antenna (dipole, inv-vee, etc), or a
mismatch due to lack of symmetry in placement of the feedline.

One commonly-used secondary purpose of a balun is to use its step-up/down
ratio to match a high-Z balanced antenna to a low-Z unbalanced feedline. 
We see this most often when a folded dipole antenna (~300 ohms) is fed
with 75-ohm coax via a 4:1 balun (commonly made out of a half-wave loop
of coax).  Another common use is when a folded dipole driven element in a
VHF Yagi antenna (~200 ohms) is fed via a 4:1 balun with 50-ohm coax.

Baluns are most commonly used at the antenna's feedpoint, and the only
case I can think of where you might use a 4:1 balun at the transmitter
end of the feedline is where you have a folded dipole antenna fed with
300-ohm balanced line.  Elecraft transmitters use a 50-ohm unbalanced
output, so a short 50-ohm coax from your K1/K2 transmitter output to a
4:1 balun to the 300-ohm feeders would allow the transmitter to "see"
75-ohms impedance at the folded dipole's resonant frequency ( and a
resultant 1.5:1 SWR)

Other than the folded dipole fed with 300-ohm line case, it doesn't make
much sense to me to use a balun at the transmitter end of the feedline,
which I believe is your intended use.

If you are planning to use a doublet antenna on all bands (fed with
450-ohm balanced line),  a balun will NOT work in this application.

73, de Earl, K6SE