[TowerTalk] OCF dipole and "good" balun
jim at audiosystemsgroup.com
Tue Oct 30 23:22:17 EST 2007
On Tue, 30 Oct 2007 22:55:31 -0500, K4SAV wrote:
>The balun you need for this antenna is a 4 to 1 current mode balun.
All the 4:1 baluns I know about are TRANSFORMERS. They pass all the
transmitter power through the core of balun (transformer). Maybe you
know something I don't know. :)
A current mode balun is a COMMON MODE CHOKE. It works by adding a
large lossy impedance in series with the common mode. It works best
if the impedance is very large and resistive (lossy). Choke baluns
work by reducing the common mode current to a very small value
(thanks to their high common mode impedance). If they don't, they're
much more likely to burn up.
Off-center fed antennas are inherently unbalanced, thus they will
generate considerable common mode voltage. To work, the choke balun
must have a VERY HIGH common mode impedance to reduce the current to
a sufficiently small value that it doesn't overheat and fail.
Choke baluns work best if they're wound with coax. Choke baluns
wound with parallel conductors put roughly 30% of the transmitter
power in the core as leakage flux. That power will cause heating in
a lossy core (and the choke won't work very well unless the core is
lossy, so that it can reduce the common mode current to near zero).
Low loss cores (#61) solve the heating problem for low power (100
watts), but their impedance is mostly inductive on the lower HF
bands, and the Q is so large that they're very narrow on the higher
bands. Inductive chokes are sensitive to line length.
Bottom line -- OCF antennas need very high resistance choke baluns
wound on coax if you want to take the feedline out of the equation.
Jim Brown K9YC
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