On Sep 2, 7:26*am, Cecil Moore wrote:
wrote:
Yes and I have to partially take back what I said; a balun CAN double
as a CM "choke" and a CM choke can double as a balun. If
one wishes to balance an unbalanced line with a CM choke, then the
impedance of the CM choke must match the source and the load, which
makes the CM choke a balun and no longer a choke :-)

Actually, a balun without a large choking impedance
is not very useful for ham antenna system applications.
The method that a W2DU balun uses to balance currents
at a dipole feedpoint *IS* nothing more than a large
choking impedance which discourages common-mode current
from flowing through the higher-the-better impedance.
I usually refer to the W2DU balun as a choke-balun.
--
73, Cecil *http://www.w5dxp.com

I disagree. There is no choking, there is merely a polarity and phase
shift between the "primary" and "secondary" of the 1:1 balun
"transformer".

The best way I can easily express this is to consider a 1:1 240V 60 Hz
isolation transformer (yes, the balun acts theoretically in the same
manner). At the unbalanced input you connect 240V on one terminal and
earthed neutral on the other terminal. Now consider the isolated
secondary each feeding an equal resistive load to the center tap of
the secondary (analagous to radiation resistance). Across each load
we see 120V, 180 degrees out of phase. This is a balanced
transmission. Correct, this is not RF but it illustrates what the
balun is supposed to be doing. There is NO choking function at all if
it is working properly. There cannot be a choke when impedances are
matched, only when they are significantly mismatched.