On Sep 4, 7:51*am, Cecil Moore wrote:
wrote:
Again, you seem to have the IEEE definitions of "differential"
and "common-mode" exactly reversed in your head.

Really? Do you believe the currents in a resonant 1/2 wave dipole are
common mode or differential mode?

Assume it is a 1/2WL wire in free space. Where
is the common reference?

There is none unless you choose to define one. You could go halfway
down the output choke and call it "common refrerence" or "isolated
ground" or whatever you want to call it. Since there usually no need
to reference it axcept perhaps for tutorial purposes, I wouldn't call
it anything. You could also reference it to earth ground but only if
you physically connected it to the aforementioned centertap.

I've heard antenna currents called "common-mode"
currents but neither Kraus nor Balanis call those
currents "common-mode". They are usually called
"antenna currents" (in phase, radiating) vs
"transmission line currents" (out of phase, non-
radiating).

In any case, differential currents on a transmission
line are 180 degrees out of phase and ideally, non-
radiating. Common-mode currents on a transmission
line are in phase and radiate. The currents in a
folded dipole are in phase and radiate.

The ideal transmission line is common mode and does not radiate
because the fields cancel as you said earlier. The dipole antenna is
ALSO common mode but the fileds do NOT cancel because the conductors
are physically 180 degrees apart from each other so they cannot
interfere with each other; instead the fields radiate into free space
rather than cancel each other out. It is rather simple really. It is
correct to call antenna currents "common mode currents". If the
currents on a transmission line are differential, how would (COULD)
they be converted to common mode currents on the antenna? We would
need a 180 degree phase shift somewhere. Answer: the currents on both
the transmission line and the antenna are common mode.

Within a ferrite toroid wired in a 1:1 current-choke-
balun configuration, common-mode current induces flux
in the toroid with virtually none from differential mode.
If the device is made out of turns of coax, the differential
currents never see the choking impedance.

Which explains why you should call it a balun and not a choke. If the
balun and source impedances match there should be no choking
impedance; maximum power trnasfer should occur.

--
73, Cecil *http://www.w5dxp.com