Unfortunately that test doesn't subject the balun to the maximum CM
stress it might experience in a typical application - at worst it
subjects the balun to a CM voltage equal to the full differential-mode
voltage at the 200 Ohm point. However in an OCFD, for example, the CM
voltage could easily be as much as four times the differential mode
voltage appearing at the 200 Ohm feedpoint.
The reason is that the impedance looking into the two sides of the
dipole are individually reactive - capacitive on the short side and
inductive on the long side - even though the "composite" impedance at
the feedpoint is purely resistive. And those reactive paths can cause
the feedpoint to float to a very high CM voltage.
Steve G3TXQ
On 22/11/2015 13:27, Michael Tope wrote:
BTW, it seems one could get a good feel for common-mode performance
using E73M's back-to-back method by alternately shorting one of the
two balanced nodes to ground. This is especially important when power
testing as it provides a measure of how much power the balun can
handle when subjected to maximum common mode stress.
73, Mike W4EF............
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