[TowerTalk] 4 port network analyzers for balun measurements

[Steve Hunt]

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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