[TowerTalk] balun testing

[Jim Lux]

On 11/22/15 9:53 AM, Steve Hunt wrote:
> 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

I assume that one could measure "withstand voltage" and "withstand 
current".. so I suspect that the question isn't about "breakdown", but 
rather "thermal power handling"

So the question is really sort of two parts:
1) what's the loss in the balun (in whatever configuration)
2) Where is that heat generated, and does it get dissipated adequately

And, then, providing  way for a user to say "in configuration X (e.g.OCF 
dipole) this is the loss".

The symmetric back to back scheme deals with the dissipation, mostly, I 
assume from resistive losses in the coax. With symmetry, I'd assume that 
the flux in the core is fairly small.

It should be possible to figure out a test fixture which puts a lot of 
asymmetry in the system.  Whether it's a realistic representation of an 
actual antenna probably isn't as important as whether it's a good way to 
measure the thermal handling.

What about driving the balanced side of the balun with an unbalanced 
input: treat it like a transformer, drive one terminal, ground the 
other, load the unbalanced port with something suitable (which probably 
isn't 50 ohms).



>
>
>
> 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.
>>
>