[TowerTalk] balun testing

[Michael Tope]

On 11/22/2015 11:26 AM, Jim Lux wrote:
> 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).
>
>

To take into account the feedpoint imbalance alluded to by G3TXQ, I 
suppose you could insert the lumped equivalent series Xc of the short 
leg of the antenna (from a NEC simulation) in series with one leg of the 
back-to-back balun connection and then put the lumped equivalent Xl of 
the long leg of the antenna in series with the other leg of the 
back-to-back balun connection. You might need to do some analysis to 
determine if the lumped inductor Q contributed significant loss (or 
check for inductor heating during power testing), but otherwise I don't 
see why this wouldn't be would be a pretty good simulation of the common 
mode effects of the unbalanced feed. You might still want to alternately 
short either leg of the secondary (i.e. 200 ohm side) of the 2nd balun 
to ground in order to force the common mode voltage at secondary of the 
1st balun to its respective maximums.

For power testing you would just connect the output of the 2nd balun to 
a high power dummy load and then look for heating in the 1st balun 
(again alternately shorting the two legs of the 2nd balun's 200 ohm 
balanced winding to maximize common mode stress).

What you would be doing is creating an antenna load simulator of sorts.

73, Mike W4EF..................