[TowerTalk] balun testing

[Jim Lux]

On 11/25/15 11:06 AM, Michael Tope wrote:
> 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.

I wonder if you even need the reactive component.  What about a 200 ohm 
resistor on one side and a short on the other?

I suppose with reactive components one can get unbalanced circulating 
currents that are higher..

I happen to have some NEC models here over a wide band. Let's consider a 
6 meter long dipole, but with 2 meters on one side and 4 on the other.

the short side is 13-205j (roughly)
The long side is 130+210j (roughly)


Shifting the feed over a bit to get 200 ohms..
short side 8.5-296j
long side 196+295j