[TowerTalk] balun testing

[Michael Tope]

On 11/25/2015 11:34 AM, Jim Lux wrote:
> 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
>

The reactances would both be in series with the load resistance, so by 
including them in the loop, I think the common-mode voltage at the 
output of each balun secondary leg would be highest since in the 
worst-case (i.e. grounding one of the reactive elements where it 
connects to the 200 ohm load resistor) it would be the load current x 
the series impedance. In the worst case the series impedance would be 
the load resistance (200 ohms) plus the load reactance (j300 ohms). If 
that doesn't burn it up with rated power, then I think you have yourself 
a pretty good OCF antenna balun.

73, Mike W4EF