[TowerTalk] balun testing

[Jim Lux]

On 11/25/15 12:21 PM, Steve Hunt wrote:
> You must take account of the reactive components. If you have only a
> resistive load, no matter how much you unbalance it you can never
> generate a CM voltage across the balun higher than the differential-mode
> output voltage.

That would depend on whether the voltage is important beyond power
dissipation.  You could test breakdown voltage in another (more 
convenient) way, e.g. hi-pot type testing.
(or if you wanted a "realistic test load" for applications engineering 
advice, beyond the "testing" aspect)


It does occur to me that with a tuned circuit on the output, the 
circulating current could be higher than would be there if it was just a 
resistive load (like driving a waveguide resonant ring, or a compact loop).

A real antenna (that would be fed by a balun, not a compact loop) isn't 
particularly high Q device, though.. the resonant rise is a factor of 2 
or 3.

Maybe it would be more convenient, though.  If you were claiming a 1.5kW 
rating, it would probably be easier to use a 1.5kW amp and resonant rise 
to get the voltage and current up, than to use a 10kW amp and a 
resistive load.



>
> With the reactive components representative of an OCFD you get a
> "voltage multiplication" effect which can generate significantly higher
> voltages. Taking your impedance values:
>
> 1kW into the 204.5 Ohm load (8.5+196) would mean a differential mode
> balun output voltage of 452v rms. But with the reactive components
> present, the feedpoint would float upwards to 669v rms.
>
> That higher voltage represents a 2.2 times higher balun power
> dissipation than you could achieve with a completely unbalanced 200 Ohm
> resistive load.

Hmmm.. but is balun power dissipation proportional to voltage? (or 
voltage squared)

Or is it a "heat dissipated in the wires" which is more a current flow 
phenomenon.





  More extreme degrees of OCFD offset - for example the
> 20%/80% that is sometimes recommended - result in even higher voltages.
>
> All of this is calculable. If you know the unbalanced load impedances,
> you could test using a purely restive load but then make the necessary
> corrections to allow for the unbalanced case.
>
> Steve G3TXQ
>
>
>
>
>
> On 25/11/2015 19:34, Jim Lux wrote:
>>
>> 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
>>
>>
>>
>>
>>
>>
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