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Re: [TowerTalk] balun testing

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Subject: Re: [TowerTalk] balun testing
From: Jim Lux <jimlux@earthlink.net>
Date: Wed, 25 Nov 2015 13:16:09 -0800
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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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