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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