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).
On 22/11/2015 13:27, Michael Tope wrote:
BTW, it seems one could get a good feel for common-mode performance
using E73M's back-to-back method by alternately shorting one of the
two balanced nodes to ground. This is especially important when power
testing as it provides a measure of how much power the balun can
handle when subjected to maximum common mode stress.
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