>
> Of course, how much power gets dissipated depends a lot on the specific
> application. But, is there some sort of standard test condition that would
> be representative?
I dunno. There's no "typical" or "standard" common mode source and load
impedances, and they're harder for hams to calculate and control. I think
the best you can do is publish the impedance and the safe allowable average
power dissipation at given ambient temperatures: Then people can
put together an EZNEC model or measure the residual current with a
calibrated ammeter and short duty cycle, and calculate the dissipation and
compare if they want.
And if we're not willing to do calculations or in-situ measurements, every
once and a while there's may be some pathological common mode situation
where a choke blows up even if it's good enough for 99% of applications
involving legal power (or even 3dB or 6dB illegality).
If several people report that happening for a tower mounted yagi, that balun
was a bad design. But if it happens on other antennas, a post-mortem may
reveal that there was no reasonable choke design to keep that from
happening. For example, I've been working on a short hatted dipole kind of
like some offerings from Force 12, etc. (this is really short: 12 feet on
40m). Say you wanted to install that vertically and dress the coax down
through the lower tube and out the middle of the bottom cap hat where you
grounded it for lightning protection.
To make everything work OK, you install a "really good" choke there ... say
the 5k we've been bandying around.
I'm pretty sure you could blow it up with legal power no problem. I don't
remember exactly from the models, but I think 8k of choking impedance with a
fairly resistive choke like you might get from #43 was still dissipating a
couple dB in that service, and the really practical choke to use at that
point on an antenna would need to be I dunno... 30k? 50k? Not
impossible, maybe, with a string of 10k chokes, but well outside of what
people would tend to sell or recommend for your yagi.
That antenna is just too strong of a common mode source for chokes that
would take the common mode stress of a more typical antenna system with 10kW
applied.
Given the high Q, large stored energy systems we can practically build in
our backyards, it's not too hard to cook up a system that shows that "a lot
of common mode impedance" is all relative. I admit it's a pathological
situation. But sometimes that happens :-)
73
Dan
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