> This thread has wandered away from the choke between amp and exciter or
> whatever(never would do this but that has been beat to death )to what
> choke impedances are and the measurement errors that can be encountered.
Which is probably beyond a general interest or benefit.
> One thing that has always bugged me is very very few of these discussions
> ever seem to consider what impedance the choke is working between. Seem to
> me if you dont know that it really does not matter what the choke
> impedance is at some frequency.
That is the best point yet! We arbitrarily assign a target impedance, with
no idea what the system common mode impedance is. Desktop common mode
impedance, which is a place where the choke really should never be used
anyway, is most likely very low.
Oddly enough if we have common mode on coaxial cables between the amp and
radio or other places on the desk, and we add a series shield impedance,
common mode can only shift more to an even worse place!!
We should never have it, but if we do, it is better left in short heavy
coaxial cable shields.
> Which leads me to ask - what is wrong with measuring the attenuation with
> a two port VNA with the choke simply in series between port 1 and port 2
> ?(of course calibrating out any cables if used) This is basically a "VI"
> measurement and seems to me to give valid measure of the atteuation if
> the load impedance presented to the choke is 50 ohms and the choke is
> driven from a 50 ohm source.
The problem with that, Hank, is the real world can be anything, but is
almost never likely to be 50 j0 ohms. This is a problem with all baluns and
chokes. The world is full of complex problems and variables, so there is no
single accurate answer.
For example, there is a limited range of conditions where an air-core choke
balun reduces common mode. Outside of that, the air core choke can increase
common mode. No one mentions that in articles about winding air-core chokes.
Our largest resource on common mode and baluns never looked once at common
mode system impedance, and right at the very start showed a gross
misunderstanding of the cause of unbalance. Virtually all that book looked
at was SWR and power under matched conditions with floating loads. We worry
about exact numbers being within 10%, while we assign numbers to systems
that are over the place. In the meantime, important basics are completely
overlooked or misunderstood.
73 Tom
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UR RST IS ... ... ..9 QSB QSB - hw? BK
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