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Re: Topband: Chokes, et. al. - what is the attenuation ?

To: "pfizenmayer" <pfizenmayer@q.com>, <topband@contesting.com>
Subject: Re: Topband: Chokes, et. al. - what is the attenuation ?
From: "Tom W8JI" <w8ji@w8ji.com>
Reply-to: Tom W8JI <w8ji@w8ji.com>
Date: Sat, 14 Jul 2012 16:12:04 -0400
List-post: <topband@contesting.com">mailto:topband@contesting.com>
> 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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