Topband: S21 impedance measurements

[shristov]

On 7/14/2012 11:14 AM, pfizenmayer wrote:
 
> 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


If one wishes to look at this as a simple voltage divider,
then the end of the souce cable should be terminated in 50 Ohms,
assuming that the unknown impedance is substantially higher.

If there is a capacitance from the choke to the ground (true most of the time),
then the problem is not so simple anymore, and the circuit should be considered
to have a PI (ot T) topology, and be measured accordingly.

And if the choke length is an appreciable part of the wavelength (also true most of the time),
then the problem becomes even more complex...

In both cases it becomes impossible to uniquely define the "choke impedance".

Fortunately enough, the perfectly adequate choking action may still exist,
regardless of the impossibility to characterize it uniquely.
That is, choke operation becomes dependent on surrounding circuits.


73,

Sinisa  YT1NT, VE3EA