TopBand: Re: Measuring impedances through a filter

[Dave_K9NX]

At 10:00 PM 6/24/98 +1000, P & V Nesbit wrote:
>
>At 09:42 23/06/1998 +0000, Tom W8JI wrote (re using a filter ahead of an
>impedance analyzer):
>
>>A bandpass filter acts like a transmission line of some 
>>characteristic Zo and some length that generally changes with 
>>frequency. It will not only  mess up all the readings, it will mess 
>>them up different amounts on different frequencies.
>
>Quite true, however this problem is easily resolved. Assuming the filter is
>home brew, feed the component values into an analysis program like ARRL
>Radio Designer, and try different terminations at the output end until you
>get the same impedance at the input end as what you measured. The
>termination at the output end will then be the same as the actual
>terminating impedance.
>
>Not elegant, but quick and effective.

Pete et all

The above approach will work (all though I'm not sure I'd call it quick) as
long as you "believe" you are dealing with a purely resistive load. If you
assume that the impeadnce being measured is actually a complex (R+jX) load,
then simulating it by pluging known values of R and L (or R and C) at the
other end of the filter is an entirely different "guessing" game.  A better
approach might be to calculate the " filtered" R+jX from the measured Z and
SWR reading then if you know the values of the componenets in the filter
run that through "backwards" through Radio Designer or some other circuit
simulation program to get the exact  R+jX, This elimintes the need to have
a wide range of R, L and C comp on hand to do the substitution and frees
you from having to make  measurements only at "resonanace".

Dave 
K9NX

--
FAQ on WWW:               http://www.contesting.com/topband.html
Submissions:              topband@contesting.com
Administrative requests:  topband-REQUEST@contesting.com
Problems:                 owner-topband@contesting.com