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Hi all,

I post this here since probably here lurks a good amount of smith
chart and transmission lines wizards.

I was trying to make a simple two stub filter with coaxial lines,
basicly it would serve as 88-108 MHz notch with a working frequency of
70 MHz.
The filter is like this:

length A
rig--|---------------|--ant
| |
b | | b
| |



The two "b" stubs are 64 cm cellflex 1/2 inch, with the antenna
analyzer they measure R=0 X=21 at 70 MHz, almost as they should per a
quick smith chart check, they should be 1/4 lambda at 98 MHz, open at
the end.
Now with two of these stubs the lenght A that gives a 50 ohm match
should be 0.378 lambda as per the smith chart calculation.

In real life with that value for A the only 50 ohm match (substituting
a dummy load for ant and the analyzer on the "rig" port) is around 36
MHz.

What doesn't seem to agree between theory and practice is that
measuring any "b" stub in parallel with the dummy load shows an
impedance of about R=11 and X=11, while on the smith chart this should
be R=11 and X=21 (obviously all at 70 MHz). Why it does measure right
alone and wrong with a coaxial "T" adapter and the dummy load in
parallel?
Of course in real life I'm assuming a Vf=0.88 for the cellflex cable
and checking measures with the analyzer.

What could be wrong?
Any hint is appreciated

73 de Frank IZ8DWF

Hi Frank

At 70 MHz, when the open stub presents a reactance of -J21 across the 50
ohm "dummy load", the resultant R-Jx is a serious mismatch to the 50 ohm "A"
line. Your Smith Chart calculations for the length of "A" may be incorrect.
Try making "A"
about 1/10th lambda.

Jerry KD6JDJ