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coax filter dilemma
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 |
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