In message 8lehk.213$GI.77@trnddc05, Jerry
writes

"Frank" wrote in message
news:_Jchk.905$nu6.310@edtnps83...

"Frank" wrote in message
news:CDchk.904$nu6.498@edtnps83...
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

PS the single shunt stub of 86.8 degrees calculates to 11.7 - j 21 ohms,
when in parallel with a 50 ohm load (At 70 MHz).

Frank

Hi Frank

My calculations indicate that the length of "A" (50 ohm coax) should be
close to 48.6 degrees between two identical open stubs with a 50 ohm
termiantion on the 50 ohm line when I use your 11.7 -J21 impedance. So, my
recommendation stands. Try a little shorter "A" if impedance match at 70
MHz is the objective.


I never was an expert on Smith Chart calculations, so I would adopt a
somewhat less precise (and definitely more suck-it-and-see) approach.

(1) Make 'A' a quarterwave at the centre of the FM band. This can be
done by temporarily connecting it as a simple shunt stub, and snipping
for a notch centred at around 98MHz.

(2) Make up stub 'B1' by temporarily connecting it as a simple shunt
stub, and snipping for a notch centred at around 93MHz.

Now, at 70MHz, the frequency response will be rolling off into the 93MHz
notch, and the RLR will be getting rapidly worse. It will be a
capacitive mismatch, so it can be corrected by adding shunt inductance
in parallel with the stub. This can be an actual inductor, or a parallel
stub with its end short circuited.

[The advantage of a stub is that you don't need additional screening. It
can be tuned by pushing a shorting pin through the coax. The short can
later be made permanent.]

So,

(3) Tune the shunt inductor/stub for best match and lowest through loss
at 70MHz.

(4) Make up stub 'B2' by repeating (2) and (3), but for (say) 103MHz.

(5) Connect up the complete filter, with the 'matched' stubs separated
by the quarterwave 'A'.

You should now have a filter with minimal loss and a good match at
70MHz, but with two deep notches at 93 and 103MHz. If you are happy with
the results, finalize any temporary short circuits etc. If you are not
happy, you can still make a few tweaks to the tuning. If all else fails,
it costs virtually nothing to replace a bit of coax which is too short.
--
Ian