"Ian Jackson" wrote in message
...
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

Hi Ian

I have had just enough experience with filter design to know that I dont
want to get involved with someone elses choice of which to use.
I am pretty sure that you are as "expert" with Smith Chart use as you care
to be. I make no claim to being a Smith Chart expert. But, I have alot
of experience using them.

I suspect that you know that impedances move along the lines (circles) of
constant R when pure reactance is added in series. And impedances move
along lines of constant conductance when reactance is added in shunt.
The chart with both lines of constant Resistance and constant Conductance
can be made by overlaying a second Smith Chart on the other with the line of
zero reactance aligned and the hi R flipped to overlay the low R of the
other chart.

Any impedance, with a REAL resistance will plot somewhere on the Smith
Chart. The outer boundrie of the Chart is marked in wavelengths. A line
drawn from the chart center thru the plotted impedance intersects the outer
boundry to identify a starting point. When that "load impedance " is seen
thru a length of lossles transmission line, it will plot somewhere on a
circle whoes center is the chart center and which passes thru the plotted
impedance.

In Frank's case, the where the load impedance is 11 -J21, the chart shows
that 0.135 lambda of line length is required to move the load impedance to
where it intersects the circle of constant Admittance, where the second stub
moves the impedance to a good match.

Jerry KD6JDJ