On Jul 21, 11:37 am, wrote:
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

First, I apologize that I don't have time to look in detail at the
following ten postings in this thread; I did look over them quickly.
But two quick comments: First, I believe the "A" length should be
about 44 electrical degrees at 70MHz to get a 50 ohm match at 70MHz.
Second, if I were doing this and wanted to seriously attenuate
88-108MHz, I'd make a small L-C filter. At these frequencies, such a
filter should be much more compact than a coax stub filter for some
given performance. Of course, if there were specific frequencies I
wanted to get rid of, I'd put zeros on them. Coaxial cavities are
nice if you need very high Q around 100MHz, but stubs of even half-
inch coax are not particularly high Q--expect Qu about 400 at 70MHz.
A 47.7nH coil will give you your desired X=21 ohms at 70MHz, and you
can get that inductance with about three turns of #10 AWG (2.5mm),
spaced two wire diameters center to center, at about 9mm ID, and the Q
will be just about the same as the coax (in obviously a much smaller
space). But even better, a filter with max 1dB attenuation from
68-72MHz and min 55dB attenuation above 88Mhz (close to 100dB at
98MHz) can be built with four small coils and some C0G capacitors in a
shielded volume perhaps 4cm x 4cm x 15cm. (Schematic is available...)

Cheers,
Tom

On Jul 22, 10:41 am, K7ITM wrote:
....
wanted to get rid of, I'd put zeros on them. Coaxial cavities are
nice if you need very high Q around 100MHz, but stubs of even half-
inch coax are not particularly high Q--expect Qu about 400 at 70MHz.
....
I gave the Qu about 400 number based on 1/2" ID of the outer
conductor, but I guess 1/2" CellFlex is actually about 0.33" ID of the
outer. Based on that, or based on the listed attenuation specs for
the line in this frequency range, I'd have to revise my Qu estimate
down to no more than 300.

Cheers,
Tom