On 23 Lug, 02:50, "Frank" wrote:

Francesco, You can build a notch filter with two stubs, and get a good
match, and rejection. The velocity factor of Cellflex 1/2" is 0.82, as per:http://www2.rfsworld.com/RFS_Edition...able_30-46.pdf
The lengths of the shunt, open stubs, are reasonably critical within +/-
1cm.

I have used a Genesys optimization program with the physical parameters of
the Cellflex coax. The open shunt stub lengths are 63 cm. The series
stub is 41.3 cm. The insertion loss at 70 MHz is 0.4 db. The return
loss
at 70 MHz is 20 db. Rejection over the whole FM band is 20 db.


ok, with cut and try arrived at similar results, shunts are 64 cm and
series is 47 cm, the antenna analyzer is happy, I'm not so happy
because I don't understand where's my original error.

Using the Smith Chart to design a filter is tricky, since you are dealing
with
multiple frequencies, and three variables.

well, yes, but actually the design started with the shorts at 98 MHz
which are easy, then the only unknown variable becomes the series stub
that gives a 50 ohm match at 70 MHz, I still don't get why the
matching section appear to be 3/8 wl on a smith chart and actually is
1/8 like if I inverted the sense of load/generator (yes, I worked with
admittances for shunt stubs).


Let me know if you would like to see my results, will be glad to e-mail them
as JPEGs.

sure, my email address is valid, thank you very much.

73
Francesco IZ8DWF