Francesco, before you go, may I ask what is the purpose of the filter?
Is it to prevent the local FM stations interfering with reception? Or is
it to stop any unexpected transmitter spurious signals from being
transmitted in the FM band?

It is to prevent desensing from strong local FM stations. Noise can go
up to S7 when beaming the local transmitters. An LC filter could be
easily made but that would need to be switched out when in TX (or made
to pass a few hundred watts), I just thougth a couple of stubs could
be easier to made. I think I was wrong.

73
Francesco IZ8DWF

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.

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

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

73,

Frank (VE6CB)