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)

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

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

Francesco, I will be e-mailing my results for your interest. We seem to
have arrived at the same conclusions.

I think I understand where you are confused. Initially, starting at the
center of the Smith Chart, the shunt open stub moves down, along the
constant conductance circle (0.2S). At 0.175 WL at 70 MHz (0.25
WL at 98 MHz) - 62 cm, the input impedance is 10 - j 20 ohms.
Next; the series section of transmission line moves clockwise along
the 10 ohm resistance circle. At 0.125 WL (70 MHz) - 44 cm, the impedance
reaches 10 + j 20. The final shunt section then moves the impedance
to the center of the Smith Chart along the 0.2 S circle.

73,

Frank (VE6CB)

"Frank" wrote in message
news:HMKhk.1277$nu6.889@edtnps83...
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

Francesco, I will be e-mailing my results for your interest. We seem to
have arrived at the same conclusions.

I think I understand where you are confused. Initially, starting at the
center of the Smith Chart, the shunt open stub moves down, along the
constant conductance circle (0.2S). At 0.175 WL at 70 MHz (0.25
WL at 98 MHz) - 62 cm, the input impedance is 10 - j 20 ohms.
Next; the series section of transmission line moves clockwise along
the 10 ohm resistance circle. At 0.125 WL (70 MHz) - 44 cm, the
impedance
reaches 10 + j 20. The final shunt section then moves the impedance
to the center of the Smith Chart along the 0.2 S circle.

73,

Frank (VE6CB)

Hi Frank

When describing the path on the Smith Chart from the "load" Z to the "rig"
Z, you write "along the 10 ohm resistance circle". I would have refered
to that circle as the circle of constant VSWR.

Jerry KD6JDJ

Hi Frank

When describing the path on the Smith Chart from the "load" Z to the
"rig" Z, you write "along the 10 ohm resistance circle". I would have
refered to that circle as the circle of constant VSWR.

Jerry KD6JDJ

Of course you are correct Jerry. I realized what I had done after I posted
the comments.

Frank, VE6CB

"Frank" wrote in message
news:RH1ik.1379$%b7.1159@edtnps82...
Hi Frank

When describing the path on the Smith Chart from the "load" Z to the
"rig" Z, you write "along the 10 ohm resistance circle". I would have
refered to that circle as the circle of constant VSWR.

Jerry KD6JDJ

Of course you are correct Jerry. I realized what I had done after I
posted
the comments.

Frank, VE6CB

Hi Frank

My reply was delayed because I had anticipated that someone might ask you
for clarification on Smith Chart use. I was sure you'd clear up the
confusion. I even thought I might learn something new. Smith Chart use is
so valuable to my thinking that I just couldnt let this thread finish
without checking in with you.

Jerry KD6JDJ