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Ian Jackson[_2_] · July 23rd 08, 08:58 AM posted to rec.radio.amateur.antenna

In message
,
writes
On 22 Lug, 21:49, Ian Jackson
wrote:
In message
,
writes

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.

No, you ARE correct. For non-experts like me, simple stub filters are
often very easy to make. This is especially true if you need to pass
only one frequency, and reject another. The one thing that you must do
at the 'pass' frequency is to tune out the unwanted reactance of the
rejection stub by adding a shunt inductor (where the 'stop' frequency is
on the HF side of the pass) or a shunt capacitor (where the 'stop'
frequency is on the LF side of the pass).

It helps if you should have (or have assess to) some test equipment
which shows you a swept frequency response and impedance match, but you
can also tune them using individual frequency measurements.

One great advantage of coax stub filters is that, unlike LC filters, you
do not need to screen each section. You can coil up the coax neatly, and
secure it with cable ties. However, it is sometimes a good idea to
ensure that the coax outers are completely 'RF dead' by bonding one to
another at several places (ie more than simply where one piece of coax
is connected to another). Of course, you can put the whole filter in a
metal box, and make sure that coax braid is connected to the box in
several places (and especially at the open end of the shunt stubs).

Of course, you can also make 'hybrid' filters which make use of the best
features of LC circuits and stub circuits. I made such a filter for use
with an FM CB radio which I had modified for use at the HF end of 10m. I
found that I was still radiating (and receiving) some signal on the
original CB band. The only practical way to eliminate this problem was
to use an external filter which rejected 27 to 28MHz, but passed 29.3 to
29.7MHz. I did this by using two LC 'suck-blow' (stop-pass) filters in
individual metal boxes, separated by a quarterwave of coax (coiled up,
of course).

On the internet, there is quite a lot of information about coaxial stub
filters (try a Google search). Anyway, may I wish you success, whatever
filter you choose to use!
--
Ian

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