Im trying to build a preamp for the 902-928 band, I bought several
surplus cellular preamps
in a auction, these had 824-855mhz ceramic filters. I took the filter
out and the noise floor
took off, i need to replace the bandpass with one in the 902-928 mhz
range. I have some
semi rigid coax .080 and of various sizes of small hardline .250 -
..540. In the past ive seem articles on making preamps & filters using
semi rigid coax but cant find any now that im ready to start building a
ATV receiver.

Depending on the type of ceramic filter, some can be moved either up or down
(yep, down) in frequency with a dremel and diamond wheel. I have done this
when designing AMPS transceivers for a different frequency range (lower)
when the lead time on sample filters from Murata in a different range was
too long. Moving them relatively small amounts, the bandwidth stays quite
close. I think I went down something like 15-20 MHz. Moving them as far as
you want far may be (probably is) a stretch You are trying to go from the
cellular base receive to the ham band. Depending on what the amp is between
the filter and output, the amp may need some tweeking for this difference in
frequency. IF you had some mobile receive (45 MHz higher) filters, going
that far would be easier, but I'm sure you don't.

A Comb-line ceramic filter is a block that has holes in it. It is like a
box with 1/4 wave rods in a line except the box is filled with ceramic.
It is about 1/4 inch thick, maybe 1/2 inch 'high' and long enough for as
many hole-resonators as it has in a line. Each hole is plated inside and is
a "shortened quarter wave transmission line" resonator. The "ground" or
"shield" equivalent of the transmission line resonators is the two plated
sides of the block. It is filed with a high-K dielectric having a very good
temperature characteristics and therefore the 1/4 wave is very short -- the
virtue of these filters.
Ours typically had three poles (holes-resonators) for the receive and 5
for the exciter filter.
One end (the 'bottom' or the shorted end of the quarter wave resonators)
is all metal plating and holes in a line. The opposite end (the resonator
open end) has little or no plating and may even show the effects of grinding
to tune it. However, some are constructed with this end mounted down and
therefore covered by the mounting bracket which will probably be soldered or
glued on and allow limited access. Coupling for the desired bandwidth and
ripple is achieved by the spacing of the resonator-holes in the solid high-k
ceramic dielectric therefore, you can't buck about with coupling and must
take what you get after moving.
To move up, you remove resonator from the "open" end, shortening it. To
move down, you remove small areas of the ground plating at the base of the
resonator-holes. I used the Dishall method and they came right on.

If the filters are comb-line with top coupling for input and output, moving
them *up* far (by shortening the resonator -- which *is* the hole), I think
will be hard because you will loose coupling between resonator and coupling
tab when you shorten the resonator...however you could regain coupling by
attaching a wire to the coupling tab and run it down the hole (the "correct"
amount) to proper get coupling back.
73, Steve K9DCI






wrote in message
oups.com...
Im trying to build a preamp for the 902-928 band, I bought several
surplus cellular preamps
in a auction, these had 824-855mhz ceramic filters. I took the filter
out and the noise floor
took off, i need to replace the bandpass with one in the 902-928 mhz
range. I have some
semi rigid coax .080 and of various sizes of small hardline .250 -
.540. In the past ive seem articles on making preamps & filters using
semi rigid coax but cant find any now that im ready to start building a
ATV receiver.