> Thanks to Bob, Skip, Jeff and everyone who assisted me with
> my Circulator question. You have given me much to think about
> as to "IF" I want to install one on my 2-Meter machine. Let
> me throw this question out as you guys have been there and
> done that! Under what circumstances would you use a
> circulator on your 2-meter repeater?
Ideally, you should always use an isolator. Some sites require one in the
terms of the lease. Good practice says to use one no matter what. Does
everyone use one? No.
> Here is my setting: A
> strong RF Site with a 152.480 Paging Transmitter,FM Radio
> Station on 103.5. My ham repeater on 147.210 is getting hit
> by the Paging and I suspect the FM Station.
How close are the repsective antennas?
> There is a
> crystal filter on my ham receiver for 152.480, a good one
> furnished by the paging company.
Maybe I'm misunderstanding, but how are you using a crystal filter on
152.480 on your 2m receiver?
> After going through the
> duplexers and crystal filter the sensitivity is on the RX is
> naturally down a little. I also have a VHF Community Repeater
> that is receiving on 160 MHz and TX on 153. Although that
> antenna is lower than the Ham Antenna it talks better. I know
> there are a lot of variables, I have good TX RX Duplexers on
> both units, a Stationmaster on the commercial and a G-7 on
> the Ham which hopefully will become a stationmaster soon. I
> just called the paging company and made them come down as
> they were splattering a lot of the high band receivers at the
> tower.
If the paging company doesn't have, at a minimum, a pass cavity on their
transmitter, make them put one on. Although I hate to use such
generalizations, I've seen plenty of paging companies that are so anxious to
eek out every fraction of a dB of ERP that they often run without any
filtering (and in many cases, operate their transmitters in excess of their
rated power).
> They installed another large CAN (Filter of some type
> on top of the cabinet and it looks like the leads go to the
> exciter, small RG-58 type cable, compared to the 1/2"
> hardline on the output.
Filtering between the exciter and PA is only partially effective. It
attenuates garbage the exciter puts out (which would normally be amplified
by the PA), but doesn't reduce broadband noise or spurious products
generated by the PA itself. The reason they put the filter between the
exciter is to save that fraction of a dB of ERP that they would have lost if
they put the cavity after the PA.
> I was grabbing at straws with the
> circulator thinking that maybe the FM Station was coming back
> down my antenna although I can't measure it.
A strong FM signal will create problems. The easy and cheap fix for that is
to put something like a DCI window filter or similiar window filter after
your duplexer such that both tx and rx frequencies will pass.
--- Jeff
---------
Jeff DePolo WN3A
Broadcast and Communications Consultant
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