At 6/3/2005 10:28 AM, you wrote:
>1 5/8" Hardline Coaxial Stubs cut to notch the repeater frequency
>only. But you will get some loss on either side of the notch and the
>notch will be 19-24 dB deep.
A sharper notch can be achieved by using notch cavity filters.
Here's what we did with a Kenwood TM-241 remote base at a comm site:
-split TX & RX paths inside the radio; added separate RX coax coming out of
radio.
-TX goes through isolator & LPF to TX antenna
-RX connected to separate RX antenna on top of tower (where only RX
antennas are allowed). Also added DCI BPF, 10 dB attenuator & notch cavity
to notch out the busiest 2 meter repeater at the site (too many to notch
all of them).
Adding the 10 dB pad only dropped the effective sensitivity by about 3 dB
due to the low noise figure of the TM-241's RX combined with the high noise
floor of 2 meters in SoCal.
We also had an IF overload problem: off-channel signal would get through
the 1st IF filter & IM with the on-channel signal, causing the squelch to
close even though the adjacent-channel signal was filtered out by the 2nd
IF filter. The problem was partially resolved by cutting the gain down in
the 1st IF, giving the 1st IF amp a little more headroom. It's possible
the 1st IF filters were bad - didn't have the time/equipment to check at
the time.
The TM-241 makes for a lousy remote base radio as the front end still gets
overloaded at times even with all the filtering & padding we added, but at
least it's usable now. It was chosen because it's one of the few radios
that works with the Doug Hall RBI interface.
Bob NO6B
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