> -----Original Message-----
> From: John Lloyd
>
> Richard,
>
> Have you tried installing a low pass filter between your 2 Mtr duplexer
> and the antenna feed line?
> I am talking about the large low pass filters that were found in the old
> VHF Motorola base stations that were about 1 inch in diameter and about
> 14 inches long.
No, I haven't tried one of those. Although, that's a good idea. And it
happens that I have one available to try.
> I have a similar situation where my 2 mtr antenna is also at the same
> elevation as numerous other 800 Mhz repeater antennas and the addition
> of a low pass filter in my 2 mtr repeater feed line cured the problem
> like you describe.
>
> The low pass filter reduces the 800 mhz energy an additional 60 db or so
> to help isolate your 2 mtr repeater from the 800 mhz repeaters.
Well, I was kinda thinking that the bandpass cavity would accomplish the
same thing? Maybe both inline might have a positive effect.
> I also have a dual isolator on the output of my transmitter before it
> goes into my 6 cavity celwave BpBr 2 mtr duplexer.
As mentioned in an earlier post the problem I found regarding using an
isolator is that it only acts like a one way valve around the VHF range. It
will be transparent to 800MHz signals. An isolator would be most effective
if I was dealing with several VHF transmitter antennas in the near field of
the 2m antenna.
When I look at the spectrum plot at the tx port when I have a bandpass
cavity inline there are NO VHF signals visible within the 150-174 range.
There's also no UHF stuff visible. The bandpass filter does a good job of
reducing the VHF & UHF stuff down into the grass. Although, there aren't
any VHF antennas on this tower so the signals that are present are from
distant towers so there not really that strong to begin with.
> You will probably need both the low pass filter in your feed line and
> dual isolator on your transmitter to solve this kind of problem.
>
> Your other solution will be to move your antenna out of the other
> antennas main RF field.
Well, that's what I'm thinking but I thought I'd see if anyone out there had
a "magic bullet" solution without having to relocate the antenna. Although,
perhaps moving the PD220 antenna from the top of the tower would be a good
investment anyway in keeping it intact during lightning season (hi, hi).
> John, K7JL
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