This seems like a perplexing problem. Maybe someone can point me in
the right direction.
I have a TPD 1554, 4-can VHF duplexer. It provides about 77 db of
isolation with 1.4 db of loss. This is just about what the spec says,
so that all looks good.
I happen to have some extra PD-1173 VHF band pass cans, so I'm
thinking of adding a can on each side to get the rejection up to 85
db or so for use on a VHF repeater (Exec II base station at 25
watts).
The bandpass can by its self shows about 0.5 db loss and 8 db of
rejection at 600 KHz. The curve on the spectrum analyzer looks good
and the return loss is very good (-40).
Here is where it gets messed up. I added a can on the duplexer on the
radio side (let's just say the RX side). The loss goes up to 1.9 db
as expected and the return loss is still good. But when I go to
measure the rejection, it now shows only 73 db. How can this be?
Everything uses double sheilded cable and attenuator pads are used on
the input and output to the analyzer.
It seems like the rejection should be additive, but for some reason
it's not showing up that way. Anyone know why?
Dwayne Kincaid
WD8OYG
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