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Re: Another interesting preamp story (desense and diplexers)



I had a desense problem with my system, but with a slight twist to the usual
mode J (VU) mode issues that we've all seen.

Using seperate beams for 2 and 70. The 2mtr uplink was flattening the 70cms
preamp (internal preamp in a BNOS linear, mounted at the base of the short
mast).
The problem was not the third harmonic of the 2mtr TX, as I have a 5 cavity
helical filter for 2 mtrs between the PA and antenna (to get rid of strong
pager transmissions at 154 and 139MHz). It was the clean 2mtr signal that
was getting into the 70cms preamp and causing desense.
This was cured by using a duplexer in the way described in previous emails
(ie. in the 70cms circuit between the antenna and preamp/PA,  with a 50r
termination on the 2mtr port, just for peace of mind).

Jules
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