> There's good reason a "hot air" alarm probably went off for many
> of you when reading the article.
I'm trying to figure out what his closing paragraph is trying to say:
"A duplexer is working correctly when the sensitivity of the receiver is not
degraded when the transmitter becomes active. There are test procedures to
check this out, but the explanation of these tests is beyond the scope of
this article. [OK, I'll agree with him so far] However, should you hear a
slow oscillation of the transmitter when it turns on and off (a rate of
about 1-2 Hz rate on weak signals), then you do have duplexer
desensitization."
What is this 1-2 Hz oscillation he's talking about?
His use of "quasi-simulcast" is what we used to call "sloppycast".
Basically the transmitters are not time-aligned as far as the launch time of
the audio from each site. Furthermore, the transmitters may or not be
locked to a high stability reference (UHSO TCXO/OCXO, GPS, Rubidium, etc.),
so there may be carrier frequency errors as well resulting in audible beat
products.
Several of my co-channel ham repeaters are sloppycasted, but with accurate
carrier frequency matching (using Rubidium reference oscillators). There is
some minor distortion in areas where the signal levels from two transmitters
are comparable due to the lack of AF time-alignment, but none of the users
have ever commented on it. Still bugs ME though - will have to do something
about that eventually...
I question whether or not the Icom radios they used for "links" are
certificated for use as fixed station equipment. We've had local instances
of field agents from the Commission shutting down Part 22 and Part 90
base/repeater stations operating using mobile radios. In one case, a Mitrek
had such bad LO leakage that it was getting into another receiver several
miles away, which is what prompted the FCC interference investigation.
--- Jeff
