Tim,
You are to be commended for thinking about possible interference to
others; I wish all repeater owners were so conscientious!
Even if the duplexer was perfectly tuned on a network analyzer, the PA
may become unstable when the power is lowered below a certain point.
This instability usually causes spurious emissions which definitely WILL
cause interference to other stations. This is not the same as intermod.
Now, when you add a detuned (or incorrectly tuned) duplexer to this
scenario, it can make the PA even more unstable. The source impedance
of a solid-state PA varies a lot as the drive level is changed, and the
load impedance of a duplexer varies with frequency and tuning. With two
instabilities operating together, it's a wonder that we can achieve a
stable system.
The fact that you had a desense problem suggests that your duplexer is
improperly tuned, or it may simply be inadequate for your transmitter
power and receive sensitivity. Desense is frequently caused by leaky
jumpers between the repeater and the duplexer; these should always be
fabricated from double-shielded cable and high-quality connectors
without adapters. Lowering the transmitter power to cure desense
doesn't always work, and sometimes the increased spurs make desense
worse as power drops.
You can stabilize the load impedance on the PA by adding a ferrite
isolator between the PA and the duplexer input. This will greatly
reduce the possibility of intermod interference to other stations.
However, I don't view your detuned duplexer as "causing" intermod. It
may certainly make your station more susceptible to intermod, but a
ferrite isolator is the important preventive device.
73, Eric Lemmon WB6FLY
"Tim S." wrote:
>
> Pretty sure I know the answer to this.
>
> But can a transmitter keying into a duplexer that's detuned cause intermod
> interference?
>
> I have a transmitter that I turned down the power because it had desense on
> it. Turning the power down fixed the desense but now I am wondering since
> its still transmitting into a poor load, can't that cause some intermod
> interference to other repeaters?...
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