> I am not sure why an isolator would cause intermod. Usually
> there are not active or non-linear components in them
An isolator IS a non-linear device.
> and
> they are often used to prevent intermod by preventing outside
> signals from coming in thru the feedline into the transmitter.
As a nonlinear device, they will naturally generate harmonics on their own,
and likewise act as mixers (however lossy), which is why filtering after an
isolator is necessary.
> In the past commerical sites would often require an isolator
> for this reason with strong transmitters close by. In better
> repeater equipment an isolator was built in.
Yes. The isolator "isolates" the transmitter from the antenna by diverting
incoming (from the antenna) RF into the reject load rather than letting it
get back into the transmitter as a way of preventing IM from occuring. But
in some instances, that's more of an "added bonus" than the real reason the
isolator was built in to some stations - to protect the PA from anomalies
that would cause high VSWR that could damage the PA or lead to instability
issues.
My concern, in the instant case, is that when the isolator is carrying
high-level RF from the transmitter will mix with other signals coming back
down from the antenna. I might try a two-tone IM text on an isolator to get
a handle on what might be expected.
--- Jeff
