Syllabus Sections:-
8c Intermodulation
8c.1 Understand how to identify whether the
distant transmitter or the local receiver is producing
intermodulation products.
Intermodulation
occurs when a strong signal, on a frequency that the receiver
is NOT designed for, gets into the receiver, and is so strong
that it alters the DC operating characteristics of one of the
stages. The modulation of that strong signal then gets
superimposed on the wanted signal.
When the
distant transmitter is causing the problem, it is because the
signal causing the interference is actually (and wrongly)
being produced and radiated for instance harmonics, and
general spuri, that a transmitter should not be radiating.
If you
are the owner of the transmitter you would need to carry out
tests to verify that the transmitter is clean. This will
involve the use of a wavemeter (or, if you have shed loads of
money, a spectrum analyser!). If the transmitter is "dirty",
then you have to take steps to rectify that.
When the
transmitter is clean, and the obligatory low pass filter is
correctly fitted, (AFTER the VSWR meter and before the antenna
feeder), the problem will be the distant receiver receiving a
very strong signal from you on a frequency that it SHOULD
discriminate against, but can't, and it will be suffering from
intermodulation.
It is
possible to have a clean transmitter, and a receiver that is
not suffering from intermodulation, but still experiencing
interference.
This can
occur because the incoming interference (not that strong) is
beating against a harmonic of the receiver local oscillator to
produce an IF frequency. Cure? TX move frequency a little.
Very similar effect from the incoming beating with the
oscillator on it's fundamental frequency, but producing an IF
the other side. (i.e if the receiver normally operates with
the oscillator HIGHER than the received frequency, the
incoming TX ABOVE the oscillator may beat with it to produce
an IF)
Again,
the "rusty bolt" effect. A clean transmitter excites nearby
metalwork where there is an accidental semiconductor junction
(The rusty bolt). This then rectifies the strong RF that is
flying around the metalwork, produces harmonics, and these get
radiated. Difficult to cure!!
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