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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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