Intermodulation

Intermodulation is the mixing of two or more signals that produce a third
or fourth frequency that is undesired. All radio communication sites produce
intermodulation no matter how good the design is. However, the fact
that there are intermodulation products produced does not mean there is a
problem.
Just what is intermodulation and how does it go about calculating an
intermodulation product, IMD? Various intermodulation products are
shown in the following for reference. The values used are simplistic in
nature so facilitate the examples. In each of the examples, A
880 MHz,
B
45 MHz, C
931 MHz, and D is the intermodulation product. The example
listed in the following does not represent all the perturbations possible.
Second order: A  B
D (925 MHz)
A  B
D (835 MHz)
Third order: A  2B
D (970 MHz)
A  2B
D (790 MHz)
A  B  C
D (1856 MHz)
A  B  C
D (1766 MHz)
Fifth order: 2A  2B  C
D (739 MHz)
The various products that make up the mixing equation to determine the
order of the potential intermodulation. All too often when you conduct an
intermodulation study for a cell site, there are numerous potential problems
identified in the report. The key concept to remember is that the intermodulation
report you are most likely looking at does not take into account
power, modulation, or physical separation between the source and the victim,
to mention a few. Therefore the intermodulation report should be used
as a prerequisite for any site visit so you have some potential candidates to
investigate.

Intermodulation can also be caused by your own equipment through bad
connectors, antennas, or faulty grounding systems. However, the majority of
the intermodulation problems encountered were a result of a problem in the
antenna system for the site and well within the control of the operator to
fix.
Just how you go about isolating an intermodulation problem is part art
and part science. I prefer the scientific approach because it is consistent and
methodical in nature.
The biggest step is identifying the actual problem; the rest of the steps
will fall in line. Therefore it is recommended that the following procedure be
utilized for intermodulation site investigations. 601