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The Intermodulation Phenomenon
Intermodulation interference occurs when two or more transmitters, transmitting at different frequencies, are coupled, resulting
in the mixing of signals. This results in the presence of additional frequencies (intermodulation products) that are either sum or
difference of the transmitted frequencies. If intermodulation products fall within the bandwidth of a receiver, intermodulation
interference may occur.
Due to the collocation of multiple transmitters at the same radio site, intermodulation is a common problem, but is easily
prevented with careful design. The use of an Intermodulation calculator, such as the free GL Communications Intermod Calculator,
will easily determine where intermodulation might occur, allowing appropriate action to be taken early in the design stage. This can
prevent the need for system modification after the system is installed and operating.
Intermodulation Analysis
When 2 or more antennas operating at different frequencies are installed on the same platform – i.e. rooftop, antenna tower,
or water tower - transmitter coupling can occur resulting in intermodulation interference. Wherever antennas will be placed in close
proximity to other antennas, intermodulation analysis should be performed. This analysis can determine the potential for
intermodulation distortion before new antennas are installed.
Once the potential for intermodulation has been identified, intermodulation interference can be prevented at the design stage.
Appropriate selection of frequencies, adequate antenna spacing, transmitter shielding, or filtering at the transmitter or receiver are
a few methods for preventing intermodulation interference.
For an overview of intermodulation reduction techniques, see the paper, Brief Overview of Intermodulation Interference Reduction Techniques, by Vijay Kulkarni.
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