7.9 Intermodulation Products

An additional interference problem that is significant at VHF and UHF frequencies is caused by IMPs. Any two frequencies that are allowed to mix (beat) in a nonlinear device will generate an additional set of frequencies. These additional frequencies are called IMPs. If these products happen to fall within the pass band of the receiver, then an IMP problem exists.

The first source of IMPs is generated by the transmitter itself (including spurious and harmonics) and caused by the nonlinearity of the transmitter. They can be reduced by the transmit branching filters. The intermodulation products that are normally considered are those resulting from two carrier frequencies rather than those generated internally to the equipment. The ITU plans should ensure that the frequencies chosen minimize this effect, but different systems at a site can result in problem IMPs.

The second source of intermodulation products are generated within the receiver input stage due to the nonlinearity of the mixers, for example. Twooff-channel signals can be mixed in the receiver to produce an IMP that corresponds to the wanted receiver frequency. This effect can be reduced by having narrowband receive filters.

Finally IMPs can be generated by any nonlinear physical device at the site, such as a rusty tower and fence. This is called the "rusty bolt effect." Metallic surfaces that have been oxidized behave in a nonlinear way like a diode. The metal structure can then become a radiating device (antenna), resulting in the IMPs.

If one mixes (modulates)...