 Ever
since the days of Marconi and spark gap radio the wireless radio
equipment has been plagued by radio interference (RFI). In the early
days before the radio channels were congested, you still had to
put up with noise from thunderstorms, errant transmitters, lousy
receivers, and primitive equipment.
 In
today’s modern technology, there are the same problems from
yesteryear, along with crowded channels, crowded radio sites, and
everything today seems to be powered by batteries and radiates wireless
electrical energy. Often RFI is not even recognized as a problem
that may be seriously affecting a given communications system.
Inter-modulation products are the emissions at
frequencies generated by the combination of two or more frequencies
in a non-linear device, such as the output stage of a transmitter,
or the input stage of a receiver.
Types and Sources of Intermodulation Interference
 Actual generated intermodulation products will usually fall into one of the following categories:
Transmitter generated intermod
The transmitted signal from one or more transmitters is received at the output of another transmitter generally via this transmitter’s antenna. This received signal must be of sufficient strength to mix with the transmitter’s own carrier in the non-linear final amplifier. This newly mixed signal is then amplified and transmitted along with the desired carrier. This is the most common type of intermod and also the easiest to cure.
Receiver generated intermod
 Transmitted RF energy can sometimes generate a voltage in a nearby receiver’s RF amplifier, biasing a transistor into a non-linear state that allows it to act as a mixer, which in turn generates the unwanted IM products. This undesired signal can be received via the antenna input or can sometimes be coupled directly into the receiver through the case. This is also a common source for generation of IM products. Receivers can also be desensitized by intermod. When that happens a strong intermodulation product actually received in the front end can overdrive the receiver, causing the automatic gain control to reduce the gain even if the intermod is on a frequency other than the desired received frequency.
Externally generated intermod
 Loose mechanical connections or dissimilar or corroded metal connections form non-linear electrical junctions, which act as "diodes" or mixers. When these devices are excited by sufficient level of one or more signals they generate IM products. Major offenders are tower section joints (especially if they are heavily corroded), broken welding beads, improperly seated or corroded connectors in the RF chain, metal buildings, and chain-link or barbed-wire fences. These are by far the most difficult sources to identify. Once they are identified, many conventional solutions exist that will usually eliminate them.
Resolution and Avoidance of Interference
 If your work is diagnostic in nature, meaning that you are attempting to solve an existing problem, you will need to identify both the type and actual source of the problem. Listen to the traffic on your system. Listen to the complaints from your system users. Continually assess whether or not you observe any resolution of these symptoms. If your research is preventative in nature you will need a great deal of information on how equipment is to be installed at the site. This pre-screening for potential problems can allow you to take into account the existing filtering already in place in order to assess the likelihood of any potential intermodulation products. You can also make wise filtering and design recommendations prior to placing the system in operation, which will minimize the likelihood of new intermodulation products being generated.
If intermod is present at a site, it will be eliminated by increased isolation between the components used to generate the product and the equipment in which the product is being generated. If your newly proposed frequencies are components in predicted possible intermod generation, they too must be treated by increasing isolation between the source of participating radiation and the location where the mixing occurs. Also, it should be remembered that the resolution of problems that do materialize is based on well-established techniques, which have been thoroughly proven. The vast majority of such problems are remedied by the use of relatively simple filters and traps installed on the appropriate equipment and with proper grounding techniques. Spurious radiation products, which are generated in the input stage of a receiver, may be suppressed by the use of a notch filter(s) at the input terminals to attenuate the undesired components of other frequencies which contribute to the spurious product. Alternately, when a large number of components are involved, or when the exact nature of the interfering signals is not clearly defined, a narrow band pass filter may be used to selectively pass only the desired received frequency. Likewise, spurious radiation components generated in the output of transmitting equipment may be reduced to levels which to not cause objectionable interference by the use of notch and/or band pass filters on the transmitter output. The careful analysis of any problem which might arise will allow the logical application of the principles of good engineering practice.
 This isolation (attenuation of an undesired signal to prevent its mixing) can be achieved in one or more ways. Increasing the physical separation between antennas and the judicious use of appropriate filtering apparatus are the most common methods of treatment. Increased vertical separation will be much more efficient than increased horizontal antenna separation. This is due to the fact that most antennas have substantial nulls in their radiation patterns immediately above and below. It is typical that for a given desired isolation between two antennas at least eight times the vertical separation distance is required if you wish to achieve the same isolation by horizontal separation. Also, for a fixed separation between antennas, isolation will be higher for a higher frequency.
Typically measured isolations are shown below in the following graphs:
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