The quickest way to cure most frequency related problems is to move frequency. On unlicensed units this may prove to be a problem if the offending transmitter is working in the centre of the band - and it's not a system you can change (as the obvious is to move the two system frequencies to each side of the band).
If moving frequency is not an option (crystal controlled, or there are just too many units) then moving location may be the next route to take. With radio telemetry the possibility of either extending the antenna cable or signal cables (or both) is highly possible. If extending the antenna cable then make sure to use a good quality double-shielded coax to limit the amount of coupling to the offending system via the coax itself. Imaging problems are seldom cured with location.
If frequency and location are not options then the next may be filters. These come in three flavours, antenna, cavity and crystal.
Using directional antennas is probably the easiest implementation of antennas as filters although it is limited in use as directional antennas only allow use in, well, one direction! If a simple one-to-one link than this is a superb answer as well as switching to horizontal polarisation often sorts out many issues.
Antennas can be made to be more than directive. They can also be made to resonate more sharply (extremely small bandwidth) or notch (reject) certain frequencies. The actual methods are a little too complex to go into here, but there are many designs available from most decent antenna manufacturers.
Antennas as passive filters is not a well known or commonly accepted method of curing interference, but a rather inexpensive one which makes it surprising their use as filters is not more widespread. It is probably because the antenna system itself is relatively tricky to set up to deliberately cancel the offending frequency (as well as the direction it comes from).
The cavity filter is more widely known and is simply a tuned circuit with a reasonably sharp centre of resonance i.e. will limit the energy from transmitters that are a reasonable distance away in frequency (more than 1%). Some are also fitted with 'notching' circuits to further reduce the offending frequency. However, it needs to be said that these types of filters do not cure interference from transmitters only a few hundred kHz away. The minimum is usually 0.5MHz per 100MHz spread between working and offending frequencies for cavities to be truly effective. Cavities are excellent for curing image related interference.
Ferro-resonant splitters can be employed for working/problem frequency span of as low as 50KHz/100MHz, but their costs may be prohibitive. Also, the line between the antenna change-over switch and the receiver input needs to be brought out and fed through the device as it can only allow signal through in one direction. Not all radio modules will allow such a modification so choose the make carefully.
The final method is to use crystal filters set to the working frequency but, as with the ferro-resonance splitters, the receiver line needs to be available for feeding through the filter. The reason here is the crystal usually cannot handle the power of the transmitter. The costs of such filters may also prove to be prohibitive so careful investigation must be done to prove this the only method.
Just to re-cap; Image related interference is best cured with antennas and/or cavity filters. Curing overloading interference from nearby users is best accomplished, in order of decreasing spread between working and offending frequencies, by using notch cavities, ferro-resonance splitters, and finally crystal filters.
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