Q: What is intermodulation? What causes intermods?
A: Intermodulation generally only comes up in conversation when discussing interference. To understand about intermodulation products, or intermods, you need to know a little bit about transmitter and receiver design.
Transmitters and receivers are designed to work at a particular frequency. High frequencies, such as those used in UHF equipment are hard to generate accurately. The designer uses clever circuitry to generate these frequencies from a lower reference source and then electronically multiply this up to the transmission frequency. In most transmitters this reference would come from a crystal. With some equipment the reference is synthesized by electronics. Very simply it is something that vibrates at a fixed frequency to generate the radio waves.
Watching the effect of a stone dropping in water would be one way of thinking about radio waves. If you dropped in a second stone, notice what happens when both lots of waves meet – they interfere with each other. With radio this interference can be calculated mathmatically, but the effect can often be like that of a third transmitter. Sometimes the unwanted effect generated is outside of the range of your equipment, in which case it is not a problem. It becomes a problem when the effect is within the range of your equipment.
Q: What does this mean?
A: It could mean that your receiver is overwhelmed by these new signals, and it will fail to pick up its own transmitter properly. It could mean that this new signal is the same frequency as another receiver you are using, which is something else you don’t want. Another possibility is that your intermods are causing interference to someone else’s receiver – or theirs to yours. Sometimes the relationship could be very obscure. An example would be a transmitter of yours, along with a transmitter of someone else, could together cause interference to yet a third user.
Q: What can I do to prevent intermods?
A: Prevention is better than cure. Manufacturers of multi channel systems often arrange their frequency sets in “banks”. All of “bank 1” would be an intermod free set, and so would “bank 2”. Mixing frequencies from the 2 banks however could cause intermod problems. The Radio Microphones page has the frequencies that are intermod free in the commonly used bands.
For talkback, http://www.jfmg.co.uk JFMG licence sets of frequencies known to be intermod free at the same site.
The strength of the signal will depend on how close the transmitters are to each other, move them further apart – particularly those that are on "hot standby" on the table.
If you can’t move them apart turn the power down. For a radio mic without obvious adjustment coiling the aerial up, or putting it on a person could make a big difference, as human bodies are good signal absorbers
At an OB site it can be very difficult to convince talkback users to move their transmitters further apart or reduce the power on their base stations. The signal doesn’t stop at the receiver’s aerial though so only use enough power to do the job. Use an attenuator on the aerial output cable. Make the feeder cable longer – it may introduce enough losses to eliminate the problem. Change the transmitter aerial Polarisation. Do all of those.
Remember that problems could be caused by the presence of more than two frequencies so don’t assume that the first one you turn off is the cause of the problem.