There are three primary ways that other wireless systems and TV channels cause interference. The first is a direct frequency conflict. If two wireless systems are on the same frequency, usually neither system will be usable unless the other is turned off. This problem is more common than might be expected, especially when all the equipment is the same type. This is because most systems have only a limited number of standard frequencies, or a set of standard frequencies in the case of multichannel equipment. It should be remembered that the other systems could be some distance away - in another house of worship down the street, in another nearby studio, or at another club in the vicinity.
TV transmitters can also be a powerful source of direct interference if the wireless frequency falls within the assigned channel of a local TV station. This problem sometimes occurs because wireless frequencies are listed in MHz and it is not always obvious within which TV channel a particular frequency falls.
A second way other wireless systems can cause interference is when their operating frequencies are too close to the frequency of the desired signal. In this situation, the wireless receiver might not be able to reject the signal from the other transmitter and interference ranging from minor to very serious can result. If the frequency separation is increased, the interference will eventually stop. However, the amount of separation required depends heavily upon the characteristics of the wireless receiver and several other factors.
The third way other wireless systems and TV transmitters can cause interference is intermodulation, or "intermod." This is one of the most common problems with wireless, yet it is often not well understood. Intermodulation occurs when strong signals present at the receiver input mix together in various combinations, sometimes producing an undesired output signal that is on or near the operating frequency of the receiver. When this happens, the effect on the system is similar to the results of direct interference.
One difficulty with intermodulation is that the frequency combinations that will cause trouble are not immediately obvious. That is, a careful examination of a list of active TV channels and the wireless systems involved will usually reveal potential direct interference problems and frequencies that are too close together. However, the frequency combinations that will create intermodulation problems are far from obvious or intuitive. This sometimes tempts even experienced wireless users into ignoring the potential problem, often to their regret.
Although not as prevalent as the types of interference mentioned above, some other interference sources should be considered. One is direct interference on the image frequency of a receiver. While very dependent upon the characteristics of the receiver, all wireless receivers exhibit at least some degree of undesired response at the image frequency. For this reason, care should be taken that there not be another wireless system or TV channel frequency on the image frequency of a wireless receiver.
There are some other minor interference sources which can affect wireless reception under certain relatively unusual circumstances. Because these very rarely cause actual problems, it is not necessary to consider them in selecting wireless frequencies unless there is reason to suspect a particular problem. One example would be if the wireless equipment is to be used within about five miles (8 km) of a powerful TV or FM radio transmitter.
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