Frequency Selection and Multiple System Use
A word of caution! Quite often, wireless Mic Systems (and wireless guitar/bass systems) operate on the same frequencies as TV channels. In general, VHF systems work on TV channels 2 - 13, and UHF systems on TV channels 14 - 69. So, when ordering a wireless mic, make sure you check the channel against your location to ensure your wireless mic systems avoid interference with local TV channels.
Wireless Mic Systems send out a radio signal which is picked up by the receiver. Each wireless system transmits this radio signal on a unique frequency or channel, and this is an important issue to consider when purchasing wireless systems. Using multiple wireless systems together requires that careful attention be paid to frequency or channel selection. Multiple people in the same band/group cannot use the same frequency/channel. As the number of wireless systems used in your performance increases, the transmitted signals interact with each other to create "intermodulation distortion" or intermod for short. If careful attention is not paid to channel selection, this intermod becomes a sort of interference which may cause wireless mic systems that work fine individually, to not work when used all at once. The key is to keep careful track of your operating frequencies to ensure they are all compatible.