Wireless microphones   
Wireless microphone systems make use of low power FM radio transmitter-receiver systems. 
The basic technology is the same as that used in commercial radio broadcast applications but makes use of miniaturized components. 

A microphone which is plugged into a small transmitter thereby transmitting its signal via FM radio signal to a receiver. Useful where cables are not possible or desirable.  Very common in film production and live performance work.
 
In the case of hand-held microphones the battery powered transmitter circuitry resides in the microphone body while a small lavalier microphone may be worn and connected to a small pocket-sized case which can be worn by the performer.  The radio receiver system holds the antenna and circuitry which decodes and transduces the radio signals sent by the microphone into electrical signals which can then be mixed and amplified in the usual fashion. 
 
The radio signal transmitted by a wireless microphone radiates in all directions and is susceptible to dropout due to frequency cancellations, transmission loss due to the distance between the transmitter and receiver as well as interference from other radio broadcasts. Objects in the path between transmitter and receiver can also be the cause of reception problems, particularly metal objects such as lighting instruments or stage props which can create multiple reflections of the radio waves and increase the possibility of dropout due to cancellations. 

Placement of the receiving antenna can minimize these problems. It is not always possible to run the receiving system at the mixer location. In some cases it might be necessary to place the antenna and receiver closer to the transmitting microphone in order to optimize the amount of signal received. Diversity reception makes use of multiple antennae and can improve signal reception and compensate for dead spots in a space but require more expensive circuitry to decode and combine the multiple sources. 
 
Wireless microphones are usually incompatible with receivers from other systems or manufacturers. This is a result of differences in noise reduction techniques and the frequencies used. Always use matched sets of microphones and receivers.  Antenna cable is always coaxial type and should not exceed cable runs of 30 meters. If cable runs over 30 meters are necessary using a radio frequency preamplifier will boost the signal and compensate for signal loss over the long cable.