The next consideration is output power of the microphone transmitter. The output power of any wireless mic system is governed by FCC rules, which have evolved over the years and now state that the transmitters can’t operate at a power level in excess of 50mw (milliwatts) and may not cause "harmful interference" at that power level. Most wireless microphone manufacturers today produce professional unlicensed systems that use a transmitter with at least 10mw of power. More expensive systems offer up to 250mw of power. The FCC decided, after extensive testing, that 50mw would give acceptable range in most situations and limit interference with other systems in the area. An important point to remember here is that more power doesn’t necessarily equate to better performance. In fact, more transmitter power, especially in an enclosed space, could create problems, such as multi-path interference, where the signal bounces off walls before reaching the receiver. Less power, in some circumstances, may create less interference and fewer dropouts or loss of signal. Using a higher-powered transmitter also drains batteries at a faster rate.
Transmitters come in a variety of shapes and sizes, depending on their use. The transmitters can be a belt pack, to which a lavalier mic is attached, or a handheld unit. These handheld wireless microphones can be all in one, microphone and transmitter together, or can make use of a plug-on transmitter. Using the correct plug-on transmitter, just about any mic can be turned into a wireless handheld microphone. The plug-on transmitter can also be used on a boom mic to free the boom operator from being attached to a camera or mixer, making his or her job much easier. When using a condenser microphone, the plug-on transmitter needs to be capable of supplying phantom power for the mic. Not all plug-on transmitters have phantom power. Condenser microphones operate within a range of 9 to 52 volts, so it’s important to have a transmitter that supplies the correct voltage for your particular microphone. The most commonly required phantom power for condenser microphones is 48 volts. On the other hand, if you use only handheld dynamic microphones with your plug-on transmitter, then phantom power isn’t required.
Belt pack transmitters either will have the lavalier microphone permanently attached via a short cable or have a multi-pin connector to which a microphone can be attached. Since most lavalier microphones are electret condenser mics, the multi-pin connector on belt pack transmitters supplies power to the microphone in addition to carrying the audio signal. With the proper adapter cable, belt pack transmitters can also be used to transmit the output signal from an audio mixer to a camera, creating what’s known as a "camera hop," allowing the cameraman and audio operator to function without the mixer being physically connected to the camera. Wireless belt pack transmitters have an antenna that extends from the unit and varies in length based on the operating frequency. All-in-one handheld mic/transmitter units typically will have an internal antenna that doesn’t show.
Transmitters are small so they can be worn on a belt or easily handheld. Receivers are normally larger and are attached to the camera or stored in an audio mixer bag where their output is connected to the mixer. Operation of most wireless systems is pretty simple. The frequency of the transmitter and the receiver must be the same. On frequency-agile systems, some manufacturers use numbers and letters to indicate steps in the transmitting/receiving frequency. So, your frequency for the day might be "B-8" or a similar indication that selects the frequency in megahertz and kilohertz. These alphanumeric indications are easier to remember than 468.970 MHz.
Some professional wireless mic receivers have the capability to "scan" the operating band to find open frequencies. This function can be extremely useful in the field, where you might be operating multiple systems in an unfamiliar location. If your system has this scanning feature, it should be performed at the beginning of a shoot at each location in which your system is used and even checked during downtimes to ensure conditions haven’t changed.