Multipath

Multipath, or more accurately, multipath propagation, is often a problem with wireless microphone systems that do not use diversity receivers. Multipath can cause signal dropouts, resulting in brief periods where the audio from the transmitter becomes noisy or is lost completely. The dropouts will occur in localized small areas as the person using the wireless transmitter moves about.

In most locations where wireless systems are used, there are numerous objects that can reflect radio signals. The radio signal from a wireless transmitter is reflected from these objects and scatters throughout the area in a complicated manner. At the receiver, radio signals from the transmitter arrive via a number of different paths. The actual signal picked up by the receiver antenna is the combination of all of these different signals.

If one path is longer than another, the signals from these two paths will have different phases. In addition, when radio signals reflect, their phase reverses. At the receiver antenna, some of the arriving signals add to the total and some subtract from the total. A dropout occurs when the sum of all the arriving signals is low, and the receiver does not have a strong enough signal to provide good audio. In effect, some signals cancel out other signals and the result is a very weak combined signal at the receiving antenna.

Generally, the locations where dropouts are experienced are small, usually only a few inches across. If the person using the transmitter moves slightly, the signal will come back and good audio will be obtained again. Because the dropouts are caused by signal cancellation, they can occur at distances far less than the maximum range of the system.

Diversity receivers are able to avoid dropouts due to multipath because they have two antennas and two receiver channels. Special circuits in the receiver select the audio from the antenna and receiver channel with the better signal. Because the chances that there will be simultaneous dropouts at both antennas are extremely low, diversity receivers provide almost complete immunity to dropouts due to multipath.

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