Technical Topics for Wireless Mics

This section goes into detail on some of the more technical topics relating to wireless microphones.

Gain Structure

Just as in audio systems, gain structure and noise are extremely important for radio frequency devices. In particular, RF systems are very susceptable to various kinds of noise and interference, including thermal noise, intermodulation, distortion, and other kinds of interference. Great care must be taken to ensure that the minimum amount of noise is injected into the RF signal path.

If an active antenna or an antenna amplifier is used, it must be located as close to the antenna as is possible, and it must provide enough gain to overcome the loss in the coaxial cable. If the amplifier is placed at the receiver end of the coax cable, more noise will be added to the signal, which will cause distortion.

Gain structure must be considered when using antenna distribition systems as well. If using a passive distribution system, care should be taken to balance the outputs such that each receiver sees the same amount of attenuation from antenna to receiver. When using an active distribution system, follow the manufacturers directions.

It is worth noting that both antenna distribution systems and antennas themselves need not match the manufacturer of the wireless system, unless the antenna has an onboard amplifier that requires a specific power source, or if the antenna splitter has a bandpass filter in it.

Signal Polarization

All radio signals tend to have a polarization to them. There are generally three types of polarization: linear, circular, and elliptical. The vast majority of wireless microphone systems have small whip antennas, and thus we say they they are linearly polarized. Log Periodic Diple Arrays (commonly referred to as “shark fin” antennas) and Yagi-Uda antennas also exhibit linear polarization. For best performance, a receive antenna must have the same orientation as the transmit antenna in a linearly polarized system.

Circularly polarized antennas are also used in wireless systems, especially in systems where extremely high performance is required. Circularly polarized antennas tend to pick up linearly polarized signals in all directions. Additionally, it is relatively wasy to achieve high gains in a helical antenna.

In the real world, polarization is not as important as some make it out to be. Because of the many reflections in a building, the polarization of a signal tends to get smeared out and thus linear antennas tend to work very well regardless of orientation.

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