Friday, May 1, 2015

A logical question concerning wireless microphone performance is the transmission range. Unfortunately, the answer is complex and is more than a simple distance measurement. Ultimately, the receiver must be able to pick up a "useable" signal from the transmitter. "Useable" means that the strength of the desired signal is within the sensitivity range of the receiver, is sufficiently stronger than (or different from) undesirable signals/RF (radio frequency) noise, and can produce an acceptable signal-to-noise ratio at the audio output of the receiver. Elements that affect usability are the transmitter/antenna, the transmission path, the receiver/antenna, and the local RFI (Radio Frequency Interference.) Some characteristics of these elements are controllable, some are not.

The RF transmission path is characterized by distance, intervening obstructions, and propagation effects. Losses due to these characteristics are generally frequency dependent: the higher the frequency - the greater the loss. See the chart above.

Important transmitter characteristics are power output and antenna efficiency. Maximum radiated power is limited by government regulations and battery capability. Antenna efficiency is limited by size and design. The efficiency of typical wireless transmitter antennas is fairly low, about 10% or less. This means that for a 50 mW (milliwatt) transmitter the effective radiated power (ERP) is less than 5 mW. This may be further attenuated by proximity to the human body or other objects that absorb/deflect RF.

Important receiver characteristics are antenna efficiency, receiver sensitivity, and the ability of the receiver to reject unwanted signals and noise. Antenna efficiency is again limited by size and design. Receiver antennas tend to be much more efficient than transmitter antennas since they can be made large enough to be effectively tuned to the proper frequency band. Other receiver characteristics are limited by design. Both elements are limited by cost.

Once the operating frequency is chosen, only the path length and antenna locations are controllable. These are usually limited by the application itself. Under good conditions (line-of-sight) at a distance of about 100 feet, the field strength of the signal from a 50 mW (milliwatt) transmitter is on the order of 1000 uV/m (microvolts), well within the range of sensitivity of a typical receiver.

Finally, RFI is characterized by its spectrum, that is, its distribution of amplitude and frequency. It typically consists of both broadband noise and discrete frequencies. However, its strength can be comparable to, or greater than, the desired RF signal in poor conditions. Except for a few predictable sources, RFI is largely uncontrollable.

Rather than quote a specific maximum operating distance, most manufacturers of wireless microphone systems give a "typical" range. For systems of the type discussed here (10-50 mW, VHF or UHF) the typical range may vary from 100 feet to 1000 feet. The lower number represents a moderately severe environment while the upper figure might be achieved in absolute ideal conditions, like the salt flats in Utah. Extremely poor conditions, like the middle of New York City, could result in a range of only 50 feet or less.

Remember: it is impossible to accurately predict the range of an arbitrary wireless microphone system in an arbitrary application.