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Author:Tymine Date:2010/11/1 19:14:35

All wireless microphones must overcome problems of transmission, especially when interference muddies or blocks a signal from the transmitter to the receiver. Transmission is affected by the wavelength of the signal and the type of antennae used in the receivers (receivers are discussed in the next section below).

The wavelength of the signal in wireless microphones is broadly designated as using radio waves (Radio Frequency or RF) or InfraRed (IR). The former is often called FM wireless and has two distinct ranges of government approved frequencies: VHF (Very High Frequency) and UHF (Ultra High Frequency). We will examine each of these in turn.

VHF(Very High Frequency) radio signals in the range of 49MHz to 216MHz are the least expensive solution to wireless transmission. This is because in the VHF range your transmission works with a single antenna and does not require diversity antennae (discussed below).

The FCC divides VHF into a low band (49MHz to108MHz) and a high band (169MHz to 216MHz). Low band VHF is used by cordless telephones, walkie-talkies, radio controlled toys, television channels 2 through 6, and wireless assistive listening systems.

High band VHF is FCC approved for wireless microphone users. The first part of this high band spectrum, from 169MHz to 172MHz, includes eight specific frequencies, known as low band VHF. These are often referred to as "traveling frequencies" because broadcast television is excluded from them. The primary users of this band include businesses and government operations such as digital paging services, hydroelectric power stations, and the U.S. Coast Guard.

The downside of wireless microphones in this low band VHF range is radio frequency interference (RFI). Although broadcast television is excluded from the low band VHF range, the volume of traffic on these frequencies, and the spacing between them, makes using wireless microphones subject to interference. The interference problem becomes virtually intolerable when using three or more wireless microphones in this range.

The high band VHF range, 169MHz to 216MHz, is designated by the US government for broadcast and commercial film/video production. Within the high band VHF range audio transmission is significantly improved. Interference is less than that of low frequency radio waves. As a result, wireless microphones can be made with manageable antenna sizes that produce good results in almost any part of the US.

The Ultra High Frequency (UHF) range contains several bands portions of the electromagnetic spectrum from 470MHz to 806MHz that have been set aside by US government for wireless microphone systems.

The shorter wavelength of the UHF radio waves creates higher energy transmissions that punch through interference. Greater bandwidth is allowed for UHF signals (eight times more than the High Band VHF), permitting a larger number of available frequencies without compromising the intervals between frequencies. This allows more systems to operate simultaneously - a significant benefit in complex setups and concert applications. By the way, you can also run both UHF and VHF systems in the same location without mutual interference.

A drawback of the shorter UHF wavelength in wireless microphones is a reduced range for a UHF signal compared to a VHF signal. In addition, UHF transmitters work better when there is a "line-of-sight" between transmitter and receiver operation. Both of these limitations have to do with the way that the shorter wavelengths of the UHF band reflect from small metal objects and move around obstacles such as corners or intervening barriers. The susceptibility of UHF transmitters to interference and dropouts is best addressed by diversity receivers discussed below.

Historically the price differential between the VHF wireless microphones and the UHF wireless microphones was large and made the VHF wireless microphones popular for people with limited budgets. However, it's now possible to purchase basic UHF microphone systems at prices comparable to VHF systems which gives UHF wireless microphones a distinct advantage for pro-audio uses.


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