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FREQUENCY BANDS
FOR WIRELESS SYSTEMS
Existing wireless microphone systems transmit and
receive on a specific radio frequency, called the operating
frequency. Individual radio frequencies are found in
frequency "bands" which are specific ranges of frequencies.
Use of radio frequencies in the United States is
regulated by the FCC (Federal Communication
Commission). The FCC has designated certain bands of
frequencies and certain frequencies in those bands for use
by wireless microphones, as well as by other services. In
the US, the frequencies used for wireless audio systems
may be grouped into four general bands or ranges:
low-band VHF (49-108 MHz), high-band VHF (169-216
MHz), low-band UHF (450-806 MHz) and high-band UHF
(900-952 MHz). VHF stands for "Very High Frequency,"
UHF stands for "Ultra High Frequency." (See Figure 3-1.)
The FCC further determines who can operate in
each band and who has priority if more than one user
is operating. "Primary" users include licensed
broadcasters (radio and television) and commercial
communications services (2-way radio, pagers, and
cellular telephones). Wireless microphone operators
are always considered to be "secondary" users. In
general, priority is given to primary users: secondary
users may not interfere with primary users, and
secondary users may be subject to interference from
primary users.
On the subject of licensing, it should be noted that
while manufacturers must be licensed by the FCC to
sell wireless equipment, it is the responsibility of the
operator to observe FCC regulations regarding their
actual use.
We will briefly describe each band and its
advantages and disadvantages for wireless
microphone system operation, based on the designated
users of the band, the physical characteristics of the
band, and the regulatory limitations of the band.
THE VHF BAND
At the beginning of the low-band VHF range is the 49
MHz region, used not only by wireless microphones but
also by cordless telephones, walkie-talkies, and radio
controlled toys. 54-72 MHz is occupied by VHF television
channels 2-4. The 72 MHz area is used by "assistive
listening" type wireless microphone systems. 76-88 MHz
is assigned to VHF television channels 5 and 6. At the
top, 88-108 MHz is the commercial FM radio broadcast
band. (See Figure 3-2.) All of these regions have been
used at one time or another for wireless microphone
systems. Allowable deviation limits (typically up to
15KHz) can accommodate high-fidelity audio (the same
as for FM broadcast). The propagation of these waves
through the air is very good, as is their ability to pass
through many non-metallic substances (a result of their
relatively long wavelength). The most attractive feature of
operation in this band is low equipment cost.
Except for assistive listening systems, however,
low-band VHF is not recommended for serious
applications. Due to the large number of primary and
secondary users, and high levels of general radio
frequency (RF) "noise," this band is prone to
interference from many sources. Transmitter power is
limited to less than 50 mW (except in the 72-76 MHz
range where up to 1 watt is allowed for assistive
listening systems). Finally, the minimum proper
antenna size for units in this range can be over one
meter long (one quarter of a five meter wave), which
can severely limit portability and/or efficiency.
Next is the high-band VHF range, widely used for
professional applications, in which quality systems are
available at a variety of prices. In the US, the
high-band VHF range is divided into two bands, which
are available to wireless microphone users. The first of
these, from 169-172 MHz, includes eight specific
frequencies designated by the FCC (Part 90.263b or
just "Part 90") for wireless microphone use by general
business. These frequencies are often referred to as
Selection
and Operation
of W
ireless Microphone Systems
22
C
H A P T E R
3
Wireless System Operation
Figure 3-1: wireless frequency bands in the United States
Figure 3-2: VHF allocations in the United States (30-300 MHz)