Two-way radios can operate on many different frequencies, and these
frequencies are assigned differently in different countries. Typically
channelized operations are used, so that operators need not tune equipment to a
particular frequency but instead can use one or more pre-selected frequencies,
easily chosen by a dial, a pushbutton or other means. For example, in the
United States, there is a block of 5 channels (pre-selected radio frequencies)
are allocated to the Multiple Use Radio System. A different block of 22
channels are assigned, collectively, to the General Mobile Radio Service and
Family Radio Service. The citizen's band radio service
(""CB"") has 40 channels.
In an analog, conventional system, (the simplest type of system) a
frequency or channel serves as a physical medium or link carrying communicated
information. The performance of a radio system is partly dependent on the
characteristics of frequency band used. The selection of a frequency for a
two-way radio system is affected, in part, by:[18]
Government licensing and regulations.
local congestion or availability of frequencies.
terrain, since radio signals travel differently in forests and urban
viewsheds.
the presence of noise, interference, or intermodulation.
sky wave interference below 50–60 MHz and tropospheric
bending at VHF.
In the US, some frequencies require approval of a frequency coordination
committee.
A channel number is just a shorthand notation for a frequency. It is,
for instance, easier to remember "Channel 1" than to remember
"26.965 MHz" (US CB Channel 1) or "462.5625 MHz" (FRS/GMRS
channel 1), or "156.05 MHz" (Marine channel 1). It is necessary to
identify which radio service is under discussion when specifying a frequency by
its channel number. Organizations such as electric power utilities or police
departments may have several assigned frequencies in use with arbitrarily
assigned channel numbers. For example, one police department's "Channel
1" might be known to another department as "Channel 3" or may
not even be available. Public service agencies have an interest in maintaining
some common frequencies for inter-area or inter-service coordination in
emergencies
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