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Welcome to our Conventional Radio System
information page. This page is designed to give you an overview of
conventional public safety radio technology, systems, concepts,
and links to where you can learn more about conventional radio
communications.
The information is divided into the following
sections:
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Conventional
Radio System Overview
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Conventional
Simplex Radio Systems
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Conventional
Duplex Radio Systems
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Conventional
Radio Resources
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Conventional Radio System Overview
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In a conventional radio system, a frequency is used for each
channel of communication. To hear transmissions
for what you want to listen to, you must have the specific
frequency for that channel programmed into your police
scanner. For example, a local police department’s
dispatch channel would be on one frequency and their car-to-car channel
would be on another separate frequency.
In small towns/cities, police and fire agencies will
have several frequencies that they will use for daily operations.
You will find a separate frequency for dispatch, car-to-car. For fire
departments, they may have dispatch,
response coordination, on-scene "fireground" tactical,
all on one frequencies.
In large towns/cities, police and fire agencies will have several frequencies that they will use for daily operations.
You will find a separate frequency for dispatch,
car-to-car, tactical use, detectives, and so on. For fire
departments,
you will find separate frequencies for dispatch,
response coordination, on-scene "fireground" tactical
use, paramedics, and so on.
Conventional radio systems can operate in one of
two ways; Simplex and Duplex.
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Conventional
Simplex Radio Systems
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In a simplex radio system, also referred to as "Direct,"
all users transmit and receive on one frequency, but cannot talk at
the same time. This is the way
everybody did it many years ago. A lot of organizations and
agencies still do (especially smaller ones).
Simplex is also used
for local tactical communications - for example,
"Fireground"
communications at a fire or police units on the scene of a call. Since VHF and UHF radio signals are
generally limited to line of sight, range is short. It is possible for a scanner listener (or system user) to be able to
hear one side of a conversation, but not the other because one
station or unit is in range of you, and the other is not.
Simplex systems may also be a two-frequency system. In a
two-frequency system, one frequency is for output (base talks out)
and the other is for input (mobiles talk back).
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Conventional
Duplex Radio
Systems |
There are two types of Duplex Systems; Semi-Duplex and Full-Duplex.
In a semi-duplex, one or more users (usually dispatch) can transmit
and receive at the same time. In a full-duplex, all users can
transmit and receive at the same time just like talking on the
telephone.
Repeated Duplex
Commonly referred to as "repeater" or
"repeated". In a repeated radio system, there is a
special radio called a
repeater that is normally on a mountaintop or other high location that listens on one
channel (the input) and retransmits whatever it receives onto
another channel (the output) in real time. Field radios (mobiles) transmit
on the input frequency and listen to the output frequency
(dispatch). Since
the repeater is normally located on a high location, range is
generally much greater than a simplex radio system; anyone in
range of the repeater can talk to anyone else who is also in range
of the repeater - even if they are not in range of each other. For a scanner user, you
normally only need to monitor the output frequency (dispatch) because the
repeater retransmits all the communications. If you can hear the
repeater, you hear everybody. If you are out of range of the
repeater, you are out of communications. In extremely simple
terms, all trunked systems are repeated duplex.
Non-Repeated Duplex
In a non-repeated duplex radio system, some
users transmit on one channel and some on a different. Normally,
the dispatcher transmits on one channel (output) and the field personnel
talk on another (input). There is no repeater so one channel does
not get repeater onto the other.
The California Highway Patrol does it in most of the state. For
example, in my area (Baldwin Park CHP office), the dispatchers
transmit on 42.8800 MHz and the mobiles transmit on 42.6600 MHz. The
only way to hear both sides of the conversation is to monitor both
channels. In
the current CHP mobile radios, the radio scans between the base
and mobile channels so an officer can hear another car a mile down
the freeway. In the previous generation of CHP mobile radios,
there were two receivers, one listening to the base and the other
to the mobile channel.
One of the disadvantages of a non-repeated
radio system is that a mobile user can not tell that there is
another station transmitting on the input channel. If a second
user starts transmitting, generally no one is heard. The LA County Sheriff's
Department normally operates its dispatch channels in non-repeated mode. They
actually do have repeaters, but have decided to not have them
repeat most of the time. Because of the previously mentioned
problem of one deputy not being able to tell that there is another
person transmitting on the input channel, LASO has their repeaters
(that are not repeating) transmit a "busy" tone on the
output whenever there is someone transmitting on the input as heard
on this audio file. That
way one deputy knows that the channel is busy and he should not
transmit.
### Special thanks to Jim Walls for his input in
the above information ###
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Conventional
Radio Resources |
An
extensive and concise overview of public safety radio communication systems and
much more can be found in books listed on our Public
Safety Scanning Books page.
Other Informational Resources:
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