The scope of this page is to provide you a
basic overview of how radio bands and frequencies apply to radio
scanning. This section is not meant to be an explanation into how
the radio spectrum works, from which radio bands and frequencies
A technical explanation into how the radio spectrum works
would be well beyond the scope of this book. Besides, you do not
need to know all that technical mumbo-jumbo to enjoy scanning.
however, you are interested in learning the technical aspects of
how the radio spectrum works, check out the tutorial on how
the radio spectrum works.
information contained on this page is broken down into the
Every country has its own rules governing usage. In the US, the FCC is the
government agency that handles issues regarding radio
transmissions, usage, and licenses for all non-military radio
Telecommunications Bureau (WTB) is the branch of the FCC that
handles nearly all FCC domestic wireless telecommunications
programs and policies. To learn more about the wireless
communication services, click here.
The National Telecommunications &
Information Administrationís (NTIA)
Office of Spectrum Management (OSM)
is responsible for managing the Federal Government's use of the
radio frequency spectrum.
The US Radio Frequency Spectrum Allocation
establishes which radio services operate in a given frequency band.
There are thirty different radio services in over 450 separate
You can view the US Radio Frequency Allocation
Chart (in pdf format) here.
The chart graphically partitions the radio frequency spectrum,
extending from 9 kHz to 300 GHz, into over 450 frequency bands, and
uses distinct colors to distinguish the allocations for the thirty
different radio services. For more information, see: Basic
Elements of Spectrum Management.
A frequency is a path or "channel"
used for communications. It is a spot on a radio band identified by
its number. To hear transmissions for what you want to listen to,
you must have the specific frequency for that channel programmed
into your scanner.
For example, my local police departmentís
(Covina PD, CA) main dispatch frequency is 154.7250 MHz.
In busy/populated areas, police and fire
agencies will most likely have several frequencies that they use
for daily operations. You will most likely find a separate
frequency for dispatch, car-to-car, tactical use, detectives, and
My local police department has a secondary frequency for
car-to-car/tactical communications. In addition, there is a
separate channel for investigators and mutual aid.
agencies, you will most likely find a separate frequency for
dispatch, response coordination, on-scene "fireground"
tactical use, paramedics, and so on.
In trunk radio systems, frequencies are not
assigned to a particular use, they are assigned in a "pool"and
usage is based on availability. See our trunk radio page for a
complete overview of how trunk radio systems utilize frequencies.
A channel designation is used to identify a
frequency by name rather than by number. For example, Channel-1
(dispatch) and Channel-2, Frequency-1 and Frequency-2, or Blue
channel, Red channel, Primary and Secondary, etc.
Continuous Tone Coded Squelch System
(Continuous Tone Coded
Squelch System) is a system that is
used to avoid interference between separate agencies within close
proximity to each other using the same frequency.
Each radio for a
particular agency is programmed with a CTCSS code so that only
those radios can hear and talk with each other and not a
The system involves an industry standard set
of sub-audible tones for controlling radios and associated
equipment. The sub-audible tone is added to the transmitted signal.
The receiving radio is then set up to listen for this specific tone
in the received and demodulated audio. If the matching tone is
present, the squelch is opened up, allowing the audio to pass
through to the speaker.
If the tone is not present, then the radio
remains silent, even though there is a signal on the frequency.
This allows two or more agencies to use the same frequency
(generally on a repeater), but not hear each other's conversations.
CTCSS is also called "PL Tone" and
is the Motorola Proprietary name for CTCSS. Several other companies
have marketed their own brands of CTCSS under different names
including "Channel Guard", "Quiet Channel",
"Quiet Mode", and "Private Mode".
DCS on the other hand stands for Digitally Controlled Squelch. It is also called DPL or Digital Private Line
and Digital Channel Guard. DCS is a digital code that is sent to
open the squelch just like the tone does in CTCSS.
Some newer high-end scanners come with a CTCSS
DCS/PL decoder built in it. If an agency is the only user in the
area, there's no need to worry about a PL tone to filter out other
There are advantages however, like in busy metropolitan
areas where CTCSS helps keep a lot of unwanted junk out of your
speaker. Without CTCSS or DCS, your scanner will pick up a lot of
unwanted stuff you just don't want to hear.
On the flip side, I
live in eastern Los Angeles County where many public safety
agencies reside and have never experienced problems without CTCSS
For a more detailed explanation of CTCSS/PL & DPL
Tone Codes, see the Delaware Repeater Associationís article: CTCSS,
PL, Tone Squelch, and Other Necessary Evils.