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Radio Frequenecy Band Overview - Police-Scanner.info

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 result. 

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. 

If, 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

The information contained on this page is broken down into the following topics:

  • The FCC (Federal Communications Commission)

  • Radio Frequencies

  • Radio Frequency Allocations

  • Radio Frequency Bands

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The FCC (Federal Communications Commission)

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 systems. 

The Wireless 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 frequency bands.

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.

About Radio Frequencies

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 so on. 

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. 

For fire 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.

Channel Designations

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 (CTCSS)

CTCSS (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 neighboring agency.

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 users. 

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 capability. 

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.

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