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Interoperable Communications Technology Glossary


700 MHz: A radio, system or channel that operates within the range of the 794-815 MHz. These systems may or may not be trunked radio systems.

800 MHz: A radio, system or channel that operates within the range of the 839-860 MHz. These systems may or may not be trunked radio systems.


Amplification: The process of increasing the strength of a radio signal.


Backward compatibility: Ability of new units to operate within an "old" system infrastructure or to directly intercommunicate with an "old" unit.

Band pass filter: A filter that allows a certain range of frequencies to pass but which will reject frequencies below and above the desired range.

Band Plan: A plan to allocate different frequencies within a range for specific purposes and users.

Bandpass: The frequency range that a receiver is currently tuning or that a filter permits to pass through it.

Bandwidth: The difference between the limiting frequencies of a continuous frequency band. Typically measured in kilohertz. May be considered the amount in kilohertz required for a single communications channel.


Call Sign: A group of letters and numbers used to identify a station and the country authorizing its operation.

Calling Frequency: An agreed-upon frequency where stations attempt to contact each other; once contact is made, stations move to a working frequency.

Car to car: To communicate with another station without using a repeater. To transmit and receive on the same frequency. Also referred to as direct or simplex.

Carrier: The un modulated output of a radio transmitter.

Channel Integrator: A fairly new and sophisticated, process/device or system which takes several inbound radio signals from a variety of bands and electronically interconnects them on the “outbound side” to one or more other otherwise incompatible radio channels. Examples: JPS ACU-1000 switch and Ma/Com’s “Network First” switch

Channel: The frequency on which a radio transmission takes place, or the input and output frequency pair used by a repeater station.

Co-channel Interference: Interference from stations on frequencies adjacent to the desired signal.

Console  Patch: Buttons or icons in the dispatcher’s radio console that (when properly selected) permit the ability to “patch” or connect two dissimilar regular radio channels or “trunked radio talkgroup” together for a specific conversation. (Example: Patching the VHF Fire Channel to the UHF Street Department Channel)

Control Point: The physical location from which a radio station’s functions (setting frequency, turning the station off and on, etc.) are controlled.

Cross Band Repeater: A single device which receives inbound radio traffic on one channel in one band (say MINSEF on VHF @ 155.475 MHz) and rebroadcasts it out on another channel in another band (say NPSPAC InterOp Channel 1 at 866.0125 MHz) and vice-versa


Dead Zone: A region where a radio signal cannot be received due to propagation difficulties.

Decibel: The ratio between two power levels on a logarithmic scale. A 3 decibel increase is a doubling of power; a 20 decibel increase is a power increase of 100 times.

Digital & Analog: The methods of modulating a radio signal as it travels through the air. Example: Digital signals are the speaker’s words turned into a series of 1’s and 0’s, which are then transmitted through the air, received at the other end and then reconstituted back into the audio sounds of the speaker’s words. Analog signals are the speaker’s words formed into wave forms and the wave forms are then sent through the air and can be output through a loudspeaker at the receiving end. Generally, analog wave forms require a wider radio channel bandwidth than a digital transmission carrying the same sound, but digitally.

Direct: To communicate with another station without using a repeater. To transmit and receive on the same frequency. Also referred to as simplex; or car to car.

Duplex: To transmit on one frequency while listening for replies on another.


Effective Radiated power: The output of a transmitter multiplied by the gain of an antenna.

Encryption: In digital radio systems, all radio transmissions are “digitized” (broken down into a formatted series of 1’os and 0’s according to some logic mutually known to all radios on that system) at a minimal, standards based level. In an “encrypted” system, these digital bits are re-coded one again based on a secret code known only to the radios within the system that have been programmed with that same secret code. In older analog radio systems, if encryption was employed, analog voice sounds were digitized, and then sent over the analog channel as an analog wave form to the distant radio, which had to have the similar decryption program in it to unscramble what the transmitting radio sent. This technique dramatically reduced the effective range of the otherwise analog signal, whereas with modern encryption employed over already digital systems, there is no degradation in signal quality or range.


Feedline: The cable connecting a radio to an antenna.

Filter: A circuit or device that will allow certain frequencies to pass while rejecting others.

Fixed Station: A station that always operates from a constant, specified land location.


Gain: The apparent increase in the strength of a signal radiated or received by an antenna caused by the antenna having better performance in some directions than others.

Gigahertz: Unit equal to 1000 megahertz or 1,000,000 kilohertz

Ground Wave: A radio wave propagated along the surface of the Earth.

Ground: A connection to a point of zero voltage, like the Earth.


Hard  Patch: A system whereby a dedicated radio is installed to take what it hears and always patch it over to another radio system channel or talk group, and (sometimes) vice-versa. (Example: The Street Department’s UHF radio channel is “hard patched” to a talk group in the police department’s trunked radio system which could be called “streets”).

Hertz: One complete cycle of a radio wave per second.

Heterodyne: A high pitched "whistle" sound caused by two carriers interfering with each other. The pitch of the "whistle" depends on the frequency difference between the carriers.

High VHF: 150-174 MHz


Input Frequency: The frequency on which a repeater station listens for signals to retransmit.

Intermod: Short for "intermodulation," this means false or spurious signals produced by two or more signals mixing in a receiver or repeater station.


Kilohertz: Unit equal to 1000 hertz.

Kilowatt: Unit equal to 1000 watts of transmitter power.


Low Band: A system or channel that operates in the 39-45 MHz band.

Low UHF: 450-470 MHz

Low VHF: 30-50 MHz


Megahertz: Unit equal to 1,000,000 hertz or 1000 kilohertz.

Microwave: A “point to point” radio system that transmits signals (often data) from one fixed point (microwave dish) direct, in a line of sight mode, to another fixed point. Often microwave is implemented to “bypass” expensive, monthly recurring costs for “data lines” leased from the phone company.

Mobile Station: A two-way radio unit installed in a car, boat, plane, etc., and used while in motion or at various stops.

Multiband Antenna: An antenna suitable for operation on several different bands of frequencies.


Nextel Re-Banding: An FCC mandated process (begun in about 2005 and lasting several years in several “waves”) under which numerous public safety licensees operating radio systems in the approximately 821 and 866 MHz were relocated (at Nextel’s expense) on the frequency spectrum to a lower spot (but still within 800 MHz), so as to create one contiguous segment of the spectrum for such public safety licensees as well as to create a buffer zone between public safety frequencies and Nextel frequencies. Due to the lack of such a buffer zone, there had been numerous instances of harmful interference between Nextel systems and public safety 800 MHz systems.

Notch Filter: A circuit that takes a small "slice" out of the bandpass tuned by a receiver; this is useful for reducing interference from narrow bandwidth signals.

NPSPAC: A radio, system or channel that operates in the 800-815 & 845-860 MHz bands. (National Public Safety Planning Advisory Committee), formerly sub group of 800 MHz channels set aside from the bulk of earlier allocated public safety 800 MHz channels. NPSPAC channels also operate under more stringent FCC rules regarding power output by base stations, so as to permit more channel re-use across a geographic area such as a state.


Omnidirectional Antenna: An antenna that transmits and receives equally well in all directions.

Output Frequency: The frequency on which a repeater station will retransmit signals it hears on its input frequency.


P25: APCO Project-25 Digital Standard, also referred to as Common Air Interface (CAI). APCO Project 25, or "P-25" for short, is the public safety industry standard developed by the APCO International (Association of Public Safety Communications Officials) to provide a radio solution that would allow different agencies using different radio systems to interoperate according to a public safety industry standard and not by system manufacturer.

Patch: A control center subsystem that permits a mobile or portable radio on one channel to communicate with one or more radios on a different channel through the dispatch console.

Priority Channel: A channel a scanner will immediately switch to when a signal is present.

Propagation: The process of how a radio signal travels. 

PTT: Abbreviation for "Push-to-Talk," the switch on a subscriber unit which, when pressed, causes the subscriber unit to transmit.


Radio Extender/Vehicular Repeater: The use of a hand-held portable radio to talk to the vehicle which the operator is associated with, and then the vehicle radio re-broadcasts the communication into the main radio system, and vice-versa. The portable is usually on a different radio band than the main vehicle radio to which it is connected back in the vehicle. AKA Pac R/T, “Pack Rat” or Vehicular Repeater.

Repeater: A base transmitter/receiver that exists for the purpose of receiving inbound radio traffic on one channel in a band and re-broadcasting it out at base station power on a companion channel in that same band. Example: A VHF repeated channel has field radios talking in on a channel like 156.015 MHZ and the base repeater re-broadcasting the signal back out on 155.79 MHz, to which all radios on that channel are selected for hearing the repeater output.


Satellite (or voting) Receiver System: A set (more than one) of receive only devices placed out in the field at various locations to pick up weaker inbound radio signals and bring them into the “head end” via phone lines, microwave or other means to present them to the dispatcher and/or to the repeater to be re-broadcast out.

Simplex: To transmit and receive on the same frequency. Also referred to as direct; or car to car.

Simulcast: The process of having a radio signal transmitted simultaneously from multiple transmitters in different locations so as to increase the area over which that radio signal can be heard, as well as the chances of that radio signal effectively penetrating dense buildings.

Skip: Any type of sky wave propagation via ionospheric refraction.

Squelch Tail: A brief bit of noise heard between the end of a radio transmission and the reactivation of the receiver’s squelch circuit.

Squelch: A circuit in a radio receiver that quiets the receiver until the strength of a received signal exceeds a specified level.


Talkgroup: A selection on a trunked radio’s “channel selector” switch which defines a grouping of persons, users, agencies or radios as a place for them to “meet and talk” via radio. Example: “Police Dispatch South” talkgroup would be where all “dispatchable” police units in an agency’s “south district” would monitor to hear calls dispatched and responded to. Some refer to talkgroups as “virtual channels” in that they only really exist in the radio system when they need to be used, and the rest of the time they only exist on paper, or in a series of instructions programmed into the trunked radios and system.

Telemetry: One-way radio transmissions used for tracking and measurement data.

Time-Out: To transmit too long in a single transmission, causing a repeater’s timer circuit to stop further transmissions.

Trunked: A computer managed radio system with multiple radio channels. For each transmission, the computer assigns a channel to the participants for the duration of that transaction.


UHF High: The frequency range from 760 to 860 MHz. 760-806 MHz radios are often called 700 MHz radios, 806 to 860 MHz radios are often called 800 MHz radios. Radios that cover the entire spectrum are often called dual band radios.

UHF Low: The frequency range from 450 to 470 MHz.

UHF: A system or channel in the Ultra High Frequency range around 450-460 MHz. 


Very High Frequencies: The frequency range from 30 to 300 MHz.

VHF High Band: The frequency range from 150 to 175 MHz. Most Public Safety radios operate in the 150-160 MHz range.

VHF Low Band: The frequency range from 30 to 50 MHz.

VHF: A radio, radio system or channel that operates in the Very High Frequency band around 150 Megahertz (MHz). If your FCC license says something like 155.250 MHz that’s a VHF channel.

Vocoder (Abbreviation for voice-coder): A device that usually consists of a speech analyzer, which converts analog speech waveforms into narrowband digital signals, and a speech synthesizer, which converts the digital signals into artificial speech sounds.


Yagi: A directional antenna consisting of a dipole connected to the receiver or transmitter and two additional elements, a slightly longer reflector and a slightly shorter director. Electromagnetic coupling between the elements focuses maximum power (or reception) in the direction of the director.