The simplest radio is the analog radio that transmits and receives on the same frequency. All radios build from this model. Imagine two people standing apart, each with a simple radio and antenna. When one talks, the other listens; and vise versa.
Radio operators wanted to get more distance from their equipment. Repeaters came into service. A repeater can rebroadcast (or repeat) a transmission from a higher tower, at a higher power and over a longer range. The radio operator will transmit on frequency “A” while the repeater listens on frequency “A”. The repeater rebroadcasts the transmission on frequency “B”. Other radio operators will tune to frequency “B” to hear the broadcast. Each radio will transmit on one frequency while receive on a second.
Analog radios transmit the operator’s voice directly by modulating either the frequency or amplitude depending on the mode. Digital radios encode the operator’s voice into a binary pattern. The binary pattern modulates the radio signal. This allows a digital radio to receive the binary pattern and convert it back to voice more clearly than an analog radio. Interference to the digital radio signal is less likely to influence the quality of the signal. Digital radio signals can also carry non-voice information. These include radio handset identification, unit numbers and location information.
In the past, public safety departments would acquire many different frequencies to cover all their projected needs. They may set aside frequencies for major incident coordination, training, and secondary activities. Unfortunately, these were also seldom events that occurred only a few times a year. This lead to a waste of resources to maintain the frequencies and the additional equipment. A trunked radio system separates the concept of a radio channel from a specific frequency. One frequency is designated as the control channel while all the other frequencies are open. All radios monitor the control frequency to get digital control signals from the central coordinating system. A user would never listen to this frequency as it is all digital control signals for the radio to use. A user may dial a channel called “dispatch”. The radio checks to see which frequency is currently assigned to the dispatch channel, and then tunes that frequency for the radio operator. Meanwhile, the radio will monitor the control channel in the event the dispatch channel changes frequency. If a radio is set to a channel that has no traffic, there will not be a frequency assigned. A trunked radio system may have only 20 radio frequencies to serve 100 channels. The radio of frequencies to channels really depends on how often each channel is expected to be used.
The frequency spectrum is divided into many bands, or areas of use. Frequency ranges can be assigned to any number of uses, such as: maritime, aeronautical, amateur, broadcast, fixed or mobile stations, land mobile, satellite, public safety and private/business. The NTIA frequency allocation chart shows all of these bands color coded. Some bands have multiple uses yet they’re always very similar, such as mobile satellite and fixed satellite.
The two way radio bands most used in emergency management are private land mobile (which includes public safety and business), amateur and to a lesser extent FRS, GMRS, and CB.
Satellite is another form of radio with a highly focused antenna. We’ll talk more about satellite frequencies in that section.
- Federal Communications Commission. (2010). Public safety and homeland security highlights. Retrieved from http://www.broadband.gov/issues/public-safety.html
- National Telecommications and Information Administration. (2003). U.S. Frequency Allocation Chart. Retrieved from http://www.ntia.doc.gov/osmhome/allochrt.html
- Radio Reference. (n.d.). Trunking basics. Retrieved from http://wiki.radioreference.com/index.php/Trunking_Basics