A two-way radio is a radio that can both transmit and receive radio waves (a transceiver), unlike a broadcast receiver which only receives content. It is an audio (sound) transceiver, a transmitter and receiver in one unit, used for bidirectional person-to-person voice communication with other users with similar radios.
Two-way radios are available in stationary (base station), mobile (installed in vehicles), and hand-held portable models. Hand-held two-way radios are often called walkie-talkies, handie-talkies or hand-helds.
Two-way radios are used by groups of geographically separated people who need to keep in continuous voice Radios de comunicacion such as aircraft pilots and air traffic controllers, ship captains and harbormasters, emergency services personnel like firefighters, police officers, and ambulance paramedics, taxi and delivery services, soldiers and military units, fast food and warehouse employees, and radio amateurs.
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Two-way radio systems usually use a single radio channel and operate in a half-duplex mode: only one user on the channel can transmit at a time, so users in a user group must take turns talking. The radio is normally in receive mode so the user can hear all other transmissions on the channel.
When the user wants to talk, they press a “push-to-talk” button, which turns off the receiver and turns on the transmitter; when the button is released, the receiver is activated again. Multiple channels are provided so separate user groups can communicate in the same area without interfering with each other.
Other two-way radio systems operate in full-duplex mode, in which both parties can talk simultaneously. This requires either two separate radio channels or channel sharing methods such as time-division duplex (TDD) to carry the two directions of the conversation simultaneously on a single radio frequency. A cell phone is an example of a full-duplex two-way radio.
During a phone call, the phone communicates with the cell tower over two radio channels; an incoming one to carry the remote party’s voice to the user, and an outgoing one to carry the user’s voice to the remote party.
Installation of receivers and transmitters at the same fixed location allowed exchange of messages wirelessly. As early as 1907, two-way telegraphy traffic across the Atlantic Ocean was commercially available. By 1912, commercial and military ships carried both transmitters and receivers, allowing two-way communication in close to real-time with a ship that was out of sight of land.
The first truly mobile two-way radio equipment was developed in Australia in 1923 by Senior Constable Frederick William Downie of the Victorian Police. The Victoria Police were the first in the world to use wireless communication in cars, putting an end to the inefficient status reports via public telephone boxes which had been used until that time. The first sets took up the entire back seat of the Lancia patrol cars.
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As radio equipment became more powerful, compact, and easier to use, smaller vehicles had two-way radio communication equipment installed. Installation of radio equipment in aircraft allowed scouts to report back observations in real-time, not requiring the pilot to drop messages to troops on the ground below or to land and make a personal report.
In 1933, the Bayonne, New Jersey police department successfully operated a two-way system between a central fixed station and radio transceivers installed in police cars; this allowed rapidly directing police response in emergencies. During World War II walkie-talkie hand-held radio transceivers were extensively used by air and ground troops, both by the Allies and the Axis.
Early two-way schemes allowed only one station to transmit at a time while others listened since all signals were on the same radio frequency – this was called “simplex” mode. Code and voice operations required a simple communication protocol to allow all stations to cooperate in using the single radio channel so that one station’s transmissions were not obscured by another’s.
By using receivers and transmitters tuned to different frequencies and solving the problems introduced by operation of a receiver immediately next to a transmitter, simultaneous transmission and reception was possible at each end of a radio link, in so-called “full duplex” mode.