Two-way radios are radios which can receive and transmit, in contrast to broadcast receivers which can only receive. They are available in stationary, mobile and hand-held configurations. The hand-held versions of two-way radios are often called walkie-talkies. Cell phones  are also considered two-way radios, as they can both transmit and receive data simultaneously.

1. History:
The first two-way radio technology was available as early as 1907. In 1912, military and commercial ships were equipped with two-way radios. The first mobile two-way radios were introduced in 1923 in Australia by a Senior Constable of the Victorian Police. The first units were installed in Victorian police cars and took up the entire back seat. In 1933, the technology spread to the United States, where the Bayonne, New Jersey police department installed two-way radios into its police cars. They were also widely used amongst ground and air troops during World War II. Today, two-way radios are compact and easy to use for a variety of personal and commercial purposes.

2. Function:
Two-way radios are used to facilitate contact between two or more parties when visibility or distance is an impeding factor. They are practical devices for staying connected to a group. Police and fire stations, security personnel and even soldiers in the military utilize two-way radios.

3. Types:
There are many different types of two-way radios. Each family has a variety of sub-types and specific radio models. Conventional two-way radios operate on a fixed RF channel, with the proper channel being chosen by the user. Trunked two-way radio systems automatically pick up the physical frequency channel. Simplex two-way radios use a single channel for receiving and transmission. Aircraft and Marine systems are usually simplex radios. Duplex two-way radios transmit and receive on two different channels and typically require a repeater or base box to function.

Push-to-talk two-way radios are usually found on modern cell phones and walkie-talkies. They feature a button that, when pushed in or in one direction, allows transmission. When the button is not pushed or pushed in an opposite direction, messages may be received. Analog, digital and hybrid are other types of two-way radios.

4. Frequencies
Two-way radios are capable of operating on many different radio frequencies. These frequencies are assigned by the country in which the radio is being used. Each country assigns frequencies based upon international agreements to different two-way services. In the United States, for example, there is an assigned block of 22 different channels. A channel is a shorthand term for a specific frequency.

5. Formats
Two-way radios are available in two primary formats–Ultra High Frequency (UHF) and Very High Frequency (VHF). A UHF station requires 3,000,000 watts to operate over a range of 60 miles, while a VHF station uses only 100,000 watts over 60 miles. Outdoor usage is best suited to VHF. If the radio will be used mostly indoors, UHF may be the best choice. Typically, the lower the frequency, the more effective it is for long range use.

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