History of radio § Broadcasting

                            

                                 Two way voice communication

A two-way radio is an audio transceiver, a receiver and transmitter in the same device, used for bidirectional person-to-person voice communication. An older term for this mode of communication is radio telephony. The radio link may be half-duplex, as in a walkie-talkie, using a single radio channel in which only one radio can transmit at a time, so different users take turns talking, pressing a "push to talk" button on their radio which switches off the receiver and switches on the transmitter. Or the radio link may be full duplex, a bidirectional link using two radio channels so both people can talk at the same time, as in a cell phone.

·         Cell phone – a portable wireless telephone that is connected to the telephone network by radio signals exchanged with a local antenna at a cellular base station (cell tower). The service area covered by the provider is divided into small geographical areas called "cells", each served by a separate base station antenna and multichannel transceiver. All the cell phones in a cell communicate with this antenna on separate frequency channels, assigned from a common pool of frequencies.

The purpose of cellular organization is to conserve radio bandwidth by frequency reuse. Low power transmitters are used so the radio waves used in a cell do not travel far beyond the cell, allowing the same frequencies to be reused in geographically separated cells. When a user carrying a cellphone crosses from one cell to another, his phone is automatically "handed off" seamlessly to the new antenna and assigned new frequencies. Cellphones have a highly automated full duplex digital transceiver using OFDM modulation working in the UHF or microwave band using two digital radio channels, each carrying one direction of the bidirectional conversation, as well as a control channel that handles dialing calls and "handing off" the phone to another cell tower. Existing 2G, 3G, and 4Gnetworks use frequencies in the low microwave range, between 700 MHz and 3 GHz. The cell phone transmitter usually has two power levels which it switches between when needed: 0.6 W when near a cell tower, and 3 W when farther away, while cell tower channel transmitter power is 50 W. Current generation phones, called smartphones, have many functions besides making telephone calls, and therefore have several other radio transmitters and receivers that connect them with other networks: usually a WiFi modem, a Bluetooth modem, and a GPS receiver.

o    G cellular network – next generation cellular networks which began deployment in 2019. Their major advantage is much higher data rates than previous cellular networks, up to 10 Gbps; 100 times faster than the previous cellular technology, 4G LTE. The higher data rates are achieved by using higher frequency radio waves in or near the millimeter wave band, around 28 and 39 GHz. Since millimeter waves are absorbed by atmospheric gases they have shorter range than microwaves. Therefore 5G cells will be the size of a city block, smaller than the cells in previous cellular networks which could be many miles across. Instead of a large cell base station and antenna tower, 5G networks will have many small antennas attached to utility poles and buildings.

·         Satellite phone (satphone) – a portable wireless telephone similar to a cell phone, connected to the telephone network through a radio link to an orbiting communications satellite instead of through cell towers. They are more expensive than cell phones; but their advantage is that, unlike a cell phone which is limited to areas covered by cell towers, satphones can be used over most or all of the geographical area of the Earth. In order for the phone to communicate with a satellite using a small omnidirectional antenna, first generation systems use satellites in low earth orbit, about 400–700 miles (640–1,100 km) above the surface. With an orbital period of about 100 minutes a satellite can only be in view of a phone for about 7 minutes, so the call is "handed off" to another satellite when one passes beyond the local horizon. Therefore large numbers of satellites, about 66, are required to ensure that at least one satellite is in view continuously from each point on Earth. Other satphone systems use satellites in geostationary orbit in which only a few satellites are needed, but these cannot be used at high latitudes because of terrestrial interference.

·         Cordless phone- a landline telephone in which the handset is portable and communicates with the rest of the phone by a short range full duplex radio link, instead of being attached by a cord. Both the handset and the base station have low power FM radio transceivers operating in the UHF band that handle the short range bidirectional radio link.

·         Land mobile radio system – short range mobile or portable half-duplex radio transceivers operating in the VHF or UHF band that can be used without a license. They are often installed in vehicles, with the mobile units communicating with a dispatcher at a fixed base station. Special systems with reserved frequencies are used by first responder services; police, fire, ambulance, and emergency services and other government services. Other systems are made for use by commercial firms such as taxi and delivery services. VHF systems use channels in the range 30–50 MHz and 150–172 MHz. UHF systems use the 450–470 MHz band and in some areas the 470–512 MHz range. In general, VHF systems have longer range than UHF but require longer antennas. AM or FM modulation is mainly used, but digital systems such as DMR are being introduced. Radiated power is typically limited to 4 watts.^[7] These systems have a fairly limited range, usually 3 to 20 miles (4.8 to 32 km) depending on terrain. Repeaters installed on tall buildings, hills or mountain peaks are often used to increase the range, when it is desired to cover a larger area than line-of-sight. Examples of land mobile systems are CB, FRS, GMRS, and MURS. Modern digital systems, called trunked radio systems, have a digital channel management system using a control channel which automatically assigns frequency channels to user groups.

·         Walkie-talkie – a battery powered portable handheld half-duplex two-way radio, used in land mobile radio systems.

·         Airband – radio system used by aircraft pilots to talk to other aircraft and ground-based air traffic controllers. This vital system is the main communication channel for air traffic control. For most communication in overland flights in air corridors a VHF-AM system using channels between 108–137 MHz in the VHFband are used. This system has a typical transmission range of 200 miles (320 km) for aircraft flying at cruising altitude. For flights in more remote areas, such as transoceanic airline flights, aircraft use the HF band or channels on the Inmarsat or Iridium satphone satellites. Military aircraft also use a dedicated UHF-AM band from 225.0–399.95 MHz.

·         Marine radio – medium range transceivers on ships, used for ship-to-ship, ship-to-air and ship-to-shore communication with harbormasters They use FM channels between 156 and 174 MHz in the VHF band with up to 25 watts power, giving them a range of about 60 miles (97 km). Some channels are half-duplex and some are full-duplex, to be compatible with the telephone network, to allow users to make telephone calls through a marine operator.

·         Amateur radio – long range half-duplex two way radio used by hobbyists for noncommercial purposes: recreational radio contacts with other amateurs, volunteer emergency communication during disasters, contests, and experimentation. Radio amateurs must hold an amateur radio license and are given a unique callsign that must be used as an identifier in transmissions. Amateur radio is restricted to small frequency bands, the amateur radio bands, spaced throughout the radio spectrum from 136 kHz to 2.4 GHz. Within these bands amateurs are allowed freedom to transmit on any frequency with a wide variety of modulation methods. In addition to radiotelephony, amateurs are the only radio operators still using obsolete Morse code radiotelegraphy.