Two-way radio
Overview
 
A two-way radio is a radio
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...

 that can both transmit
Transmitter
In electronics and telecommunications a transmitter or radio transmitter is an electronic device which, with the aid of an antenna, produces radio waves. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating...

 and receive (a transceiver
Transceiver
A transceiver is a device comprising both a transmitter and a receiver which are combined and share common circuitry or a single housing. When no circuitry is common between transmit and receive functions, the device is a transmitter-receiver. The term originated in the early 1920s...

), unlike a broadcast receiver which only receives content. The term refers to a personal radio transceiver that allows the operator to have a two-way conversation with other similar radios operating on the same radio frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...

 (channel). Two-way radios are available in mobile
Mobile Radio
This article is about professional equipment. For mobile radios used in amateur radio, see amateur radio mobile operation. Mobile radio or mobiles refer to wireless communications systems and devices which are based on radio frequencies, and where the path of communications is movable on either...

, stationary base
Base station
The term base station can be used in the context of land surveying and wireless communications.- Land surveying :In the context of external land surveying, a base station is a GPS receiver at an accurately-known fixed location which is used to derive correction information for nearby portable GPS...

 and hand-held portable configurations.
Encyclopedia
A two-way radio is a radio
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...

 that can both transmit
Transmitter
In electronics and telecommunications a transmitter or radio transmitter is an electronic device which, with the aid of an antenna, produces radio waves. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating...

 and receive (a transceiver
Transceiver
A transceiver is a device comprising both a transmitter and a receiver which are combined and share common circuitry or a single housing. When no circuitry is common between transmit and receive functions, the device is a transmitter-receiver. The term originated in the early 1920s...

), unlike a broadcast receiver which only receives content. The term refers to a personal radio transceiver that allows the operator to have a two-way conversation with other similar radios operating on the same radio frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...

 (channel). Two-way radios are available in mobile
Mobile Radio
This article is about professional equipment. For mobile radios used in amateur radio, see amateur radio mobile operation. Mobile radio or mobiles refer to wireless communications systems and devices which are based on radio frequencies, and where the path of communications is movable on either...

, stationary base
Base station
The term base station can be used in the context of land surveying and wireless communications.- Land surveying :In the context of external land surveying, a base station is a GPS receiver at an accurately-known fixed location which is used to derive correction information for nearby portable GPS...

 and hand-held portable configurations. Hand-held radios are often called walkie-talkie
Walkie-talkie
A walkie-talkie is a hand-held, portable, two-way radio transceiver. Its development during the Second World War has been variously credited to Donald L. Hings, radio engineer Alfred J. Gross, and engineering teams at Motorola...

s or handie-talkies. Two-way radio systems usually operate in a half-duplex
Duplex (telecommunications)
A duplex communication system is a system composed of two connected parties or devices that can communicate with one another in both directions. The term multiplexing is used when describing communication between more than two parties or devices....

 mode; that is, the operator can talk, or he can listen, but not at the same time. A push-to-talk
Push to talk
Push-to-talk , also known as Press-to-Transmit, is a method of conversing on half-duplex communication lines, including two-way radio, using a momentary button to switch from voice reception mode to transmit mode....

 or Press To Transmit button activates the transmitter; when it is released the receiver is active. However a mobile phone
Mobile phone
A mobile phone is a device which can make and receive telephone calls over a radio link whilst moving around a wide geographic area. It does so by connecting to a cellular network provided by a mobile network operator...

 or cellular telephone is an example of a two-way radio that both transmits and receives at the same time (called full-duplex
Duplex (telecommunications)
A duplex communication system is a system composed of two connected parties or devices that can communicate with one another in both directions. The term multiplexing is used when describing communication between more than two parties or devices....

 mode). It uses two different radio frequencies (channels) to carry the two directions of the conversation simultaneously.

History

Installation of receivers and transmitters at the same fixed location allowed exchange of messages wirelessly. As early as 1907, two-way telegraphy
Telegraphy
Telegraphy is the long-distance transmission of messages via some form of signalling technology. Telegraphy requires messages to be converted to a code which is known to both sender and receiver...

 traffic across the Atlantic Ocean
Atlantic Ocean
The Atlantic Ocean is the second-largest of the world's oceanic divisions. With a total area of about , it covers approximately 20% of the Earth's surface and about 26% of its water surface area...

 was commercially available. By 1912 commercial and military ships carried both transmitter
Transmitter
In electronics and telecommunications a transmitter or radio transmitter is an electronic device which, with the aid of an antenna, produces radio waves. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating...

s and receivers
Receiver (radio)
A radio receiver converts signals from a radio antenna to a usable form. It uses electronic filters to separate a wanted radio frequency signal from all other signals, the electronic amplifier increases the level suitable for further processing, and finally recovers the desired information through...

, allowing two-way communication in close to real-time with a ship
Ship
Since the end of the age of sail a ship has been any large buoyant marine vessel. Ships are generally distinguished from boats based on size and cargo or passenger capacity. Ships are used on lakes, seas, and rivers for a variety of activities, such as the transport of people or goods, fishing,...

 that was out of sight of land.

The first truly mobile two-way radio was developed in Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...

 in 1923 by Senior Constable Frederick William Downie of the Victorian Police. The Victoria Police
Victoria Police
Victoria Police is the primary law enforcement agency of Victoria, Australia. , the Victoria Police has over 12,190 sworn members, along with over 400 recruits, reservists and Protective Service Officers, and over 2,900 civilian staff across 393 police stations.-Early history: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
Lancia
Lancia Automobiles S.p.A. is an Italian automobile manufacturer founded in 1906 by Vincenzo Lancia and which became part of the Fiat Group in 1969. The company has a long history of producing distinctive cars and also has a strong rally heritage. Some modern Lancias are seen as presenting a more...

 patrol cars.

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
Aviator
An aviator is a person who flies an aircraft. The first recorded use of the term was in 1887, as a variation of 'aviation', from the Latin avis , coined in 1863 by G. de la Landelle in Aviation Ou Navigation Aérienne...

 to drop messages to troops on the ground below or to land and make a personal report.

In 1933, the Bayonne, New Jersey
Bayonne, New Jersey
Bayonne is a city in Hudson County, New Jersey, United States. Located in the Gateway Region, Bayonne is a peninsula that is situated between Newark Bay to the west, the Kill van Kull to the south, and New York Bay to the east...

 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
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

 hand-held radio transceivers
Walkie-talkie
A walkie-talkie is a hand-held, portable, two-way radio transceiver. Its development during the Second World War has been variously credited to Donald L. Hings, radio engineer Alfred J. Gross, and engineering teams at Motorola...

 were extensively used by air and ground troops, both by the Allies and the Nazis.

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.

Early two-way schemes required training operators to learn and use Morse code
Morse code
Morse code is a method of transmitting textual information as a series of on-off tones, lights, or clicks that can be directly understood by a skilled listener or observer without special equipment...

; in ship-board installations the radio operating officer typically had no other duties than handling radio messages. When voice transmission became possible, dedicated operators were no longer required and two-way use became more common. Today's two-way mobile radio equipment is nearly as simple to use as a household telephone, from the point of view of operating personnel, thereby making two-way communications a useful tool in a wide range of personal, commercial and military roles.

Types

There is an array of two-way radio technologies, systems, and types. There are families of radio types and each family has differing sub-groups and specific radio models. Some of these types are listed below.

Conventional

Conventional radios operate on fixed RF channels. In the case of radios with multiple channels, they operate on one channel at a time. The proper channel is selected by a user. The user operates a channel selector or buttons on the radio control panel to pick the channel.

In multi-channel systems, channels are used for separate purposes. A channel may be reserved for a specific function or for a geographic area. In a functional channel system, one channel may allow City of Springfield road repair crews to talk to the City of Springfield's road maintenance office. A second channel may allow road repair crews to communicate with state highway department crews. In a geographic system, a taxi company may use one channel to communicate in the Boston, Massachusetts area and a second channel when taxis are in Providence, Rhode Island. In marine radio operations, one channel is used as an emergency and calling channel, so that stations may make contact then move to a separate working channel for continued communication.

Motorola uses the term mode to refer to channels on some conventional two-way radio models. In this use, a mode consists of a radio frequency channel and all channel-dependent options such as selective calling.
Scanning in conventional radios

Some conventional radios scan more than one channel. That is, the receiver searches more than one channel for a valid transmission. A valid transmission may be a radio channel with any signal or a combination of a radio channel with a specific CTCSS (or Selective calling
Selective calling
In a conventional, analog two-way radio system, a standard radio has noise squelch or carrier squelch which allows a radio to receive all transmissions. Selective calling is used to address a subset of all two-way radios on a single radio frequency channel...

) code.

There are a wide variety of scan configurations which vary from one system to another. Some radios have scan features that receive the primary selected channel at full volume and other channels in a scan list at reduced volume. This helps the user distinguish between the primary channel and others without looking at the radio control panel. An overview:
  • A scanning feature can be defined and preset: when in scanning mode, a predetermined set of channels is scanned. Channels are not changeable by the radio user.
  • Some radios allow an option for user-selected scan: this allows either lockout of pre-selected channels or adding channels to a scan list by the operator. The radio may revert to a default scan list each time it is powered off or may permanently store the most recent changes.

In professional radios, scan features are programmable and have many options. Scan features can affect system latency. If the radio has a twenty channel scan list and some channels have CTCSS, it can take several seconds to search the entire list. The radio must stop on each channel with a signal and check for a valid CTCSS before resuming scanning. This can cause missed messages.

For this reason, scan features are either not used or scan lists are intentionally kept short in emergency applications. Part of APCO Project 16
APCO-16
Project 16 or APCO Project 16 was a standard development effort started in the 1970s by the Association of Public-Safety Communications Officials-International , a trade association of mostly police and fire service providers...

 set standards for channel access times and delays caused by system overhead. Scan features can further increase these delays. One study said delays of longer than 0.4 seconds (400 milliseconds) in emergency services are not recommended. No delay from user push-to-talk until the user's voice is heard in your radio's speaker is an unattainable ideal.
Talk-back on scan

Some conventional radios use, or have an option for, a talk-back-on-scan function. If the user transmits when the radio is in a scan mode, it may transmit on the last channel received instead of the selected channel. This may allow users of multi-channel radios to reply to the last message without looking at the radio to see which channel it was on. Without this feature, the user would have to use the channel selector to switch to the channel where the last message occurred. (This option can cause confusion and users must be trained to understand this feature.)

This is an incomplete list of some conventional radio types:
  • Marine VHF radio
    Marine VHF radio
    Marine VHF radio is installed on all large ships and most seagoing small craft. It is used for a wide variety of purposes, including summoning rescue services and communicating with harbours, locks, bridges and marinas, and operates in the VHF frequency range, between 156 to 174 MHz...

  • Family Radio Service
    Family Radio Service
    The Family Radio Service is an improved walkie talkie radio system authorized in the United States since 1996. This personal radio service uses channelized frequencies in the ultra high frequency band. It does not suffer the interference effects found on citizens' band at 27 MHz, or the...

  • UNICOM
    UNICOM
    Universal Communications or as known by its abbreviation, UNICOM, is an air-ground communication facility operated by a private agency to provide advisory service at uncontrolled aerodromes and airports.-Description:...

  • Amateur Radio
    Amateur radio
    Amateur radio is the use of designated radio frequency spectrum for purposes of private recreation, non-commercial exchange of messages, wireless experimentation, self-training, and emergency communication...


Trunked

In a trunked radio system, the system logic automatically picks the physical radio frequency channel. There is a protocol that defines a relationship between the radios and the radio backbone which supports them. The protocol allows channel assignments to happen automatically.

Digital trunked systems may carry simultaneous conversations on one physical channel. In the case of a digital trunked radio system, the system also manages time slots on a single physical channel. The function of carrying simultaneous conversations over a single channel is called multiplexing
Multiplexing
The multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred...

.

Instead of channels, radios are related by groups which may be called, groups, talk groups, or divided into a hierarchy such as fleet and subfleet, or agency-fleet-subfleet. These can be thought of as virtual channels which appear and disappear as conversations occur.

Systems make arrangements for handshaking and connections between radios by one of these two methods:
  • A computer assigns channels over a dedicated control channel. The control channel sends a continual data stream. All radios in the system monitor the data stream until commanded by the computer to join a conversation on an assigned channel.
  • Electronics embedded in each radio communicate using a protocol of tones or data in order to establish a conversation, (scan-based).


If all physical channels are busy, some systems include a protocol to queue or stack pending requests until a channel becomes available.

Some trunked radios scan more than one talk group or agency-fleet-subfleet.

Visual clues a radio may be trunked include the 1) lack of a squelch knob or adjustment, 2) no monitor button or switch, and 3) a chirp (made infamous by Nextel) showing the channel is available and ready at the moment the push-to-talk is pressed.

This is an incomplete list of some trunked radio types:
  • Motorola Dimetra
    Dimetra
    DIMETRA IP is the brand name under which Motorola markets its implementation of the TETRA digital radio communications standard.-Overview:Tetra is a scalable radio network technology typically used by emergency services and other Government agencies...

  • Logic Trunked Radio
    Logic Trunked Radio
    Logic Trunked Radio is a system developed in the late 1970s by the E. F. Johnson Company.LTR is distinguished from some other common trunked radio systems in that it does not have a dedicated control channel. Each repeater has its own controller and all of these controllers are coordinated together...

  • EDACS
    EDACS
    EDACS stands for The Enhanced Digital Access Communication System, a radio communications protocol and product family invented in the General Electric Corporation in the mid 1980s.- History :...


Simplex

Simplex channel systems use a single channel for transmit and receive. This is typical of aircraft VHF AM and marine radios. Simplex systems are often legacy systems that have existed for years or decades. The architecture allows old radios to work with new ones in a single network. In the case of all ships worldwide or all aircraft worldwide, the large number of radios installed, (the installed base,) can take decades to upgrade. Simplex systems often use open architectures that allow any radio meeting basic standards to be compatible with the entire system.
  • Advantage: as the simplest system configuration, there is reliability from the fact that only two radios are needed to establish communication between them.
  • Disadvantages: The simplex configuration offers communication over the shortest range or distance because mobile units must be in effective range of each other. The available channel bandwidth limits the number of simultaneous conversations, since "dead" air time cannot be easily used for additional communication.

Duplex

Duplex channel systems transmit and receive on different discrete channels. This defines systems where equipment cannot communicate without some infrastructure such as a repeater
Radio repeater
A radio repeater is a combination of a radio receiver and a radio transmitter that receives a weak or low-level signal and retransmits it at a higher level or higher power, so that the signal can cover longer distances without degradation. This article refers to professional, commercial, and...

, base station
Base station
The term base station can be used in the context of land surveying and wireless communications.- Land surveying :In the context of external land surveying, a base station is a GPS receiver at an accurately-known fixed location which is used to derive correction information for nearby portable GPS...

 or Talk-Through Base. Most common in the US is a repeater configuration where a base station is configured to re-transmit the audio received from mobile units. This makes the mobiles, or hand-helds, able to communicate amongst one another anywhere within reception range of the base station or repeater. Typically the base or repeater station has a high antenna, which allows greater range, compared with a ground vehicle or hand-held transceiver.

Duplex systems can be divided into two types. The term half-duplex refers to systems where use of a push-to-talk switch is required to communicate. Full duplex refers to systems like mobile telephones with a capability to simultaneously receive and transmit.
  • Advantage: duplex channels usually allow repeater operation which extends range (in most cases due to increased transmit power and improved aerial location / height) - especially where hand-held radios are in use.
  • Disadvantage: If a radio cannot reach the repeater, it cannot communicate.

Hybrid simplex/duplex

Some systems use a mix of the two where radios use duplex as a default but can communicate simplex on the base station channel if out-of-range. In the US, the capability to talk simplex on a duplex channel with a repeater is sometimes called talk-around, direct, or car-to-car.

Push-to-talk

In one Motorola system, a Special Products microphone was created with a rocker-style push-to-talk button. The microphone looked like a normal mobile microphone except that the button rocked either up or down instead of pressing in. Rocking the switch in one direction transmitted duplex on a repeater; the other transmitted simplex on car-to-car.

In two-way radios with headsets, a push-to-talk button may be included on a cord or wireless electronics box clipped to the user's clothing. In an ambulance or aircraft, a button may be present where the corded headset plugs in to the radio wiring. Dispatch consoles often have a hand-operated push-to-talk buttons along with a foot switch or pedal. If the dispatcher's hands are on a computer keyboard, the user can step on the foot pedal to transmit. Some systems have muting so the dispatcher can be on a telephone call and the caller cannot hear what is said over the radio. Their headset microphone will mute if they transmit. This relieves the dispatcher of explaining every radio message to a caller.

In some circumstances, voice-operated transmit (VOX) is used in place of a push-to-talk button. Possible uses are handicapped users who cannot push a button, Amateur radio
Amateur radio
Amateur radio is the use of designated radio frequency spectrum for purposes of private recreation, non-commercial exchange of messages, wireless experimentation, self-training, and emergency communication...

 operators, firefighters, crane operators, or others performing critical tasks where hands must be free but communication is still necessary.

Analog versus digital

One example of analog radios are AM aircraft radios used to communicate with control towers and air traffic controllers. Another is a Family Radio Service walkie talkie. Equipment is less complex than digital.
  • Advantage: In high-quality equipment, better ability to communicate in cases where a received signal is weak or noisy.
  • Disadvantage: Only one conversation at a time can occur on each channel.


Examples of digital communication are APCO Project 25
P25
Project 25 is a suite of standards for digital radio communications for use by federal, state/province and local public safety agencies in North America to enable them to communicate with other agencies and mutual aid response teams in emergencies...

, a standard for digital public safety radios, Nextel's iDEN
Iden
For other uses of the word, see Iden The Iden was an English automobile manufactured from 1904 until 1907. Designed by George Iden, formerly of Daimler, they were four-cylinder 10/17 hp and 25/35 hp shaft-driven cars; each came with "Idens's frictionless radial gearbox"....

, Motorola's MOTOTRBO, and NXDN
NXDN
NXDN is a Common Air Interface technical protocol for mobile communications. It was developed jointly by Icom Incorporated and Kenwood Corporation. was formed in order to promote the NXDN protocol in North and South America...

 implemented by Icom as IDAS and by Kenwood as NEXEDGE.
  • Advantage: More simultaneous talking paths are possible and information such as unit ID, status buttons, or text messages can be embedded into a single digital radio channel.
  • Disadvantage: Radios must be designed to the same, compatible standard, radios can become obsolete quickly, cost more to purchase, and are more complicated.

Data over two-way radio

In some cases, two-way radio is used to communicate analog or digital data. Systems can be simplex or duplex and may employ selective calling features such as CTCSS. In full-duplex systems, data can be sent real-time between two points. In simplex or half-duplex, data can be sent with a time lag between many points.

Some two-way digital systems carry both audio and data over a single data stream. Systems of this type include NXDN
NXDN
NXDN is a Common Air Interface technical protocol for mobile communications. It was developed jointly by Icom Incorporated and Kenwood Corporation. was formed in order to promote the NXDN protocol in North and South America...

 and APCO Project 25
P25
Project 25 is a suite of standards for digital radio communications for use by federal, state/province and local public safety agencies in North America to enable them to communicate with other agencies and mutual aid response teams in emergencies...

. The method of encoding and decoding the audio stream is called a codec
Codec
A codec is a device or computer program capable of encoding or decoding a digital data stream or signal. The word codec is a portmanteau of "compressor-decompressor" or, more commonly, "coder-decoder"...

, such as the AMBE
Advanced Multi-Band Excitation
Advanced Multi-Band Excitation is a proprietary speech coding standard developed by Digital Voice Systems, Inc.- Overview :AMBE is a codebook-based vocoder that operates at bitrates of between 2 and 9.6 kbit/s, and at a sampling rate of 8 kHz in 20-ms frames...

 family of codecs.

After market GPS tracking and mobile messaging devices can be interfaced with popular two-way radio models providing a range of features.

Analog

Analog systems may communicate a single condition, such as water level in a livestock tank. A transmitter at the tank site continually sends a signal with a constant tone. The tone would change in pitch to indicate the tank's water level. A meter at the remote end would vary, corresponding to the pitch, to indicate the amount of water present in the livestock tank. Similar methods can be used to telemeter any analog condition. This type of radio system serves a purpose equivalent to a four-to-twenty milliampere loop. In the US, mid-band 72-76 MHz or UHF 450-470 MHz interstitial channels are often used for these systems. Some systems multiplex telemetry of several analog conditions by limiting each to a separate range of tone pitches, for example.

Digital

Digital systems may communicate text from computer-aided dispatch (CAD). For example, a display in a tow truck may give a textual location for a call and any related details. The tow truck driver may press an acknowledge button, sending data in the opposite direction and flagging the call as received by the driver. They can be used for analog telemetry systems, such as livestock tank levels, as described above. Analog conditions are translated into data words. Some systems send radio paging messages which can either 1) beep a paging receiver, 2) send a numeric message, or 3) send a text message.

Digital systems typically use data rates in the 1,200-19,200 kilobit-per-second rates and may employ modulation schemes such as frequency shift keying, audio frequency shift keying, or quadrature phase shift keying to encode characters. Modern equipment have the same capabilities to carry data as are found in Internet Protocol. Working within the system's protocol constraints, virtually anything can be sent or received.

Engineered versus not engineered

Engineered systems are designed to perform close to a specification or standard. They are designed as systems with all equipment matched to perform together. For example, a modern, local government two-way radio system in the US may be designed to provide 95% area coverage in an urban area. System designers use radio frequency models, terrain models, and signal propagation modeling software in an attempt to accurately estimate where radios will work within a defined geographic area. The models help designers choose equipment, equipment locations, antennas
Antenna (radio)
An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...

, and estimate how well signals will penetrate buildings. These models will be backed-up by drive testing and actual field signal level measurements. Designers adjust antenna patterns, add or move equipment sites, and design antenna networks in a way that will accomplish the intended level of performance.

Some systems are not engineered. Legacy systems are existing systems which were never designed to meet a system performance objective. They may have started with a base station and a group of mobile radios. Over a period of years, they have equipment added on in a building block style. Legacy systems may perform adequately even though they were not professionally designed as a coherent system. A user may purchase and locate a base station with an expectation that similar systems used in the past worked acceptably. A City Road Department may have a system that works acceptably, so the Parks Department may build a new similar system and find it equally usable. General Mobile Radio Service
General Mobile Radio Service
The General Mobile Radio Service is a licensed land-mobile FM UHF radio service in the United States available for short-distance two-way communication. It is intended for use by an adult individual who possesses a valid GMRS license, as well as his or her immediate family members...

 systems are not usually engineered.

Options, duty cycle, and configuration

1940s tube-type land mobile two way radios often had one channel and were carrier squelch
Squelch
In telecommunications, squelch is a circuit function that acts to suppress the audio output of a receiver in the absence of a sufficiently strong desired input signal.-Carrier squelch:...

. Because radios were costly and there were fewer radio users, it might be the case that no one else nearby used the same channel. A transmit and receive crystal had to be ordered for the desired channel frequency, then the radio had to be tuned or aligned to work on the channel. 12-volt mobile, tube-type radios drew several amperes on standby and tens-of-amperes on transmit. Equipment worked ideally when new. The performance of vacuum tubes gradually degraded over time. US regulations required an indicator lamp showing the transmitter had power applied and was ready to transmit and a second indicator, (usually red,) that showed the transmitter was on. In radios with options, wire jumpers and discrete components were used to select options. To change a setting, the technician soldered an option jumper wire then made any corresponding adjustments.

The trend is toward increasing complexity. Modern radios can have capacities over 100 channels and are synthesized: the internal electronics in modern radios operate over a range of frequencies with no tuning adjustments. High-end models may have several hundred optional settings and require a computer and software to configure. Sometimes, controls on the radio are referred to as programmable. By changing configuration settings, a system designer could choose to set up a button on the radio's control panel to either:
  • turn scan on or off,
  • alert another mobile radio, (selective calling),
  • turn on an outside speaker, or
  • select repeater locations.

Microprocessor-based radios can draw less than 0.2 amperes on standby and up to tens-of-amperes on high-powered, 100 watt transmitters.

Base stations, repeaters
Radio repeater
A radio repeater is a combination of a radio receiver and a radio transmitter that receives a weak or low-level signal and retransmits it at a higher level or higher power, so that the signal can cover longer distances without degradation. This article refers to professional, commercial, and...

, and high-quality mobile radios often have specifications that include a duty cycle
Duty cycle
In engineering, the duty cycle of a machine or system is the time that it spends in an active state as a fraction of the total time under consideration....

. A repeater should always be continuous duty. This means the radio is designed to transmit in a continuous broadcast without transmitter overheating and failure. Mobile radios used in emergency equipment are rated for continuous duty use. This is necessary because any one of an entire fleet of ambulances, for example, could be pressed into service as command post at a major incident.

Time delay is always associated with radio systems, but it is apparent in spacecraft communications. For Apollo program and Space Shuttle
Space Shuttle
The Space Shuttle was a manned orbital rocket and spacecraft system operated by NASA on 135 missions from 1981 to 2011. The system combined rocket launch, orbital spacecraft, and re-entry spaceplane with modular add-ons...

, Quindar tones
Quindar tones
Quindar Tones, most often referred to as the "beeps" that were heard during the American Apollo space missions, were a means by which remote transmitters on Earth were turned on and off so that the Capsule communicator could communicate with the crews of the spacecraft...

 were applied for operator assistance.

Life of equipment

Though the general life term for the two way radio is 5 to 7 years and 1 to 2 years for its accessories but still the usage, atmosphere and environment plays a major role to decide its life term. There are so many speculations on the life term of two way radios and their accessories i.e. batteries, chargers, head set etc.

In government systems, equipment may be replaced based on budgeting rather than any plan or expected service life. Funding in government agencies may be cyclical or sporadic. Managers may replace computing systems, vehicles, or budget computer and vehicle support costs while ignoring two-way radio equipment. Equipment may remain in use even though maintenance costs are unreasonable when viewed from an efficiency standpoint.

Different system elements will have differing service lifetimes. These may be affected by who uses the equipment. An individual contacted at one county government agency claimed equipment used by 24-hour services wears out much faster than equipment used by those who work in positions staffed eight hours a day.

One document says "seven years" is beyond the expected lifetime of walkie-talkies in police service. Batteries are cited as needing replacement more often. Twelve-year-old dispatch consoles mentioned in the same document were identified as usable. These were compared to problematic 21-year-old consoles used elsewhere in the same system.

Another source says system backbone equipment like consoles and base stations are expected to have a fifteen year life. Mobile radios are expected to last ten years. Walkie talkies typically last eight. In a State of California document, the Department of General Services reports expected service life for a communications console used in the Department of Forestry and Fire Protection is 10 years.

Two-way radio frequencies

Two-way radios can operate on many different frequencies
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...

, and these frequencies are assigned differently in different countries. Typically channelized operations are used, so that operators need not tune equipment to a particular frequency but instead can use one or more pre-selected frequencies, easily chosen by a pushbutton or other means. For example, in the United States, there is a block of 22 channels (pre-selected radio frequencies) assigned, collectively, to the General Mobile Radio Service
General Mobile Radio Service
The General Mobile Radio Service is a licensed land-mobile FM UHF radio service in the United States available for short-distance two-way communication. It is intended for use by an adult individual who possesses a valid GMRS license, as well as his or her immediate family members...

 and Family Radio Service
Family Radio Service
The Family Radio Service is an improved walkie talkie radio system authorized in the United States since 1996. This personal radio service uses channelized frequencies in the ultra high frequency band. It does not suffer the interference effects found on citizens' band at 27 MHz, or the...

.

In an analog, conventional system, (the simplest type of system,) a frequency or channel serves as a physical medium or link carrying communicated information. The performance of a radio system is partly dependent on the characteristics of frequency band used. The selection of a frequency for a two-way radio system is affected, in part, by:
  • government licensing and regulations.
  • local congestion or availability of frequencies.
  • terrain, since radio signals travel differently in forests and urban viewsheds.
  • the presence of noise, interference, or intermodulation.
  • sky wave interference below 50-60 MHz and tropospheric
    Troposphere
    The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 80% of the atmosphere's mass and 99% of its water vapor and aerosols....

     bending at VHF.
  • in the US, some frequencies require approval of a frequency coordination committee.


A channel number is just a shorthand notation for a frequency. It is, for instance, easier to remember "Channel 1" than to remember "26.965 MHz" (CB Channel 1) or "462.5625 MHz" (FRS/GMRS channel 1), or "156.05 MHz" (Marine channel 1). It is necessary to identify which radio service is under discussion when specifying a frequency by its channel number. Organizations, such as electric power utilities or police departments, may have several assigned frequencies in use with arbitrarily assigned channel numbers. For example, one police department's "Channel 1" might be known to another department as "Channel 3" or may not even be available. Public service agencies have an interest in maintaining some common frequencies for inter-area and/or inter-service coordination in emergencies.

Each country allocates radio frequencies to different two-way services, in accordance with international agreements. In the United States some examples of two-way services are: Citizen's Band, FRS
Family Radio Service
The Family Radio Service is an improved walkie talkie radio system authorized in the United States since 1996. This personal radio service uses channelized frequencies in the ultra high frequency band. It does not suffer the interference effects found on citizens' band at 27 MHz, or the...

, GMRS
General Mobile Radio Service
The General Mobile Radio Service is a licensed land-mobile FM UHF radio service in the United States available for short-distance two-way communication. It is intended for use by an adult individual who possesses a valid GMRS license, as well as his or her immediate family members...

, MURS
Multi-Use Radio Service
In the United States, the Multi-Use Radio Service is an unlicensed two-way radio service similar to Citizens Band . Established by the U.S. Federal Communications Commission in the fall of 2000, MURS created a radio service allowing for unlicensed operation, with a power limit of 2 watts...

, and BRS.

Amateur radio
Amateur radio
Amateur radio is the use of designated radio frequency spectrum for purposes of private recreation, non-commercial exchange of messages, wireless experimentation, self-training, and emergency communication...

 operators nearly always use frequencies rather than channel numbers, since there is no regulatory or operating requirement for fixed channels in this context. Even amateur radio equipment will have "memory" features to allow rapidly setting the transmitter and receiver to favorite frequencies.

UHF versus VHF

The most common two-way radio systems operate in the VHF
Very high frequency
Very high frequency is the radio frequency range from 30 MHz to 300 MHz. Frequencies immediately below VHF are denoted High frequency , and the next higher frequencies are known as Ultra high frequency...

 and UHF
Ultra high frequency
Ultra-High Frequency designates the ITU Radio frequency range of electromagnetic waves between 300 MHz and 3 GHz , also known as the decimetre band or decimetre wave as the wavelengths range from one to ten decimetres...

 parts of the radio spectrum
Radio spectrum
Radio spectrum refers to the part of the electromagnetic spectrum corresponding to radio frequencies – that is, frequencies lower than around 300 GHz ....

. Because this part of the spectrum is heavily used for broadcasting
Broadcasting
Broadcasting is the distribution of audio and video content to a dispersed audience via any audio visual medium. Receiving parties may include the general public or a relatively large subset of thereof...

 and multiple competing uses, spectrum management
Spectrum management
Spectrum management is the process of regulating the use of radio frequencies to promote efficient use and gain a net social benefit.The term radio spectrum typically refers to the full frequency range from 3 kHz to 300 GHz that may be used for wireless communication...

 has become an important activity of governments to regulate radio users in the interests of efficient and non-interfering use of radio. Both bands are widely applied for different users.

UHF has a shorter wavelength which makes it easier for the signal to find its way through rugged terrain or the inside of a building. The longer wavelength of VHF means it can transmit further under ideal conditions. For most applications, lower radio frequencies are better for longer range. A broadcasting TV station illustrates this. A typical VHF station operates at about 100,000 watts and has a coverage radius range of about 60 miles. A UHF station with a 60-mile coverage radius requires transmitting at 3,000,000 watts.

If an application requires working mostly outdoors, a VHF radio is probably the best choice, especially if a base station radio indoors is used and an external antenna is added. The higher the antenna is placed, the further the radio can transmit and receive. One exception to using a VHF radio outdoors is if it is used it in a heavily wooded or rugged area. Under these conditions a UHF radio may be able to transmit better though the terrain (unless the VHF antenna is raised above the terrain).

If the radios are used mainly inside buildings, then UHF is likely the best solution since its shorter wavelength travels through the building better. There are also repeaters that can be installed that relay a UHF signal to increase the communication distance.

There are more available channels with UHF, so in more populated areas UHF may be less likely to have interference from other systems. Since the range of UHF is also not as far as VHF under most conditions, there is less chance of distant radios interfering with the signal.

Range

The useful direct range of a two-way radio system depends on radio propagation
Radio propagation
Radio propagation is the behavior of radio waves when they are transmitted, or propagated from one point on the Earth to another, or into various parts of the atmosphere...

 conditions, which are a function of frequency, antenna height and characteristics, atmospheric noise, reflection and refraction within the atmosphere, transmitter power and receiver sensitivity, and required signal-to-noise ratio for the chosen modulation method. An engineered two-way radio system will calculate the coverage of any given base station with an estimate of the reliability of the communication at that range. Two-way systems operating in the VHF and UHF bands, where many land mobile systems operate, rely on line-of-sight propagation for the reliable coverage area. The "shadowing" effect of tall buildings may block reception in areas within the line-of-sight range which can be achieved in open countryside free of obstructions. The approximate line-of-sight distance to the radio horizon can be estimated from : horizon in kilometers = 3.569 times the square root of the antenna height in meters.

There are other factors that affect the range of a two-way radio such as weather, exact frequency used, and obstructions.

Other two-way radio devices

Not all two way radios are hand-held devices. The same technology that is used in two way radios can be placed in other radio forms. An example of this is a wireless callbox. A wireless callbox is a device that can be used for voice communication at security gates and doors. Not only can they be used to talk to people at these entry points, personnel can remotely unlock the door so the visitor can enter. There are also customer service callboxes that can be placed around a business that a customer can use to summon help from a two way radio equipped store employee.

Another use of two-way radio technology is for a wireless PA system. A wireless PA is essentially a one-way two way radio that enables broadcasting messages from handheld two-way radios or base station intercoms.

See also

  • Astro (Motorola)
    Astro (Motorola)
    Motorola Astro is the name brand given to two-way radios from the manufacturer Motorola that are capable of digital voice transmission. Early models used the VSELP vocoder while the latest models use the IMBE and AMBE+2 vocoders which are specified by and thus compatible with the APCO-25 Common...

  • Digital Mobile Radio
    Digital Mobile Radio
    Digital mobile radio is an open digital radio standard for professional mobile radio users specified in the European Telecommunications Standards Institute Standards TS 102 361 parts 1 to 4 and used in products sold in all regions of the world. The applicable ETSI compliance standards are EN 300...

  • GE Marc V
  • Project 25
  • Quik Call I
  • Specialized Mobile Radio
    Specialized Mobile Radio
    Specialized Mobile Radio may be an analog or digital trunked two-way radio system, operated by a service in the VHF, 220, UHF, 700, 800 or 900 MHz bands. Some systems with advanced features are referred to as an Enhanced Specialized Mobile Radio, . Specialized Mobile Radio is a term defined...

  • P25 Phase 2 Forum, for more information on P25 Phase 2

External Links

http://www.p25phase2.com, P25 Phase 2 Forum

http://dmrassociation.org, DMR (Digital Mobile Radio) Association

http://www.tetramou.com, TETRA Association

http://www.project25.org, Project 25 Technology Interest Group
The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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