Mobile radio

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This article is about professional equipment. For mobile radios used in Amateur Radio, see Mobile rig.

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 end. There are a variety of views about what constitutes mobile equipment. For US licensing purposes, mobiles may include hand-carried, (sometimes called portable), equipment. An obsolete term is radiophone. ^[1]

A sales person or radio repair shop would understand the word mobile to mean vehicle-mounted: a transmitter-receiver used for radio communications from a vehicle. Mobile radios are mounted to a motor vehicle usually with the microphone and control panel in reach of the driver. In the US, such a device is typically run off of the host vehicle's 12 volt electrical system.

Some mobile radios are mounted in aircraft, (aeronautical mobile), shipboard, (maritime mobile), on motorcycles, or railroad locomotives. Power may vary with each platform. For example, a mobile radio installed in a locomotive would run off of 72- or 30-volt DC power. A large ship with 117V AC power might have a base station mounted on the ship's bridge.

[edit] Disambiguation: Two-way versus telephone

The distinction between radiotelephones and two-way radio is becoming blurred as the two technologies merge. The backbone or infrastructure supporting the system defines which category or taxonomy applies. A parallel to this concept is the convergence of computing and telephones.

Radiotelephones are full-duplex (simultaneous talk and listen), circuit switched, and primarily communicate with telephones connected to the public switched telephone network. The connection sets up based on the user dialing. The connection is taken down when the end button is pressed. They run on telephony-based infrastructure such as AMPS or GSM.

Two-way radio is primarily a dispatch tool intended to communicate half-duplex, push-to-talk, and primarily intended to communicate with other radios. These systems run on push-to-talk-based infrastructure such as Nextel's iDEN, Specialized Mobile Radio (SMR), MPT-1327, or Enhanced Specialized Mobile Radio (ESMR). Modern two-way radio systems may have full-duplex telephone capability.

[edit] History

Early users of mobile radio equipment included transportation and government. Railroads used MF communications to improve safety. Instead of hanging out of a locomotive cab and grabbing train orders while rolling past a station, voice communications with rolling trains became possible. Radios linked the caboose with the locomotive cab. Early police systems were initially one way using MF frequencies above the AM broadcast band, (1.7 MHz). Some early systems talked back to dispatch on a 30-50 MHz link, (called crossband).

Early mobile radios were amplitude modulation. Electrical noise showed frequency modulation was superior for its ability to work around automobile generators and ignition noise. Early tube type radios used dynamotors - essentially a twelve volt motor that turned a generator to make high voltage B+. Some early mobile rigs were the size of a suitcase or had separate boxes for the transmitter and receiver. Later technology evolved to use vibrators and solid-state power supplies in order to make high voltage for the tubes. These circuits included inverters which changed the 6V or 12V DC to AC which could be passed through a transformer to make high voltage. The high voltage side of the transformer was rectified to make DC for the vacuum tubes, (called valves in British English). A common trait of tube type mobile rigs was their heavy weight partly caused by every design including iron-core transformers in power supplies. High voltage supplies were inefficient and tube filaments added to current demands, taxing vehicle electrical systems. Sometimes, a generator upgrade was needed to support the current required for a tube type mobile radio.

Examples of US 1950s-1960s tube-type mobile radios with no transistors:

• Motorola FMTRU-140D (dynamotor powered)
• Motorola Twin-V, named for its vibrator power supply
• General Electric Progress Line
• Kaar Engineering Model 501

US equipment had similar traits partly dictated by Federal Communications Commission (FCC) regulations. The requirement that unauthorized persons be prohibited from using the radio transmitter meant many radios were wired so they would not transmit unless the ignition was on. Persons without a key to the car could not transmit. Equipment had to be type accepted, a form of approval, by the FCC. In order to be type accepted, the radio set had to be equipped with an indicator light, usually green or yellow, that showed power was applied and the radio was ready to transmit. Mobile radios were also required to have a lamp showing the transmitter was on, (usually a red one). These traits continue in the design of modern radios. Early tube type radios operated on 50 KHz channel spacing with plus-or-minus fifteen kilohertz frequency deviation.

In the early 1970s, the California Department of Forestry requirement for 6-frequency transmit, 3-frequency receive, scanning, and a 5-tone burst encoder was unusual, leading to this custom-built mobile radio.

Solid state equipment arrived in the 1960s, with more efficient circuitry and smaller size. Channel spacing narrowed to 25-30 KHz with transmitter deviation dropping to plus-or-minus five kilohertz. By the mid 1970s, tube-type power amplifiers had been replaced with high-power transistors. From the 1960s to the 1980s, large system users with specialized requirements often had custom built radios designed for their unique systems. Systems with multiple-CTCSS tone encoders and more than two channels were unusual. Manufacturers of mobile radios built customized equipment for large radio fleets such as the California Department of Forestry and the California Highway Patrol.

Examples of US hybrid partially solid state mobile radios:

• Motorola Motrac
• General Electric Transistorized Progress Line
• General Electric MASTR Professional
• RCA 1000-series

[edit] Today

Modern equipment may have 100 or more channels, be microprocessor controlled, and have built-in options such as unit ID. A computer and software may be required to configure the mobile radio. Menus of options may be several levels deep and offer a complicated array of possibilities. Some mobile radios have alphanumeric displays that translate channel numbers (F1, F2) to a phrase more meaningful to the user, (Providence base, Boston base). Channel spacing narrowed again to 12.5-15 KHz with transmitter deviation dropping to plus-or-minus 2.5 kilohertz. Newer radios are trending toward smaller sizes.

Examples of US microprocessor-controlled mobile radios:

• Motorola Astro Digital Spectra W9
• Kenwood TK-690

[edit] Details

Commercial and professional mobile radios are often purchased from an equipment supplier or dealer whose staff will install the equipment into the user's vehicles. Large fleet users may buy radios directly from an equipment manufacturer.

A modern mobile radio consists of a radio transmitter-receiver, housed in a single box, and a microphone with a push-to-talk button. Each installation would also have a vehicle-mounted antenna connected to the transmitter-receiver by a coaxial cable. Some models may have an external, separate speaker which can be positioned and oriented facing the driver to overcome ambient road noise present when driving. The installer would have to locate this equipment in a way that does not interfere with the vehicle's sun roof, electronic engine management system, vehicle stability computer, or air bags.

Mobile radios installed on motorcycles are subject to extreme vibration and weather. Professional radios designed for use on motorcycles are weather and vibration resistant. Some are mounted on shock mount systems that reduce the radio's exposure to vibration imparted by the motorcycle's modal, or resonant, shaking.

Some mobile radios use noise-canceling microphones or headsets. At speeds over 100 MPH, the ambient road and wind noise can make radio communications difficult to understand. For example, California Highway Patrol mobile radios have noise-canceling microphones which reduce road and siren noise heard by the dispatcher. Most fire engines and radios in heavy equipment use noise-canceling headsets. These protect the occupant's hearing and reduce background noise in the transmitted audio. Noise-canceling microphones require the operator speak directly into the front of the microphone. Hole arrays in the back of the microphone pick up ambient noise. This is applied, out-of-phase, to the back of the microphone, effectively reducing or canceling any sound which is present both in front and back of the microphone. Ideally, only the voice present on the front side of the microphone goes out on the air.

Some radios are equipped with transmitter timers which limit the length of a transmission. A bane of push-to-talk systems is the stuck microphone: a radio locked on transmit which disrupts communications on a two-way radio system. One example of the problem occurred in a car with a concealed two-way radio installation where the microphone and coiled cord were hidden inside the glove box. An operator tossed the mike into the glove box and shut it, pressing the push-to-talk button and locking the transmitter on. On taxi systems, a driver may be upset when a dispatcher assigns a call (s)he wanted to another driver and may deliberately hold the transmit button down. Radios with time-limits transmit for the maximum amount of time, usually 30- or 60-seconds, after which the transmitter automatically turns off and a loud tone comes out of the radio speaker. The volume level of the tone on some radios is loud and cannot be adjusted. As soon as the push-to-talk button is released, the tone stops and the timer resets.

The best quality equipment may meet some primitive US military specifications for exposure to temperature ranges, vibration, and humidity. Specifications for selectivity and intermodulation distortion should be as high as possible, especially if the equipment is to be used for life-safety purposes or in an urban environment. For modern mobile radios, these figures should be in the 80-110 db range if the radio is used in a city.

[edit] Antenna

Mobile radios universally have an associated antenna. The most common antennas are wire whips which protrude vertically from the vehicle. Physics defines antenna lengths: length relates to frequency. 25-50 MHz antennas may be over nine feet long for a quarter wavelength. A 900 Mhz antenna may be about five inches for a quarter wavelength. A transit bus may have a ruggedized antenna, which looks like a white plastic blade or fin, on its roof. Some vehicles with concealed radio installations have antennas designed to look like the original AM/FM antenna, a rearview mirror, or may be installed inside windows, or hidden on the floor pan or underside of a vehicle. Aircraft antennas look like blades or fins, the size and shape being determined by frequencies used. Microwave antennas may look like flat panels on the aircraft's skin. Temporary installations may have antennas which clip on to vehicle parts or are attached to steel body parts by a strong permanent magnet.

Antennas can be costly since they are usually not included in maintenance contracts for mobile radio fleets. Some types of vehicles in 24-hour use, with stiff suspensions, tall heights, or rough diesel engine idle vibrations may destroy antennas quickly. The location and type of antenna can affect system performance drastically. Large fleets usually test a few vehicles before making a commitment to a certain antenna location or type.

US Occupational Safety and Health Administration guidelines for non-ionizing radio energy generally say the radio antenna must be two feet from any vehicle occupants. (Read the OSHA guidelines before attempting to install an antenna.) This rule of thumb is intended to result in passengers being exposed to safe levels of radio fields in the event the radio transmits.

[edit] Notes

1. ^ Cited in many references including on escutcheons and silk-screened face plates on 1960s Motorola products including early HT-200 and Dispatcher-series mobiles. Later HT-200s dropped the term. It was also seen on some Kaar Engineering mobile products. One example is the title on a Special Products service manual Model T31BAT-3100B-SP3 'Dispatcher' Radiophone 25-54 MC 12 W RF Power 12VDC, State of California Radio Communications System, (Chicago: Motorola Communications Division, 1965). The Oxford English Dictionary conflicts with some uses, saying radiophone is interchangeable with radiotelephone. The 11th edition of Newton's Telecom Dictionary says radiophone is an, "obsolete term."