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A two-way radio is simply a radio that can both transmit and receive (a transceiver). A push-to-talk button is usually present to activate the transmitter. Portable two-way radios are often called walkie-talkies or handie-talkies. Two-way radios are also available in mobile and base configurations. An example of a two-way radio that both transmits and receives at the same time (or full-duplex) is a mobile phone or cellular telephone, which uses two different radio frequencies to carry the two directions of the conversation simultaneously.

Contents 1 Two-way radio frequencies 2 Types 2.1 Conventional versus trunked 2.1.1 Trunked 2.1.2 Conventional 2.1.2.1 Scanning in conventional radios 2.1.2.2 Talk-back on scan 2.2 Simplex versus duplex channels 2.3 Analog versus digital 2.4 Engineered versus not engineered 2.5 Options, duty cycle, life span and configuration

[edit] Two-way radio frequencies

Two-way radios can operate on many different frequencies, and these frequencies are assigned differently in different countries. For example, in the United States, there is a block of 22 channels assigned, collectively, to the General Mobile Radio Service and Family Radio Service. Of these 22 channels, channels 8-14 are FRS, channels 15-22 are GMRS, and channels 1-7 are shared between the two services.

Citizens Band Radio or CB also uses a numbered-channel scheme, with channels numbered from 1-40.

The CB channels have no direct relationship, however, to the channels found on GMRS and FRS radios, or any other channel-numbering scheme; 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 55.25 MHz (TV Channel 1) or "156.05 MHz" (Marine channel 1). If no specific context is in place, it is useful to say what radio service you are talking about when specifying a frequency by its channel number.

Another form of 2-way radio service is Multi-Use Radio Service (MURS). MURS uses 5 channels on the VHF band, 2 of which are shared with the business band radio service (more on this below). MURS, like FRS, does not require a license to use, and has a maximum power limit of 2 watts TPO (Transmitter Power Output), 4 times the power limit of FRS radios.

The business band radio service uses the same VHF frequency ranges as MURS, and the same UHF frequency ranges as GMRS & FRS. Business band radio service also requires a license to use, like GMRS, and has several frequencies specifically assigned to it (usually designated as "color dot" frequencies, such as "red dot" (151.625 mHz)). As its name suggests, it is used by business and other commercial interests.

[edit] 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.

[edit] Conventional versus trunked

[edit] Trunked

Main article: Trunked radio system

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 multiplex.

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, 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:

• Logic Trunked Radio
• EDACS

[edit] 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 to 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.

[edit] Scanning in conventional radios

Some conventional radios scan more than one channel. That is, the receivers 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 other signaling) 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.

• 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.

[edit] 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
• Family Radio Service
• UNICOM

[edit] Simplex versus duplex channels

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 system.

• Advantage: as the simplest system configuration, there is reliability from the fact that only two radios are needed to establish communication between them.
• Disadvantage: The simplex configuration offers communication over the shortest range or distance.

Duplex channel systems transmit and receive on different discrete channels. This defines systems where equipment cannot communicate without some infrastructure such as a repeater or base station. 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. This is a much larger area than a simplex system allows.

• Advantage: duplex channels usually allow repeater operation which extends range - especially where hand-held radios are in use.
• Disadvantage: If a radio cannot reach the repeater, it cannot communicate.

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 called talk-around, direct, or car-to-car. In one Motorola system, a Special Products microphone was created with a rocker-style push-to-talk button. The button rocked either up or down. Pushing in one direction transmitted on repeater; the other transmitted on car-to-car.

[edit] 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, a standard for digital public safety radios, and Nextel's iDEN.

• 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.

[edit] Engineered versus not engineered

Engineered systems are designed to perform close to a specification or standard. They are designed as integral systems with all equipment matched to perform together. For example, a cellular telephone system may be designed to provide 90% 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, 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. Government radio systems often specify 95% engineered coverage or greater.

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 systems are not usually engineered.

[edit] Options, duty cycle, life span and configuration

1940s tube-type land mobile two way radios often had one channel and were carrier squelch. Because radios were viewed as 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. There are so many speculations on the life term of two way radios & their accessories i.e batteries, chargers,head set etc. Though the general life term for the two way radio is 5 to 7 years & 1 to 2 years for its accessories but still the usage, atmosphere & environment plays a major role to decide its life term. A system designer could chose to an 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, and high-quality mobile radios often have specifications that include a duty cycle. 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.