ANTENNAS AND THEIR CHARACTERISTICS

An antenna is a device specifically designed to transfer radio signals from a transmitter and radiate that signal into the atmosphere. How far the signal travels is affected by a number of factors, of which the antenna plays a key role.

OMNIDIRECTIONAL ANTENNAS

An Omnidirectional antenna radiates the radio signal in a 360° pattern. Omnidirectional antennas are used when the antenna can be located near the center of the coverage area. With this type of antenna, the range will be about the same in all directions.

DIRECTIONAL ANTENNAS

Because antennas are used in diverse geographic areas, it is often desirable to change their radiation pattern to make them more directional to best suit the user’s specific coverage area requirements. There are three common types of directional antennas:

• Cardioid
• Unidirectional
• Bidirectional

A characteristic of directional antennas is referred to as gain. Gain is the antenna’s ability to increase its effective radiated power and expand the normal coverage area. Antennas that focus, or concentrate, radiated signals in one direction are basically sending a stronger signal in that direction. By doing this, there is a resultant gain in signal strength in the direction of the concentration. There is also a loss in signal strength in all other directions.

CARDIOID ANTENNAS

The cardioid antenna produces a heart-shaped radiation pattern that is more concentrated in one direction, allowing a longer maximum range. Cardioid antennas are used when the antenna will be located near the edge of the coverage area. This concentration of energy in one direction reduces the coverage in the opposite direction. This tends to reject radio signals being received from that direction. Thus reducing the amount of interference and unwanted signals.

UNIDIRECTIONAL ANTENNAS

A unidirectional antenna generates an even more focused radiation pattern which aims the radio signal in a narrowband in one direction. Unidirectional antennas are used when a narrow strip, or corridor area, requires coverage.

BI-DIRECTIONAL ANTENNAS

A bi-directional antenna increases coverage perpendicular to the plane of the antenna. A long strip of highway or railroad right of way can be covered by a bi-directional antenna.

TOWERS

Towers may be used to raise antennas to the height necessary to cover the required geographical area with radio signals. Antennas also may be raised by mounting them on buildings or by placing the antenna tower on top of a hill or mountain. The location and height of the tower, along with the power of the signal, determine the range of coverage.

Towers are often used to support multiple antenna systems. Government regulations may require that towers over a certain height be properly painted and have lights to avoid a possible air navigational hazard. User’s may also be required to notify government agencies of all proposed tower construction and obtain appropriate licenses and permits.

TRANSMISSION LINES

The base station is connected to the antenna using a transmission cable and is usually located as close to the antenna site as possible. The transmission line is designed to efficiently carry radio signals between a base station and its antenna. A transmission line is made of outer and inner conductors called coaxial cables. Attenuation, or signal loss, is one of the most important considerations in transmission lines. All transmission lines dissipate some power, which results in a partial loss of signal strength.

Because of attenuation, the transmission line length also dictates its diameter, since the larger the line diameter, the less signal loss and the longer the line can be. This means that the further the base station is from the antenna site, the bigger the transmission cable should be to minimize the amount of signal loss.

THE ELECTROMAGNETIC SPECTRUM

The electromagnetic spectrum is the total range of frequencies of electromagnetic radiation. It extends from the lowest audio waves (15 Hz) to the highest light waves (900,000 GHz). To make it more manageable, the electromagnetic spectrum is divided into bands.

RADIO FREQUENCY BANDS

The part of the spectrum we are concerned with are the frequency bands used for radio communications. These are:

• VHF Low band (25 MHz - 50 MHz)
• VHF High band (136 MHz - 174 MHz)
• UHF Band (403 MHz - 512 MHz)
• 800/900 MHz Bands

The characteristic behavior of frequencies in different bands in the spectrum are important when choosing a frequency for a radio system.

FREQUENCY BAND CHARACTERISTICS

The frequency band characteristics most important to radio communications include:

• Propagation
• Noise Interference
• Range

PROPAGATION

The atmosphere that surrounds the earth acts to attenuate and refract radio signals, just as it does light. Just how much it is affected depends on the frequency. As a general rule, the lower the frequency, the less the attenuation, or loss of signal. Lower frequency radio waves travel better through fog or dust than higher frequency waves. Low frequency AM broadcast radio signals will travel far beyond the horizon and can be reflected back to earth for reception at great distances.

Higher frequency television or FM commercial broadcast stations are absorbed by the earth's atmosphere and are therefore limited to line of sight transmission. Below 300 Kilo Hertz, the characteristics are just the opposite. Here, radio waves follow the curvature of the earth for great distances. This type of propagation is called a ground wave. Radio communications over distances up to several thousand miles are possible by making use of low frequency ground waves.

Above 300 kHz to about 30 MHz, the ionosphere will sometimes reflect and/or refract the radio signals. When returned to earth, they are received hundreds or even thousands of miles away. This is called skip, and these radio signals are called sky waves. The frequencies from 30 MHz to about 900 MHz are the most suitable for two-way radio transmission. Generally, this range of radio frequencies is characterized by line-of-sight propagation.

NOISE AND INTERFERENCE

Electromagnetic noise interference comes from machines and engines. VHF low band signals are very susceptible to noise interference because noise occurs in the lower frequency ranges. VHF high band has minimal susceptibility to noise interference. UHF and 800/900 MHz signals have virtually no susceptibility to noise interference. From this, we can see that low band is not a good choice in high noise areas.

RANGE

In rural areas, VHF low band signals have the best range because they tend to follow the curvature of the earth. VHF high band signals have good range characteristics. The UHF and 800/900 MHz bands have only fair range characteristics because signals in this range can be attenuated by foliage and other terrain found in rural areas. From this, we can see that the VHF bands are the best choices for use in rural areas.

In suburban areas, VHF low band signals have the good range characteristics. VHF high band signals have excellent range characteristics. The UHF band has good range characteristics, and the 800/900 MHz bands have only fair range characteristics.

In urban areas, VHF low band signals have poor range because they cannot penetrate buildings. Also, urban areas tend to have high levels of noise interference, which affects low band signals. VHF high band signals have good range characteristics and the UHF band and 800/900 MHz bands have excellent range characteristics because their signals can bounce off of, and penetrate buildings. From this, you can see that low band is not a good choice in urban areas.

SELECTING THE RIGHT BAND

There are many compromises in selecting a frequency band. To aid you, there are two general rules that apply:

As the frequency increases, range decreases, but so does the ambient electrical noise.

Reflections from buildings increase with frequency.

The result is that low band VHF is generally more suited for large rural areas. High band VHF is generally more suited for suburban and mixed urban and rural areas. The UHF and 800/900 MHz bands are in general most suited for urban usage.

SPECTRUM MANAGEMENT

Because frequency spectrum is an infinite resource, and the number of users in many areas is high, many radio channels are becoming crowded. Channel loading is a term used to describe the number of users assigned to the same frequency. Channel loading is so heavy in some areas that additional users are no longer allowed on particular channels, or frequencies. The use of channels is authorized and licensed by government agencies in most countries.

In the United States, the FCC, or Federal Communications Commission, is in charge of regulating communications by radio (both commercial broadcast and two-way), television, wire, satellite, and cable. International regulations fall under the jurisdiction of the ITU, or International Telecommunications Union.

In all cases, a license to operate radio equipment is required and must be applied for with the appropriate governing body. The license is granted to operate on a particular frequency, or set of frequencies, with specific eligibility rules that must be met. Spectrum management and conservation is a global issue. Not only are government agencies using regulations to help conserve and manage our spectrum, but they are also encouraging the adoption of new technologies—technologies that have the potential to make our use of the spectrum more efficient.

FACTORS AFFECTING RANGE AND COVERAGE

The factors that have the greatest impact on range and coverage include.

• The frequency band of the radio system
• The power output of the radio transmitter
• The type of antenna being used
• The height and location of the antenna
• The terrain the system will be operating in
• The amount of electromagnetic noise in the general area of operation

Each of these are prime factors in determining how far we can communicate and to where we can communicate.

ANTENNA HEIGHT AND LOCATION

Antennas radiate energy in all directions, much like a light bulb radiates light. A mobile radio moving further away from the transmitting antenna receives less and less radio frequency energy. The distance from the antenna to the furthest point of communications can take place is known as a radio system's range. As a radio moves away from the base antenna, at some point it receives too little radio signal for reception, or it is unable to generate enough energy from its antenna to talk back to the base station. When this occurs, that radio has reached the limit of its range.

If you think of range as the radius of a circle, the circle itself will be the coverage area. In a radio system, this circle drawn on a map indicates the usable area of the radio system. The range of a radio system is affected by many factors.

The most critical coverage factor is base station antenna height and location. This is because the range of a radio system is theoretically limited to the radio horizon as seen by the radio antenna. Thus, the range of a radio system, for the most part, depends upon the base station antenna. Basically, the higher up in the air that antenna can be installed, the greater an area will be covered.

TERRAIN

Because the radio waves follow a line-of-sight path, terrain variations can also cause difficulties in communications. Hills and valleys create shadows in a coverage area. These shadow areas are often called communication holes. Tall buildings can also have the same effect on coverage.

In such cases, if we raise the antenna height, we can eliminate most of the holes. Thus, antenna height, to a large extent, also cures terrain problems.

NOISE

Another factor affecting range and coverage is noise. By noise, we mean electrical interference of all types. Some causes of electrical noise are power lines, neon signs, electric motors, and other radio systems. As a radio moves away from the base station antenna, it eventually receives too little energy for effective communications reception. Noise problems at these limits of the coverage area, called fringe areas, can be severe. The radio signal is simply too weak compared to the noise signal. In the fringe areas, increasing transmitter power can increase the energy level at the receiving radio's antenna, and will help to overcome the noise.

Another way to overcome this problem is through the use of gain antennas. Recall that gain represents an antenna's ability to increase it's effective related power. This is done by channeling the antenna's normal radiation pattern in a particular direction and thus extending the distance it will cover. The use of a high gain antenna ensures better reception in the fringe areas of a system.

RADIO SYSTEMS

The majority of radio systems that are in use today are conventional systems. These include:

• Unit-To-Unit
• Central Dispatch
• Repeater Systems

For many users, these systems provide the best combination of functionality and cost-effectiveness.

UNIT-TO-UNIT SYSTEMS

The most basic type of conventional system provides unit-to-unit communications. This type of system allows field personnel to communicate directly with each other.

In unit-to-unit systems, users are equipped with two-way radios, either mobile or portable, so that each person has the ability to both talk and listen. However, because these radios transmit and receive on the same frequency, a user cannot talk and listen simultaneously. Regardless of the number of radios in the system, only one radio user can talk at a time.

CENTRAL DISPATCH

In this type of system, a communication's link is established between a central dispatcher and radio users in the field. When the dispatcher wants to call one or more field units, he or she transmits the message over the radio system. Everyone with radios tuned to the same frequency hears the message from the dispatcher. Likewise, field personnel can initiate a conversation or respond to the dispatcher.

REPEATERS

Recall that a repeater is a special type of base station. When the repeater receives a radio signal, it immediately retransmits that signal. Since the repeater is a base station which has a much higher transmitter output power than a mobile or portable radio, a repeater is an excellent way to extend the coverage range of a radio system.

Many people who need and use radio communications cannot afford, or justify, the cost of their repeater system. One way to get the benefits of a repeater system without the expense is by using a community repeater. A community repeater is a repeater system that can be shared by many users.

TRUNKING SYSTEMS

Trunking improves calling efficiency, with the most obvious benefit of a trunked radio system being its ability to minimize user waiting times.

A Motorola trunking system consists of trunking repeaters, a central controller, and field units.

TRUNKING REPEATERS

A trunked radio system begins with a series of repeaters, it is possible for a Motorola trunked system to have up to 28 repeaters. One of the repeaters is assigned the duty of transmitting and receiving data information. This repeater is called the control channel. All other repeaters are designated as voice channels.

CENTRAL CONTROLLER

The central controller is a computer that acts much like a traffic cop. It processes all inbound and outbound data. When a request for a voice channel is received, it assigns an available repeater.

FIELD UNITS

In a trunked system, users are divided into groups. Each user group may consist of a control station at the dispatch point and a mixture of mobile and portable radios. All trunked radios are capable of switching to the frequencies of the repeaters in the system. They also contain a unique code word, which identifies each unit and indicates to which user groups the radio belongs.

HOW A TRUNKED SYSTEM WORKS

Assume a trunking system is in the idle mode and repeater number one is the designated control channel. The idle mode is when no users are talking and all their radios are monitoring the data being sent out by the control channel. During the idle mode, the central controller consistently sends out data signals over the control channel. All user radios receive these data signals, so they know which channel to monitor as the control channel. The central controller is also monitoring the control channel to see if anyone wants to make a call. All of this is done automatically.

If a radio unit in user group B initiates a call, when the microphone is keyed, a burst of data identifying that individual radio and its user group is automatically sent to the control channel indicating a unit in user group B is making a call request.

First, the call request is sent through the control channel to the central controller.

Next, the central controller processes the call request and assigns one of the idle voice channels to the entire user group. This channel assignment is transmitted in the form of another burst of data, back to the radios over the control channel.

Next, each radio in the user group automatically switches to the assigned voice channel frequency and listens to the message.

This is all done in less than half a second and in time to receive the caller's first words.

All other radios not designated as members of user group B ignore the data and continue to monitor the control channel.

Finally, when the call is completed, the radios in user group B switch back to the control channel frequency and once again continue to receive the data signals from the central controller.

Similarly, if someone in user group A initiates a call to transmit a message, the control channel assigns one of the idle repeaters to the radios in user group A.

Because each user group is assigned a different repeater with a different frequency, user group A will not hear user group B's transmissions, and vice-versa.

Trunked radio systems are often used by government agencies where different departments, such as police and fire, can use the same infrastructure, but operate as different user groups.

Trunking can also be used for shared or public use. In these applications, different companies —like taxi cabs, construction companies, and delivery trucks - can be assigned as different user groups that use the same radio system, and are charged for their use of the service.

PAGING SYSTEMS

A radio paging system provides one-way communications to selective individuals. Pagers are FM receivers that alert a person that someone wants to communicate with him or her. The person wearing the pager is alerted when someone needs to get in touch with them. Unlike a two-way radio system, the people who do receive the paging message must respond by phone, or in person, since the pagers cannot talk back.

PAGING SYSTEM COMPONENTS

A paging system consists of a paging encoder, a radio transmitter, and individual pagers with a built-in radio receiver and decoder. Each pager has a specific address code that is unique to that pager—much like a mailing address that has a number, a street name, and a zip code.

HOW PAGING SYSTEMS WORK

To send a page, the caller enters the user's address code into the paging encoder. This code is transmitted over the air, much like a voice message, except it is digital information rather than voice. Each pager has a receiver and a digital decoder with its specific address code programmed into it.

The pager compares the received code to its address code, if they match, the page is received and the user is alerted. If the codes do not match, the page is not allowed to pass through the decoder and nothing happens. Pagers can be programmed with individual, group or multiple code assignments. For example, a doctor may have a pager with an individual code so his/her office can contact him/her, and a group code that allows him/her and other members of an emergency team to be alerted via a single page during emergency response situations.

TYPES OF PAGERS

There are a variety of pagers available which include:

Basic pagers, which are tone-only pagers that simply sound an alert.

Alphanumeric pagers that alert the user and display a message, such as the telephone number to call, or an action to take.

Tone and voice pagers, which alert the user and then deliver a short voice message.

Paging systems are for people who are on the move, and they can be a cost-effective and efficient means of communications in the right circumstances.

TELEPHONE INTERCONNECT

Most two-way radio systems can be enhanced by a telephone interconnect option. Telephone interconnect allows the mobile or portable radio user to place and receive standard telephone calls via the two-way radio system. In a conventional system, telephone interconnect requires a special piece of equipment called a patch, or interconnect device. This allows connection of the telephone lines to the base station or repeater. In addition, the mobile or portable radios must also have telephone interconnect capability.

In trunked systems, a telephone interconnect terminal is connected to the central controller, which routes any telephone calls to the appropriate repeater. A radio initiated telephone call is routed through the assigned repeater, the central controller, the interconnect device and is completed over standard landline service. A landline user can call a mobile or portable by dialing a number to access the interconnect terminal and by adding the mobile/portable identification number. The central controller then assigns a repeater to the call and sends the signal to the appropriate radio. It is also possible for a landline user to call an entire user group.

STAT-ALERT CONTROL SIGNALING

Control signaling technology uses the voice channel to quickly send and receive short bursts of data containing pre-defined information without tying up the voice channel. In conventional two-way radio systems, RF channels are most commonly used to carry voice messages. However, these same RF channels can also be used to carry digital signal transmissions.

To send and receive digital Stat-Alert messages, the radios must contain digital encoding and decoding circuitry.

A typical voice message may be 15 to 20 seconds long by the time the caller identifies him or herself, states the message, and receives an acknowledgement that the message was received and understood. That same message can be sent in fractions of a second as a digital signal. In addition, since digital signals have built-in error detection and correction capability, the messages are always accurately received.

When a Stat-Alert message is sent, it also includes the sending unit's identification. When the base station receives the message, if there are no errors, it automatically sends back an acknowledgement signal to the sending unit. If there is an error, or if the message did not get through, the sending unit will not receive the acknowledgement and will automatically retransmit the message until it receives the acknowledgement.

STAT-ALERT PROVIDES THE FOLLOWING FEATURES

Vehicle ID - which displays the sending unit's identification on the dispatch control unit.

Status Change - which automatically displays the unit's status, such as "at job site", on the dispatcher's screen.

Emergency Alarm - which by pressing a button sends an emergency signal which alerts the dispatcher to a critical situation.

Call Alert - which lets the dispatcher leave a page if the driver is away from the vehicle.

Voice Selective Calling - which allows the dispatcher to send voice messages to selected individual radios in the field.

VOICE SECURITY

Once voice information is transmitted over an RF channel, it is hard to intercept by almost anyone with an inexpensive scanner, as well as other users on the same frequencies. A radio user transmitting sensitive information must either accept the risks or avoid using the radio system—which is not always practical.

A solution is Motorola's SECURENET, the most sophisticated form of digital encryption commercially available for two-way radio. Digital encryption is similar to Stat-Alert signaling, except the actual voice message is sent as a digital signal.

Instead of sending two-way voice messages as an analog signal, SECURENET first converts the analog voice signal to a digital signal.

Once the signal is in a digital format, SECURENET uses an electronic code key to encrypt, or code, the digital signal. When the encryption is completed, the digital coded message is transmitted.

Receiving radios have an electronic code key that checks for the correct code and then decrypts, or decodes, the message. The radio converts the original digital signal back to an analog voice signal, and the listener only hears the intended voice message.

The SECURENET receiving radio can respond to the encrypted call using the same digital encryption process.

With SECURENET encryption, anyone listening in on the channel will hear indistinguishable digital noise. The actual message is undetectable without the proper decoding circuitry and encryption algorithm. SECURENET equipped radios can operate in either the clear mode, for normal voice transmissions, or in the encrypted mode for secure voice transmissions.

DATA COMMUNICATIONS

Data communications provide a means of extending a user's centralized computer system, and its associated data files, to people in the field. Because of this capability, data communications can provide users with unique applications that go far beyond the realm of normal two-way radio communications. This includes:

• Database inquiry and update
• Data entry by field personnel
• User Status Updates and Messaging
• Computer-Aided Dispatch
• Report Writing

DATA COMMUNICATIONS SYSTEM COMPONENTS

Within a typical data communications system, there are five major components:

• End-User Terminals
• Radio Communications Equipment
• Data Communications Infrastructure Equipment
• Customer's Computer
• Application Software required to support the data communications system

The End User Terminals are all microprocessor based and consist of a keyboard, a display screen, an RF modem, and a data radio. The data radio may either be an integral part of the terminal or a separate two-way radio. Both mobile and portable data terminals are available.

HOW DATA COMMUNICATIONS WORKS

To see how a data system works, we'll follow a typical radio communications application. For example, a police officer stops a vehicle and wants to do a license check to see if there are any outstanding warrants, or if the vehicle is stolen.

In a traditional voice system, the officer would call the dispatcher and give him or her, the license number. The dispatcher would then go to a computer and look up the information. The dispatcher would then call back the police officer and verbally provide the information found in the computer files.

This may take several minutes, and depending on the amount of voice traffic on the system, could take even longer. In a dangerous situation, even a few minutes is a long time.

In a data communications system, the officer simply enters the license plate number on the terminal and hits the inquiry button. The radio sends the data request to the base station, which routes it through the data infrastructure directly to the computer. The computer accesses the appropriate file and automatically sends the information back through the system directly to the officer's data terminal, which displays the information on the screen. This occurs in only a few seconds.

This is only one typical data communications application. There are many other areas in which data communications can be an important system enhancement.

WIDE AREA COVERAGE

Wide area coverage systems are required when radio users must operate in remote areas. Both Simulcast and Spectra-Tac wide area coverage systems can be used on both conventional and trunked systems.

SPECTRA-TAC SYSTEMS

Spectra-Tac is designed to improve inbound coverage from mobile or portable radios to the base station. Spectra-Tac receivers are strategically placed throughout the coverage area. These receivers pick up the radio signal and feed it back to the central location over phone lines or microwave.

Because multiple receivers may pick up the same signal, a voting switch determines which receiver has the signal with the best audio quality. That signal is relayed to the central site either over phone lines or by microwave.

SIMULCAST

Simulcast utilizes multiple remote transmitter and receiver sites to extend coverage of the system. Each remote site uses repeaters with identical frequencies to those at the prime site. A dedicated microwave or fiber optic link is required for inter-site communications.

Because these sites are all linked together, whenever a user transmits a signal at any one site, all the sites automatically retransmit the signal simultaneously. In this manner, the entire coverage area receives the transmitted message.