#1 - INTRODUCTION TO 2-WAY RADIOS

INTRODUCTION TO 2-WAY RADIOS

A radio uses electromagnetic waves to send information across the air. This is accomplished by producing an electrical signal that moves back and forth, or oscillates, at a rapid rate. The rate at which a radio signal oscillates back and forth is called its frequency and is measured in Hertz. Most radio frequencies are in millions of cycles, or Megahertz, per second.

WHY PEOPLE NEED RADIOS

The primary function of virtually all radio systems is to communicate with people on the move. The two main reasons people do this are for economic reasons and for safety reasons. Economic reasons include ways in which two-way radios can be used to: Cut operational and business costs Increase profitability Improve customer service Gain a competitive edge Safety reasons for using two-way radios include: Quickly response to emergency situations Keep in touch with people in potentially dangerous situations Provide backup support when needed The common denominator to both economic and safety reasons is that radio users have improved control over the resources they manage.

HOW A BASIC RADIO SYSTEM WORKS

A transmitter is used to produce, and amplify a radio signal, which is combined, or modulated, with a voice signal from a microphone. The modulated radio signal is sent to an antenna which radiates the signal into space. The radiated signal is picked up by a receiving antenna, and sent to a receiver. Here the radio signal is processed back into the original audio signal, which is fed into a speaker so that the original voice message can be heard.

MODULATION TECHNIQUES

Combining an audio signal with a radio signal is called modulation. In this process, the voice signal is impressed on a radio frequency, or RF signal, called the RF carrier. The RF carrier is usually modulated by a voice message. However, it can also be modulated by tone signals, or digital information. In this manner, data can also be transmitted via two-way radio. Two basic types of modulation are Amplitude Modulation (AM) and Frequency Modulation (FM). FM is the most popular form of modulation in two-way radio systems.

RADIO SYSTEM COMPONENTS

Two-way radio equipment can be classified as either fixed, mobile, or portable. Fixed equipment is located at a central site such as an office or a headquarters, and usually consists of a base station, microphone, and an antenna. Fixed stations can have three types of control—local, extended local, and remote. A local control base station is used when the dispatcher’s position is close to the antenna site. An extended local control base station is used when the dispatch point is up to 1000 feet from the antenna site. The transmitter and receiver are located near the antenna site. The radio’s controls are in a separate unit at the dispatcher’s position, and connected to the radio equipment using a wire line cable. A remote control base station is used when the antenna site is located more than 1000 feet from the dispatch position. Leased telephone lines may be used to connect the radio equipment with the dispatcher’s control unit. A mobile unit is a radio that is mounted in either the trunk or under the dash of a vehicle. A portable unit is a battery-powered radio that is small enough to be carried by a person.

RADIO SYSTEM OPERATION

The most basic radio system is a simplex system consisting of radio units operating on a single frequency. Because everyone transmits and receives on the same frequency, users cannot talk and listen at the same time. Simplex means transmission in one direction at a time. In a two-frequency simplex system, the transmitters operate on one frequency, and the receivers on another. This is used for dispatcher control of the system. The mobile and portable units cannot talk to, or hear, each other because their receivers are operating on a different frequency than their transmitters. The dispatcher is the only person who can hear all traffic. A repeater is a type of base station remotely located from the system’s base of operations. When the repeater receives a signal from a radio unit, it acts as a relay and retransmits the signal. The repeater operation is called duplex because it can receive and transmit at the same time. A repeater is a type of base station remotely located from the system’s base of operations. When the repeater receives a signal from a radio unit, it acts as a relay and retransmits the signal. The repeater operation is called duplex because it can receive and transmit at the same time. A repeater is an excellent way to extend the range of a radio system. When the repeater receives a signal from a mobile, or portable, its transmitter is turned on and the message repeated. This is typically done at a higher power, and from a better antenna height, than the originating mobile or portable unit. Radios on a repeater system can have an additional frequency that enables them to talk with one another when out of range of the repeater system. This is called repeater talkaround.

SQUELCH CONTROL

Squelch circuitry is used to reduce, or eliminate, unwanted signals and noise before they are heard in the speaker. There are two types of squelch... carrier squelch and coded squelch. Carrier, or noise compensated squelch, is used to prevent noise from being heard in the speaker when there are no transmissions taking place. Coded squelch allows listeners to hear only those messages intended for them. The two most common types of coded squelch are tone coded squelch, called Private-Line, and digital coded squelch, called Digital Private Line. It is important to remember that while coded squelch systems eliminate the annoyance of listening to other transmissions, they do not provide privacy. Each radio is supplied with a switch that defeats the coded squelch circuit. This is necessary since users must listen to be sure there is no other voice traffic on the channel before transmitting. Otherwise, a transmission already in progress could be disrupted by someone else transmitting at the same time.