Radio Communications

Essentials

Module 2: Two-Way Radios: How they Work!

Ray Ryan

A cell phone is a radio

A wireless mouse is a radio

A garage door opener is a radio

A keyless lock for a car door is a radio

Cable and Satellite TV are both radios

A speed pass for a toll bridge is a radio

A security tag in a department store is a

radio

You are surrounded by radios and probably don’t even realize it.

Radios Are Everywhere . . .

Here’s how it works

Electrical and magnetic fields are formed when

electricity moves through an antenna.

That energy will radiate from one antenna to another.

Transmitter

Receiver

Modulation is the result of combining the

audio signal with the RF carrier.

The result is either amplitude modulation

(AM) or frequency modulation(FM).

P25 Phase 2 digital modulation uses

a combination of both AM and FM

modulation to convey more

information in each ‘symbol.’

Modulation

Radio Frequencies

•VHF

• 30 Mhz to 300 Mhz

•VHF Low Band (Typically just called Low Band)

• 30 Mhz to 50 Mhz

•VHF High (Typically just called VHF)

•UHF

• 300 Mhz to 3 Ghz

•Typically referring to 450 to 470 Mhz

•700 Mhz

•698 to 806 Mhz

•Became available with the Digital Television Transition

•800 Mhz

•806 to 869 Mhz

•Rebanding

Benefits and Limitations

Low Band VHF UHF 700/800

Simplex Repeated Repeated Repeated

Single

Frequency

Frequencies

are Not

Paired

Paired

Frequencies

Paired

Frequencies

Coverage

Area

Largest Smallest

Range Greatest Least

Building

Penetration Worst Best

All radios have the same basic

elements

Microphone Modulator

RF

Power

Amplifier

Antenna

Sensitive Audio Amplifier

RF

Oscillator

Audio Power

Amplifier

Speaker

Tuner Demodulator

Sensitive

RF

Amplifier

TRANSMITTER

RECEIVER

Switch

Simplex

All radios transmit and receive on the

same frequency.

39.9 MHz

Console

Phone Line

Receive audio is fed to the transmitter

Microphon

e

Modulator

RF

Power

Amplifie

r

Antenna

Audio

Amplifier

RF

Oscillator

Audio Power

Amplifier

Speaker

Tuner Demodulator Sensitive

RF

Amplifier

151 MHz

159 MHz

It transmits what it receives . . .

That’s why it’s called a

“Repeater”

A special filter, called a

“Duplexer” replaces the switch.

This allow the transmitter and

receiver to operate at the same

time

Filter

To Consoles

Repeater

Audio

Radios transmit and receive on different frequencies.

Repeaters

Portable typically transmits and receives on a different frequency band.

851 MHz

159 MHz

806 MHz

Vehicular Repeater

851 Mhz

Audio

Audio

Console

Vehicular Repeaters

•Available in all frequency bands

•Can be used either in-band or cross band

•Can be used with conventional and

trunked radio systems

•Can be used as stand alone repeater from

the vehicle

Repeater

#2

Public

Works Repeater

#1

Fire

Repeater

#3

Ambulanc

e

Repeater

#4

Police

Dispatc

h

Console

Typical Conventional Network To talk with fire, you must first select the fire

module on the console then press the Push-to-Talk

button.

To talk with Police, you must first select the Police

module on the console then press the Push-to-Talk

button.

Sele

ct

Unsele

ct

The audio from the selected

repeater will be heard on the select

speaker. The audio from the three

other repeaters will be heard on the

unselect speaker.

Receiver Voters

Aux Receiver Aux Receiver

Voter

Repeater #2

Repeater

#1

Repeater #3

Repeater

#4

75%

95% 5%

0% Simulcast

Controller

Simulcast Network

151

151

151

151

159

Console

Connects multiple sites together to work

as one single repeater.

Dispatchers can transmit to a vehicle

without needing to know which site the

receive audio is coming from.

Provides for a larger coverage area.

Simulcast Benefits

Simulcast Disadvantage

This area of darker blue

is refereed to as the

Simulcast

Overlap Zone

Setting of the Amplitude

and Phase of the RF and

Audio Signals from Both

Sites is Critical.

Trunked Radio Networks

Trunking automates the communications

process, much like a cell phone network.

Provides the best user experience; users

can focus on their work instead of radio.

Trunked systems are flexible, reliable, and

resource efficient.

Radio sites and repeaters become shared

resources available to all users.

Law Enforcement

Talk-Group

122

Fire

Talk-Group

214

Public Works

Engineers

Talk-Group

657

Public Works

Construction

Talk-Group

655

Emergency

Medical Talk-

Group

549

Emergency

Services

Talk-Group

100 Inter-Op

Talk-Group for

Public Works

656 Inter-Ops

Talk-Group

1025

Talk Groups

Talk Groups

Your mobile or portable communicate on a

Talk-Group not a Channel.

You can create as many Talk-Groups as

needed. It’s software not hardware.

User specific Talk-Groups provide for

regular work without overlap.

Shared Talk-Groups provide for

interoperability to work together.

A Control Channel

Several working channels

The Trunking Controller

A Trunked Radio Site has the

following equipment:

Control

Channel

Channel

#1

Channel

#2

Channel

#3

Trunking Controller

Trunked Radio Site

Trunking Process

All radios begin by

monitoring the

control channel.

Talk Group

#122

Talk Group

#433 Talk Group

#214

Contro

l

Chann

el#1

Chann

el#2

Chann

el#3

Trunking Controller

When radio 1221 presses

the PTT button, a request is

sent on the control channel

for a working channel.

The Trunking Controller tells

everyone on talk-group 122

to go to channel #1.

The operator of radio 1221

will hear a beep from the

speaker which tells him/her

that they can now begin to

speak.

Units 1222 and 1223 will

both go to channel #1 and

their squelches will open so

that they can hear the audio

from unit 1221

1221 1222

1223 3025 3026

3027

Police Fire

6054 6055

6056

Sheriff

Voice channels are

assigned to groups

as they are

needed.

Trunking Process

Talk Group

#122

Talk Group

#433 Talk Group

#214

Control Voice

#1

Voice

#2

Voice

#3

Trunking Controller

Channels can be assigned to Talk Group 122

at both Site 1 and Site 2.

Multi-Site Trunking

Talk Group

#122

Talk Group

#122

Site 2 Site 1

Network

Controller

Trunking Efficiency

The number of channels at a site is a function

of the call volume rather than the number of

talk-groups.

Users may roam from area to area without

changing talk groups.

Users may be assigned their own talk-group

and have shared talk-groups with other

agencies to improve interoperability.

Trunking Reliability

If a channel stops working, the controller

removes the channel from service and

sends an alarm to the technicians.

Since the users communicate on talk

groups rather that specific channels, they

never know a channel has been taken out

of service.

If a site loses connection with the network

controller, it will still operate as a stand

alone site.

Radio Networks

Combining the building blocks provides the

features and coverage area required.

Complexity, User Experience, and Cost are the

trade offs in designing a radio system.

Technical Complexity

Co

st

Co

st