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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