Physics & RF
B09 Radio Comms Fundamentals — Study Guide
This guide covers Radio Comms Fundamentals as part of Physics & RF. Start with the vocabulary, then try to answer each question before revealing the answer. Connect these facts to a real radio link or electrical circuit and practice the subject in the arcade when you are ready.
Acronyms first
| Term | Meaning |
|---|---|
| RF | Radio Frequency |
| Tx | Transmit / Transmitter |
| Rx | Receive / Receiver |
| IF | Intermediate Frequency |
| ADC | Analog to Digital Converter |
| HDX | Half-Duplex |
| FDX | Full-Duplex |
| CTCSS | Continuous Tone-Coded Squelch System |
| DCS | Digital-Coded Squelch |
| DTMF | Dual Tone Multi-Frequency |
| SELCALL | Selective Calling |
| FSK | Frequency Shift Keying |
| COTS | Commercial Off-the-Shelf |
| FRS | Family Radio Service |
| GMRS | General Mobile Radio Service |
| TMRS | Trunked Mobile Radio System |
| RETRANS | Retransmission |
| COMSEC | Communications Security |
| NFM | Narrowband FM |
| LOS | Line of Sight |
Key facts
Radio communication typically uses the RF spectrum from about…
3 kHz to 300 GHz
A radio link connects a transmitter to a receiver using…
electromagnetic (RF) energy through free space
How many basic areas make up a radio link?
7
Trace a radio link from sender to receiver.
Transmitter (Tx) → Transmit Antenna → Propagation Path (Free Space) → Receive Antenna → Receiver (Rx)
Which component generates and modulates the RF signal?
Transmitter (Tx)
Which component captures and demodulates the signal back into usable audio or data?
Receiver (Rx)
The transmit antenna converts…
electrical energy into radio waves
The receive antenna converts…
radio waves into electrical energy
Three antenna factors that strongly affect link quality:
type, gain and orientation
Who made the first human-voice radio transmission (1900) and is called the father of radio broadcasting?
Reginald Fessenden
Fessenden's first radio broadcast of music and speech happened on…
Christmas Eve 1906
A heterodyne mixes two frequencies to produce…
their sum and difference
Scenario: an incoming 100 MHz signal is mixed with a 110.7 MHz local oscillator. Which new frequencies come out?
210.7 MHz and 10.7 MHz
Why does a superheterodyne convert RF down to a fixed IF?
Amplifying and filtering is easier and cheaper at one fixed lower frequency
In a heterodyne receiver, the second 'known' frequency comes from…
an oscillator inside the receiver
Which component shifts the signal down to the intermediate frequency?
Mixer
Modulation ____ information onto a radio wave; demodulation ____ it.
puts / takes it back off
Which mode lets only ONE direction flow — like TV and commercial broadcast radio?
Simplex
Which mode allows both directions, but only one at a time, on the SAME frequency?
Half-duplex
Which mode allows both directions simultaneously using TWO different frequencies?
Full-duplex
Scenario: two soldiers on walkie-talkies must say 'over' and release push-to-talk before the other can speak. Which mode?
Half-duplex
Scenario: you're on a cell phone call and both people can talk over each other. Which mode?
Full-duplex
Scenario: an AM station broadcasts to thousands of car radios that never transmit back. Which mode?
Simplex
'Duplex' literally means…
two paths
The term 'Full-Duplex' means the same thing as 'Duplex'.
True
A squelch circuit…
mutes receiver audio when the signal is too weak or noisy
Which squelch opens based only on signal strength, often set with a knob?
Carrier (noise) squelch
Which is the MOST common squelch control technology?
CTCSS
CTCSS superimposes one of how many low-pitch tones?
50
CTCSS tones range from…
67 to 257 Hz
CTCSS tones are called ____ because you don't really hear them.
sub-audible
Which squelch sends a continuous stream of FSK digital data instead of a tone?
DCS
Scenario: two groups share a channel, but your radio stays silent unless it hears your group's 100 Hz tone. What's at work?
CTCSS
Scenario: dispatch dials a 2–7 digit string to alert one radio, like dialing a phone. Which system?
DTMF selective calling
The DTMF keypad is laid out as a…
4×4 matrix (rows = low tones, columns = high tones)
DTMF selective calling uses how many digits?
2 to 7
Which system sends a short burst of tone sequence (~0.5–2 s) so only one programmed unit opens up?
SELCALL
Scenario: a fire chief needs to page Engine 7 without waking every other radio on the frequency. Best tool?
SELCALL
Scenario: a unit buys Garmin GPS units and Android tablets instead of building custom devices. This is…
COTS
A major DISADVANTAGE of COTS gear:
Adversaries can buy, copy and jam the same tech
The three major types of COTS radios in the DoD:
Networked radio systems, radio repeaters, trunked mobile radio
FRS band range:
462–467 MHz (UHF)
FRS total channels:
22
GMRS total channels:
30 (22 shared with FRS + 8 repeater inputs)
FRS typical range:
0.5–2 miles
GMRS handheld range (repeater up to 25 mi):
1–5 miles
Channel spacing for FRS, GMRS and Part 90 LMR:
12.5 kHz
Which needs NO license (FCC Part 95 Subpart B)?
FRS
Which needs an FCC license under Part 95 Subpart E?
GMRS
Part 90 Private Land Mobile Radio band ranges:
150–174 MHz (VHF) and 421–470 MHz (UHF)
Scenario: a family going hiking wants cheap radios with no license. Pick:
FRS
Scenario: a utility company runs its own private radio network for its crews. Which license?
FCC Part 90 Private Land Mobile Radio
A radio repeater uses…
two frequencies: input and output
Repeater vs Army RETRANS: which is soldier-operated, mobile and encrypted?
RETRANS
COMSEC hides the fact that you are transmitting.
False
In a trunked radio system, what picks a free channel when you press talk?
The site/system/trunking controller
Trunked radio users are organized into…
talk groups
Why is trunking more efficient than fixed channels?
Channels return to a shared pool and get reused when free
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