T2 · Published 26 August 2026 · By The Ham Desk

Modulation, Bandwidth and What a Repeater Actually Does

A carrier on its own carries nothing. This unit covers how information is put onto it, how much spectrum each mode eats, and the division of labour that lets a 5 W handheld in a valley talk to a station 40 km away.

Band 2 m
Study time 14
Level Core
questions 10

Waveform and antenna diagram for Radio and Signals Fundamentals — Radio and Signals Fundamentals — waveform overlay and radiation pattern

Pre-flight briefing

Modulation in one sentence

Modulation varies some property of a radio-frequency carrier - its amplitude, its frequency or its phase - in step with a voice, a tone or a data stream. Amplitude modulation moves the envelope, frequency modulation shifts the carrier up and down, and CW is simply the carrier switched on and off in Morse. Single sideband is amplitude modulation with the carrier and one sideband removed, which is why it is so much more efficient than plain AM.

Each choice trades bandwidth against robustness. CW squeezes into about 150 Hz, SSB into 2.4 to 3 kHz, and wideband FM on 2 metres occupies roughly 16 kHz because Carson's rule adds the peak deviation to the highest modulating tone and doubles the sum.

Bandwidth is the real currency

Two stations can share a band only if their signals do not overlap: the number of conversations a band can hold is its width divided by the bandwidth of one signal. Narrow modes therefore get narrow sub-bands, which is why 2 metre FM is confined to the upper part of the band and weak-signal modes such as CW and SSB live near the bottom edge.

The repeater bargain

A repeater listens on one frequency and simultaneously transmits on another, from a high site with a good antenna. Your handheld transmits on the repeater's input; the repeater retransmits on its output, which you listen to. The frequency difference is the offset: 600 kHz on 2 metres, 5 MHz on 70 centimetres, 1.6 MHz on 1.25 metres.

Offsets are negative below 147 MHz and positive above it, which is why a 2 metre repeater output of 146.940 MHz has an input of 146.340 MHz, while 147.120 MHz pairs with 147.720 MHz. Many repeaters also require a CTCSS tone - a sub-audible tone transmitted with your voice - to open their receiver, which keeps co-channel repeaters from keying each other up.

Terms you must be able to define
Term	What the examiner wants to hear
Carrier	The steady radio-frequency signal that gets modulated; by itself it conveys no information.
Deviation	How far an FM carrier is shifted from its resting frequency by the modulating signal; 5 kHz is the standard for 2 m voice.
Offset	The fixed frequency difference between a repeater's input and output, e.g. 600 kHz on 2 m.
CTCSS	Continuous Tone-Coded Squelch System: a sub-audible tone sent with your transmission to open a repeater's receiver.
Simplex	Transmitting and receiving on the same frequency, with no repeater in between.

Question deck 10 questions

T1B01 Recall

What does modulation do to a radio carrier?

T1B02 Recall

Which of these voice or telegraphy modes occupies the narrowest bandwidth?

T1B03 Core

What is the standard peak deviation for FM voice on the 2 metre band?

T1B04 Recall

What is the standard repeater offset on the 2 metre band?

T1B05 Core

A repeater output is 147.120 MHz. Assuming standard offset, what is its input frequency?

T1B06 Core

Why do many repeaters require a CTCSS tone?

T1B07 Recall

What is the national 2 metre simplex calling frequency?

T1B08 Core

What is the essential difference between simplex and duplex operation?

T1B09 Stretch

Using Carson's rule, roughly what bandwidth does a 2 metre FM voice signal occupy?

T1B10 Stretch

Why does the FM sub-band sit at the top of the 2 metre allocation while CW and SSB sit at the bottom?

Reference sheet

The explanations for all ten questions sit on this sheet. Pick an answer on the left and the matching card lights up.

T1B01 Explanation 1

Key A — It varies one of the carrier's characteristics to carry information

Amplitude, frequency or phase is varied in step with the information; the carrier itself is unchanged in purpose but no longer a bare tone. Power and polarisation are set by the transmitter and the antenna, not by the modulation.

Rule 47 CFR §97.3(c) - definitions of the emission types

T1B02 Explanation 2

Key D — CW

A CW signal needs only enough width for the keying sidebands - about 150 Hz. SSB needs 2.4-3 kHz, AM about 6 kHz, and wide FM about 16 kHz.

Rule 47 CFR §97.305 - emission types permitted in each band segment

T1B03 Explanation 3

Key C — 5 kHz

2 metre FM voice uses 5 kHz peak deviation, giving an occupied bandwidth of about 16 kHz. The 2.5 kHz figure belongs to narrowband FM used in some other services, and 500 Hz is closer to the shift used by RTTY.

Rule 47 CFR §97.307(f)(3) - 2 m FM bandwidth limit

T1B04 Explanation 4

Key B — 600 kHz

2 metres uses 600 kHz, 70 centimetres uses 5 MHz and 1.25 metres uses 1.6 MHz. Getting this wrong means you transmit outside the repeater's input passband and nobody hears you.

Rule 47 CFR §97.205 - repeater station operation

T1B05 Explanation 5

Key A — 147.720 MHz

Above 147 MHz the standard 2 metre offset is positive, so add 600 kHz: 147.120 + 0.600 = 147.720 MHz. A negative offset would put the input at 146.520 MHz, which is the national simplex calling frequency and not a repeater input.

Rule 47 CFR §97.205(b) - repeater frequency coordination

T1B06 Explanation 6

Key D — So co-channel repeaters do not key each other up and users do not hear unrelated traffic

A CTCSS tone is a sub-audible signal sent with your transmission; the repeater's receiver only opens when it hears its own tone. The tone is not encryption - anyone can listen - and it does not affect power or offset.

Rule 47 CFR §97.205 - repeater control and access

T1B07 Explanation 7

Key C — 146.520 MHz

146.520 MHz is the national 2 metre FM simplex calling frequency. 144.200 MHz is the SSB calling frequency, 146.940 MHz is a typical repeater output, and 147.120 MHz is another repeater output.

Rule 47 CFR §97.301(a) - band plan convention, not a rule citation

T1B08 Explanation 8

Key B — Simplex uses one frequency for both directions; duplex uses two

Duplex simply means transmitting and receiving on different frequencies, which is what makes a repeater possible. Mode and band have nothing to do with it.

Rule 47 CFR §97.3(a)(16) and (c) - definitions

T1B09 Explanation 9

Key A — About 16 kHz

Carson's rule is twice the sum of peak deviation and the highest modulating frequency: 2 x (5 kHz + 3 kHz) = 16 kHz. That is why 2 metre FM channels are spaced 15 to 20 kHz apart.

Rule 47 CFR §97.307(f)(3) - emission bandwidth limits

T1B10 Explanation 10

Key A — Wide FM needs more room, and the band plan gives narrow weak-signal modes the quiet bottom edge

It is a coordination convention rather than a propagation effect: 144.0-144.5 MHz is reserved for CW and weak-signal work, SSB calling is at 144.200 MHz, and FM activity starts higher up. Keeping the wide signals together leaves the bottom of the band usable for long-distance contacts.

Rule 47 CFR §97.101(a) - good amateur practice and band plans

How to use this unit

Work the ten questions in one sitting with the reference sheet covered, then uncover it and read every explanation - including the ones you got right. The wording of the real exam is different, so learn the reasoning rather than the answer letters.