This group covers how voice and other information is placed on a radio signal, which modulation types are used where, and how much spectrum each signal occupies. Knowing the typical bandwidth of each mode lets you reason out most of the questions.
Types of modulation
Modulation is the process of impressing information on a carrier wave. The three basic ways are to vary the carrier’s amplitude, frequency or phase.
- Amplitude modulation (AM) varies the strength of the carrier. Single sideband (SSB) is a form of AM in which the carrier and one of the two sidebands are removed, so all transmitter power goes into the useful part of the signal.
- Frequency modulation (FM) varies the carrier’s frequency in step with the audio. Phase modulation (PM) varies the phase and sounds very similar; the two are often grouped as “FM or PM”.
- CW (continuous wave) is a carrier keyed on and off to send Morse code.
- PSK (phase shift keying) is a digital method, not an AM mode.
Where each mode is used
- FM or PM is used for VHF and UHF voice repeaters and for VHF packet radio, where audio tones are fed to an FM transmitter.
- SSB is the choice for long-distance, weak-signal contacts on VHF and UHF, as well as HF voice. Its narrow bandwidth and lack of a wasted carrier let it get through when FM would be lost in the noise.
- Fast-scan TV uses AM video, and slow-scan TV sends still pictures in a voice-sized channel.
Sideband convention
On 160, 80 and 40 meters, operators use lower sideband (LSB). On 20 meters and higher, including 10 meters, VHF and UHF, they use upper sideband (USB). Suppressed and inverted sideband are not conventions you choose between.
Bandwidth comparison
| Signal | Approximate bandwidth |
|---|---|
| CW | about 150 Hz |
| SSB voice | about 3 kHz |
| Slow-scan TV | about 3 kHz |
| FM voice (VHF repeaters) | 10 to 15 kHz |
| AM fast-scan TV | about 6 MHz |
A narrower signal fits more stations into a band and puts more power into less spectrum, which helps weak signals. CW is the narrowest of the common modes. FM voice is much wider than SSB, and fast-scan TV is enormously wider, which is why it is practical only on the 70 cm band and above.
SSB versus FM
SSB advantages: narrower bandwidth and better weak-signal performance. SSB disadvantages: it is harder to tune in correctly, because a slight frequency error changes the pitch of voices.
FM advantages: simple tuning and good audio quality for strong local signals. FM disadvantage: its receiver captures the strongest signal, so only one signal can be received at a time, and it uses more bandwidth. Neither mode is notably more susceptible to high SWR; that is a feed line and antenna matter.
Key facts to memorize
- SSB is a form of amplitude modulation with carrier and one sideband removed.
- VHF/UHF repeaters and VHF packet use FM or PM.
- Weak-signal VHF/UHF voice uses SSB.
- Upper sideband is used on 10 meters, VHF and UHF; lower sideband on 160, 80 and 40 meters.
- CW bandwidth is about 150 Hz; SSB voice about 3 kHz; FM repeater voice 10 to 15 kHz; fast-scan TV about 6 MHz.
- CW is the narrowest of the common signal types.
- FM disadvantage versus SSB: only one signal can be received at a time.
Memory hooks
- Bandwidth steps up in order: CW (150 Hz) then SSB (3 kHz) then FM (about 15 kHz) then TV (6 MHz).
- “Upper above 10 MHz”: USB on 20 meters and every higher band, including 10 meters, VHF and UHF; LSB on 160, 80 and 40 meters.