Modulation modes compared
AM, FM, SSB and CW — what each one varies, how much bandwidth it needs, and when you would choose it.
7 minute read
A bare carrier carries no information. Modulation is how you put information onto it, and the mode you choose determines your bandwidth, your noise performance and your effective range.
The four you must know
CW — Continuous Wave
The carrier is simply switched on and off to form Morse characters.
- Bandwidth: around 100–150 Hz — by far the narrowest
- Strength: all the power is in one narrow slice, so it gets through when nothing else will
- Cost: you have to know Morse
AM — Amplitude Modulation
The amplitude of the carrier varies with the audio signal. This produces a carrier plus two sidebands, each carrying the same information.
- Bandwidth: roughly 6 kHz for voice
- Weakness: most of the transmitted power sits in the carrier, which carries no information at all. Amplitude noise — ignition, lightning, switching — lands directly on the signal.
SSB — Single Sideband
A refinement of AM: suppress the carrier and one sideband, transmit only the other.
- Bandwidth: about 2.4–3 kHz, roughly half of AM
- Strength: all transmitted power carries information, so it is far more efficient. This is why SSB dominates HF voice work.
- Convention: lower sideband below 10 MHz, upper sideband above. USB is also standard on VHF/UHF SSB.
FM — Frequency Modulation
The instantaneous frequency of the carrier varies with the audio. The amplitude stays constant.
- Bandwidth: around 10–16 kHz for voice — the widest of the four
- Strength: because the information is in frequency, not amplitude, a limiter stage can strip amplitude noise away entirely before detection. FM sounds clean or it sounds like nothing.
- The capture effect: the stronger of two signals on the same frequency completely suppresses the weaker one.
Choosing a mode
| You want | Use |
|---|---|
| Maximum range from minimum power | CW |
| Efficient HF voice | SSB |
| Clean local VHF/UHF voice, repeaters | FM |
| Historical or specialist AM operation | AM |
The bandwidth principle
Narrower is not automatically better — it is a trade. Narrow modes concentrate power and get through poor conditions; wide modes carry more information and sound better when conditions are good. FM sounds excellent on a strong local repeater and useless at the edge of range, where SSB would still be workable.
For the exam, remember the ordering: CW narrowest, then SSB, then AM, then FM widest.