Before this:What is a radio wave?Anatomy of a signal
Analog modulation — AM, FM, SSB
Key takeaways Analog modulation puts a voice on a carrier by varying one of its properties. AM varies the carrier’s amplitude (simple, used for aviation and shortwave). FM varies its frequency (noise-resistant, used for broadcast and most analog two-way voice). SSB strips AM down to a single sideband with no carrier — half the bandwidth, all the power on the information, the backbone of long-distance HF voice. The same three properties — amplitude, frequency, phase — return as the basis of digital modulation.
Digital modes are built on ideas first used in analog radio, so it pays to meet the classics. All three change the carrier in step with the audio; they differ in which property they change.
Amplitude modulation (AM)
In AM, the audio rides on the carrier’s amplitude (strength) while its frequency stays put. Louder audio → bigger swings in carrier height. It’s the oldest and simplest scheme — easy to build and easy to demodulate.
Its weakness is that noise and interference are also amplitude variations, so AM picks up static readily. Yet it survives where simplicity or a special property matters: shortwave broadcast and aviation VHF airband. Aviation keeps AM partly because if two aircraft transmit at once you hear both (a beat tone), alerting the controller — whereas FM would let the stronger one silently mask the other.
Frequency modulation (FM)
In FM, the audio varies the carrier’s frequency while amplitude stays constant. The amount the frequency swings is the deviation — wider deviation carries louder/richer audio but uses more bandwidth.
Because the information is in frequency, not amplitude, FM can ignore amplitude noise — a receiver just tracks the frequency wiggle and discards strength changes. That’s why FM sounds clean and is everywhere: FM broadcast (wide deviation, ~200 kHz wide) and narrowband FM two-way voice (~12.5 kHz), the analog cousin of the digital voice systems GopherTrunk decodes. FM also has a capture effect: the strongest signal on a channel takes over, suppressing weaker ones.
How wide is an FM signal? A handy estimate (Carson’s rule) is bandwidth ≈ 2 × (deviation + top audio frequency). Narrowband two-way FM with ~2.5 kHz deviation and 3 kHz audio works out to 2 × (2.5 + 3) = 11 kHz — which is why it fits a 12.5 kHz channel. Wide-deviation FM broadcast (~75 kHz deviation, 15 kHz audio) needs 2 × (75 + 15) = 180 kHz — hence the ~200 kHz channels. More deviation buys better noise immunity at the cost of bandwidth.
Single sideband (SSB)
A plain AM signal is wasteful: it sends a carrier (no information) plus two mirror- image sidebands (the same information twice). SSB throws away the carrier and one sideband, transmitting just one sideband — either upper (USB) or lower (LSB).
The payoff is big: about half the bandwidth and all the power on the information, so a modest SSB transmitter reaches across continents on HF. That’s why amateurs and long-distance services favour it. The cost is precision — without a carrier to lock onto, the receiver must be tuned exactly, or voices sound like Donald Duck.
How they look and sound
| Mode | On the waterfall | Sounds like |
|---|---|---|
| AM | Central carrier spike + symmetric sidebands | Voice with audible static |
| FM | A wider block (width follows deviation) | Clean voice, hiss when no signal |
| SSB | Offset blob, no carrier spike | Clear voice only when tuned exactly |
Recognising these footprints on the spectrum and waterfall is a practical skill — it tells you what you’ve found before you decode it.
Where analog still lives
Despite the move to digital, analog is far from gone: FM broadcast, aviation AM airband, marine VHF FM, amateur SSB on HF, weather radio, and plenty of legacy two-way narrowband FM. Many of these aren’t GopherTrunk’s trunked-voice specialty, but understanding them makes the digital versions — which vary the very same carrier properties — far easier to grasp.
Quick check: why does FM resist static better than AM?
Recap
- AM varies amplitude — simple, static-prone, used for aviation and shortwave.
- FM varies frequency — noise-resistant, used for broadcast and analog two-way.
- SSB is AM with the carrier and one sideband removed — half the bandwidth, ideal for long-distance HF.
- Each has a recognisable waterfall footprint and sound.
- The same amplitude/frequency/phase ideas power digital modulation next.
Next: the bridge to digital — how symbol rate (baud) relates to bitrate.
Frequently asked questions
What is the difference between AM and FM?
AM (amplitude modulation) varies the carrier’s strength to carry the audio, keeping its frequency fixed. FM (frequency modulation) varies the carrier’s frequency instead, keeping its strength fixed. FM is more resistant to amplitude noise (static), which is why it sounds cleaner for voice and music; AM is simpler and still used where many signals share a band, like aviation.
Why does aviation still use AM?
Aircraft VHF airband uses AM partly for historical reasons and partly because of a useful property — when two stations transmit at once, AM lets you hear both (a “heterodyne”), so a controller notices a collision. With FM, the stronger signal would simply capture the channel and mask the weaker one.
What is SSB and why is it efficient?
SSB (single sideband) is a refined form of AM that removes the carrier and one of the two redundant sidebands, transmitting only what’s needed. That puts all the power into the information and uses about half the bandwidth, so SSB carries far over HF with modest power. The trade-off is that the receiver must be tuned precisely or the voice sounds distorted.
How can I tell AM, FM, and SSB apart on a waterfall?
AM shows a central carrier spike with symmetric sidebands. FM is a wider block whose width depends on deviation. SSB is an offset blob of energy with no carrier spike, sitting to one side of where the carrier would be. With practice each has a recognisable footprint.