Modulation Concepts
Part of Radio
Modulation is the process of impressing information onto a radio carrier wave — without it, a transmitter can only radiate a constant tone carrying no message.
Why This Matters
A radio transmitter without modulation is like a lighthouse without a signal pattern — it broadcasts presence but conveys no information. Modulation is the encoding method that allows voice, music, data, and Morse code to travel across the electromagnetic spectrum. Understanding modulation means understanding how information gets into a radio wave, how it is extracted at the receiver, and why different modulation methods are suited to different applications.
For a rebuilding civilization, choosing the right modulation method for each application is a practical engineering decision with real consequences. Amplitude modulation (AM) is simpler to implement and receives with crystal-set simplicity but uses bandwidth inefficiently. Single-sideband (SSB) is six times more efficient in power and spectrum but requires more complex circuits. Frequency modulation (FM) is noise-resistant and high-fidelity but requires wider bandwidth. Continuous wave (CW, Morse code) is the most robust of all — communicating through noise and weak signals that would make voice communication impossible.
In a world with limited manufacturing capability, your modulation choices will be constrained by what equipment you can build or salvage. Starting with the simplest (CW and AM), understanding their physics, and progressing to more capable modes as your manufacturing base develops is the natural path.
Continuous Wave (CW) and Keyed Signals
The simplest modulation is no modulation at all — just turning the carrier on and off in patterns that encode information. This is CW (continuous wave) operation, and when keyed in Morse code patterns, it is the most robust communication mode ever devised.
When you key a transmitter — close a switch that connects the oscillator to the antenna — the radio frequency carrier goes on. When you open the switch, it goes off. The receiver hears a tone when the carrier is present, silence when it is absent. By listening to the pattern of tones and silences, the receiving operator decodes the Morse code message.
Why is CW so robust? Because the receiver needs only to distinguish “carrier present” from “carrier absent.” You can filter the received signal to a very narrow bandwidth — as little as 50–200 Hz — without losing any information (since there is no audio frequency content, only timing). Narrow bandwidth means less noise power enters the receiver, improving signal-to-noise ratio. A CW signal is readable through interference and noise conditions where a voice signal is completely unintelligible.
CW transmitters are also simpler than voice transmitters: a stable oscillator, a keying circuit, and a power amplifier. The keying circuit need only cleanly switch the amplifier on and off — no audio circuits, no modulator, no linearity requirements.
Amplitude Modulation (AM)
AM impresses the audio signal onto the carrier by varying the carrier’s amplitude (instantaneous power level). The carrier frequency stays constant; its strength rises and falls at audio frequencies. When you speak into an AM microphone, the carrier power varies from silence (carrier only, called the “unmodulated carrier”) to maximum (carrier plus audio content, called “100% modulation”).
Mathematically: the AM signal is v(t) = A[1 + m·cos(2πf_audio·t)]·cos(2πf_carrier·t), where A is carrier amplitude, m is modulation index (0 to 1 for standard AM), f_audio is the audio frequency, and f_carrier is the carrier frequency. When this expression is expanded, it produces three frequency components: the carrier at f_carrier, and two sidebands at f_carrier + f_audio and f_carrier - f_audio.
The two sidebands each contain identical information — both carry the same audio signal. The carrier carries no information at all, just power. So in standard AM, approximately two-thirds of the transmitted power goes into the carrier (wasted as information goes) and only one-third into the information-carrying sidebands. This power inefficiency is a fundamental limitation of AM.
Modulation depth (m × 100%) should be as high as possible without overmodulation (m > 1). At 100% modulation, the carrier amplitude doubles at audio peaks and drops to zero at audio troughs. Overmodulation clips the troughs below zero, creating distortion and spurious emissions on adjacent frequencies — a serious problem that interferes with other stations.
Single Sideband (SSB)
SSB suppresses the carrier and one sideband, transmitting only the remaining sideband. This eliminates the wasted carrier power and the duplicate sideband. For a given audio signal and peak transmitter power, SSB delivers approximately 9 dB (8×) more effective signal than AM. In practice, SSB contacts are possible over distances that AM cannot bridge with the same equipment.
The tradeoff: more complex circuitry, precise frequency setting (the suppressed carrier frequency must be accurately restored in the receiver to correctly demodulate the audio), and no compatibility with simple crystal detectors. SSB requires a product detector in the receiver — a circuit that reinserts a locally-generated carrier at the correct frequency before demodulating.
Upper sideband (USB) transmits the frequencies above the carrier. Lower sideband (LSB) transmits the frequencies below. Internationally, USB is standard for frequencies above 10 MHz; LSB is standard below 10 MHz. Use USB for 20m, 15m, 10m amateur bands; LSB for 80m, 40m.
SSB transmitters require linear amplifiers — the final amplifier stage must faithfully reproduce amplitude variations in the signal without distorting. AM transmitters also need linearity in the modulated stage. CW transmitters have no linearity requirement (the amplifier is either fully on or off).
Frequency Modulation (FM)
FM varies the carrier frequency in proportion to the audio signal, while the carrier amplitude remains constant. Speak louder, the frequency deviation increases; speak at a higher pitch, the frequency changes faster. The receiver, designed to respond to frequency variations rather than amplitude, reproduces the audio.
FM has an inherent noise advantage over AM: atmospheric noise and most interference arrives as amplitude variations (random amplitude modulation). An FM receiver includes a limiter that clips all amplitude variations before the detector, so the noise never reaches the audio output. This is why FM sounds cleaner than AM.
The required bandwidth for FM is larger than AM. Standard FM broadcasting uses ±75 kHz deviation, requiring about 200 kHz of spectrum. Narrowband FM (NBFM), used in two-way radios, uses ±5 kHz deviation and fits in 25 kHz. For post-collapse community radios, narrowband FM on VHF (2 meters, 144–148 MHz) provides excellent local communication with common salvaged equipment.
FM receivers need more complex circuitry than AM receivers: a limiter, and a discriminator or ratio detector circuit rather than a simple envelope detector. However, FM receivers are mass-produced and widely salvageable. Building FM capability from scratch is more challenging than AM but not beyond a well-equipped electronics workshop.
Choosing Modulation for Your Application
CW (Morse code): use for long-distance communication, emergency contacts, and any situation where signal levels are marginal. Requires operator training. The most achievable with homebrew equipment.
AM voice: use for local and regional broadcast, simple voice communication where receivers may be simple crystal sets or basic AM radios. Lower spectral efficiency but widest compatibility.
SSB voice: use for long-distance voice communication where power efficiency and spectrum efficiency matter. Requires compatible transceivers on both ends. The standard for HF amateur and marine communication.
FM: use for local VHF communication where you have or can salvage compatible equipment. The standard for community portable radio networks. Requires VHF equipment that is harder to build from scratch than HF.
Data/digital modes: Morse code is the original digital mode. More advanced digital modes (RTTY, PSK31, Winlink) require computer interfaces but provide tremendous capability — error correction, store-and-forward messaging, weak-signal performance far beyond voice. If you have computing capability, digital modes become very attractive.