Signal School
Modulation Manual

Lesson 1: Analog Modulation

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What Is Modulation?

Modulation is the process of encoding information (like your voice) onto a carrier wave so it can be transmitted over radio. In analog modulation, this is done by continuously varying aspects of the carrier wave in proportion to the audio signal (like your voice or music).

There are three common analog modulation types in amateur radio:

AM – Amplitude Modulation

How It Works:

  • The height (amplitude) of the wave changes based on the sound input.

  • The frequency stays constant; only how "tall" the wave is varies.

Waveform Diagram:

Imagine a smooth sine wave where the peaks and valleys grow or shrink to match the loudness of your voice. It’s like stretching a ribbon up and down while keeping the spacing between folds the same.

Pros:

  • Easy to understand and demodulate

  • Compatible with old radios and simple circuits

  • Used on the AM broadcast band (e.g. 530–1700 kHz)

Cons:

  • Takes up more bandwidth

  • Doesn’t work well for long-distance HF communication

FM – Frequency Modulation

How It Works:

  • The frequency of the wave is varied with your audio input, while the amplitude stays constant.

  • Louder sounds = faster frequency shifts

Waveform Diagram:

Think of the wave getting more “squished” or “stretched” based on pitch or volume.

Pros:

  • Very clear audio, especially on voice

  • Much more resistant to static and noise

  • Great for short-range communication (VHF/UHF)

Cons:

  • Takes up more bandwidth

  • Doesn’t work well for long-distance HF communication

SSB – Single Sideband

How It Works:

  • SSB is a form of AM where unnecessary parts of the signal are removed:

    • Only one sideband is transmitted (USB or LSB)

    • The carrier and opposite sideband are suppressed

Spectrum Comparison:

AM: Uses both side bands - Is the carrier - Bandwidth used = ~6 kHz

SSB: Uses one side band - is not the carrier - Bandwidth used = ~2.4 kHz

  • USB - Upper Side Band (common above 10 MHz)

  • LSB - Lower Side Band (common below 10 MHz)

Pros:

  • Highly efficient – less power, less bandwidth

  • Ideal for long-distance HF communication

  • Most common mode for voice on HF bands

Cons:

  • Sounds unnatural or “mushy” without proper tuning

  • Requires a special receiver to decode correctly

Audio Samples (If Supported)

(If you're viewing this on a platform that supports audio, search or try the following):

  • YouTube: “AM vs FM vs SSB audio comparison”

  • Sites like www.sigidwiki.com often have audio clips.

You’ll notice:

  • AM sounds warm but staticky

  • FM sounds crisp and clean

  • SSB sounds thinner or robotic, but gets through in weak conditions

Summary Chart

Modulation

Use Case

 Pros Cons
AM Shortwave, old gear Simple, traditional Noisy, inefficient
FM
VHF/UHF repeaters
 Clear, noise resistant Short range, wide bandwidth
SSB HF voice comms Efficient, long-distance Harder to tune, sounds odd

Want to Try It?

Use a WebSDR (like http://websdr.org) and tune to an HF band

Try selecting “AM,” “LSB,” “USB,” or “FM” on the same signal to hear the difference

Use an RTL-SDR dongle to scan live signals on VHF/UHF bands