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What is the difference between AM, FM, SSB, and CW?

PostedApril 8, 2023
UpdatedAugust 1, 2023
ByHam Radio FAQ

AM, FM, SSB, and CW are different types of modulation used in radio communication. Each has its unique characteristics, advantages, and disadvantages, making them suitable for various applications in amateur radio.

Amplitude Modulation (AM)

Amplitude Modulation (AM) is the oldest and simplest modulation technique, where the amplitude (or strength) of a carrier wave is varied according to the modulating signal (usually an audio signal) while keeping the carrier wave’s frequency and phase constant. This results in a transmitted signal that consists of the carrier wave and two sidebands, which contain the modulating signal’s information.

Advantages of AM

  • Simple to generate and demodulate
  • Compatible with older radio equipment
  • Can be easily monitored on SSB receivers

Disadvantages of AM

  • Inefficient use of power (only about 33% of the transmitted power carries the actual information)
  • Occupies a larger bandwidth compared to other modulation types
  • More susceptible to noise and interference

In amateur radio, AM is primarily used on the HF bands and occasionally on the VHF and UHF bands for voice communication.

Frequency Modulation (FM)

Frequency Modulation (FM) is a modulation technique in which the carrier wave’s frequency is varied according to the modulating signal, while the amplitude remains constant. FM has the advantage of being less susceptible to noise and interference compared to AM, making it the dominant mode for VHF and UHF communication.

Advantages of FM

  • Better signal-to-noise ratio compared to AM
  • Less susceptible to noise and interference
  • Relatively easy to generate and demodulate

Disadvantages of FM

  • Occupies a larger bandwidth compared to SSB and CW
  • Less suitable for long-distance communication on HF bands due to propagation characteristics

In amateur radio, FM is widely used for local communication on VHF and UHF bands, especially for repeater operation and handheld radio communication.

Single Sideband (SSB)

Single Sideband (SSB) is a form of Amplitude Modulation (AM) where one of the sidebands and the carrier wave are removed, leaving only one sideband (either the upper or lower sideband) to carry the information. This makes SSB more efficient in terms of power usage and bandwidth compared to traditional AM.

Advantages of SSB

  • More efficient use of power and bandwidth compared to AM and FM
  • Better signal-to-noise ratio than AM
  • Suitable for long-distance communication on HF bands

Disadvantages of SSB

  • More complex to generate and demodulate compared to AM and FM
  • Requires more precise tuning for reception

In amateur radio, SSB is the most popular mode for voice communication on the HF bands and is also used on VHF and UHF bands for weak-signal work and long-distance communication.

Continuous Wave (CW)

Continuous Wave (CW) is not a modulation technique like AM, FM, or SSB, but rather a mode of operation where the carrier wave is turned on and off according to a predefined pattern, typically to transmit Morse code. Since CW transmits information by simply turning the carrier wave on and off, it is considered the most basic and efficient form of radio communication.

Advantages of CW

  • Highly efficient in terms of power usage and bandwidth
  • Can be transmitted and received with simple equipment
  • Suitable for long-distance communication, even with low power

Disadvantages of CW

  • Requires knowledge of Morse code for communication
  • Slower communication compared to voice or digital modes

In amateur radio, CW remains popular among operators, particularly for long-distance communication on HF bands, weak-signal work, and as a foundation for learning and understanding radio communication principles.

Each of these modulation types serves a specific purpose in amateur radio, offering different advantages depending on the situation and desired communication range.

To summarize:

  • AM (Amplitude Modulation) is the oldest and simplest modulation technique. It is used primarily on the HF bands for voice communication but is less efficient and more susceptible to noise compared to other modulation types.
  • FM (Frequency Modulation) is the dominant mode for VHF and UHF communication due to its better signal-to-noise ratio and resistance to interference. It is widely used for local communication, repeater operation, and handheld radio communication.
  • SSB (Single Sideband) is the most popular mode for voice communication on the HF bands. It is more efficient in terms of power usage and bandwidth compared to AM and FM and is suitable for long-distance communication.
  • CW (Continuous Wave) is the most basic and efficient form of radio communication, used primarily for long-distance communication on HF bands and weak-signal work. It requires knowledge of Morse code for communication.

By understanding the differences between AM, FM, SSB, and CW, amateur radio operators can choose the most appropriate modulation type for their communication needs, whether they’re focused on local communication, long-distance contacts, or weak-signal work. Each modulation type has its unique set of advantages and disadvantages, so it’s essential for operators to be familiar with the various options and make informed decisions based on their specific requirements and goals.

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