AM - Amplitude modulation

AM, which stands for Amplitude Modulation, is one of the oldest forms of modulation used in amateur radio and radio communications. Although AM plays a less central role in modern amateur radio than it once did, it is still used in certain situations and by enthusiasts.


1. Basic principle of AM

  • Amplitude modulation works by varying the amplitude (strength) of a carrier signal according to the modulating signal (e.g. speech or music).
  • The carrier signal remains at a constant frequency, while its strength varies according to the modulation signal.
  • The modulation produces two sidebands - the upper sideband (USB) and the lower sideband (LSB) - which are symmetrical about the carrier frequency and both contain the same information.


2. Frequency spectrum and bandwidth

  • AM requires a relatively large bandwidth. The total bandwidth of an AM signal is approximately twice the highest frequency of the modulation signal.
  • For example: If the highest frequency of the modulation (e.g. the highest voice frequency) is 3 kHz, the AM signal requires a bandwidth of about 6 kHz (3 kHz for the upper sideband, 3 kHz for the lower sideband).


3. use in amateur radio

  • Historical significance: In the early days of amateur radio (before the 1950s), AM was the dominant modulation for voice communication.
  • Current use: AM is still used in modern amateur radio, especially on the lower shortwave bands (e.g. 160 metres and 80 metres) and by enthusiasts who appreciate the retro charm of classic AM transmission. AM is also used for short and medium wave broadcasting.
  • Nostalgia and experimentation: Some radio amateurs use AM for nostalgic reasons or for special types of communication, such as using older, classic radio equipment.


4. Advantages and disadvantages

  • Advantages:
    • Simplicity: AM is technically simpler to implement than other modulation modes such as SSB. Many older radios are designed for AM.
    • Wide acceptance: Because AM is widely used in broadcasting, AM signals can be decoded by many receivers, including older ones.
  • Disadvantages:
    • Inefficiency: AM is inefficient compared to other types of modulation such as SSB, as much of the transmit power is wasted in the carrier and one of the two redundant sidebands.
    • Larger bandwidth: AM requires more bandwidth than SSB, which reduces the number of simultaneous connections that can be made in the same frequency range.
    • Susceptibility to interference: AM is relatively susceptible to interference and noise, as changes in signal strength (e.g. due to interference) directly affect the modulation.


5. Technical details

  • Modulation: In AM modulation, the audio signal is mixed with the carrier signal, resulting in a signal that varies in amplitude according to the audio signal.
  • Demodulation: An AM signal is received by filtering out the two sidebands and extracting the modulation signal. A simple diode detector can be used to extract the audio signal from an AM signal.


6. AM vs. SSB in Amateur Radio

  • While AM used to be the standard method for voice transmission, SSB has become more popular due to its efficiency and narrower bandwidth. Nevertheless, AM remains a nostalgic and occasionally used mode in amateur radio.


In summary, AM is a historic and still used modulation in amateur radio, which has retained its niche in broadcasting due to its simplicity and widespread use. Despite its disadvantages in terms of efficiency and susceptibility to interference, AM remains relevant in amateur radio for certain applications and under certain conditions.