What is AM (Amplitude Modulation)?

Short Answer

AM (Amplitude Modulation) is a type of modulation in which the amplitude of a high-frequency carrier wave is varied according to the strength of the information signal, such as voice or music. The frequency and phase of the carrier wave remain unchanged.

AM is mainly used in AM radio broadcasting, aviation communication, and long-distance transmission. It is simple, easy to generate, and can travel long distances, especially through ground and sky wave propagation.

Detailed Explanation :

AM (Amplitude Modulation)

Amplitude Modulation, commonly known as AM, is one of the earliest and simplest methods of sending information over long distances using radio waves. In AM, the amplitude (height) of a high-frequency carrier wave is changed in proportion to the instantaneous value of a low-frequency message signal. The message signal may be sound, speech, music, or any other information that needs to be transmitted.

The carrier wave is a high-frequency electromagnetic wave that helps the low-frequency audio signal travel long distances. Since low-frequency waves cannot be transmitted effectively due to poor range and large antenna requirements, the process of modulation becomes essential. AM is an important technique used in communication systems throughout history and is still widely used today.

How AM Works

To understand AM, imagine two waves:

  1. Carrier wave
    • High frequency
    • Constant amplitude
    • Used to “carry” the message
  2. Message or baseband signal
    • Low frequency
    • Contains actual information

In amplitude modulation:

  • The amplitude (height) of the carrier wave changes based on the message signal.
  • When the message signal is strong, the amplitude of the carrier increases.
  • When the message signal is weak, the amplitude decreases.
  • The frequency and phase of the carrier wave remain the same throughout.

This produces a new wave called the AM wave, which contains the information in its amplitude variations.

Mathematical Expression of AM

Although detailed math is not required at school level, AM can be expressed as:

Where:

  •  = carrier amplitude
  •  = message signal
  •  = carrier angular frequency

This shows how the message modifies the amplitude of the carrier.

Need for Amplitude Modulation

AM is required because:

  • Low-frequency audio signals cannot travel long distances.
  • Without modulation, signals mix and interfere.
  • Antenna size becomes too large for low frequencies.
  • High-frequency waves travel better and carry information more effectively.

By modulating, communication becomes more reliable and practical.

Sidebands in AM

When the carrier is modulated, two new frequencies are produced:

  1. Upper Sideband (USB) = carrier frequency + message frequency
  2. Lower Sideband (LSB) = carrier frequency – message frequency

The carrier, USB, and LSB together form the complete AM signal.

Sidebands carry the actual information.

Bandwidth of AM

Bandwidth of AM is given by:

Where  is the highest frequency of the message signal.

Example:
If the highest audio frequency is 5 kHz → bandwidth needed = 10 kHz.

Modulation Index

Modulation index tells how much the amplitude of the carrier changes.
It is the ratio of message amplitude to carrier amplitude.

  • If the index is too low, modulation is weak.
  • If the index is too high, distortion occurs.

Proper modulation index ensures a clear signal.

Applications of AM

AM has been widely used for many decades and continues to play an important role in communication.

  1. AM Radio Broadcasting

Medium-wave and long-wave radio use amplitude modulation for:

  • News
  • Music
  • Talk shows
  • Sports broadcasting

AM radio can reach very long distances, especially at night using sky waves.

  1. Aviation Communication

Aircraft pilots and air traffic control use AM for clear and reliable communication.

  1. Two-Way Radio

Emergency services, police and military communication systems used AM earlier.

  1. Rural and Remote Broadcasting

AM signals can cover large geographical areas.

Advantages of AM

  • Simple and inexpensive to generate
  • Easy to receive and demodulate
  • Can cover long distances
  • Suitable for communication in remote areas
  • Works even with low signal strength

Limitations of AM

Although useful, AM has some disadvantages:

  • Easily affected by noise (static, electrical interference)
  • Poor sound quality compared to FM
  • Less efficient in power usage
  • Requires larger bandwidth

Despite these limitations, AM remains important for communication systems.

Demodulation of AM

At the receiver’s end, the original message must be extracted from the AM wave.
This process is called demodulation.

AM receivers use:

  • Envelope detectors
  • Diode detectors

These detect the varying amplitude and recover the audio signal.

AM in Modern Technology

Although digital systems are more common today, AM is still used in:

  • Aircraft communication
  • Remote control systems
  • Emergency broadcasting
  • Amateur (HAM) radio

It remains valuable because of its long-range capability and simple technology.

Conclusion

AM (Amplitude Modulation) is the method of varying the amplitude of a high-frequency carrier wave according to the information signal. It allows audio signals to travel long distances, supports broadcasting, and provides reliable communication even in remote areas. Although modern digital techniques are more advanced, AM remains an essential and foundational concept in wireless communication.