6.3.4 Applications Of Conventional AM

Although conventional amplitude modulation (full-carrier AM) has largely been replaced in modern communications systems by more spectrally and power-efficient techniques, it remains in use in several important applications where its particular characteristics offer operational advantages. Conventional AM remains in use in the following applications:

Aviation VHF voice communications. AM is used almost universally for civil and military airband communications in the VHF band (118–137 MHz). The principal reason is that AM does not exhibit the capture effect associated with FM (see Section 6.2.2.7). If two aircraft transmit simultaneously on the same frequency, an FM receiver will typically lock onto the stronger signal and suppress the weaker one entirely. An AM receiver, by contrast, will reproduce both signals as an audible heterodyne, alerting the listener to the presence of interference. In safety-critical air traffic control environments, this behavior is preferred because it reduces the risk of an unnoticed blocked transmission.
Legacy medium-wave and shortwave broadcasting. Conventional AM remains widely used for MF and HF broadcasting. The primary reasons are historical infrastructure, wide receiver compatibility, and the simplicity and low cost of envelope-detection receivers. Although digital broadcasting systems offer improved audio quality and spectral efficiency, large installed bases of AM receivers and long-established transmitter networks continue to support conventional AM broadcasting in many regions.
Military HF radios. While modern military HF systems predominantly employ single-sideband (SSB) and digital modulation schemes, conventional AM is still supported in many tactical and legacy platforms. AM can be advantageous for interoperability with older equipment, for training, and in certain net configurations where simple envelope detection or compatibility with allied systems is required. Its robustness to slight frequency offsets also makes it tolerant of less precise oscillators in austere conditions.
Maritime HF radios. Maritime services historically used full AM for ship-to-ship and ship-to-shore communications, and although SSB is now standard for long-range HF voice, AM is still encountered in legacy equipment and in some regional services. As with aviation, interoperability and compatibility with older receivers remain important considerations.

Despite its continued use in certain safety-critical and legacy systems, conventional AM is inherently inefficient in both bandwidth and transmitted power. For this reason, most modern voice and data systems employ suppressed-carrier or digital modulation techniques that make more effective use of available spectrum and energy. The development of these improved schemes builds directly on the principles established by conventional AM and is examined in the following sections. Let’s look first at the benefits of making full AM more power- and bandwidth-efficient.