Ever wondered about pilots’ and airport control towers’ communications? What kind of radio communication do they use? Aviation radio traditionally utilizes AM (Amplitude Modulation) for communications instead of FM (Frequency Modulation).

Why Do Aircraft Use AM Frequencies Instead of FM?

The capture effect of FM would prevent weaker stations from being blocked out by receiving stronger stations. VHF AM is the frequency used by aviation voice radios. AM allows multiple stations to be received on the same channel.

Did you know? – The first use of AM radio in aviation was in 1920 when a U.S. Navy seaplane used a new AM radio compass to locate and fly to a Navy ship nearly 100 miles offshore.

Why is AM used over FM, when FM is capable of broadcasting a superior, stronger signal?

I’ll answer these questions and others I’ve often heard in regard to the topic of aircraft/tower communications.

What Radio Frequencies Do Aircraft Use?

Aircraft use AM frequencies for air-to-ground communications because AM is more resistant to noise and interference than FM is. The reason for this is that in AM, the amplitude (or strength) of the radio wave is varied to convey information, whereas in FM the frequency is varied. 

When a radio wave is affected by noise or interference, it can cause the frequency to shift, which can make the transmission harder to understand. With AM, the noise or interference is less likely to affect the amplitude of the signal, so the transmission is still relatively easy to understand.

Another reason why AM is often used in air-to-ground communications is because it allows for simpler, more reliable equipment to be used, which is critical in the demanding and harsh environment of an aircraft.

Other reasons for the use of AM frequencies for air-to-ground communications include:

  • To Avoid the “Capture Effect”

The “capture effect” is when a stronger radio signal overpowers a weaker signal. This obviously presents a problem for aircraft and ground crew in their efforts to communicate. The use of AM frequencies in aviation communications allows for multiple signals transmitted on the same channel to be heard.

  • For Greater Range

AM signals are able to travel greater distances than FM signals are. Range capabilities depend upon the wavelength used, so it makes sense that AM frequencies (with their longer waves/lower frequencies) would be able to pass through obstacles, resulting in a greater range.

Perhaps you’ve had the opportunity to notice that “old school” FM radio stations don’t broadcast nearly as far as AM radio stations do. In addition, AM’s longer waves are able to reflect off of the stratosphere, allowing for even greater range.

Related: How High do Planes Fly?

Why Do AM Frequencies Have a Greater Range Than FM Frequencies?

AM frequencies on the ground have a greater range because they are able to better penetrate through obstacles such as buildings, trees, and hills.

For aviation purposes, where obstacles are not as often a problem, this also holds because AM frequencies are able to change their amplitude (or strength) in order to better transmit. Additionally, AM frequencies are able to travel further distances due to their longer wavelength, which allows them to better bounce off the ionosphere and travel long distances.

In contrast, FM frequencies, though they feature a stronger signal, have a shorter wavelength and are more susceptible to interference from obstacles, making their range more limited.

What is the Advantage of Using AM Frequencies over FM Frequencies for Aviation?

AM frequencies have several advantages over FM frequencies when it comes to aviation, including:

  • Higher power output: AM signals can be transmitted at a higher power output than FM signals, allowing for better signal strength and clarity.
  • More efficient use of bandwidth: AM signals use less bandwidth than FM signals, making it possible to transmit more information using the same amount of frequency spectrum.
  • Greater range: AM signals can travel further distances than FM signals, making them more suitable for long-range communication between aircraft and ground control.
  • Better penetration through atmospheric interference: AM signals can better penetrate through atmospheric interference such as thunderstorms and clouds, ensuring clear communication even in adverse weather conditions.
  • Better compatibility with older equipment: Many older aircraft and ground stations still use AM frequencies, making them more widely compatible than FM frequencies.

Why Are FM Frequencies More Immune to Noise than AM Frequencies?

FM frequencies are more immune to noise than AM frequencies because of the way they are modulated. With AM, the amplitude of the carrier wave is varied to convey the information, whereas with FM, the frequency of the carrier wave is varied.

When there is noise present, it tends to affect the amplitude of the signal, causing the information to become distorted or lost. With FM, the noise tends to affect the frequency of the signal less, allowing the information to remain more intact. Additionally, FM uses a much wider frequency range than AM, which allows for greater immunity to noise.

What Is the Best Frequency to Listen to Aircraft?

The best frequency to listen to aircraft depends on the type of aircraft and the location you are in. In general, the most commonly used frequency for commercial air traffic control is between 118.000 MHz and 137.000 MHz. However, specific frequencies may vary depending on the location and the type of aircraft, such as military aircraft, private aircraft, and emergency services.

To find the best frequency to listen to aircraft in your area, consult an aeronautical frequency directory or check online resources.

Does the Military use AM or FM Frequencies?

Military personnel use both AM and FM frequencies for their communications. AM frequencies are typically used for long-range communication, such as for air-to-ground or ground-to-ground communication.

FM frequencies are used for shorter-range communication. Additionally, the military also uses other types of modulation such as digital modulation, spread spectrum, and other, more advanced techniques.

Does Civilian Aviation Use AM or FM Frequencies?

Civilian aviation primarily uses AM frequencies for communication between aircraft and ground control. AM allows for clearer and more robust communication, even in areas with high levels of interference.

Some aircraft also use VHF (very high frequency) FM frequencies for communication within the aircraft and with other aircraft in the vicinity.

So, Why Does Aircraft Use AM Frequencies to Communicate Instead of FM?

The main reason aircraft use AM (amplitude modulation) frequencies for air traffic control and navigation is because AM signals are more resistant to interference from static and noise.

Moreover, AM signals are able to penetrate through atmospheric interference and clouds better than FM (frequency modulation) signals. This makes it more reliable for aviation communication to control towers and to other planes, especially in low visibility or bad weather conditions.

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