Waves, frequencies and ranges

Aircraft travel from one place to another at a height of 11km; some of them, even at the speed of sound, therefore, it is necessary to have a communication channel between them and their surroundings. This medium is the radio.

It was invented by an electronics engineer called Guillermo Marconi in 1901, two years previous to the Wright brothers’ first ever flight. Radio is indispensable to aviation.

But how does it work? How is the radio used in aeroplanes? Here’s what we tell you.

How does radio works

Radio is based on the transmission of electromagnetic waves to the atmosphere. These waves travel at the speed of light (300 thousand kilometres per second) and form a specific pattern.

Basically, the radio transmitter converts our voice to electrical signals.

Afterwards, the aerial transmitter converts those signals to electromagnetic waves and broadcasts them into space.

Finally, the radio receptor decodes the waves and sends them back to their original pattern, recovering the information and generating the audio once again.

Radio wave characteristics

A radio wave has two main characteristics, frequency and wave length.

  • Its frequency is the number of complete cycles per second, i.e., the number of complete waves the receptor can capture. The unit of measure for frequency is the Hertz (Hz) and it is represented by f.
  • The length of wave is the distance between two crests of the wave. A high frequency means small lengths of wave and vice versa. The unit of measure is the metre (m) and it is represented by the Greek letter, lambda λ.

Wave modulation

According to its function, there are two methods of modulation of radio waves: amplitude modulation (AM) and frequency modulation (FM).

In aviation, radio works with the AM method or amplitude modulation. It uses less bandwidth and is the first one ever used; however, it is also the one which is mostly affected by interference and hence has a poorer audio quality.

FM or frequency modulation is what is used daily when we play the radio in our cars and enjoy the latest hits.

Frequencies used in aviation

In aviation we do not use all the frequencies for the same purpose:

  • Instrumental landing system (ILS) uses frequencies reserved for this:
    from 108MHz to 112MHz.
  • From 108MHz to 118MHZ: these frequencies are used for VOR navigation.
  • VHF Communication uses frequencies from 118MHz to 137MHz.

Communication between longer distances uses other frequencies such as HF.

Range of radio communication in aviation

VHF frequencies propagate in a straight line through the atmosphere but are limited by the line of sight, i.e., the waves do not follow the curvature of the Earth.

This is the reason why It is necessary to install signal repeaters in the surface of our planet

Additionally, pilots can make use of a formula to find out if they are within sufficient range to talk to the station they are contacting.

Range in nautical miles = 1.23 x (√h1 + √h2), h1 being the height of the airplane and h2 the height of the station, both expressed in feet.