Amplitude modulated radio frequencies are grouped into three bands according to the wavelength of their carrier frequencies. The carrier frequency chosen depends to a large extent on the distance between the broadcasting station and the target listeners.

1. Long wave (low frequency).
All transmission whose carrier frequencies are less than 400 kHz are generally classified as long wave. At a frequency of 100 kHz, a quarter-wavelength antenna is 750 meters high.

Such an antenna poses several problems such as vulnerability to high winds and danger to low flying aircraft. Long wave broadcasting stations therefore use an electromagnetically short antenna which necessarily limits their reach to a few tens of kilometers because the short antenna has only the ground wave.

2. Medium wave.
Carrier frequencies in the range 300 kHz to 3MHz are regarded as medium wave. The height of the antenna becomes more manageable and the possibility of using the sky wave to reach distant audiences is a reality. Generally, it is used for local area broadcasting.

3. Short wave.
Short wave generally refers to carrier frequencies between 3MHz and 30MHz. The wavelengths under consideration are between 100 meters and 1 meter. Antenna structures can be constructed to give specified directional properties.

Most of the energy can be put into the sky wave and the signal can be bounced off the ionosphere (the layer of ionized gas that surrounds the Earth) to reach receivers halfway round the world. A very severe problem is encountered in short wave transmission, that is, the signal tends to fade from time to time.

This phenomenon is caused by the multiple paths by which the signal can reach the receiver. It is clear that if two signals reach the receiver by different paths such that their phase angles are 180 degrees apart they will cancel each other.

The ionosphere sometimes experiences severe turbulence due mainly to radiation from the Sun. Short wave transmission is therefore at its best during the hours of darkness.

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