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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