Frequencies
The Concept of frequencies
The word 'frequency' can be defined as a 'the number of vibrations per unit in time'. For example, if you were to stand on the end of a pier and count the waves passing underneath, then the number of waves that passed every minute would be the wave's frequency in minutes.
In radio engineering, the electrical oscillations in a circuit are described in the same way, but the frequencies of radio waves are very much quicker and are measured in thousands of waves, (or cycles), per second. As these figures are so large, they become awkward to manage and were abbreviated to kilo-cycles, (meaning a thousand cycles per second), or Mega-cycles, (meaning a million cycles per second). This phrase, 'cycles' has now been replaced by the term, 'Hertz', (from Heinrich Hertz who was one of the pioneers of radio), but the meaning is the same.
1 kilo-cycle = 1 kiloHertz =
1,000 cycles per second.
1 Mega-cycle = 1 MegaHertz =
1, 000, 000 cycles per second
1 Giga-cycle = 1 GigaHertz =
1,000,000,000 cycles per second
Associated with 'frequency' is 'wavelength', which is the distance between the crest of one wave and the crest of the next.
The Relationship between Frequency and Wavelength
The relationship between wavelength and frequency is fixed. If you alter the frequency, you alter the wavelength and vice versa. This is because all radio waves travel at the same speed, (approximately the speed of light or about 300,000,000 metres/sec). So, if a series of waves have a long wavelength, only a few can pass in a given length of time and they will have a 'low frequency'. If the waves have a short wavelength, then far more will pass in the same length of time and they will have a 'high frequency'.
Some radio broadcasting station still quote 'wavelength' rather than frequency when talking about their transmissions.
Frequency Designations
As radio frequencies cover a large range, for convenience they have been divided up into different 'frequency bands' as shown.
Very Low Frequency |
(V.L.F) |
3 kHz - 30 kHz |
Low Frequency |
(I..F) |
30 kHz -300 kHz |
Medium Frequency |
(M.F.) |
300 kHz - 3 MHz |
High Frequency |
(H.F.) |
3MHz - 30 MHz |
Very High Frequency |
(V.H.F.) |
30 MHz - 300 MHz |
Ultra High Frequency |
(U.H.F) |
300 MHz - 3 GHz |
Super High Frequency |
(S.H.F) |
3 GHz - 30 GHz |
Extra High Frequency |
(E.H.F) |
30 GHz + |
Bandwidth
The bandwidth of a transmission is the space that it requires in the frequency range, and depends upon the type of communication taking place. High qualityaudio transmissions, (such as music),require a bandwidth of about 8 kHz,radiotelephone about 3 kHz and digital data even less. If all transmission channels were given a bandwidth of 10 kHz, (to avoid interference), it can be seen that VLF only has room for 2.7 channels, whereas SHF has room for 2.7 million.
