Background
The electromagnetic spectrum and the piano keyboard
The programme compares the electromagnetic spectrum to a piano keyboard. There are low frequencies and high frequencies. However, the analogy goes further than this. Each octave on a piano keyboard is a doubling of frequency. This means that the frequency goes up significantly in just a few octaves. A similar effect is seen in the electromagnetic spectrum. Diagrams of the electromagnetic spectrum usually use a logarithmic scale. The useful frequency bands increase in multiples rather than in a linear way.
This logarithmic scale can be illustrated on the piano. It might seem that the top octave covers the same range of frequencies as the bottom octave. However, this is not the case. The bottom octave might be 64 Hz to 128 Hz, whereas the top octave is 2048 Hz to 4096 Hz (32 times the width of the bottom octave).
Similarly, in diagrams of the electromagnetic spectrum, it may looks as if the VHF region covers the same range of frequencies as the long wave region, but this is not the case. Each represents a tenfold increase in frequency, but the VHF region covers a much broader band of frequencies because the starting frequency is 1000 times higher. Thus VHF has a greater bandwidth by a factor of 1000. This effectively means that the VHF band can carry more information.
The piano keyboard and the electromagnetic spectrum are different in their total width. A typical piano has about 7 octaves. Even the whole range of human hearing (say, 40 Hz to 20,480 Hz) represents a factor of 512, which is 9 doublings or 9 octaves. But the electromagnetic spectrum from 100 kHz (105 Hz) to 1020 Hz represents a factor of 1015, which is 50 doublings or 50 octaves. Radio waves
The radio-wave region is divided into five bands as shown below. The figures are approximate, but there is a factor of 10 in frequency and wavelength between one region and the next. The usual names are given in the first column, and alternative names in the second column. There are attempts to give each regions a name based on its wavelength.
| usual name |
alternative name |
wavelength |
frequency |
use |
| microwave |
|
0.03 m |
10 GHz |
satellite TV; cross-country TV; telephone trunk lines; SETI |
| ultra high frequency (UHF) |
ultra short wave |
0.3 m |
1 GHz |
television broadcast |
| very high frequency (VHF) |
very short wave |
3 m |
100 MHz |
FM radio broadcast; mobile phones |
| short wave (SW) |
high frequency |
30 m |
10 MHz |
ham radio; radio control; police |
| medium wave (MW) |
medium frequency |
300 m |
1000 kHz |
AM radio broadcast |
| long wave (LW) |
low frequency |
3000 m |
100 kHz |
AM radio; ship-to-shore |
Units
- 1 Hz (hertz) is one oscillation per second.
- 1 kHz (kilohertz) is 1000 hertz.
- 1 MHz (megahertz) is 1,000,000 hertz.
- 1 GHz (gigahertz) is 1,000,000,000 hertz.
The parabolic dish
The dishes used for radio telescopes, satellite receivers and microwave receivers have the shape of a parabola in cross section. The graph of y = x2 is a parabola. A parabolic dish has a very special property: all the rays that arrive parallel to its axis are reflected and focused to the same point. This is why it is used for collecting the parallel rays that arrive from deep space. A 5-metre dish will collect all the rays from an area of 20 m2 and reflect them all into the receiver at its focus.
