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To transmit signals, it is necessary to convert, encode or modulate the signal into a form of energy that will travel through copper cables, fibre optics or free space.
Radio Signals in Free Space
- To transmit and receive radio signals, an antenna is needed.
- The ideal size for an antenna is half a wavelength long.
- Antennas longer than a few hundred metres are too big to be practicable. This sets a lower limit on the range of easily useable frequencies.
- Antennas smaller than around a millimetre are hard to make. This sets an upper limit on the range of frequencies that can be used easily.
- Information signals are often at a low frequency. For example Audio covers the range from 20 to 20 000Hz.
- At 20 000 Hz, the radio wavelength would equal C / F = 3x108 / 20000 = 15 kilometres. This is much too long to be transmitted using an antenna of a reasonable size. The solution is to use a carrier wave with a much shorter wavelength.
- The carrier wave is a radio frequency signal at a frequency chosen such that the size of the antenna is convenient.
- FM broadcast radio uses the 100MHz band.
This choice was not by chance.
The wavelength is 3 metres and a half wave antenna is 1.5 metres long.
The quarter wave 75cm antenna is just right to mount on a car and work efficiently.
This makes VHF FM car radio convenient.
- On its own, the carrier contains no information.
- The carrier has to be modified to allow the information signal to be sent. This is called MODULATION.
- The carrier wave is modified in a manner that is proportional to the information signal.
- AM - The amplitude of the carrier is modified.
- FM - The frequency of the carrier is modified.
- There are several digital modulation methods. These achieve the same goal without the disadvantages of analogue AM and FM.
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