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9. VHF PROPAGATION

9.1 RADIO WAVES

If an alternating current of suitably high frequency is fed to a transmitting aerial,
the energy is not confined to the metal of the aerial but radiates out into space in

the form of electro-magnetic waves (radio waves). This radiation of energy

through space comprises alternating electrical and magnetic fields at right angles
to each other. The amplitude of each field varies (oscillates) between zero and a

maximum value, at the same frequency as the alternating current in the aerial.

Polarization.

The term polarization is used to describe the direction or plane of oscillation of

the electrical field of an electro-magnetic wave. For instance a vertical
transmitting aerial produces (mainly) a vertically polarized radio wave, with the

electrical field (E) oscillations occurring in the vertical plane and the magnetic

field (H) oscillations in the horizontal plane. For efficient reception, the receiving
aerial should also be vertical. If the transmitting aerial is horizontal, the receiving

aerial should also be horizontal.

The electric and magnetic fields oscillate at right angles to each other and both are
at right angles to the direction of propagation (or travel) of the radio wave.

Speed of Propagation.
Radio waves travel at the speed of light. The speed is virtually constant and is:

300,000,000 metres per second

or
162,000 nautical miles per second

Wavelength.
The wavelength of a radio wave can be defined as the distance travelled by the

radio signal during the transmission of one cycle. Wavelength is normally

expressed in metres unless it is less than one metre, when centimetres or

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