Chapter

7

VHF Propagation

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7 VHF Propagation

Propagation VHF 7

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VHF Propagation 7

Introduction

RadioWaves

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 electromagnetic 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.

Figure 7.1 Vertical polarization

The term polarization is used to describe the direction or plane of oscillation of the electrical field of an electromagnetic 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. Figure 7.1 shows the peak values E and H of the electric and magnetic fields of a vertically polarized 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 millimetres are used.

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