Propagation and antennas

Introduction

Radio communication depends on the radiation of electromagnetic waves from the transmitting antenna. The electromagnetic waves are created by the alternating RF currents in the antenna, which arise from the coupling of the output of the transmitter into the antenna system.

The transmitted signal may be regarded as a succession of concentric spheres of ever-increasing radius, each one a unit of one wavelength apart, formed by forces moving outwards from the antenna. These hypothetical spherical surfaces, called 'wave-fronts', approximate to plane surfaces at great distances.

There are two inseparable fields associated with the transmitted signal, an 'electric field' (E) due to voltage changes and a 'magnetic field' (H) due to current changes, and these always remain at right-angles to one another and to the direction of propagation as the wave proceeds. The oscillations of each field are in phase and the ratio of their amplitudes remains constant. The lines of force in the electric field run in the plane of the transmitting antenna in the same way as would longitude lines on a globe having the antenna along its axis. The electric field is measured by the change of potential per unit distance, and this value is termed the 'field strength'.

Fig 7.1 The fields radiated from a transmitting antenna. (a) The expanding spherical wave-front consists of alternate reversals of electric field, with which are associated simultaneous reversals of the magnetic field at right angles to It, as shown in (b) and (c). The dotted arcs represent nulls. The lower diagrams should be interpreted as though they have been rotated through 90° of arc, so that the magnetic field lines are perpendicular to the page

The two fields are constantly changing in magnitude and reverse in direction with every half-cycle of the transmitted carrier. As shown in Fig 7.1, successive wave-fronts passing a suitably-placed second antenna induce in it a received signal which follows all the changes carried by the field and therefore reproduces the character of the transmitted signal. The field strength at the receiving antenna may range from less than 1mV/m to greater than l00mV/m.

Waves are said to be 'polarised' in the direction of (parallel to) the electric lines of force. Normally the polarisation is parallel to the length of the antenna, ie a horizontal antenna produces horizontally polarised waves. In order to receive maximum signal strength. The receiving antenna must be orientated to the same polarisation. In practice, particularly at VHF, the polarisation may be modified by factors such as abnormal weather conditions and reflection from the ionosphere. However, since even a 90º difference in polarisation will result in a 3dB (half an S-point) lower signal at the receiver, these losses need only concern those working with very low signal levels.

The electromagnetic wave is an alternating quantity. Its wavelength (λ) is the distance, in the direction of propagation, between points where the intensity of the field is similar in magnitude and sign, ie the distance travelled in free space to complete one cycle. Therefore:

velocity = frequency x wavelength

c = f x λ

where c is the velocity of propagation which for electromagnetic waves in free space is approximately 300,000,000m/s (186,000 miles/s). Therefore: