ception. 12 Terrestrial broadcasting stations tend to use physical components should be large, approximately 1/4 either vertical or horizontal polarisation. Satellite broad- wavelength for optimum radiation. This is why antennas casting stations use circular polarisation. The polarisation are physically large in comparison with their operational of a wave is sometimes 'twisted' as it propagates through wavelength. space. This twisting is caused by interfering electric Energy radiates from an r.f. source or transmitter in all or magnetic fields. It is particularly noticeable near directions. If you imagine a spherical surface surrounding steel-structured buildings where aerials are mounted at the transmitter, then the interior of the surface would be odd angles to the vertical and horizontal planes to com- 'illuminated' with radiated energy, just like the inside of pensate for these effects. a globular lamp-shade. The illumination is not necessarily uniform, however, since all transmitters are, to some Field strength extent, directional. The strength of a radio wave can be expressed in terms of If the r.f. source is sinusoidal, then the electric and the strength of its electric field or by the strength of its magnetic fields will also be varying sinusoidally at any magnetic field. You should recall that these are measured point in the radiation field. Now it is difficult to depict in units of volts per metre and amperes per metre re- a propagating electromagnetic field but some of its im- spectively. For a sinusoidally varying field it is customary portant properties can be identified. To do this we con- to quote r.m.s. values Erms and Hrms. What is the physical sider propagation in a particular direction on a straight significance of Erms? This is numerically equal to the line connecting a transmitter to a distant receiver as r.m.s. voltage induced in a conductor of length 1 m when shown in Figure 2.1-10. You will see that this line co- a perpendicular electromagnetic wave sweeps over the incides with the z-direc