he intensity of the "illumination" falling on propagation. a receiving aerial. The direction of the electric field determines the A plane wave expands outwards as it travels through polarisation of a plane wave. space from a point source. As a result, the power density At any point, the ratio of the electric and magnetic falls off with increasing distance from the source. If you fields is the same and equal to the wave impedance. have studied any optics then you will be familiar with the This impedance is 377 S approximately. Basic features of radio communication systems CHAPTER 2. 1 Luminous pattern area Luminous pattern area Fig. 2.1-11 Radiation patterns from a car headlamp: (a) top view; (b) side view. The product Erms X Hrms gives the power density of its forward light into a main beam or main lobe. Again, the wave. this 'gain' in light intensity has been achieved by confining The power density falls off as the square of the dis- the available light into a narrower beam of illumination; tance from the r.f. source. there has been no overall gain in light output from the To obtain optimum signal reception from free space bulb. a receiving aerial should be set for the correct There are also optimum sizes and distances for the polarisation and be suitably located with regard to placement of reflectors and lenses. These are dictated by height and direction. the physical size of the bulb, the desired gain intensity of the main beam or main lobe, the required width of the main beam and the requirement to suppress minor or 2.1.5 Antennas and aerials spurious light lobes which consume energy and cause unnecessary glare to oncoming motorists. 2.1.5.1 Introduction A car headlamp (Figure 2.1-11) has two main light-emitting patterns: a horizontal pattern and a verti- An antenna or aerial is a structure, usually made from cal pattern. The horizontal pattern (Figure 2.1-11(a)) is good conducting material, that has been designed to have a bird's eye view of the illumination pattern. A plot of the a shape and size SO that it