 will provide an efficient means horizontal pattern is called a polar diagram. The vertical of transmitting or receiving electromagnetic signals or azimuth pattern (Figure 2.1-11( is the pattern seen through free space. Many of the principles used in the by an observer standing to one side of the lamp. The construction of antennas can be easily understood by vertical pattern is sometimes called the end-fire pattern. analogy to the headlamp of your car (see Figure 2.1-11). Both light patterns must be considered because modern An isotropic light source is a light source which radi- headlamp reflectors tend to be elliptical and affect ates light equally in all directions. The radiation pattern emitted light in the horizontal and vertical planes from an isotropic light source can be altered by placing differently. a reflecting mirror on one side of the light source. This is In the above description, light has been assumed to carried out in car headlamps where a quasi-parabolic travel from bulb to free space but the effect is equally reflecting mirror (reflector) is placed behind a bulb to true for light travelling in the opposite direction, i.e. the increase the light intensity of the lamp in the forward system is bi-directional. It can be used either for trans- direction. The reflector has therefore produced a change mitting light from the bulb or for receiving external light in the directivity of the light source. The increase or 'gain' at the point source usually occupied by the bulb filament. of light intensity in the forward direction has been gained This can be easily verified by shining an external light at the expense of losing light at the back of the lamp. This source through the lens and the reflector in the opposite gain is not a 'true gain' because total light energy from the direction from which light had emerged, and seeing it lamp has not been increased; light energy has only been converge on the bulb source. re-directed to produce an intensity gain in the forward Many of the principles introduced abo