for Yagi arrays operating in the TV band are shown in Table 2.1-1. These figures have been taken The Yagi-Uda aerial array shown in Figure 2.1-20 is one from a well known catalogue but some of the terms need of the most commonly used antenna arrays. It is used explanation. extensively for the reception of TV signals and can be 'Number of elements' means the total number of di- seen on the roofs of most houses. The Yagi array is an rectors, folded dipoles and reflectors used in the antenna system designed with very similar principles to array. For example, if the number of elements in an the car headlamp system described in Section 2.1.5.1. Its array is 10, the array includes eight directors, one main elements are a folded dipole, a reflector, and folded dipole and one reflector. 16 Earlier on, we said that the impedance of a folded dipole aerial was 292 0 yet now we say that this antenna is designed to operate with a 75 system. This apparent discrepancy arises because the use of reflector and directors loads the folded dipole and causes its impedance to fall. Ju- dicious director spacing is then used to set the array to the required impedance. There is a class of Yagi arrays known as Log Periodic Yagis. These have greater bandwidths because the directors are spaced differently. They do cover the entire TV bands but their gain is a compromise between frequency bandwidth and gain. Basic features of radio communication systems CHAPTER 2. 1 'Forward gain' is the maximum 'gain' which the an- Balanced Balanced tenna can provide with respect to an isotropic antenna transformer aerial. A maximum aerial gain of 10 dB means that Balanced the antenna will provide 10 times the 'gain' you would get from an isotropic aerial when the array is Balanced pointed in its maximum gain direction. transmission line 'Front to back ratio' is the difference in gain between the direction of maximum antenna gain and the minimum direction of gain which is usually in the opposite direction. This ratio is important because it Balanced to provi