lements (+0.5 dB) ratio (+2 dB) angle (+3°) Side lobes Maximum Main lobe response directivity elements which serve as 'electrical lenses' to Side lobes concentrate the signal into a more clearly defined beam. The number of directors per array varies according to the Fig. 2.1-20 Yagi-Uda array: (a) physical arrangement; (b) radiation gain required from the aerial. The length of directors and pattern. the spacing between them are also dependent on the number of elements used in the array. In general, gain Solution increases with the number of directors, but greater gain Substituting Equation 2.1.8 in Equation 2.1.19 yields needs more careful alignment with the transmitting sta- tion and requires that the antenna be more sturdily erms nwu.moA Erms 377 cos a mounted otherwise its pointing direction will waver in high winds which can cause fluctuations in the received signal strength. The Yagi array is usually designed to be connected to 100 x 10-6 X 10-5 X cos 60° a 75 O transmission line. 16 Yagi arrays suitable for oper- ation over the entire TV band can be obtained commer- = 1.68 uV cially, but these broadband arrays are usually designed to 'trade off' bandwidth against aerial gain. Broadband Yagi arrays are extremely useful for mobile reception where 2.1.6 Antenna arrays minimum space and convenience are of importance. (You often see them on top of mobile caravans.) Domestic Yagi 2.1.6.1 Introduction arrays are usually designed to provide greater gain but with a more restricted operational frequency band. The latter is Antenna arrays are used to shape and concentrate energy not a disadvantage because TV stations operating from in required patterns. One of the more common domestic a common transmitting site confine their broadcasts to arrays is the Yagi-Uda array used for the reception of well defined frequency bands. The common practice for television signals. domestic Yagi arrays is to use three or more designs (scaled in size) to provide reception for the complete TV band. 2.1.6.2 Yagi-Uda array Typical values 