STANDING-WAVE ANTENNAS 



27 



of a dipole is to use an artificial ground plane. It 

 usually takes the form of a number of grounded 

 rods spreading radially from the base of the antenna 



It is essentially a center-fed half-wave dipole with a 

 parasitic counterpart "dummy" (see Section 3.5) 

 in its immediate neighborhood and connected to the 

 latter at the ends of the dipole. The induced current 

 in the dummy has the same distribution as, and is in 

 phase \^'ith, that of the primary dipole. Hence the 

 radiation pattern is essentially that of a simple half- 

 wave dipole. The radiation resistance is four times 

 that of the ordinary dipole. 



3.2.5 



Multiple Half-Wave 

 Long Antennas 



For an antenna of length equal to an integral 

 FiGiiRE 9. Quarter-uave dipole with artificial ground. number, n, of half Wavelengths, the radiation field is 



(Figiu-e 9). If the antenna is a quarter- wa\e dipole given by: 



the effect of the artificial ground is to produce an 1. ?) is odd: 



image quarter-wave dipole; the radiation resistance 

 and the radiation pattern of the system are those of a 

 half- wave dipole. 



X/2- 



c 



INPUT 



D 



2. n is even: 



Es = 



607, 



?OS I COS d I 



\2 / 



sin d 



60/i 

 d 



sm 



(?-«) 



(15) 



(16) 



sin d 



where d is the radial distance to a field point and 

 li is the input current at the center of one of the 

 half-wave elements. 



The radiation patterns are illustrated in Figure 11 

 2. Folded dipole. Another variant of the dipole for the doublet, n = 1 (the half-wave dipole), and 



Figure 10. Folded dipole. 



antenna is the folded dipole, shown in Figure 10. ii = 2, 3, 4. 



/ 



I 

 \ 



n = 2 



n:3 



Figure 11. Antenna radiation patterns (relative field strength). 



