POLY ROD ANTENNAS 



839 



fined almost entirely within the rod, and the phase velocity approaches that 

 in an unbounded medium of the same dielectric constant. By choosing 



intermediate values of 



D 



can be varied between these limits. 



3. Endpire Antennas 



We consider a linear array of isotropic radiators, infinite in number but 

 so closely spaced as to occupy a finite length. We assume that the radia- 

 tors are uniformly excited from a feed line, a transmission line parallel to 

 the array phasing the various elements according to phase velocity on the 

 line. The radiation pattern is given by*^ 



sin 7r(p cos 6 — 0) 

 7r(p cos 0-/3) 



(1) 



0.2 0.4 



0.8 1.0 1.2 



Fig. 2 — Normalized phase velocity for a cjlindrical dielectric wire. 



where r = relative field strength 



6 = angle with respect to the array axis 

 p = length of array in free space wavelengths 

 27r/3 = phase shift in radians in the feed line from one end of array to 

 the other end. 

 The pattern is symmetrical about the array axis. 



Plotted from (1), Fig. 3 shows the pattern of a six w^avelength radiator, 

 p = 6, for selected values of p. When /3 = p (= 6 in this case), phase 

 velocity along the feed line is equal to free space velocity, and the resulting 

 pattern is endfire. With /3 = p + 0.5 (^ 6.5) the pattern remains endfire 

 and the major lobe becomes sharper. For (8 < p and /3 > p + 0.5 (as shown 

 by /3 = 5.0, 5.5, 7.0) the pattern deteriorates into a forward conical beam. 



6 R. M. Foster, B. S. T. J., Vol. 5, p. 307, 1926. 



