POLY ROD ANTENNAS 851 



present, the usable bandwidth is therefore limited primarily by the frequency 

 response of the coupling arrangements from polyrod to waveguide or coaxial 

 line. 



We have been exclusively concerned so far with plane polarized radiation. 

 A circularly symmetrical polyrod such as in Fig. 8 can be used equally well 

 to radiate circularly polarized waves. To do this, the polyrod is fed from 

 a waveguide in which circulary polarized dominant waves are generated by 

 means of a 90° phase shift section.^- 



The effect of dielectric loss upon polyrod performance is shown in Fig. 12, 

 to be compared with Fig. 7. The power factors are: Styramic, 0.0005; 

 hard rubber, 0.003; acetate butyrate, 0.020; pol3^styrene, 0.0002. Mate- 

 rials having power factors less than 0.001 are satisfactory for polyrod an- 

 tennas. 



Figure 13 shows the crosstalk between adjacent polyrods, that is, the 

 power received in one radiator when the other is energized. For polyrods 

 pointing in the same direction, separations greater than a wavelength insure 

 low mutual coupling. This makes the polyrod attractive as the element 

 in broadside arrays. Proximity to other undriven polyrods affects the gain 

 and directional pattern to a greater extent, as shown in Fig. 9. 



More generally, the performance of a polyrod is affected by proximity 

 to any metallic or dielectric objects. The gain and pattern must be deter- 

 mined empirically for each new configuration. It has been found that a 

 metal rod can be plated parallel to a polyrod without seriously affecting its 

 behavior so long as a separation of a wavelength or more is maintained. 

 Sheets of dielectric material can be brought even closer without adverse 

 effect so long as large surfaces are not in direct contact with the polyrod. 

 These and other experiences suggest that the polyrod is relatively unaffected 

 by nearby objects. 



Tests have been made of the effect of fresh and salt water in the form of 

 a spray or solid stream playing on a polyrod. Provided that puddles do not 

 formi on the surface, as can happen with rectangular polyrods, the effect is 

 a decrease of 1 to 2 db in gain under the worst conditions. 



In conclusion, for microwave applications involving moderate gains of 

 15 to 20 dh, the polyrod assumes a convenient physical form and displays 

 high electrical efficiency. It is less subject to disturbance by nearby ob- 

 jects than might be expected. It is especially useful as an element in broad- 

 side arrays. As an example of such arrays. Fig. 14 shows a 42 rod steerable 

 beam antenna used in an important type of Navy fire control radar. 



'- For a discussion of this subject, cf. A. G. Fox, "An Adjustable Waveguide Phase 

 Changer," to be published in Proc. I. R. E. 



