Polyrod Antennas 



By G. E. MUELLER and W. A. TYRRELL 



The polyrod, a new form of microwave endfire antenna, is described. This 

 consists of a properly shaped dielectric rod protruding from a metal waveguide. 

 For applications requiring moderate gain, it possesses desirable electrical and 

 mechanical properties. It is useful as a unit antenna in broadside arrays on 

 account of its low crosstalk into adjacent polyrods. This paper describes work 

 done from 1941 to 1944 at the Bell Telephone Laboratories, Holmdel, N. J. 

 Important individual contributions are acknowledged in some of the footnotes. 

 A report of this development has been withheld from earlier publication for 

 reasons of military security. 



1. Introduction 



A UNIFORM rod (or "wire") of dielectric material without metallic 

 -^ ^ boundaries is a well-known type of single conductor transmission 

 line. In this kind of waveguide, a portion of the energy travels along in 

 the space outside the rod. At discontinuities, including those caused by 

 proximity to other objects, radiation takes place. For this reason, the di- 

 electric waveguide has not become generally useful as a transmission medium, 

 this need havmg been satisfied by the hollow metal pipe. The tendency 

 toward radiation inherent in the dielectric guide is turned to advantage, 

 however, in a new form of radio antenna. Here the objective is to encourage 

 radiation from all parts of the dielectric rod. Li progressing along the rod, 

 therefore, power is gradually transferred from within the dielectric to the 

 space outside. At a point where the transfer has been effectively com- 

 pleted, the rod can be terminated abruptly. By proper design, this radiat- 

 ing structure is an endfire antenna. Since it has been most often fabricated 

 from polystyrene, it has become known as the polyrod antenna. It is 

 especially useful for microwaves. 



We must now review and examine certain features of dielectric rod trans- 

 mission and of endfire antenna theory, for their bearing on polyrod design 

 and performance. 



2. Dielectric Wire Transmission^ 



A dielectric rod can be energized with an infinite variety of transmission 



modes. These are in general hybrid waves- possessing transverse and longi- 



[ tudinal components of both E and //. We shall here be concerned only 



1 Hondros and Debye, Ann. der Fhvs., Vol. 32, pj). 465-476; J. R. Carson, S. P. Mead 

 and S. A. Schclkunoff, B.S.TJ., Vol. 15, pp. 310-333, 1936; G. C. Southworth, B.S.T.J., 

 Vol. 15, pp. 284-309, 1936; S. A. Schelkunoff, "Electromagnetic Waves," pp. 425-428, 

 D. Van Nostrand, New York, 1943. 



2 Except in the case of circular symmetry. Cf. Schelkunoff, loc. cit., pp. 154, 425. 



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