Absfmct-Expressions which representthe elevation-plane patterns of a large TEM mode coax aperture and a TMo, mode circular aperture on finite circular and square ground planes have been derived using geometrical theory of diffraction; design curves for the coax case are presented. The expressions for the field pattern were obtained by superposing the infinite ground plane solutions, the first-order diffracted field solutions and the axial caustic solutions in their respective regions of validity. The pattern analyses for the coax and circular apertures are verified experimentally.
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IXTRODUCTIONHE IKFLUESCE of bodies of general shape on antenna cha.ract,erist,ics is of int,erest for aircraft and spacecraft applications. To gain some insight into these charact,erist,ics for complex bodies, a scaled model version of the antenna is usually first investigated by examining it,s behavior in simple structure such as flat-rectangular, or circular ground planes. Once a theory which describes the scaled antenna in simple structures has been developed and verified, it will aid in understanding the full-scale antenna when it is placed in more complex structures.The geometrical theory of diffraction (GTD) [l] has been used previously to study the effect of finite ground planes on radiat.ion pabterns [2]-[5].Balanis [2] dea.ls with the radiation from apertures in a t.hin square ground plane (this analysis neglects the effects of t.he edges parallel to the plane in which radiation pat,terns are calculat,ed). Circular ground planes are considered in [ 3 ] , [4]. However, the effect of the caust.ic of diffracted rays was not included in [3], [4], and t,he effect, of the edge curvature was neglected in [3]. Papers dealing exclusively with a method which employs equivalent currents is given in [5], [SI for evaluating the fields in the vicinity of the caust,ic direction; it may be noted that. the GTD representation fails at the caustics of diffracted rays. The radiat,ion from an aperture in a 2-D rect,angular conducting cylinder is treated in [7].Irl. this paper, t,he radiat.ion fields of a large TEM mode coax and a large circular TMol mode aperture in a finite circular and squa.re ground plane are analyzed via GTI). The analysis consist,s in supplementing t,he radiation fields for the circular aperture and the coax in an infinite ground plane by the additional field contribut,ions arising from the diffraction by t.he edges of t.he finite ground plane.The infinite ground plane solut.ion yields the field radiated in t,he forward direction. This solution is readily obtained using apert.ure integration over a.n assumed aperture field dist.ribution; the result,ing field may be viewed as the field associat.ed with a ray direct.ly radiated by the a.pert,ure in the infinit>e ground plane.The effect of the finite ground plane is incorporated by employing the compact dyadic edge diffract,ion coefficient. developed by Kouyoumjian and Pathak [lo]; their dyadic diffract,ion coefficient, provides a total field which is continuous across the shad...