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This paper describes some results of an extensive theoretical and experimental work on a novel type of antenna design-a ring-shaDed dielectric antenna.It consists of two basic elements:a dielectric ring and a primary feed.One possible construction of a such antenna has a ring with rectangular cross-secttion and a waveguide-dielectric radiator serves as a primary source.The optimum dielectric ring for a maximum antenna gain gets into the second Fresnel zone. A radiation theory and experimental results for an X-band antenna model are given in the paper.It is concluded that a ring shaped dielectric antenna is an efficient directional radiator with simple and cheap construction. Fig.I illustrates the basic radiation mechanism of the ring-shaped dielectric antenna,grounded upon the Fresnel zonal theory [2] .The Drimary feed is denoted F and is located on the ring's axis at a distance d from the ring. The radii of the ring ( aI and a2 ) are equal to those of the second Fresnel zone.In the case of a metal ring ( MR )- Fig.I,b,it acts as a screen and therefore,there is no radiation from the ring's aperture.In our case ( Fig.I,c ),the dielectric ring ( DR ) compensates the antiphase jump in the second Fresnel zone and in this way it increases considerably the effective antenna aperture.Theoretically, a maximum gain of I4 dB could be achieved over the primary feed gain. The total radiation field is found by superposition of the partial fields ( Fig.2 ): RADIATION THEORYwhere E is the total antenna radiation field; E an electric field, radiated through a round aperture with a radius aI in an infinlite perfectly conducting screen; E2 -an electric field,diffracted at a conducting disc with a radius a2;E3 -an electric field due to the dielectric ring.This method is used in [I] and [3 ] for an analysis of the metal ring-shaped passive repeaters. Following ( I ),a far-field expression is obtained and the radiation pattern is given by F(G) = ( CI(@) + C2(@) -C3() ) + ( ) + S2 -S((@) )2 where (2) C1(G) = sin ( 2a,sn ) + fI VI () 3) S (9) = cos ( -a2sin29 ) f1 V(9) 4 =I cs (-2 asnI 2.I4 109