Leaky-wave antennas realized by using artificial surfaces

Within the class of electromagnetic bandgap structures, periodic surfaces printed in stratified dielectric media occupy an important role. Solutions have been devised for antenna applications, often oriented to realization of compact antennas, or to suppression of surface- and space-wave propagation for reducing diffraction lobes. From the literature, a different application also emerges which uses the same printed periodic surface technology for gain enhancement (H.D. Yang and D.R. Jackson, IEEE Trans. Ant. Propag., vol. 48, no. 4, pp. 556-564, 2000; T. Zhao et al, AP-S Symp. Digests, vol. 3, pp. 248-252, 2001). The basic mechanism to use for the gain enhancement is the excitation of weakly attenuated leaky waves, the same type as that described in the past for unbounded layered media with high dielectric contrast (D.R. Jackson et al., IEEE Trans. Ant. Propag., vol. AP-41, pp. 344-348, 1993). The excitation and leaky-wave propagation can be obtained by printing over a grounded slab a frequency selective surface (FSS). Within the bandwidth where the FSS resonates, the structure behaves as a weakly transparent parallel-plate waveguide. A point-source (dipole or slot) can thus excite a cylindrical leaky wave, thus implying a large phased aperture-field distribution on the FSS. The radiation of this large aperture produces a directive conical beam. For an appropriate design of the slab thickness, the leaky-wave phase velocity approaches infinity and the conical radiation collapses into a broadside beam. In this paper, a systematic procedure for the antenna analysis is presented, which uses a synthesis of the FSS via equivalent network, based on the data obtained by full-wave analysis.