Hard and Soft Gangbuster Surfaces

This paper presents new possible solution for realizing hard and soft surfaces by using frequency selective “gangbuster” surfaces printed on a grounded dielectric slab. The solution is technologically simple and physically intuitive. A sample of numerical results obtained from full wave analysis are presented for validating the concept. INTRODUCTION Recently, a lot of research efforts have been devoted to periodic planar surfaces used to generate new equivalent boundary conditions. These studies have stimulated various engineering applications in the field of microwave and antennas. In some applications, these surfaces constitute the modern version of transversely corrugated structures, often used to improve the performance of feed-horns. In the early nineties, the so-called “soft” and “hard” surfaces (terminology derived from acoustics) were introduced and their relationship with the classical corrugated structures were discussed [1]. The soft surface behaves like a perfectly electric conductor (PEC) for TE polarization and as a perfectly magnetic conductor (PMC) for the TM polarization; vice-versa for the hard surface. Various solutions were presented in the past for devising such surface properties, which are based on printed strips short circuited to a ground plane. This paper present new solutions for hard and soft surfaces realized by printing a “gangbuster type” frequency selective surface (FSS) [2] on a grounded slab. Depending on the direction of wave propagation and on the nature – capacitive or inductive – of the printed FSS, these structures exhibit alternatively hard or soft properties. PEC/PMC STRIP IDEAL MODEL Let us first consider the problem of a plane wave incident on a material interface located on the x-y plane. “Wave fixed” orthogonal unit vectors l , t , n are introduced, where n is the normal to the surface, l is the direction of propagation of the incident plane wave on the surface, and t is the unit vector orthogonal to l and n . We denote the polarization in terms of TE and TM component w.r.t. l (note that for this case TE-TM w.r.t. l is also TE-TM w.r.t. n ). Let us assume that the x-y plane is occupied by an anisotropic, homogeneous surface impedance defined by l H Z t H Z l E t E TE TE TM TM TM TE ˆ ˆ ˆ ˆ − = + , where TM Z and TE Z are the TM and TE impedances. An ideal soft surfaces is characterized by ∞ = TE Z and 0 = TM Z , and an ideal hard surface is characterized by 0 = TE Z and ∞ = TE Z [1]. Table I summarize the various cases, including electric and magnetic conductors. These surfaces behave as a mixture of simultaneous perfectly electric conducting (PEC) and perfectly magnetic conducting (PMC) properties. In particular, hard (soft) ideal surface imposes the annulment of both E and H fields along l ( t ). A simple model of hard and soft ideal properties can thus be obtained by an alternation of PEC and PMC strips with vanishing widths. The strips impose the simultaneous annulment of the component of both E and H along their direction. For the hard and soft cases, the strip are oriented along l (Fig. 1a) and t (Fig. 1b), respectively. TE TM HARD ∞ = TE Z 0 = TM Z SOFT 0 = TE Z ∞ = TM Z PEC 0 = TE Z 0 = TM Z PMC ∞ = TE Z ∞ = TM Z Table I: TE and TM impedance properties of ideal artificial surfaces