Mushroom surface cloaks for making struts invisible

There are many situations where electromagnetic waves are obstructed by some mechanical structure. If the structure is part of or close to an antenna, the obstruction may represent aperture blockage, causing increased sidelobes and reduced gain. The blocking cylinders can be struts supporting the subreflector in a Cassegrain antenna or the feed in a prime-focus paraboloid. The increased sidelobes and reduced gain for these cases are previously studied in detail in several articles, and some simple analytical expressions and design curves are given in (Kildal et al., 1988). The above mentioned blockage effects are a result of the forward scattered field. This is often characterized in terms of an induced field ratio (IFR). However, the blockage losses and sidelobes caused by forward scattering are proportional to the product of the IFR and the physical width (Kildal et al., 1988). Therefore, the paper (Kildal et al., 1996) chose to characterize the forward scattering in terms of this product, being referred to as the equivalent blockage width or simply the blockage width. Thereby, it is easier to compare struts of different widths and types with respect to their electrical performance and mechanical strength. For opaque objects the blockage width becomes at high frequency equal to the physical width. The blockage width becomes by the definition a complex value, where both the real part and the absolute value are representative for characterizing invisibility. We here choose the absolute value only, to save space. The equivalent blockage width is readily computed by considering a plane wave incident on an infinitely long strut, which is a two-dimensional scattering problem. In this paper we treat for simplicity only normally incident plane waves.