Convoluted array elements and reduced size unit cells for frequency-selective surfaces

When the curvature of a surface that is to be made frequency selective is not small, it modifies the array lattice and element geometry, altering the transmission performance of the surface. For a given degree of curvature, small unit cells are distorted less. The transmission response of a typical array of conducting elements has a main reflection resonance at a wavelength λ1, with further nulls at shorter wavelengths. We have attempted to reduce the unit cell dimension p as much as possible for a given reflection wavelength λ1, thereby increasing its separation from grating responses. The ratio λ1/p has been used as a figure of merit. An array of linear dipoles on a square lattice of side p is a standard of comparison, a typical case giving a figure of merit of 1.6. A Jerusalem cross array gives a value of about 4.5. By convoluting the conductors of a square loop element, or by using, as the array elements, the low order generations of space filling curves, such as that described by Hilbert, we have obtained values of λ1/p in the range 7–12. The unit cell areas are lower than that of the dipole standard by factors averaging about 40.