Experimental Investigation of Closely Packed Spiral Element Yields Narrowband Design

Arrays of conducting elements, or their complementary slots in a conducting plane have a plane wave transmission characteristic which varies with frequency. Such frequency selective surfaces (FSS) have a number of important applications in antennas and radomes and are often required to be curved. A current approach is to choose a plane FSS with the desired transmission response ideally insensitive to angle of incidence, and then to project this onto the curved surface. This projection will naturally cause a distortion of the FSS geometry with corresponding changes in the electrical performance. The effects of this distortion may be lessened by using a more closely packed array. Close packing has the additional advantage of increasing the separation of the useful array resonant bands from the onset of grating responses or, perhaps, surface waves trapped in the supporting dielectric substrate. One may define a measure of the packing density as the ratio of resonant wavelength to lattice periodicity. This parameter is of the order of 2 for simple dipole arrays, for example