Entwined spirals for ultra compact wideband frequency selective surfaces

The application of a suitable intertwining scheme to planar periodic quadrifilar spiral frequency selective surfaces (FSSs) is shown to dramatically decrease operating frequencies while greatly expanding the bandwidth. At both normal and oblique incidence the proposed intertwined spiral FSSs provide a very sub-wavelength response at wavelengths 40 times larger than the unit cell size. The high packing density of the array ensures the stability of stopband frequency response at oblique incidence. Negligible variations of the resonant frequency are observed for incidence angles up to 45°, and the bandwidth common to TE and TM incidence polarizations is about 40%.