Entwined Planar Spirals for Artificial Surfaces

Entwining planar quadrifilar spirals arranged in doubly periodic arrays enables a strong subwavelength response of the unit cell smaller than 1/40 of wavelength. It is shown that interleaving counterwound spiral arms extended into adjacent unit cells dramatically increases the equivalent capacitance while reducing the inductance. The dielectric substrate enhances this effect of the unit cell miniaturization with concurrent bandwidth expansion. The proposed topology of compact planar spiral array exhibits excellent angular and polarization stability and circular polarization selectivity in a broad frequency band. Negligible variations of the resonance frequency are demonstrated for both TE and TM polarized waves at incidence angles up to 45 with a common fractional bandwidth over 40% at the level of -10 dB.