Tuning Performance of Metamaterial-Based Frequency Selective Surfaces

A bandpass, miniaturized-element frequency selective surface with relatively high out-band rejection was recently reported whose unit cell dimensions are smaller than lambda ₀ /10. The new FSS is made up of a metallic, square loop array and a metallic grid on either side of a thin dielectric substrate. Further analysis on this structure reveals that, a very wide frequency tuning can be accomplished by inserting lumped, variable capacitors in the gap between the loops. This paper presents the tunability performance of the miniaturized-element FSS. For experimental verification, a waveguide measurement approach is chosen to lower the fabrication cost. The FSS unit cell dimensions are modified appropriately to fit within a WR90 waveguide aperture and appear as a perfect periodic surface once image theory is applied. Prototypes of the structure in the form of a waveguide flange at X-band are fabricated and then loaded with fixed-valued, chip capacitors of different values. Each waveguide sample contains eighteen capacitors mounted on it. The measurement results show that a wide tuning range, with acceptable performance, from 8.49 to 11.48 GHz can be accomplished by altering the capacitance from 0.2 to 0.05 pF.