Reduction of mode coupling effects in double layer dipole slot dichroics

The authors present some experimental and theoretical results for a selection of two layered dichroic panels. This serves to illustrate the additional problems caused by evanescent mode coupling in such multiple layer configurations, and some methods of reducing these effects. The FSS considered are all arrays of simple dipole slots. These elements offer the advantage of simplicity in construction and in basis function representation in modal analysis. They give a passband transmission characteristic and have potential applications where their polarisation discrimination is an asset. They suffer, however, from consequences of their high resonant wavelength relative to their unit cell size. Some higher order modes are only weakly evanescent in the vicinity of the passband, resulting in detrimental consequences. This situation can occur with other commonly used array geometries, particularly multiple loop elements (double square loops, concentric rings) where the inner loops are, effectively, widely spaced.