Superior dual band FSS with fractal elements

Summary form only given. Fractal geometries have been extensively implemented in the design of multiband antenna and frequency selective surface (FSS). For example, convoluted and Hilbert patch elements were first proposed to reduce the unit cell size of FSS (Parker and El Sheikh, IEE Proc.-H, 138, 19-22, 1991.) A fractal FSS with self-similar Sierpinski dipole array was shown to have two stop bands (J. Romeu and Y. Rahmat-Samii, IEEE Trans., AP-48, 1097-1104, 2000). However, its resonant frequencies are rather sensitive to incident angles and polarizations. Minkowski patch FSS and Sierpinski carpet fractals have been shown to provide two or three stop bands with dual polarization capability (J.P. Gianvittorio, et. al., IEEE Trans., AP-51, 3088-3096, 2003.) Recently miniature band-pass FSS with fractal antenna-filter-antenna arrays was designed and demonstrated with good frequency selectivity (S. Zheng, et. al., IEEE AWPL, 11, 240-243, 2012.) A dual band fractal FSS with a 3:1 reflection to pass band ratio and significantly improved resonant frequency stability was proposed by the author (T.K. Wu, Microwave Optical Tech. Letters, 54, 833-835, 2012). It is well known that a miniature FSS with a longer element length has a lower resonant frequency with other transmission nulls (i.e. grating lobes and Wood´s anomalies) at higher frequencies. As was also pointed out in the first reference, the fractal array elements with longer element length and reduced unit cell size provide solutions to the manufacturing of FSS radomes with small radius of curvature and multiband performance without the onset of grating lobes. In other words, for a given 1st resonant frequency one needs to pack as much element length as possible into the unit cell. Minkowski square loop (MSL) fractal element suits this objective very well. As was also shown in the last reference, MSL fractal FSS outperform the square loop FSS by exhibiting excellent resonant frequency stability with varying inciden- angles and polarizations. In this paper, miniature MSL fractal element FSS is shown to provide not only good resonant frequency stability but also improved frequency selectivity. Specifically, a 2:1 reflection to pass band ratio FSS characteristics will be presented with miniature MSL fractal element FSS. Finally for ease of FSS element selection purposes, the transmission performance of MSL fractal element FSS will be compared to the convoluted and other element FSS.