Design of symmetric dual Butterworth-type of frequency selective surfaces

This paper researches how to realize the Butterworth type Frequency Selective Surface(FSS) from a symmetric double screen FSS and to obtain the "rectangular" filter characteristics with "flat top" and "steep cut-off".Combined with the plane wave expansion and mutual admittance methods,the aperture array admittance and the mutual admittance between the arrays were given.Based on the method above,the effects of the FSS screen and the dielectrics media between the FSS screens on the Butterworth FSS were analyzed numerically by taking a common "Y" aperture for an example.Then,a single screen "Y" type FSS was fabricated and pasted on the both sides of media accurately by plating a copper film with thickness of 15 μm on a 500 mm×500 mm polyimide substrate.The free space method was used to test the transmission characteristics,and the simulation results are in good agreement with test ones.Furthermore,when the electrical thickness of the dielectric media was set to be in 2.15 mm,"Y" unit cell period in 7.2 mm×6.235 2 mm,arm length in 3.6 mm,and the arm width in 0.8 mm,the mutual admittance of dual screen FSS is equal to the real part of admittance for a single-screen FSS.As a result,the Butterworth type FSS is obtained successfully.Experiments show that the designed Butterworth type FSS has reduced its-3 dB bandwidth from 3.5 GHz to 2.1 GHz when the cell circle is increased by 0.6 mm.Finally,it suggests that the design of Butterworth type FSS should be to adapt a no-load cell unit with a circle less than 0.4λ on the assumption of resonant sizes,and its radio between gap length and width and the thickness of intermediate electric should be less than 5.5 and about 0.1λ,respectively.