A novel microstrip resonator, uniplanar double spiral resonant cell (UDSRC) is analytically investigated to access the controllability of its bandstop property and one hi-lo microstrip lowpass filter using UDSRCs with enhanced frequency selectivity and rejection level is also presented. The equivalent circuit corresponding to each part of UDSRC is initially proposed to describe its special bandstop property with two transmission zeros. Furthermore, analytical theories of each circuit element are introduced and the comparison of the calculated results and the fullwave-simulated ones is done to verify the proposed equivalent circuit and the analytical theories. Both the analytical investigation and parametric analysis indicate that the two transmission zeros can be controlled through tuning the primary geometrical parameters. Thus, the given property is utilized by embedding two different UDSRCs in the feed lines of the reference filter. Both the simulated and measured results indicate that the frequency selectivity and rejection level are improved effectively. The frequency selectivity of the fabricated prototype is about 65.8 dB/GHz while the stopband rejection level is more than 10dB from 2.08 GHz to 6.62 GHz. Compared with the reference filter, the performance is improved greatly while the transversal dimension of the feed line is not increased because UDSRCs are completely embedded in the feed lines.
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