Compact wide stopband SIR filter for Radio Altimeter

Presented in this paper is a very wide stopband filter of reduced size, improved stop-band rejection, and good input-output return loss for Radio Altimeter applications. To improve the stopband rejection three and four different types of stepped-impedance resonators (SIRs) are used. To further improve the wideband rejection, a notch filter is kept at the output of the filter. The simulated results on EM simulator of applied wave research (AWR) show an out of band rejection of better than 40 dB up to 18 GHz, and a return loss better than 15dB. The filter is optimized for manufacturability and 6 different types of prototypes were fabricated to address the RO-4350B dielectric constant variation and manufacturing tolerance. The paper details the test results which were compared with the simulated one. The produced filter size is 12mm × 8.5mm and the insertion loss is 1.7 dB and out of band rejection better than 30 dB up to 18 GHz. The filter is designed for the altimeter application where the out of band rejection up to 18 GHz, good phase linearity, compactness in size and good in put output return loss are highly required. Further, an attempt was made to improve the return loss of the filter by using defected ground structure to realize a resonator, the simulation shows that the return loss with DGS improved by 5 dB.