A novel dual-band ISGW cavity filter with enhanced frequency selectivity is proposed in this paper by utilizing a multi-mode coupling topology. Its cavity is designed to control the number of modes

and then the ports are determined by analyzing the coupling relationship between these selected modes. By synthesizing the coupling matrix of the filter

a non-resonating node (NRN) structure is introduced to flexibly tune the frequency of modes

which gets a dual-band and quad-band filtering response from a tri-band filter no the NRN. Furthermore

a frequency selective surface (FSS) has been newly designed as the upper surface of the cavity

which significantly improves the bad out-of-band suppression and frequency selectivity that often exists in most traditional cavity filter designs and measurements. The results show that its two center frequencies are ${f_{01}}$ = 27.50 GHz and ${f_{02}}$ = 32.92 GHz

respectively. Compared with the dual-band filter that there is no the FSS metasurface

the out-of-band suppression level is improved from measured 5 dB to 18 dB

and its finite transmission zero (FTZ) numbers is increased from measured 1 to 4 between the two designed bands. Compared with the tri-band and quad-band filter

its passband bandwidth is expanded from measured 1.17 %

1.14 %

and 1.13 % or 1.31 %

1.50 %

0.56 %

and 0.57 % to 1.71 % and 1.87 %. In addition

the filter has compact

small

and lightweight characteristics.