A mechanical tuning balanced bandpass filter based on substrate integrated waveguide (SIW) dual‐mode resonant cavity is presented. It can be accomplished by placing two metallised via‐holes with open‐loop slots on the top layer which affect the TE 102 and TE 201 modes of the square SIW cavity, respectively. Direct source‐load couplings have been utilised to achieve the high selectivity of the differential mode (DM) passband and improve the DM out of band rejection. A planar module of 60 × 60 mm 2 has been fabricated on RO5880 substrate with a dielectric constant of 2.2 which verify the design concept.