This paper presents two novel miniaturized substrate integrated waveguide (SIW) cavity-backed slot antennas (CBSAs) with multiband and broadband characteristics. The first antenna combines the modified non-resonant rectangular slot with two pairs of symmetrically placed shorting pins and fed with tapered coplanar waveguide (CPW) to SIW transition for obtaining three resonances at 8.8, 12.25, and 14.35 GHz. The 10 dB return loss bandwidths (BWs) correspond to three resonances are 4.78, 3.11, and 1.82 % with peak realized gain of 6.41, 6.31, and 7.26 dBi respectively. Later for the second antenna, an indigenous method of splitting the SIW cavity is incorporated using shorting vias to obtain broadband impedance matching. Lower and high order modes (half TE110 and TE210) of the SIW cavity are coupled together to achieve broad BW with S11 <-10 dB from 12.34-14.54 GHz (16.4 %) and a flat gain > 6.4 dBi over the operating range. © 2022 European Association for Antennas and Propagation.