In this article, a compact fully planar high gain antenna based on half-mode substrate integrated waveguide (HMSIW) cavity is presented. The design uses a novel configuration of HMSIW cavity with high length to width ratio along with tapered open edge and a pair of slot stub. The high length to width ratio of the cavity helps to excite closely spaced multiple TEy m10 cavity modes within comparatively smaller footprint due to use of the HMSIW cavity. These modes combine to give hybrid mode resonance in the cavity which helps to generate a narrow beam high gain radiation pattern of the antenna. The size of the proposed antenna is further reduced and a pair of slot stub is put along the sidewall of the cavity which helps to sustain similar hybrid mode field distribution within much smaller dimension. A size reduction of 76.7% is achieved in the proposed design configuration without degrading much of the gain performance. The proposed antenna resonates at 9.8 GHz with a gain of 7.9 dBi which is much higher than other reported HMSIW cavity antenna. The proposed antenna may find application in point to point communication, short range radar in X band. © 2019 Wiley Periodicals, Inc.