Abstract—This paper presents the design, simulation, fabrication, and measurement of a Half-Mode Substrate Integrated Waveguide (HMSIW) cavity slot antenna with a half-octagonal ring slot structure, operating in the C-band frequency range. The primary objective is to enhance impedance bandwidth while achieving significant miniaturization compared to conventional Substrate Integrated Waveguide (SIW) antennas. Simulated results demonstrate an impedance bandwidth of 18%, covering the frequency range of 5.60 GHz to 6.71 GHz, with an S11 value consistently below −10 dB. Fabrication and experimental testing validate these findings, achieving a measured impedance bandwidth of 19.4% over the range of 5.82 GHz to 7.07 GHz. The antenna exhibits a dual-directional and linear radiation pattern, radiating energy in both forward and backward directions, with a simulated gain of 2.73 dBi. The HMSIW structure achieves a 50% size reduction compared to full-mode SIW designs. These features make the proposed antenna suitable for modern telecommunications applications within the C-band, where wide bandwidth, miniaturization, and reliable linear radiation performance are essential. This advancement provides a promising solution for systems requiring high efficiency and reduced form factor without compromising performance.