- 2002-2011 IEEE. A metamaterial-inspired ultra-miniature substrate integrated waveguide (SIW) cavity-backed slot antenna (CBSA) is presented in this letter. A ramp-shaped slot is used in an SIW CBSA to force the structure to work in the first negative-order resonance. Other miniaturization elements such as ramp-shaped strips as the ground plane around the antenna and another loading patch underneath the slot antenna are used to achieve an 87% miniature antenna working at 2.1 GHz. Close agreement is observed for the proposed simulated and measured performance results and suggests maximum gain of 3.6 dBi with an 80% efficiency.