In this paper, we propose a substrate integrated waveguide(SIW) slot array antenna with a metasurface superstrate operating in the C band. SIW slot array antennas have advantages for CubeSat applications due to their low radiation loss, low transmission loss, and high isolation characteristics. However, these antennas have narrow bandwidth characteristics due to high Q factor. To compensate for this, the additional resonance mode TM10 of the metasurface superstrate has been used. An SIW slot array antenna without metasurface superstrate has dimensions of 23.5 mm×67.5 mm×0.675 mm(0.37λ0×1.05λ0×0.01λ0, λ0 at 4.67 GHz), bandwidth of 4.65～4.69 GHz (approximately 1 %), and maximum realization gain of 4.7 dBi. Using the metasurface superstrate, the size of the antenna is 23.5 mm×67.5 mm×3.7 mm(0.37λ0×1.07λ0×0.06λ0, λ0 at 4.74 GHz), the bandwidth is 4.73～4.87 GHz(3 %), and the maximum realized gain is 6.58 dBi.
