A circularly polarized (CP) $\text{4}\times\text{4}$ array antenna based on substrate integrated waveguide (SIW) technology is presented. Circular polarization is achieved by applying the sequential rotation technique (SRT) with a well-designed sequential feed network and linear-polarized array elements. The proposed $\text{4}\times\text{4}$ array has a wide axial ratio (AR) bandwidth of 14% from 18.3 to 21.1 GHz with gain $ > \!\!13\,{\rm dBic}$. Then, the array is expanded to achieve higher gains. A $\text{16}\times\text{16}\hbox{-}{\rm element}$ array is designed, fabricated, and tested. Test results show that the $\text{16}\times\text{16}$ array has an AR bandwidth of 13.8% from 18.5 to 21.25 GHz and a peak gain of 25.9 dBic at 20.5 GHz.