For high speed downlinking of payload data from small satellites, a new 4×4 aperture coupled microstrip patch array antenna has been presented. The antenna is designed for the Ku band and a peak gain of 18.0 dBi is achieved within the impedance bandwidth from 11.75 GHz to 12.75 GHz. Wide bandwidth is achieved as the patch elements are excited through E-shaped slots having asymmetric side lengths and widths. Each square patch element of the array with truncated corners and appropriately placed slots generates right hand circularly polarized (RHCP) radiation with very high crosspolarization discrimination. A corporate feed network consisting of T-junctions and quarter-wave impedance transformers is developed to feed the array elements from a single coaxial port of 50 Ω. To improve the radiation from the patches and waveguiding in the feed network, two types of Rogers substrates with different dielectric constant and thickness are considered. Our proposed microstrip patch array antenna of size 7.8 cm × 6.4 cm × 0.3 cm can perform efficiently with a downlink data rate as high as 4.6 Gbps for small satellites.