A lightweight portable planar slot array antenna at X-band for fixed satellite communications is presented in this letter. The terminal is composed of two radial-line slot antennas (RLSAs) with the slots placed on the upper layer in spiral: one for reception (RX) band with left-handed circular polarization (LHCP), and the other one for transmission (TX) band with right-handed circular polarization (RHCP). A lightweight two-layer dielectric structure is used to achieve and optimize the RLSA weight. The radiating element is a slot pair, designed to provide circular polarization. The feeding network consists of a 50-Ω SMA connector for each RLSA. A radiation efficiency of more than 70% is achieved due to the low dielectric constant substrate. Finally, measurements of the lightweight portable planar RX/TX antenna prototypes are presented and compared to simulations, where very good agreement is observed.