The high data rate and global satellite communication systems are expected to be an improvement agenda to the social and economic activities in the 21st century. The experimental high data rate (gigabit) communications satellite is required to face the technological challenges of the future growing multi-media applications. For this gigabit communications system, a Ka-band scanning spot beam antenna (SSBA) which realizes multiple beam forming is required. CRL and MELCO have been developing Ka-band active phased array antenna (APAA) for the SSBA as one of the most important and challenging technology. An APAA has been developed mainly for radar systems for many years. CRL and MELCO are investigating and developing the direct radiating APAA for communication satellites which can operate under wide frequency band in Ka-band. This paper describes the preliminary study results of the Ka-band APAA for gigabit satellite and the radiation performances of the partial experimental model of Ka-band APAA with two multiple beams to both transmit and receive signals. The partial model includes the active MMIC device such as HPA (high power amplifier), LNA (low noise amplifier) and PS (phase shifter), and multi-layered BFN (beam forming network) as key components. Performances such as radiation pattern, beam scanning performance and input–output performance have been measured. We also verified that the REV (rotating element electric-field vector) method was effective for pattern calibration. Copyright © 1999 John Wiley & Sons, Ltd.