Phased array antenna-based system degradation at wide scan angles

Phased array antennas offer a number of advantages to the National Aeronautics and Space Administration (NASA) missions compared to traditional gimbaled reflectors including electronic and vibration free beam steering, graceful degradation, smaller volume and multibeam capability. However, the monolithic microwave integrated circuit (MMIC)-based phased array antennas also present challenges to mission designers because of reduced power efficiency, higher costs, and system effects that result in link degradations. The NASA Glenn Research Center (GRC) continues to pioneer aerospace communications technologies to address the challenges of array antennas to improve efficiency, reduce costs [Romanofsky, R., et al., December 2000], and better understand system effects [R. Acosta, et al., May 2002 and S. Johnson et al., June 2002]. This paper addresses the degenerative system effects between high-rate modulated data and signal timing delays caused by antenna beam steering at wide scan angles. Conventional phase shifters used in MMIC-based phased array antennas are physically limited to 360° of phase shift. Often, depending on the size of the array and/or specified beam angle, the required phase shift of some elements may exceed 360°. In these instances, the actual phase shift obtained is the remainder of the phase request, modulo 2π. The modulo 2π result causes a delayed carrier signal radiated from the element whose phase shifter requires a phase shift greater than 360°. Multiple delayed signals radiates from the antenna depending on the size of the array and scan angle. The array antenna system performance described in this report is characterized by measuring the bit error rate (BER) of the link from modulator to demodulator through the phased array antenna across a link inside an anechoic chamber. The array antenna introduces intersymbol interference (ISI) into the link due to the modulo 2π effect of the phase shifters. The ISI is observable in the BER versus Eb/No curve as a "flaring" or additional insertion loss at high Eb/No. Theoretical calculations and computer simulations indicate as much as 3-4 dB degradation due to ISI when the data rate is an appreciable fraction of carrier frequency (>2 Gbps at K- band). Dat- a rates more characteristic of near term systems (e.g. < 1 Gbps) result in somewhat lower losses. Results presented here were obtained for 220 Mbps and 622 Mbps through a K-band (20 GHz), 91-element, phased array antenna. Discussion is provided where results agree and disagree with the simulation results for the data rate and carrier frequency used.