Coverage Ratio–Based Evaluation of Antenna Omnidirectionality for a Pair of Microstrip Patch Antennas on a 6U CubeSat

CubeSat missions increasingly rely on microwave-band communication systems, whose antennas often exhibit directional radiation patterns. As a result, multiple antennas are commonly used to improve coverage; however, a quantitative method to evaluate their performance across all spacecraft attitudes has been lacking. This paper introduces the Coverage Ratio of CubeSat Attitude (CRCA), a metric that quantifies the proportion of orientations for which the antenna gain exceeds a required threshold. CRCA is introduced and demonstrated using the S-band command antenna system of the 6U CubeSat VERTECS. The proposed metric is then used to quantitatively compare multiple antenna placement configurations, clarifying the effect of mounting faces on attitude-dependent coverage. Electromagnetic simulations and three-dimensional radiation pattern measurements using a metal CubeSat enclosure show good agreement when splitter and cable losses are taken into account. The combined radiation pattern achieves greater than −8.0 dBic in 90% of attitudes in simulation, and greater than −10.0 dBic of attitudes in 90% in measurement. Furthermore, a CRCA-based link budget analysis demonstrates that sufficient uplink margin can be conservatively maintained under tumbling conditions. The proposed CRCA framework provides a practical and generalizable approach for evaluating antenna omnidirectionality and attitude-dependent communication performance in CubeSat missions.