This book highlights the application of active array antennas in high-resolution microwave imaging radar systems. It introduces the basic principles, analytical methods, and performance parameters of active array antennas to achieve low profile, high efficiency, and lightweight. The book systematically elaborates the architecture, analysis, and engineering practice to achieve wideband, multi-band, multi-polarization, and common aperture in active array antennas. It explores hotspot technologies of digital array antennas, microwave photonic array antennas, and active packaging antennas. By...