Abstract: Aiming at the application requirements of high integration, high density, low profile, lightweight, and low cost for low-altitude surveillance active phased array radar systems, a Ku-band active phased array antenna is designed based on an antenna-on-board (AOB) architecture. By adopting RF vertical interconnection and multilayer hybrid PCB laminates, the antenna array, T/R modules, feed network, and power supply are highly integrated. The AOB scheme enables horizontal layout with vertical signal/power integration, yielding a profile height of only 3.67 mm and a weight of less than 180 g. The use of highly integrated multi-channel multifunction T/R RF chips reduces the chip count to an average of 0.25 RF chips per channel (four channels per chip), lowering cost while improving reliability. Measurements on a fabricated prototype show that the array achieves a boresight G/T value of ≥−8.5  dB/K and a boresight EIRP of ≥32.6  dBW, satisfying the design specifications. The antenna array features a simple structure, ease of fabrication, and mass-production suitability, providing technical support for the development of low-altitude surveillance phased array radars.