A novel portable 2.4 GHz antenna beam sensor has been designed and implemented. The functions of the proposed sensor are to observe the spatial radiation distribution by using vision and to measure the spatial radiation power intensity distribution. The antenna beam sensor comprises a 6 × 6 radio frequency (RF) rectenna array. Each RF rectenna is a combination of a patch antenna and a high-efficiency radio frequency–direct current (RF–DC) conversion circuit. The output of each RF rectenna is connected to an LED, and the location of the spatial beam power intensity is determined from the luminous intensity of the LED. Additionally, the spatial beam power intensity distribution is obtained by measuring the output voltage of the RF rectennas. The patch antenna size is 7 × 7 cm2, the measured reflection coefficient of the antenna is greater than 10 dB at 2.4 GHz, and the measured antenna maximum gain is 5.1 dBi. The RF–DC conversion circuit is designed with a 2.4 GHz ten-stage Cockcroft–Walton voltage multiplier rectifier circuit, which can improve the conversion efficiency and beam sensing sensitivity. The RF–DC conversion circuit size is 3.5 × 4.7 cm2. The measured reflection coefficient of the circuit is greater than 10 dB at 2.4 GHz, and the highest measured conversion efficiency is 50.38%. The overall size of the antenna beam sensor is 42 × 42 cm2 and the weight is 1.15 kg. Therefore, the sensor is light and easy to handle, which makes it easy to carry for measurement work. This proposed antenna beam sensor is useful for applications in the research of wireless power transfer, microwave energy harvesting, and antenna radiation beam behavior in air.