Compact switched-beam array antenna with a butler matrix and a folded ground structure

Abstract

A compact switched-beam array antenna, based on a switched Butler matrix with four folded ground antennas, is presented for unmanned aerial vehicle (UAV) applications. The folded ground structure, including a slotted patch radiator surrounded by multiple air-gapped ground layers, is adopted to maximize compactness. The extra ground layers provide extra capacitive coupling around the patch antenna, resulting in a down-shift of resonant frequency and a reduction in the antenna size. Also, to optimize aerial operation with a wider beam coverage, the 1 × 4 array is integrated with a switched Butler matrix controlled by a microcontroller unit (MCU). The choice of the Butler matrix reduces the complexity of beamforming circuitry and avoids the use of high-cost phase shifters requiring extra control-bit signals. Further, the array antenna is optimized for high isolation among the antenna ports and a minimal UAV body effect. Then, the proposed structure was verified at 1.96 GHz for test purposes only, and the array size, excluding the antenna case, was 2.16λo × 0.54λo × 0.07λo. The measured 10 dB impedance bandwidth for all antenna elements in the array was always better than 3.4%, and the isolation among the antenna ports was also better than 19 dB. The measured peak gain, excluding the loss of the switched Butler module, was about 9.98 dBi, on average. Lastly, the measured peak scan angles were observed at −39°, −17°, 9° and 31° according to switching modes.