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.

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A printed circuit board (PCB) implementable multi-slot loaded antenna with a low-profile air-gapped structure for both gain and efficiency enhancement is proposed. The proposed planar antenna is configured by the upper substrate with a radiation patch separated from the lower substrate consisting of a pair of matching pads and a ground plane by the air-gap. The radiation patch on the upper substrate is loaded by two rectangular slots and a single narrow slot with a symmetrically placed feed point and a shorting pin. The narrow slot on the center of the radiation patch with the shorting pin improves both gain and radiation efficiency. The pair of two additional rectangular slots increase the gain while maintaining the required air-gap very low. Further, the symmetric matching pads are integrated along the feed and shorting vias to improve the realized antenna gain for the whole operation band. As a result, the proposed structure obtains a high gain-to-volume ratio as well as the enhanced efficiency. The proposed structure was fabricated at 5.9 GHz with the overall volume of 1.08 λ0 x 1.08 λ0 x 0.05 λ0 (55 mm x 55 mm x 2.7 mm) including the whole ground plane. The measured antenna gain was 9.4 dBi at 5.9 GHz verifying the high realized gain-to-volume ratio. Also, the measured half power beamwidths in E and H planes were about 72˚ and 74˚, respectively. Lastly, the measured 10-dB impedance bandwidth of the proposed structure showed approximately 5.5 %.INDEX TERMS Antenna gain enhancement, high efficiency antenna, low-profile antenna, slot-loaded antenna, shorted patch antenna

A high gain stacked antenna based on a planar magneto-electric dipole structure is proposed. The main radiator is configured by a probe-fed patch with a symmetrically arranged pair of dipole radiation elements. Further, an additional air-gapped radiator with multiple patch elements is integrated for gain enhancement. Since both the main and stacked radiators are planar structures, the overall volume can remain low-profile regardless of the airgap. To verify the performance of the proposed structure, a single magneto-electric dipole antenna and three different types of stacked radiators were implemented at 5.8 GHz. The magneto-electric dipole antenna showed measured 10-dB impedance bandwidth and gain of 5.2% and 8.0 dBi, respectively with the overall size of 0.96 λ0 x 0.96 λ0 including a ground plane. With the additional stacked radiator having the airgap of 0.1 λ0, the maximum measured gain was increased to 9.6 dBi. Further, to verify the beamforming performances, three types of 1x8 phased array stacked structures were fabricated with a volume of 0.96 λ0 x 6.38 λ0 x 0.14 λ0 at 5.8 GHz. The measurements showed a maximum peak gain of 18.1 dBi and a half-power-beamwidth scan angle of 49˚ with a side-lobe level less than-8 dB. INDEX TERMS Antenna gain enhancement, beamforming antenna, low-profile stacked antenna, magnetoelectric dipole, phased array antenna

Electrically conformal antenna arrays (ECAA) for 2-dimensional (2-D) beamforming configured by wide beamwidth multipole radiation elements are proposed. Unlike the conventional conformal antenna array with physically curved shape, the proposed arrays are planar structures with wideangle beam steering. The wide range of beam steering with a low gain variation allows the planar array structure to achieve the electrically conformal phased array patterns. In order to verify the ECAA concept, two different types of ECAA in a 1x8 configuration using planar multipole antenna elements were fabricated at 5.8 GHz and compared to the conventional patch array. Each multipole element had halfpower-beamwidth (HPBW) wider than 110° at both E-and H-planes with the maximum HPBW of 175°. The first type of ECAA showed a measured peak gain of 10.8 dBi with a measured half power steering angle of 155° while maintaining a side lobe level (SLL) less than-10 dB. Further, the second ECAA showed a measured peak gain of 12.8 dBi with a measured half power steering angle of 144° while keeping the SLL less than-10 dB. These scan angles of the first and second types were extended about 66.7% and 54.8%, respectively from the conventional patch array with the same size. Lastly, two types of 8x8 array modules for 2-D beam steering were verified at 5.8 GHz and the first type showed a maximum peak gain of 21.0 dBi. Further, the 2-D half power steering angles in E-and H-planes of the second type were about 145° and 137°, respectively. INDEX TERMS Beamforming, electrically conformal antenna array (ECAA), phased array antenna, multipole antenna, wide angle 2-D beam steering