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 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

K-band 0°/180°phase shifter MMIC based on balanced structure which can be applied to BPSK or QPSK modulator with dc-offset cancellation is presented. The proposed balanced phase shifter consists of four Lange couplers, two quarter-wave transmission lines and four switches for open and short terminations. The proposed structure implemented in GaAs 0.15-µm low noise pHEMT process ensures stable 0/180°phase shift while common-mode signals due to the mismatch in reflections by open and short are fed to the isolation port. The proposed phase shifter is verified by the phase shift of 179 ± 1.5°with the insertion loss of 4.5 ± 0.8 dB from 17.5 to 22.5 GHz. The fabricated phase shifter size is 2.2 × 2.85 mm including test pads.Introduction: Phase shifters are one of the most widely used components in microwave applications, such as phased-arrays and communication systems. For reliable system performances, phase shifters must generate a desired relative phase shift with low insertion loss variation and good return characteristics. Thus, there have been many techniques such as a reflection type phase shifter with series and parallel LC terminating circuits [1], vector-sum based phase shifter [2] and modified rat-race structure with the Lange coupler [3]. In addition, 0/180°phase shifters configured with the combinations of directional couplers, Wilkinson dividers, or 3-dB couplers are used as BPSK modulators [4,5]. However, these structures undergo the dc-offset problem [1,2] or degradation in return characteristics [3] by common mode signals returning to the input owing to the mismatch in reflective circuitry. Thus, a new balanced phase shifter structure that generates stable 0/ 180°phase shift and cancels unwanted common-mode signal which results in dc-offset is proposed.

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