In this article, an arbitrary-phase-difference (A-PD) transdirectional (TRD) coupler is proposed and applied in the design of a flexible Butler matrix. The A-PD TRD coupler, which also features the characteristics of arbitrary power division and miniaturized size, is composed of four sections of coupled lines and four types of capacitors. Explicit equations for designing the A-PD TRD coupler were derived utilizing the even-odd mode decomposition analysis, and guidelines to obtain the circuit parameters were also given. To demonstrate the design procedure, examples of the proposed coupler are provided and the relevant studies on the coupler's electrical parameters are conducted. By combining the proposed coupler with the TRD coupled-linebased crossovers and the -45° phase shifters, a miniaturized 4 X 4 Butler matrix with flexible output ports phase differences (-25°, 155°, -115°, and 65°) is implemented. It inherits the features of the TRD coupler including A-PD and compact size. At the center frequency, the measured amplitude and phase imbalances are less than 1.4 dB and 3.2°, respectively, between the output signals. Compared to state-of-the-art A-PD-couplerbased Butler matrix, the proposed structure shows a smaller size (1.35λ _g X 1.36λ _g ) and a wider bandwidth (20%).

In the paper, a low-profile dual-band printed quadrifilar helix antenna (PQHA) with wide beamwidth is presented for UAV GPS applications. The antenna is composed of a hollow dielectric cylinder with four arms of dual-helix metal strips and a hollow dielectric ring with four pairs of circular metal strips. To provide dual-band operation, different lengths of the dual-helix metal strips are applied. By loading the circular metal strips on the dual-helix metal strips, size reduction of the antenna height is realized. Moreover, wide beamwidth is obtained. A miniaturized quad-feed network is also designed to provide equal amplitude signals with sequentially quadrature phases. To validate the proposed structure, a prototype is fabricated. The height of the antenna is 18.5 mm. The experimental results show that good impedance matching (|S11| < -18 dB) and axial ratio (< 2 dB) are obtained at GPS L1/ L2 bands. At 1.227 GHz, the measured 3-dB axial ratio beamwidths (ARBWs) are 186° and 187° at xoz and yoz planes, respectively. While the values are 167° and 163° at 1.575 GHz. The measured half-power beamwidths (HPBWs) are both more than 120° at the two bands. The results indicate that the proposed PQHA is a good candidate for UAV GPS applications.

A full-passband linear-phase band-pass filter (BPF) equalized with negative group delay circuits (NGDC) is proposed. The NGDCs are utilized to suppress the salient group delay at the edge of the passband of the traditional BPF for achieving the full-passband linear-phase characteristic. To obtain input- and output-port impedance matching, two BPFs loading NGDCs are center symmetrically configured by Wilkinson power dividers. To verify the design concept, a 3-order full-passband linear-phase BPF is designed and fabricated. From the measured results, the GD fluctuation is reduced by 74% from 0.57 to 0.15 ns, return loss is better than 10 dB, and the insertion loss variation is less than 3 dB within the frequency range from 2.142 to 2.804 GHz. The proposed BPF achieves a flat group delay in the whole passband and realizes the full-passband linear-phase characteristic.

Broadside radiation from Chern photonic topological insulators (PTIs) is studied in this work. The one-way dispersion of the topological edge mode is located in the fast wave region, making leaky-wave radiation possible. In this communication, design guidelines are presented and validated by full-wave simulations: the Berry curvature and the Chern number of photonic crystals are calculated to design a topological edge mode, then a leaky-wave antenna is proposed where a perturbation is utilized around the source location to eliminate open-stopband behavior and generate broadside radiation with high efficiency. It is demonstrated that leakage of the topological edge mode is capable of both frequency and bias scanning to generate a steerable beam across the broadside direction. Moreover, the mechanism behind the immunity of PTIs to defects is also investigated in detail. Although topological edge waves can bypass PEC-slab defects ideally, the phase change along the defect affects the radiation pattern. The switch between sum-difference beams by tuning the scale of the PEC-slab is direct evidence to verify our analysis. To sum up, the open-stopband behavior is overcome to realize broadside radiation, and the conditional robustness of PTIs in terms of antennas is clarified.

In this paper, a compact coupled-line trans-directional (TRD) coupler with wide continuous frequency tuning range and superior performance is presented. The proposed tunable TRD coupler is composed of three coupled lines, two shunt stubs, and three types of varactors. By providing suitable reverse bias voltages on these varactors, the operating frequency of the proposed TRD coupler can flexibly be adjusted as desired. Moreover, better impedance match and port isolation are maintained at all tuning center frequencies. A theoretical analysis using the even-odd mode theory is presented to clarify the influence of the design parameters on the minimum and maximum operating frequencies. It is exhibited that the proposed TRD coupler has approximately 150% frequency tuning range in ideal conditions. Due to the manufacturing limitation, a microstrip TRD coupler with a tunable frequency of 1.0-2.5 GHz (85.7%) is designed and fabricated. The measured results show that the proposed TRD coupler can be tuned from 1.0 to 2.5 GHz with more than 27 dB return loss and more than 35 dB isolation in the entire tuning range, agreeing well with the simulation results.