In this letter, a novel method of reducing mutual coupling is proposed and it used on a modified compact broadband antipodal Vivaldi antenna (AVA) array for future 5G mmWave communication application. The proposed structure consists of eight antenna elements which are fed by a 1-to-8 power divider. In order to reduce the mutual coupling between AVA array elements, multiple notch structures are added on the ground plane. Thus, the isolation between the antenna elements can achieve a maximal additional 37.3 dB enhancement, impedance bandwidth is extended slightly from 24.65-28.5 GHz to 24.55-28.5 GHz, and the gain is improved simultaneously. To verify the designed method, the proposed AVA arrays were fabricated and measured. And they show an overall size of 28.823 mm $\times$ 60 mm $\times$ 0.787 mm. The measured gain of the modified AVA array is 6.96-11.32 dB in the frequency band of future 5G mmWave communication, which is higher than the initial AVA array, whose gain is 5.34-8.5 dB.

A low-profile printed planar antipodal Vivaldi antenna (AVA) array with good performance for future 5G millimeter-wave communication applications is presented. The proposed structure is an 8 × 1 array with eight elements in E-plane, which is fed by a 1-to-8 power divider network. A novel feature called anisotropic metasurface (MS) is located at the aperture, resulting in the low cutoff frequency reduced from 24.55 GHz to 24.1 GHz for miniaturization. In addition, the phase distribution locate at the aperture of AVA array can be corrected by adding MS. Thus, the gain of AVA array can be improved from 8.5.11.2 dB to 9.35.12 dB in the frequency band of 5G millimeter-wave communication. Finally, to verify the designed methods, the proposed AVA with MS (AVA-MS) array was successfully fabricated and measured, which occupying an area of 6.11 λg × 2.93 λg × 0.08 λg. Numerical and experimental results coincide well with each other, which certifies the new method for achieving miniaturization and gain enhancement of our proposed AVA-MS array.

In this letter, a novel compact dual-band filtering power divider (DB-FPD) is proposed. It consists of a U-shaped Wilkinson power divider, two pairs of dual-resonance resonators (DRRs), and a pair of spur lines. With the simultaneous use and appropriate design of the coupled feedlines, mixed electric and magnetic couplings between the DRRs and spur-lines with different lengths, multiple transmission zeros are produced which result in two passbands with desired power division, high-frequency selectivity, good isolation, and an ultrawide stopband. A prototype DB-FPD operating at 2.3 and 3.5 GHz with 3-dB fractional bandwidths of 8% and 6% is designed, fabricated, and measured. The measured responses agree well with the design simulations, exhibiting a stopband up to 13.8 GHz (6f₀) with 20-dB rejection level.

In this brief, a direct synthesis design method of a continuously tunable wideband bandpass filtering attenuator is proposed. By adjusting the adjustable resistors, the attenuation amount that is inversely proportional to its resistance value can be obtained. In order to improve frequency selectivity, a half wavelength open-circuited stub and a parallel coupled line were loaded to achieve three pairs of transmission zeros (TZs). For the problem of low efficiency of traditional attenuators, a synthesis design method that can improve design efficiency was adopted, and a filtering attenuator with working frequency of 2.21-4.65 GHz, fractional bandwidth of 71.14%, and tunable attenuation range of 1.58-18.98 dB was simulated to verify the reliability of the method. The synthesis design outcomes correspond well with the simulation results.

A novel single-feed filtering dielectric resonator antenna (FDRA) with quasi-elliptic bandpass response is proposed in this brief. The proposed FDRA consists of a shunted slotline stepped-impedance resonator (SSIR), a rectangular dielectric resonator antenna (DRA) and a microstrip feedline with a pair of transverse arms. The antenna is excited in the fundamental TE $_{11\delta }$ mode, the transverse arm mode and the two SSIR modes. Moreover, three radiation nulls are produced near the edges of the passband by the SSIR and the transverse arm of microstip feedline, respectively. A prototype FDRA is fabricated and measured, which offers a bandwidth ( $| S_{11} | < -10$ dB) of 21.14% (3.13 - 3.87 GHz), and a peak realized gain of 5.3 dBi within passband. The realized gain shows a decline of 20 dB at passband-edge, which exhibits high selectivity.

In this paper, a terahertz (THz) dual-band bandpass filter based on spoof surface plasmon polaritons (SSPPs) with wide upper stopband suppression is proposed. The filter utilizes two types of SSPP unit cells loaded on a double-sided quasi-SSPPs transmission line to achieve dual-band filtering responses and wide upper stopband suppression simultaneously. The performance and bandwidth of the filter's passbands can be adjusted by modifying the SSPP unit cells within the dual-band filtering part. The detailed design method is provided to enhance understanding of the operating principle of the presented filter. The simulation results of the designed THz filter demonstrate its excellent dual-band filtering performance. It features a wide stopband spanning from 0.91 to 1.45 THz with |S₂₁| < -35 dB. To validate the proposed approach, a microwave filter with a similar design is implemented. Finally, the simulated and measured S-parameter responses of the filter are provided for comparison and evaluation.