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In this letter, a dual-wideband dual-circularly polarized (CP) reflectarray (RA) with 1013 dual-CP RA cells is proposed. The overall size is 161 × 161 × 1.8 mm. Composed of a metal reflection plate and a substrate printed with circuits of fan-shaped, arc-shaped and rectangular patterns, current paths with different lengths can be formed, and multiple resonance points can be introduced to expand the working bandwidth (BW). The −10 dB cross-polarization BW of cells has reached 83.67% (16.03–39.09 GHz). Center frequencies of the right-handed (RH) CP and left-handed (LH) CP reflected beams are designed to point along (ϕ, θ) = (90°, 20°) at 19 GHz and (ϕ, θ) = (90°, −20°) at 31 GHz. Specifically, the 3 dB gain BW of RHCP and LHCP reflected beams is 43.8% (7.3–27.0 GHz) and 47.1% (24.5–39.6 GHz), respectively. 1.5 dB axial ratio (AR) BWs are greater than 90.1%. The dual-wideband and dual-CP characteristics enable the proposed RA to meet the communication requirements of satellite communication transmission and reception at the same time and become a promising candidate technology for long-distance communication for K-band and Ka-band applications.

An optically transparent right-handed (RH) circularly polarized (CP) reflectarray (RA) with 3940 cells is proposed in this communication. The overall structure is composed of a 0.125-mm-thick polyethylene terephthalate (PET) film with fine metal line (FML), quartz glass with a thickness of 0.5 mm, a 0.6-mm air layer, indium tin oxide (ITO) ground, and an RHCP feed horn. The overall size of the reflection plane is $170\times 170\times 1.225$ mm. The shaped pattern of a double arrow, two diamond, and a chain with three incomplete diamonds form the circuit of unit cell. The RA cell is miniaturized ( $2.4\times 2.4$ mm) by bending the corresponding current path with a linewidth of $40~\mu $ m. To compensate for the loss induced by the FML, we increased the resolution of the reflection plane, and the measured results showed that the peak gain and aperture efficiency (AE) reached 32.5 dBic and 54.9%, respectively. The 3-dB gain bandwidth (BW) reached 23.2% (28.5–36 GHz), and the axial ratio (AR) is less than 1.5 dB across the frequency band. These advantages, such as optical transparency, high directionality, and good circular polarization characteristics, make it a potential candidate for satellite communication applications.

This communication presents a millimeter-wave (mmW) compact wideband high-gain $8\times8$ circularly polarized (CP) filtenna array with densified 16-way substrate integrated waveguide (SIW) feeding network. First, a parallel-series $2\times2$ CP patch subarray without a power distribution network is designed by exciting four and two quasi- modes, simultaneously, in a single SIW cavity. By properly setting perturbed via-slot groups, hexagonal radiating apertures, and square patches etched with a 45°-inclined center slot, three axial ratio (AR) minima are created to realize wide AR bandwidth. Additional bandpass filtering response can also be observed, which helps to realize high-selectivity CP filtenna array design. The densified 16-way feeding network mainly consists of four sequentially rotated SIW four-way power dividers, and two filtering SIW TE10-to-TE20 mode two-way power dividers. The former is constructed based on the anti-phase field distribution of TE20 SIW mode, and the latter features an additional filtering response and simple structure. An $8\times8$ CP filtenna array centered at 28 GHz was fabricated and tested. The measured results show a-10dB impedancebandwidth of 14.4%, a 3 dB AR bandwidth of 15.3%, a peak gain of up to 23.9 dBic, and high out-of-band rejection levels better than 39 dB.

A flexible endfire antenna based on spoof surface plasmon polaritons (SSPPs) is proposed at millimeter-wave band. First, due to the strong field confinement of SSPPs, it can ensure that the field distribution of the antenna is almost unchanged when it is bent, so that its impedance characteristics remain relatively stable. Then, to realize the endfire radiation, a symmetrical SSPPs transmission lines fed by a microstrip-slotline structure is proposed. Finally, an endfire antenna operates in the frequency range of 30–50 GHz, over which the average gain of this antenna is 14.9 dBi, and the overall size of this antenna is 9.2λ0×2.67λ0×0.016λ0. Besides, this antenna can still maintain good radiation performance in bending, and its maximum radiation angle changes with a linear correspondence of the bending angle. When the bending angle of the antenna changes from 0° to 30°, the maximum radiation angle increases from 0° to 24°, and the gain fluctuates within 2 dB, which shows that the flexible antenna has good stable radiation performance. The proposed antenna holds the advantages of wide bandwidth, small size and flexibility, and has a good prospect in the future flexible circuit and wearable systems.

A broadband circularly polarized (CP) strip dipole antenna array is proposed in this letter. The basic structure is a Z-shaped dipole excited by a rectangular slot. CP radiation with a narrow axial ratio (AR) bandwidth is realized at the high-frequency band first. To widen bandwidth, a pair of transverse stubs are arranged at the beginnings of the Z-shaped dipole, which greatly increases the phase difference of two orthogonal electric fields and produces one new AR minimum point at the low-frequency band. Furthermore, two short vertical stubs are connected to the Z-shaped dipole, which can control the AR minimum point of high frequency. Hence, a wide AR bandwidth can be realized by manipulating two AR minimum points separately. To achieve good impedance matching and minimize the impact on AR, two vertical slots are incorporated into the rectangular coupling slot. Finally, the proposed CP strip dipole obtains an operating bandwidth of 46${\rm{\% }}$ (19.1–30.5 GHz). Then, the single antenna is expanded into an antenna array with 64 elements to obtain high gain. The experimental results of the proposed array reveal an impedance bandwidth of 58${\rm{\% }}$ (17.6–32 GHz) and an AR bandwidth of 42.8${\rm{\% }}$ (19.25–29.75 GHz). The peak gain is 22.18 dBic. The experimental results verify good performance of the designed array.

This article presents a novel class of compact hybrid bandpass filters (BPFs) using substrate-integrated waveguide (SIW) and stripline resonators. As the stripline resonators are inserted between two SIW cavities, they are able to increase the filtering order without occupying any additional area. This is beneficial for circuit miniaturization. To verify the concept, several third-order hybrid filters with various responses are designed by combining a half-wavelength stripline resonator (HWSLR) or a full-wavelength stripline resonator (FWSLR) and two SIW resonators. Their sizes are reduced by more than 33% compared with standard third-order SIW filters. In addition, a class of fourth-order box-like hybrid filters with two flexible transmission zeros (TZs) is also proposed by using the HWSLR and FWSLR simultaneously. Compared with previously reported box-like SIW filters, a size reduction of more than 65% is obtained. Three filter prototypes are fabricated and measured for validation.