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A circularly polarized antenna with wide axial-ratio beamwidth is presented by placing two pairs of parallel dipoles in a square contour. Firstly, our study demonstrates that circular polarization can be achieved at the broadside by setting a 90 phase difference between the vertical and horizontal paired-dipoles. Secondly, the principle of wide-beamwidth circularly-polarized radiation is described under the similarity in polar and azimuthal radiation pattern for a pair of parallel dipoles. After the spacing between two parallel dipoles is studied, the similar radiation patterns in these two orthogonal planes are derived. As such, a widebeamwidth circular polarization can be achieved by placing the two pairs of parallel dipoles vertically and horizontally while setting a 90 phase difference between them. In final, a circularly-polarized printed antenna is designed and fabricated on a single dielectric substrate. The two arms of its four dipoles are formed on the lower and upper interfaces of the substrate, and they are excited by a 1-to-4 probe-to-microstrip feeding network. Experimental results show good agreement with simulated ones in terms of radiation pattern/gain, axial ratio and returned loss. In particular, the 3-dB axial ratio beamwidth at the central frequency of 1.6 GHz has extended to 126 in an angular range from to . The fabricated antenna exhibits a low-profile property with the height of 0.0043 free-space wavelength.Index Terms-Axial ratio, circularly polarized antenna, dipoles, microstrip-line feeder, wide beamwidth.

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A circularly polarized (CP) antenna with a wide axial-ratio (AR) beamwidth is presented by placing the two pairs of folded dipoles in a square contour. First, our study demonstrates that the CP radiation can be achieved at the broadside by setting a 90 • phase difference between the vertical-and horizontal-paired folded dipole radiators. Second, a radiation pattern subtraction approach is presented to explain how the radiation beamwidth of the paired folded dipoles could be appropriately widened or narrowed according to the radiation pattern subtraction of two opposite electric current densities in the folded dipoles. Using this approach, the E-plane beamwidth of a pair of folded dipoles is made certainly widened, whereas its H-plane counterpart becomes narrowed. On this basis, the Eand H-plane radiation patterns can still be made approximately identical with each other over a wide range of polar angle, even though the distance between dipoles is deliberately shortened. As such, the desired CP antenna with shortened dipole-to-dipole distance and unaffected wide AR beamwidth can be achieved using two pairs of parallel folded dipoles with shortened distance between them instead of linear dipoles in [24]. Finally, a CP printed antenna is designed and fabricated on a single dielectric substrate. Experimental results are found in good agreement with the simulated ones in terms of radiation patterns, gain, AR, efficiency, and reflection coefficient. In particular, the 3-dB AR beamwidth at the central frequency of 1.6 GHz is extended to 135 • at XZ-plane and 142 • at Y Zplane. Moreover, the overall size of the constituted CP antenna with folded dipoles is decreased to 0.43 λ0 × 0.43 λ0 (λ0 is the wavelength in free space).Index Terms-Circularly polarized antenna, folded dipole and radiation pattern subtraction, wide beamwidth. Y. Luo and Q.-X. Chu are with the

Full-color matrix devices based on perovskite light-emitting diodes (PeLEDs) formed via inkjet printing are increasingly attractive due to their tunable emission, high color purity, and low cost. A key challenge for realizing PeLED matrix devices is achieving high-quality perovskite films with a favorable emission structure via inkjet printing techniques. In this work, a narrow phase distribution, high-quality quasi-two-dimensional (quasi-2D) perovskite film without a "coffee ring" was obtained via the introduction of a phenylbutylammonium cation into the perovskite and the use of a vacuum-assisted quick-drying process. Relatively efficient emissions of red, green, and blue (RGB) uniform quasi-2D perovskite films with high photoluminescence quantum yields were cast by the inkjet printing technique. The RGB monochrome perovskite matrix devices with 120 pixel-per-inch resolution exhibited electroluminescence, with maximum external quantum efficiencies of 3.5, 3.4, and 1.0% (for red, green, and blue light emissions, respectively). Furthermore, a full-color perovskite matrix device with a color gamut of 102% (NTSC 1931) was realized. To the best of our knowledge, this is the first report of a full-color perovskite matrix device formed by inkjet printing.

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A Yagi-Uda antenna with a stepped-width reflector is presented. Totally different with the traditional ones, the proposed reflector is shorter than the driven element as a result of the stepped-width structure. To further understand its working mechanics, an equivalent circuit to a dipole with a parasitic element is employed to explain the shortened length of the stepped-width reflector. Then, the proposed stepped-width reflector, shorter than the driven element, was applied to design, fabrication and measurement of a Yagi-Uda antenna. Measured results show a good agreement with the simulated ones. In particular, the total length of the reflector employed in the Yagi-Uda antenna is 50 mm and that of the driven element is 53 mm, which verifies the shorter length of the reflector and benefits achieving a smaller size.Index Terms-stepped-width structure, a dipole with a parasitic element, equivalent circuit, Yagi-Uda antenna, miniaturized reflector.

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