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Additional file 17: Table S1. Summarised search strategies to identify the effects of L-citrulline interventions on blood pressure. (DOCX 15 kb)

Microstrip phased array has aroused interest of many researchers because of its beam agility. However, a big problem for typical microstrip array is that its main beam can only scan from about –50° to 50°, with a gain loss of 4-5 dB. Meanwhile, the relatively narrow operating bandwidth of microstrip antenna is also a problem in application. These flaws have dramatically limited its applications and spawned many studies on phased array with wide-angle scanning capability. Several methods have been proposed to broaden the scanning coverage of phased array, such as utilizing pattern-reconfigurable antenna as an element of array, taking wide-beam antenna as the element of array, and adopt metasurface as the top cladding of array. However, most of existing researches mainly focus on achieving wide-angle scanning performance within a relatively narrow bandwidth. A phased array that possesses wide-angle scanning capability at both main planes within a relatively wide bandwidth is highly desirable. In this paper, a wide-beam magnetoelectric (ME) dipole antenna is proposed. It consists of an ME dipole antenna in the form of microstrip patch and a pair of magnetic dipoles. Metallic through holes integrated with patches and ground are utilized to form magnetic currents. Extra magnetic dipoles are added to broaden the 3-dB beam-width. The simulated results reveal that the 3-dB beam-width of the proposed antenna is greater than 107° in the E-plane (9 GHz–12 GHz) and 178° in the H-plane (7 GHz–12 GHz) respectively. The impedance bandwidth of the proposed antenna is 53.26% from 7.3 GHz to 12.6 GHz (VWSR < 2). Based on the proposed antenna element, two linear phased arrays are fabricated and measured. To test the wide-angle scanning capability of the arrays, each antenna element is simply fed with alternating currents with identical amplitude and linearly increasing phases. The measured results reveal that the wide-angle scanning capability of H-plane array and E-plane array can be obtained from 9 GHz to 12 GHz. The scanning beam of the H-plane array can cover the range from -90° to 90°. The scanning beam of the E-plane array can cover the range from –70° to 70°. The impedance bandwidth of the central antenna is 27.03% for the H-plane array from 9.6 GHz to 12.6 GHz (active VWSR < 2.5) and 36.36% for the E-plane array from 9 GHz to 13 GHz (active VWSR < 2) respectively. Hence, the proposed method can be used as a reference for designing a wide-beam antenna and wide-angle scanning phased array and the designed phased arrays can be applied to X-band radar systems.

Aiming at obtaining low scattering antenna array, in this paper a novel method of integrating electromagnetic metasurface with conventional antenna is proposed. The theoretical analysis and practical implementation of this method are presented. Using this method, a novel antenna array is obtained by connecting partial unit cells of metasurface with transmission line and adopting coaxial excitations. In the radiation mode, the metasurface is excited and radiates effectively. Besides, the array has almost the same performance as the conventional array. In the scattering mode, this array demonstrates low in-band RCS due to the scattering cancellation of middle metasurface and other surrounding structures. Using this method, a 2 × 1 array, as an example, is designed and numerically analyzed. The results show that the array has the well-behaved radiation performance and low RCS property. The working principle of the proposed array is illustrated by investigating the current and resultant field. Further analysis also reveals the effecting law of metasurface unit cells in antenna's radiation and scattering performance. Therefore, flexible designs can be obtained to fit different requirements. Finally, experiments are conducted. And the good agreement between computations and measurements further verifies the validity of the proposed design. Moreover, the proposed method also features easy implementation and high integrity and can be extended to the designing of large scale array antennas.

The transmission polarization conversion metasurface has been widely concerned, because it has the advantage of being easy to be conformal with the antenna. Based on the reasonable arrangement of transmission polarization conversion units, various and complex electromagnetic functions can be realized. As the electromagnetic open window on the flight platform, the antenna is the bottleneck that restricts the decrease of radar cross section (RCS) of the whole flight platform. It is difficult to simultaneously realize the normal and efficient radiation of the antenna and the decrease of the RCS of the antenna. When the designed transmission metasurface is used in the antenna design, the radiation and scattering of the antenna can be regulated comprehensively. In this paper, a composite polarization conversion metasurface is proposed and verified. The unit cell of composite polarization conversion metasurface consists of two mirror symmetrical anisotropic metal patches in the upper layer, a dielectric layer and a polarization gate in the lower layer. When the polarization direction of the incident electromagnetic wave is perpendicular to the extension direction of the polarization gate and arrives at the composite polarization conversion surface, the conversion surface can realize the conversion from transmission linear polarization to right-hand circular polarization in a frequency range from 9.3 GHz to 10.9 GHz. When the polarization direction of the incident electromagnetic wave is parallel to the extension direction of the polarization gate, co-polarized total reflection can be realized. The chessboard arrangement metasurface is composed of composite polarization conversion unit and its mirror unit. A novel linearly polarized chessboard arrangement metasurface antenna is composed of the linearly polarized source microstrip antenna with a bandwidth of 9.4–10.7 GHz and the chessboard arrangement metasurface. By using the counter rotating cancellation characteristic of circular polarization, the chessboard arrangement metasurface antenna maintains linearly polarized radiation. Comparing with the source microstrip antenna, the linear polarization purity of chessboard arrangement metasurface antenna is improved from 9.5 GHz to 10.5 GHz. At the same time, the forward gain of the chessboard arrangement antenna increases and the radar cross section decreases. The maximum reduction is 39.2 dB. To further verify the practicability of the design and analysis, the chessboard arrangement metasurface antenna sample is fabricated and measured in microwave anechoic chamber with an Agilent 5230C network analyzer. The experimental results are in good agreement with the simulation results. This study has important reference value in the design of high gain, low RCS antenna and integrated regulation radiation and scattering of antenna.

With the rapid evolution of radar technology and mobile communication systems, polarization conversion has received much attention from academia and industry in recent years, which has the advantages of improving system performance through eliminating multipath fading. In this paper, a novel broadband reconfigurable reflective polarization convertor is designed, which combines the idea of metamaterial and the technology of micro-electro-mechanical system (MEMS) switches. The proposed structure consists of three layers: an upper metallic patches layer, a middle dielectric layer with a thickness of 2 mm, and a bottom metal plate. There are through-holes of metal connecting the upper and bottom layers. According to the simulation using HFSS software, when the MEMS switch is on, the device works with a relative bandwidth of 57.77% from 7.78 GHz to 14.10 GHz, of which the polarization conversion ratio is larger than 80%. In addition, at 7.62 GHz and 12.56 GHz, the reflected wave is a right-hand circularly polarized wave and a left-hand circularly polarized wave, respectively. When the MEMS switch is off, the reflected wave is in the same polarization, which means the device does not convert the polarization of electromagnetic wave anymore. The electromagnetic wave are decomposed into the u-v coordinate system to further understand the wideband polarization rotation. The reflection phase and the surface current distributions of the convertor are analyzed. Then, the working principle of polarization rotation is explained by analyzing the current distributions and explaining the theory from three different viewpoints. Finally, a 1225-cell (35×35) prototype is fabricated to verify the simulation results. The measured curve has three resonant frequencies and shifts towards the lower frequency slightly. The discrepancy between simulations and measurements is mainly attributed to the restriction of fabrication and measurement condition. In general, experimental results are in agreement with the simulations: when linear polarized wave is incident, the reflected wave realizes the transition from co-polarization to cross-polarization as the switch is switched from off to on. The proposed reconfigurable polarization rotation surface has advantages of broadband, low loss and ease of fabrication, which has great potential applications in antenna radiation, reducing the radar cross section and other territories in controlling electromagnetic wave dynamically.