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This letter presents a wideband three-dimensional (3-D)-printed circularly polarized (CP) ultrathin dielectric slab waveguide (DSW) Luneburg lens (LL) with multibeam application in X-band. The characteristic that DSW supports TE0 and TM0 waves to pass through with different phase constants is used to generate 90° phase difference to compose CP wave for the proposed structure. To prove the point, the prototype antenna is fabricated by 3-D printing and measured. The results show that the proposed LL supports multibeam application with 11 beams covering ±60° range in azimuth plane, and the 3 dB axial ratio bandwidth for multibeam antenna is 23.5% with reflection coefficient lower than −10 dB and gain higher than 13.7 dBi. The measured results show good coincidence with simulated ones. [ABSTRACT FROM AUTHOR]

This letter presents a simple method to design the wideband circularly polarized spherical multibeam Luneburg lens (LL) antenna by using three-dimensional (3-D)-printing. The conventional 3-D-printed spherical LL is divided into many cells and each cell is filled by the unit cell with different filling ratios, which breaks the circular symmetry of the LL. The proposed method divides the LL into many layers, and the profile surface equation of each layer is calculated by the ordinary least square method to keep the rotation symmetry of the LL. Then, according to the proposed circularly polarized implement method, the transformation of linear polarization to circular polarization can be realized by selecting a proper radius of LL. The simulated and measured results show that the axial ratios of the proposed multibeam LL antenna are less than 3 dB with good impedance performance and more than 13.8 dBi gain in a relative bandwidth of 31.6%, which demonstrates the validity and simplicity of the presented method. [ABSTRACT FROM AUTHOR]

A unidirectional planar electrically small antenna shape synthesis method is proposed through the use of characteristic mode concepts. Antenna feed does not have to be specified beforehand, and can be easily selected on the synthesized conductor suggested by the eigencurrents distribution. The afforded freedom leads to potentially successful high-directivity electrically small antenna designs. By properly choosing the starting shape and size, the synthesized antenna can well approach the maximum achievable directivity, the associated gain, and Q-factor predicted by the circular plate of the same size. [ABSTRACT FROM AUTHOR]

This letter presents a broadband 3-D-printed circularly polarized (CP) spherical Luneburg lens (LL) with the linearly polarized feed antenna for the X-band. In order to implement spatially continuous varying effective refractive index (RI), the proposed LL is divided to many unit cells and their effective RI is simulated and calculated by effective medium theory. Since the effective RI of unit cells is anisotropic for two orthogonal linearly polarized (LP) waves, the proposed LL is an LP to CP polarizer realized by properly selecting the radius of the LL. The whole structure is fabricated by low cost 3-D-printing (3DP) technic, which is fed by an LP antenna source rotated to a certain angle with respect to the LL around the normal. The measured results indicate that the voltage standing wave ratio (VSWR), axial ratio, and gain enhancement are lower than 2, less than 2.8 dB, and more than 10 dB at X-band, respectively. The results of measurement agree well with the simulation. [ABSTRACT FROM AUTHOR]

A quantity called available directivity is defined for a radiating structure in terms of its characteristic mode content. It is shown how its maximum value can be evaluated when a certain number of such modes are considered to be excited with appropriate modal excitation coefficients. The achievable directivity is found by constraining the associated quality factor. This provides a more precise way of determining the prospective directivity of a structure than the usual estimates in terms of the spherical wave modes related to the smallest sphere enclosing the structure. [ABSTRACT FROM AUTHOR]

This letter presents a novel transmission metasurface to achieve orbital angular momentum (OAM) waves at the X-band. The unit cell of the proposed metasurface consists of three dielectric layers and four metallic layers, where the metallic layer is composed of an outer metallic ring and a center quasi-snow patch. A good transmission coefficient of the unit cell is obtained at the operating frequency. Meanwhile, a full 360° phase shift can be achieved by changing the length of the quasi-snow patch. The method of generating OAM waves is studied and analyzed. Based on this method, a metasurface producing a +1-mode OAM wave is designed and simulated. To verify this concept, one prototype is fabricated, assembled, and measured. The measured results agree well with the simulated ones, showing that the +1-mode OAM wave can be generated successfully with a high gain of 14.5 dBi. Moreover, a narrow divergence angle of +/−9° is achieved in the produced OAM wave. [ABSTRACT FROM AUTHOR]

A novel polarization-reconfigurable planar antenna is presented. The antenna consists of an electronically reconfigurable polarizer integrated with a printed slot. By changing the states of the p-i-n diodes on the polarizer, the linearly polarized (LP) waves radiated by the slot can be converted into either right-hand circularly polarized (RHCP) or left-hand CP (LHCP) waves. The polarizer contains 16 unit cells arranged as a $4\times 4$ array. The antenna radiates RHCP waves if the p-i-n diodes on the top side of the polarizer are switched ON, while LHCP waves are radiated if the p-i-n diodes of the bottom side of the polarizer are switched ON instead. The physical mechanisms of the antenna are discussed and the parametric study is carried out by full-wave simulations. To verify the concept, one prototype at 2.5 GHz is designed, fabricated, and measured. Good agreement between the measured and simulated results is obtained. The antenna achieves a gain ≥8.5 dBic in both RHCP and LHCP with an aperture efficiency of 70%. Advantages of the proposed design include electronically reconfigurable polarizations for RHCP or LHCP, low profile, low cost, high isolation between the dc bias circuit and RF signals, high power handling capability, and easy extension to large-scale arrays without increasing the complexity of the dc bias circuit. To the best of our knowledge, this is the first report of an electronically polarization-reconfigurable CP antenna with a single-substrate polarizer. [ABSTRACT FROM AUTHOR]

This communication presents the complete design of a circularly polarized (CP) folded reflectarray (FRA) antenna with an integrated planar structure for the first time in the open literature. To achieve circularly polarized, a printed meander-line polarizer is designed and integrated with the linearly polarized (LP) FRA. To achieve a low-profile planar structure, an integrated $2 \times 2$ planar array is designed as the feed source instead of a horn. Thus, the whole antenna, including the feed source, LPFRA, and meander-line polarizer, can be fully integrated and fabricated using low-cost printed circuit board technology. To validate the concept, a right-handed CPFRA operating in C-band is designed, fabricated, and measured. The broadside axial ratio (AR) of the proposed CPFRA is lower than 1 dB over a bandwidth from 5.22 to 5.46 GHz. In addition, the maximum gain of 22.8 dBic is obtained at 5.38 GHz with the antenna efficiency of 27%. The antenna is promising for applications in satellite communication due to advantages of low profile, easy fabrication, low cost, and high gain. [ABSTRACT FROM AUTHOR]

This paper presents a novel shared-aperture dual-band dual-polarized (DBDP) high-gain antenna for potential applications in synthetic aperture radars (SARs). To reduce the complexity of SAR antennae, a DBDP high-gain antenna based on the concept of Fabry–Perot resonant cavity is designed. This antenna operates in both $C$ - and $X$ -bands with a frequency ratio of 1:1.8. To form two separate resonant cavities, two frequency selective surface layers are employed, leading to high flexibility in choosing desired frequencies for each band. The beam-scanning capability of this proposed antenna is also investigated, where a beam-scanning angle range of ±15° is achieved in two orthogonal polarizations. To verify this design concept, three passive antenna prototypes were designed, fabricated, and measured. One prototype has broadside radiation patterns, while the other two prototypes have frozen beam scanned to +15°. The measured results agree well with the simulated ones, showing that the high gain, high port isolation, and low cross-polarization levels are obtained in both the bands. Compared with the conventional high-gain DBDP SAR antennae, the proposed antenna has achieved a significant reduction in the complexity, mass, size, loss, and cost of the feed network. [ABSTRACT FROM AUTHOR]

This paper presents a novel sequentially rotated patch array antenna with dual circular polarizations (CP). The present design has the advantages of compact size, simple feed networks, low cross polarization, and high isolation. The radiating elements are dual linearly polarized (LP) equilateral triangular patches with hybrid feeds. The vertical polarization (VP) of the patch antenna is obtained by using an aperture-coupled feed and the horizontal polarization (HP) is obtained by using a proximity feed. These two feeds are orthogonally positioned and are printed on different PCB layers, which leads to the high isolation between these two ports and facilitates the design of the feed networks. The array antenna consists of six such dual-LP triangular patches sequentially rotated by 60 ^\circ and fed by separated feed networks. Since LP antenna elements are used, the design of the feed network is much simplified. Through sequentially varying the feeding phase by \pm 60^\circ, dual circular polarizations are obtained. The operation principle of the array antenna is also analytically explained in this paper. To verify the design concept, one dual LP equilateral triangular patch and one dual CP equilateral triangular patch array resonating at 10.5 GHz are designed, fabricated, and tested. There is a good agreement between the simulation and measurement results, both of which show that the array antenna exhibits high port isolation and good circular polarizations with low cross polarization at different planes. The proposed design technique can be applied to the design of dual CP array antennas operating at other frequency bands. [ABSTRACT FROM PUBLISHER]

This paper presents a compact-size, low-cost smart antenna with electronically switchable radiation patterns, and reconfigurable polarizations. This antenna can be dynamically switched to realize three different polarizations including two orthogonal linear polarizations and one diagonally linear polarization. By closely placing several electronically reconfigurable parasitic elements around the driven antenna, the beam switching can be achieved in any of the three polarization states. In this design, a polarization reconfigurable square patch antenna with a simple feeding network is used as the driven element. The parasitic element is composed of a printed dipole with a PIN diode. Using different combinations of PIN diode on/off states, the radiation pattern can be switched toward different directions to cover an angle range of 0° to 360° in the azimuth plane. The concept is confirmed by a series of measurements. This smart antenna has the advantages of compact size, low cost, low power consumption, reconfigurable polarizations, and beams. [ABSTRACT FROM PUBLISHER]

A wideband and dual-polarization antenna array with high efficiency and high isolation is proposed. In order to achieve the desired performance, a wideband radiating element and a full-corporate waveguide feed network are employed in the design. In particular, ${{E}}-plane waveguides are utilized for the feed-network configuration so as to suppress wave leakage between adjacent layers when the antenna is fabricated via conventional machining techniques. The operational mechanism is presented and a \text16 \times \text16 antenna array is tested for verification. Experimental results show that the bandwidths of VSWR < \text2.0 for both of the polarizations are about 17.1% (12.45–14.78 GHz) and the isolation between the two polarization ports is better than 39.5 dB. In addition, more than 70% antenna efficiency with higher than 29.4 dBi gain is achieved over the operational bandwidth. [ABSTRACT FROM PUBLISHER]

A novel dual-polarized patch antenna for ultra-wideband (UWB) applications is presented. The antenna consists of a square patch and four capacitively coupled feeds to enhance the impedance bandwidth. Each feed is formed by a vertical isosceles trapezoidal patch and a horizontal isosceles triangular patch. The four feeds are connected to the microstrip lines that are printed on the bottom layer of the grounded FR4 substrate. Two tapered baluns are utilized to excite the antenna to achieve high isolation between the ports and reduce the cross-polarization levels. In order to increase the antenna gain and reduce the backward radiation, a compact surface mounted cavity is integrated with the antenna. The antenna prototype has achieved an impedance bandwidth of 112% at (\vert S11 \vert \leq -10~dB) whereas the coupling between the two ports is below -28~dB across the operating frequency range. The measured antenna gain varies from 3.91 to 10.2 dBi for port 1 and from 3.38 to 9.21 dBi for port 2, with a 3-dB gain bandwidth of 107%. [ABSTRACT FROM PUBLISHER]