Circularly Polarized Metasurface Antenna Based on the Theory of Characteristic Mode

A low-profile and broadband metasurface antenna is proposed for circular polarization radiation. The metasurface is an array of subwavelength square patches. The modal behaviors of the proposed metasurface are investigated by using the characteristic mode theory. Two characteristic modes with the same resonant frequencies and orthogonal current distributions are chosen as operation modes. Furthermore, a hybrid feed system consisting of a cross-slot and a microstrip line is employed to excite the two orthogonal modes having a 90° phase difference to obtain circular polarization radiation. Based on these concepts, an antenna with low profile of 0.07λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> (λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> is the free-space wavelength at an operation frequency of 5.5GHz) is designed. The measurement results show that the proposed antenna has -10 dB impedance bandwidth of 4.8-6.35 GHz and 3 dB axial-ratio bandwidth of 4.85-6 GHz. Moreover, the antenna gain is 6.8-9.7 dBic in the whole axial-ratio bandwidth.

This paper proposes a novel method for enhancing the half-power beamwidth (HPBW) of an end-fire circularly polarized (CP) terminal antenna, whereby the end-fire CP antenna is designed for 6 G satellite communication. First, the CP property is realized by allocating the resonances of both In-phase mode (IM) and out-of-phase mode (OM) based on the theory of characteristic mode (TCM). The HPBW of OM and IM is enhanced by layering the radiator, which introduces interlayer currents for OM and substrate thickness for IM. Besides, by using the stepped ground edge to regulate field distribution in the radiator, the HPBW is further broadened. The proposed wide HPBW CP antenna is manufactured and measured to validate the novel design concept. The measured results show that −6 dB impedance bandwidth is 2.45 ~ 2.56 GHz, and 3 dB axial ratio (AR) bandwidth is 2.48 ~ 2.50 GHz. The measured HPBW in the xoy and yoz planes are 76° and 94°, respectively. The high normalized comprehensive coefficient (NCC), considering AR bandwidth, Gain, HPBW, and antenna size, proves that the proposed antenna is especially suitable for 6 G terminal satellite communication.

In this paper, modal analysis is used as a guide to design a circularly polarized open‐slot antenna. The design proposes an open wide slot that is fed electromagnetically by a modified feedline. Both open slot and feedline are asymmetrically placed to the edge of the substrate. The complete antenna without excitation is analyzed using characteristic mode theory to provide a physical insight into its circular polarization operation. The modal analysis proves that several nearly orthogonal modes can be generated at certain frequencies with 90° phase differences. The axial ratio (AR) corresponds to the modal analysis, which proves the validity of the modal analysis. An AR bandwidth from 3.2 to 6 GHz (61%) is achieved within an S11 bandwidth of 3.2–14 GHz (125%). The proposed antenna achieves a peak realized gain 4.3 dBi of within the AR band.

In this letter, a wideband circular polarization (CP) metasurface (MS) antenna is proposed, which is composed of a centrosymmetric structure with a mode suppressor and an asymmetric aperture-coupled feed structure separated by dielectric substrates. Based on the characteristic mode theory, the aperture-coupled feed structure consisted of an L-shaped slot and a microstrip line is employed to excite two orthogonal modes, which have a 90° phase difference to obtain CP radiation. Meanwhile, two methods have been proposed to expand CP bandwidth: one is the direct suppression of characteristic mode by decreasing the overlap frequency of operating and nonoperating mode. The other one is indirect suppression using a mode suppressor. Finally, the MS antenna with a low profile of λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> × λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> × 0.075 λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> at 5 GHz is fabricated and measured. The measured results show that the proposed antenna has a 3 dB axial ratio bandwidth of 45.2% from 4.1 to 6.5 GHz and an impedance bandwidth of 41.8% from 4.1 to 6.31 GHz, and a peak gain of 7.94 dBic at 5.9 GHz.

A circularly polarized metasurface antenna based on characteristic mode theory is proposed in this paper. The characteristic modes of the metasurface structure were solved. And two mutually orthogonal characteristic modes to realize circular polarization was determined. By adding rectangular slots on the metasurface to change the orthogonality of the mode currents, the axial ratio performance of the metasurface antenna is improved. The antenna with an overall size of 35×35×4 mm3 (0.72λ×0.72λ×0.08λ at 6.2 GHz) is presented. The simulated S 11 bandwidth and 3dB axial ratio bandwidth are 30% (5.1 GHz - 6.9 GHz) and 17.3% (5.8 GHz - 6.9 GHz), respectively.

Axial Ratio (AR) and Impedance Bandwidth (IBW) enhancement of Circular Polarized (CP) monopole antenna

This article presents a method of using modal analysis to characterize the circular polarization (CP) performance of a printed monopole antenna. The radiator and feedline are positioned on the edge of the substrate and fed accordingly at the edge, and a modified ground plane is employed. The complete antenna without excitation is analyzed using characteristic mode theory to provide physical insight into its CP operation. The modal analysis proves that several nearly orthogonal modes can be generated at certain frequencies with orthogonal phase differences as well. The axial ratio (AR) corresponds to the modal analysis which proves the validity of the modal analysis. An axial ratio bandwidth from 3.1 to 6.2 GHz (66. 7%) is achieved within an S11 bandwidth of 2.6–11 GHz (123.5%). The proposed antenna is compact with stable radiation patterns, easy to design, and fabricate without inserting parasitic patches or inserting slots on the antenna.

A new design method to significantly expand the axial ratio (AR) bandwidth of a single-fed, low-profile circularly polarized (CP) antenna without relying on any complex feed network or occupying a large transverse dimension is presented. First, four small patches arranged in chessboard pattern are utilized as the main radiator, and meanwhile, a cross slot with unequal slot lengths is adopted to excite triple sequentially orthogonal modes, aiming to attain two CP modes (AR <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> and AR <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ). Second, in order to further considerably enhance the AR bandwidth and concurrently reduce the overall size, the feeding cross-slot resonance is favorably excited and effectively coupled by the chessboard patches to produce a new slotline-based CP mode (AR <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ) located before AR <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> . Finally, an I-shaped top-loaded monopole fence, as a new degree of design freedom, is employed to collaboratively regulate these multiple resonances to form a wide AR bandwidth covering AR <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> , AR <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , and AR <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> . Results show that, on the premise of keeping the inherent compact-size and low-profile advantages of microstrip antenna, the 3 dB AR bandwidth is remarkably enhanced up to 30.1% by the combination of the three CP modes. Based on the design method, a wideband polarization-reconfigurable antenna with switchable linear polarization (LP), left-handed circular polarization (LHCP), and right-handed circular polarization (RHCP) states is further designed and experimentally validated. The measured results demonstrate that, with quite compact transverse size ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.16\lambda _{0}^{2}$ </tex-math></inline-formula> ) and low profile ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.04\lambda _{0}$ </tex-math></inline-formula> ), a wide overlapped bandwidth (27.6%) of the polarization-reconfigurable antenna is successfully accomplished.

To enhance axial ratio (AR) bandwidth, this research proposes a circularly polarized (CP) single-fed microstrip patch antenna using a circular artificial ground structure (AGS) and meandering probe. To achieve broader AR bandwidth, the circular AGS is populated with rectangular unit cells and partially cut unit cells along the circular contour, while the meandering probe is used to improve the impedance bandwidth. Simulations are performed and results compared with that of conventional rectangular-AGS antenna. The simulation results show that the circular-AGS antenna, given 62 mm circular ground plane, achieves broader impedance (5.12 - 9.00 GHz and 54%), AR (5.21 - 8.27 GHz and 45%) and gain bandwidths (3.85 - 7.00 GHz and 58.06%), in comparison with the rectangular-AGS antenna (4.50 - 7.45 GHz and 49%; 4.52 - 7.42 GHz and 21%; and 4.00 - 6.80 GHz and 51.58% for impedance, AR and gain bandwidths). The circular-AGS antenna is capable of converting linear polarization in the off-axial ratio band into circular polarization. To verify, a circular-AGS antenna prototype is fabricated and experiments undertaken. The experimental impedance, AR and gain bandwidths of the circular-AGS antenna are 47.82% (5.17 - 8.42 GHz), 43.81% (5.24 - 8.17 GHz) and 60.74% (3.75 - 7.00 GHz). The proposed circular-AGS antenna can achieve broader AR bandwidth and is thus ideal for broadband CP applications. The novelty of this research lies in the use of circular AGS to effectively enhance AR bandwidth, as opposed to rectangular AGS which is conventionally used in CP polarizers.

A novel low-profile dual-circularly polarized metasurface antenna with wideband and widebeam circular polarization (CP) radiations is proposed. It is constructed by an array of polarization-rotation metasurface (PRMS) cells and fed by a new cross aperture with four narrow slots in the ground plane. Due to the polarization-rotation property of the PRMS structure, a pair of linear-polarization (LP) aperture-coupling feedings can respectively inspire left-hand and right-hand CP (LHCP, RHCP) radiations. Furthermore, the proposed antenna has a compact structure with a low profile of 0.07λ0 at 5 GHz. Simulated results show that the 10-dB impedance bandwidths for LHCP and RHCP states are respectively 36.0% (3.78–5.58 GHz) and 24.6% (4.26–5.44 GHz). The 3-dB axial ratio (AR) bandwidth for the LHCP state is 17.4% (4.19–5.42 GHz) with large 3-dB AR beamwidths of 103.22° and 55.65° in the xoz and yoz planes, while for the RHCP state, the AR bandwidth is 17.0% (4.50–5.35 GHz) with large beamwidths of 77.93° and 99.67° at the two planes. In addition, the polarization isolation within the two AR bands is over 15 dB.

This paper reports a novel polarization-reconfigurable antenna based on the theory of characteristic mode (CM), which can electronically alter its polarization states between linear polarization (LP), left-hand circular polarization (LHCP), and right-hand circular polarization (RHCP). A square patch with an I-shaped slot is used as a main radiator, which has two LP orthogonal CMs with the same modal significance and 90° characteristic angle difference at 2.4 GHz. A simple feeding network is designed to excite the different combinations of the two modes. By properly choosing the state of the excitation, the LP, LHCP, and RHCP waves can be generated. A $1 \times 4$ antenna array is designed for the demonstration of array application, and experiments are carried out to validate the proposed design. The measured overlapping impedance bandwidth (BW) ( $\vert \text{S}_{11}\vert \le -10$ dB) and 3-dB gain BW is 2.25–2.7 GHz in the LP state. The overlapping BWs in conjunction with the 3-dB axial ratio BW are 2.16–2.65 and 2.1–2.55 GHz in the LHCP and RHCP states, respectively. The measured maximum gains are 7.2 dBi, 9.78 dBic, and 9.8 dBic for the LP, LHCP, and RHCP states, respectively. The common BW for all three polarization states is 2.25–2.55 GHz or 12.5%. Experimental results show a good performance of the proposed antenna array.

A method for the design of broadband circularly polarised (CP) patch antennas was recently proposed by the authors (Chung, K.L. and Mohan, A.S., IEEE Trans. Antennas and Propagat., vol.51, p.3239-48, 2003). Designs for both C- and Ku- band antennas made use of high permittivity materials as their grounded substrates. However, the high permittivity material suffers from a high tolerance (/spl sim/3%) in dielectric constant, making the design process more difficult. This work examines the substrate tolerance effects on the performance of the CP-EMCP antenna elements presented in the previous paper. The performance includes the input impedance characteristics (VSWR), impedance bandwidth (ZBW), antenna gain, axial ratio and axial-ratio bandwidth (ABW). Further, this paper supplements the information on the design of broadband CP-EMCP patch antennas that comprise hi-lo dielectric combinations. The tolerance effects in four extreme cases on the gain and impedance matching (VSWR) of both the C- and Ku-band designs are shown to be insignificant. However, these same effects are significant on the axial ratio and axial-ratio bandwidth in both designs. The tolerance in thickness of high permittivity materials causes a deterioration in the boresight axial ratio of singly-fed stacked patch antennas.

This paper presents a low-profile broadband dual circularly polarized antenna with flexible configuration of peripheral metasurface to implement diversified functions, for example, reduction of radar cross section (RCS). The core metasurface antenna based on the theory of characteristic mode consists of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$3\times 3$</tex> modified subwavelength square metal patches as well as an inductive microstrip side feed structure with an overall low profile of 0.06 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\lambda_{0}$</tex> at 9.6 GHz and the peripheral metasurface is a topologically optimized random array of I-shaped unit cells. Simulation results show that the bandwidth of 28.9%, the normal circular polarization axial ratio bandwidth of 19.5%, and the normal gain greater than 7.84 dBi. Meanwhile, more than 9.2 dB RCS reduction is achieved with a slightly negative influence on radiation characteristics at 8.45 GHz.

The Theory of Characteristic Modes (TCM) had its humble beginnings in the early 1970's. The beauty of TCM lies in its ability to fully characterize the radiation and scattering properties of an arbitrary object based only on the object's geometry and material properties. This ability provides valuable insights into an antenna's behavior independent of the feeding arrangement as well as providing information about how desirable radiation modes can be excited. This feature has led to its use to design integrated antennas in the High Frequency (HF) band for land vehicles, ships and aircraft. However, TCM had largely remained a specialist field until it was rediscovered for aiding the design of mobile handset antennas about a decade ago. In particular, TCM provides a powerful tool to understand and exploit excitation of the terminal chassis to enhance antenna performance. Another powerful feature of TCM is that multiple characteristic modes at a given frequency facilitate orthogonal radiation patterns, which provide effective Multiple-Input Multiple-Output (MIMO) antennas.

The Theory of Characteristic Modes (TCM) had its humble beginnings in the early 1970's. The beauty of TCM lies in its ability to fully characterize the radiation and scattering properties of an arbitrary object based only on the object's geometry and material properties. This ability provides valuable insights into an antenna's behavior independent of the feeding arrangement as well as providing information about how desirable radiation modes can be excited. This feature has led to its use to design integrated antennas in the High Frequency (HF) band for land vehicles, ships and aircraft. However, TCM had largely remained a specialist field until it was rediscovered for aiding the design of mobile handset antennas about a decade ago. In particular, TCM provides a powerful tool to understand and exploit excitation of the terminal chassis to enhance antenna performance. Another powerful feature of TCM is that multiple characteristic modes at a given frequency facilitate orthogonal radiation patterns, which provide effective Multiple-Input Multiple-Output (MIMO) antennas.