Takeshi Fukusako

The authors propose a mode conversion technique in a cavity-backed circularly polarized magneto-electric (ME) antenna in order to achieve a wide axial ratio (AR) beamwidth covering a wide frequency range. The proposed technique allows both the ME antenna element and the cavity structural parameters to be independent of each other, which would otherwise always be associated with each other. The proposed antenna demonstrated impedance bandwidth (|S 11 | ≤−10 dB) of 38.73% (1.52–2.25 GHz), 3-dB axial ratio bandwidth of 33.05% (1.49–2.08 GHz), and 3-dB AR beamwidth exceeding 214° in a single plane (xz or yz-plane) across the operating band.

This paper presents a new concept of designing circularly polarized folded reflectarray (CPFRA) antennas. By applying the dual-reflector shaping technique to the CPFRA antennas, the reflection-phase distributions of the two reflecting surfaces comprising the CPFRA antennas are modified to realize low profile, high aperture efficiency, and arbitrary aperture-field distribution simultaneously. Numerical simulations based on the geometrical optics show that the aperture efficiency can be improved up to around 100% by realizing uniform aperture-field distribution and reducing the spillover. Designs of CPFRA antennas with low side-lobe levels are also presented.

This research proposes omnidirectional circularly polarized monopole antennas (OCPA) with single-annular (SA-AMC) and dual-annular artificial magnetic conductor (DA-AMC) ground planes. The aim of the proposed antenna scheme is to circumvent the design rigidity of conventional OCPA whose inherent shortcoming is fixed bottom ground plane size. The OCPA with SA-AMC ground plane consists of a monopole element, four diagonally adjoined parasitic elements to covert linear to circular polarizations, and an SA-AMC ground plane. The OCPA with DA-AMC ground plane consists of a monopole element, four diagonally adjoined parasitic elements, and a DA-AMC ground plane. Unlike the conventional OCPA, the bottom ground plane radius ( R _g ) of the OCPA with SA- and DA-AMC ground planes can be varied between 18.4 – 188.4 mm; and 29.2 – 119.2 mm, respectively, without affecting the omnidirectionality and impedance and axial ratio (AR) bandwidths of the OCPA. Furthermore, the SA- and DA-AMC ground planes effectively improve the impedance (| S ₁₁ | ≤ –10 dB) and AR bandwidths (AR ≤ 3 dB) while reducing leakage current on the coaxial feeder cable. The measured | S ₁₁ | and AR bandwidths of the OCPA with SA-AMC ground plane, given the optimal R_g of 18.4 mm, are 8.08% (5.34 – 5.79 GHz) and 4.38% (5.36 – 5.60 GHz), while those of the OCPA with DA-AMC ground plane, given the optimal R _g of 29.2 mm, are 14.37% (5.10 – 5.89 GHz) and 19.85% (4.90 – 5.98 GHz). The novelty of this research lies in the use of SA- and DA-AMC structures to circumvent the design rigidity of the conventional OCPA. Essentially, the integration of the AMC structures offers design flexibility in the realization of OCPA.

A conical cavity-backed antenna with embedded multiple quarter-wave resonators (multiple QWRs) is proposed. The 3-dB axial ratio bandwidth (ARBW) of a cavity-backed antenna is limited by cavity resonance, undesired cavity current, and circular polarization (CP) behavior of the radiator. As a result, the cavity-backed antenna exhibits a high AR level and narrows the 3-dB ARBW, even though a high antenna gain and front-to-back ratio remain. In this study, a simple crossed-dipole was used to excite a conical cavity with four embedded multiple QWRs. The multiple QWR consists of two metallic plates and a cavity rim wall, which generate an additional current to support CP radiation. A conical cavity with embedded multiple QWRs can reduce the AR level and enlarge the 3-dB ARBW by preventing cavity resonance which creates a strong electric field inside the cavity, minimizing the effect of the undesired cavity current that radiates the cross-polarized wave, and further adding the CP mode to the antenna. The prototype realized a wide impedance bandwidth of 93.83% (1.72–4.76 GHz) and wide axial ratio bandwidth of 83.75% (1.86–4.54 GHz). The maximum gain was 9.9 dBic at 4.18 GHz. For the case where the radiator cannot be changed, this study demonstrates a cavity approach utilizing a microwave structure (multiple QWR) to reduce the AR level and maximize the 3-dB ARBW of a cavity-backed antenna without radiator modification.

This study described a technique to compensate for the difference in reflection phase characteristics yielded by a finite-size meta-surface (MS) to render them identical to the reflection characteristics analyzed using the unit cell model corresponding to the infinite MS structure. Thus, an MS with a backing conductor having notches and slots to compensate for the reflection phase affected by the complex diffraction at the edge, was proposed. The notches and slots’ mechanism inducing the inductive reactions was discussed based on the current distributions. The proposed and conventional MSs were fabricated to verify the deterioration and compensation effect; the former was consistent with the designed performance based on the unit cell analysis corresponding to the infinite structure.

This talk presents bandwidth enhancement technique of axial ratio (AR) in 1 -point-fed circularly polarized (CP) microstrip antennas using metasurface (MS). In these structures, MSs consisting of a rectangular unit cell have been used for widening the impedance bandwidth in addition to the AR bandwidth. The author called this structure artificial ground structure (AGS) for the dual purpose. The AGS has been installed underneath the patch antenna with two truncated corners, and the substrates have a relative permittivity of ε r = 2.2 and loss tangent of tan δ = 0.001 (ROGERS RT/Duroid 5880). The AGS has the function of an AMC for expanding the -10-dB S 11 bandwidth. Also, the AGS can convert the incident wave of linear polarization (LP) or elliptical polarization (EP) to CP reflected wave. This mechanism can extend the AR bandwidth. However, this expansion is sometimes limited by resonances in related to the outer circumference of the MS structure. If the circumference is rectangular, that is the dimension along x is different from that along y, an extra difference of phase causes in x- and y- components of the radiated field resulting in deteriorated AR.

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This paper presents a broadband circularly polarized patch antenna with an artificial ground structure (AGS), which has rotated rectangular unit cells with respect to the sides of the rectangular ground plane, as a polarizer. A two-element ( $2\times 1$ ) array configuration of the antenna is also presented. The rectangular unit cells are rotated by 45° with respect to the sides of a square dielectric substrate, named a diamond-shaped AGS (DAGS). The overall dimensions of the AGS are smaller than those of the ground plane. The rotated unit cells and the smaller AGS patch dimensions effectively suppress surface wave resonance on the AGS. The proposed antenna achieves a wide 3-dB axial ratio bandwidth in the boresight direction. The proposed design has a flatter gain curve in the operating frequency band compared to that for conventional designs. It is found that the array configuration outperforms the single-element structure.

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An off-line coherent base station scanner has been proposed for evaluating vehicular MIMO antennas by utilizing the reference signal present in the downlink signal from the LTE base station. However, this system does not support TDD-LTE, and post-processing takes too long. This report introduces a coherent base station scanner capable of real-time FDD and TDD demodulation; it is verified it in outdoor environments. A prototype demonstrates good results which confirm the effectiveness of the proposal.