A novel directional coupler loaded with feedback capacitances on the coupled lines is presented. Its effect of enhancing the coupling is qualitatively shown by deriving an equation for the coupling. Besides, a method to compensate for the phase difference between the even and odd modes of the coupler is presented. To demonstrate, a novel tandem 3-dB coupler consisting of the proposed coupled lines is designed and described. In addition, a waveguide (rectangular coaxial line) 8×8 HYB matrix using planar double-layer structure that is composed of the proposed tandem 3-dB couplers and branch-line couplers, which is operated in S-band, is designed and fabricated showing excellent performance.

In this paper, a measurement method for the impedance and mutual coupling of multi-antennas that we have proposed is summarized. Impedance and mutual coupling characteristics are obtained after reducing the influence of the coaxial cables by synthesizing the measured S-parameters under the condition that unbalanced currents on the outside of the coaxial cables are canceled at feed points. We apply the proposed method to two closely positioned monopole antennas mounted on a small ground plane and demonstrate the validity and effectiveness of the proposed method by simulation and experiment. The proposed method is significantly better in terms of the accuracy of the mutual coupling data. In the presented case, the errors at the resonant frequency of the antennas are only 0.5dB in amplitude and 1.8° in phase.

We propose a simple and small phase shifter for a beam-steerable base-station antenna. This phase shifter has no metallic heterojunction, and the phase shift is controlled by moving an M-shaped dielectric plate between the strip conductor and the ground plane of a strip line. We derive a design equation from the condition that at the center frequency f0, the reflection coefficient = 0. In this phase shifter, the reflection coefficient becomes minimum at f0 regardless of the movement distance, r, of the dielectric plate, and the relationship between the phase shift and r is linear. These characteristics are verified by performing simulations and measurements. The size of the M-shaped dielectric phase shifter is 0.27λ00.12λ0, where λ0 is the free-space wavelength at f0. The insertion loss is smaller than about 0.2 dB within a fractional bandwidth of 10%, and the phase shift can vary from 0 to about 80 degrees.

In this paper we propose a new far-field RCS prediction method using cylindrical or planar near-field RCS data. First we derive the relation between RCS and the scattering coefficient using physical optics technique. The far-field RCS prediction algorithm is obtained by approximating the relation using the condition of Fresnel region and the paraxial constraint of scanning angle in the case of cylindrical or planar scanning. Finally we predict the far-field RCS using measured or calculated near-field RCS data of the conducting rectangular prism or plate. The validity of the proposed algorithm is demonstrated.

In this paper, an analysis method for calculating balanced and unbalanced modes of a small antenna is summarized. Modal condactances which relate dissipated power of the antenna are directly obtained from standard S-parameters that we can measure by a 2-port network analyzer. We demonstrate the validity and effectiveness of the proposed method by simulation and measurement for a dipole antenna with unbalaned feed. The ratio of unbalanced-mode power to the total power (unbalanced-mode power ratio) calculated by the proposed method agrees precisely with that yielded by the conventional method using measured radiation patterns. Furthermore, we analyze a small loop antenna with unbalanced feed by the proposed method and show that the self-balancing characteristic appears when the loop is set in resonant state by loading capacitances or the whole length of the loop is less than 1/20th the wavelength.

A method is proposed for improving the accuracy of the characteristic basis function method (CBFM) using the multilevel approach. With this technique, CBFs taking into account multiple scattering calculated for each block (IP-CBFs; improved primary CBFs) are applied to CBFM using a multilevel approach. By using IP-CBFs, the interaction between blocks is taken into account, and thus it is possible to reduce the number of CBFs while maintaining accuracy, even if the multilevel approach is used. The radar cross section (RCS) of a cube, a cavity, and a dielectric sphere were analyzed using the proposed CBFs, and as a result it was found that accuracy is improved over the conventional method, despite no major change in the number of CBFs.

This paper proposes a dual linear-polarized open-ended waveguide subarray designed for use in phased array antennas. The proposed subarray is a one-dimensional linear array that consists of open-ended waveguide antenna elements and suspended stripline feed networks to realize vertical and horizontal polarizations. The antenna includes a novel suspended stripline-to-waveguide transition that combines double- and quad-ridge waveguides to minimize the size of the transition and enhance the port isolation. Metal posts are installed on the waveguide apertures to eliminate scan-blindness. Prototype subarrays are fabricated and tested in an array of 16 subarrays. The experimental tests and numerical simulations indicate that the prototype subarray offers a low reflection coefficient of less than -11.4dB, low cross-polarization of less than -26dB, and antenna efficiency above 69% in the frequency bandwidth of 14%.

Owing to their ultra-wideband characteristics, tapered slot antennas (TSAs) are used as element antennas in wideband phased arrays. However, when the size of a TSA is reduced in order to prevent the generation of a grating lobe during wide-angle beam scanning, the original ultra-wideband characteristics are degraded because of increased reflections from the ends of the tapered slot aperture. To overcome this difficulty, we propose a new antenna structure in which parallel-plate waveguides are added to the TSA. The advantage of this new structure is that the reflection characteristics of individual antenna elements are not degraded even if the width of the antenna aperture is very small, i.e., approximately one-half the wavelength of the highest operating frequency. In this study, we propose a procedure for designing the new antenna through numerical simulations by using the FDTD method. In addition, we verify the performance of the antenna array by experiments.

In this paper, the performance of the induced dimension reduction (IDR) method implemented along with the method of moments (MoM) is described. The MoM is based on a combined field integral equation for solving large-scale electromagnetic scattering problems involving conducting objects. The IDR method is one of Krylov subspace methods. This method was initially developed by Peter Sonneveld in 1979; it was subsequently generalized to the IDR(s) method. The method has recently attracted considerable attention in the field of computational physics. However, the performance of the IDR(s) has hardly been studied or practiced for electromagnetic wave problems. In this study, the performance of the IDR(s) is investigated and clarified by comparing the convergence property and memory requirement of the IDR(s) with those of other representative Krylov solvers such as biconjugate gradient (BiCG) methods and generalized minimal residual algorithm (GMRES). Numerical experiments reveal that the characteristics of the IDR(s) against the parameter s strongly depend on the geometry of the problem; in a problem with a complex geometry, s should be set to an adequately small value in order to avoid the "spurious convergence" which is a problem that the IDR(s) inherently holds. As for the convergence behavior, we observe that the IDR(s) has a better convergence ability than GPBiCG and GMRES(m) in a variety of problems with different complexities. Furthermore, we also confirm the IDR(s)'s inherent advantage in terms of the memory requirements over GMRES(m).

We investigate a phased array-fed dual reflector antenna applying one-dimensional beam-scanning of the center-fed type, using an elliptical aperture to provide wide area observation. The distinguishing feature of this antenna is its elliptical aperture shape, in which the aperture diameter differs between the forward satellite direction and the cross-section orthogonal to it. The shape in the plane of the forward satellite direction, which does not have a beam-scanning function, is a ring-focus Cassegrain antenna, and the shape in the plane orthogonal to that, which does have a beam-scanning function, is an imaging reflector antenna. This paper describes issues which arose during design of the elliptical aperture shape and how they were solved, and presents design results using elliptical aperture dimensions of 1600 mm × 600 mm, in which the beam width differs by more than two times in the orthogonal cross-section. The effectiveness of the antenna was verified by fabricating a prototype antenna based on the design results. Measurement results confirmed that an aperture efficiency of 50% or more could be achieved, and that a different beam width was obtained in the orthogonal plane in accordance with design values.

We propose a new configuration for phased array antennas. The proposal uses radiation pattern reconfigurable antennas as the antenna element to improve the gain on the scanning angle and to suppress the grating lobes of sparse phased array antennas. This configuration can reduce the element number because the desired gain of the total array can be achieved by using fewer elements. We demonstrate the concept by designing a radiation pattern reconfigurable Yagi-Uda antenna. PIN diode switches are added to the parasitic elements to change director and reflector. The switches of multiple array elements are concurrently controlled by just a single one-pair line. This control structure is simple and can be applied to large-scale arrays. The proposed antenna yields an element gain that almost matches the theoretical limit across about half the coverage, even if the element spacing is enlarged to 1λ. If the switch states are interchanged, the gain in the mirror direction can be increased. We design a 48-element array and compare its gain against those of normal dipole antennas. We also fabricate the proposed antenna and demonstrate radiation pattern switching.

This paper presents flexible inner-outer Krylov subspace methods, which are implemented using the fast multipole method (FMM) for solving scattering problems with mixed dielectric and conducting object. The flexible Krylov subspace methods refer to a class of methods that accept variable preconditioning. To obtain the maximum efficiency of the inner-outer methods, it is desirable to compute the inner iterations with the least possible effort. Hence, generally, inaccurate matrix-vector multiplication (MVM) is performed in the inner solver within a short computation time. This is realized by using a particular feature of the multipole techniques. The accuracy and computational cost of the FMM can be controlled by appropriately selecting the truncation number, which indicates the number of multipoles used to express far-field interactions. On the basis of the abovementioned fact, we construct a less-accurate but much cheaper version of the FMM by intentionally setting the truncation number to a sufficiently low value, and then use it for the computation of inaccurate MVM in the inner solver. However, there exists no definite rule for determining the suitable level of accuracy for the FMM within the inner solver. The main focus of this study is to clarify the relationship between the overall efficiency of the flexible inner-outer Krylov solver and the accuracy of the FMM within the inner solver. Numerical experiments reveal that there exits an optimal accuracy level for the FMM within the inner solver, and that a moderately accurate FMM operator serves as the optimal preconditioner.

An approximate formula is proposed for the equivalent susceptance of a circular iris in a parallel plate waveguide when the TEM mode cylindrical wave is incident from the center of the iris. Schwinger's variational method for a linear iris is generalized to the cylindrical case, and an approximate closed form formula is obtained which recovers the result of the linear iris when the radius of the circular iris is sufficiently larger than the wavelength. For verification of the formula, an exact integral equation is formulated and solved numerically by Galerkin's method. A comparison between them shows good agreement.

We propose a novel improved characteristic basis function method (IP-CBFM) for accurately analysing the radar cross section (RCS). This new IP-CBFM incorporates the effect of higher-order multiple scattering and has major influences in analyzing monostatic RCS (MRCS) of single incidence and bistatic RCS (BRCS) problems. We calculated the RCS of two scatterers and could confirm that the proposed IP-CBFM provided higher accuracy than the conventional method while significantly reducing the number of CBF.

This letter presents the construction and design equations of a lumped element Wilkinson divider with dual-band operation. This divider is constructed of series and parallel LC resonant circuits, and an isolation resistor. The element values can be uniquely determined by giving the two frequencies for operation as a Wilkinson divider and the load resistance. An 800 MHz/2 GHz dual-band Wilkinson divider is treated as a design example, and its operation is verified by simulation and experiment. The fabricated divider has compact dimensions of 3.564 mm2.

The method for estimating propagation loss that classifies receiving points into multiple groups by focusing on the number of reflections and diffractions, and applies a separate statistical model to each group was extended from only 2.4 GHz band to both 2.4 GHz and 5 GHz band. The extended statistical model was created from received power measurements. First, an appropriate grouping method was investigated based on the fitting error of statistical model. Non-line-of-sight (NLOS) receiving points were grouped in order of points that a wave reflected one time reaches, points that a wave reflected two times reaches, and points that a wave diffracted one time reaches. Next, the effectiveness of the proposed method was verified by comparison with conventional statistical models (one-slope, dual-slope, multi-wall, partitioned) on three office floors that differ from the environment used to create the statistical model. The average NLOS estimation error for the three evaluation environments was 4.9 dB, demonstrating that the proposed method has accuracy equal to or better than that of conventional methods.

We present design procedures for using conducting wires in A-sandwich junctions to achieve high transmission performance; bench-test results validate the procedures. The scattering characteristics of the junction are obtained by solving the electric field integral equation of volumetric equivalent currents. The transmission performance is evaluated by subtracting the scattered fields of the same-sized A-sandwich panel in order to offset the effect of edge diffraction. Optimum wire width is determined by examining transmission performance with different arrangements. The designed junction achieves high transmission performance. The measured scattering characteristics of a bench model demonstrate the validity of the presented method.

In this study, we demonstrate an acceleration of flexible generalized minimal residual algorithm (FGMRES) implemented with the method of moments and the fast multipole method (FMM), based on a combined tangential formulation. For the implementation of the FGMRES incorporated with the FMM concept, we propose a new definition of the truncation number for the FMM operator within the inner solver. The proposed truncation number provides an optimal variable preconditioner by controlling the accuracy and computational cost of the inner iteration. Moreover, to further accelerate the convergence, we introduce the concept of a multistage preconditioner. Numerical experiments reveal that our new version of FGMRES, based on the proposed truncation number for the inner solver and the multistage preconditioner, achieves outstanding acceleration of the convergence for large-scale and practical electromagnetic scattering and radiation problems with several levels of geometrical complexity.

We propose a novel phased array-fed dual-reflector antenna that reduces performance degradation caused by multiple reflection. The marked feature of the proposed configuration is that different reflector profiles are employed for the two orthogonal directions. The reflector profile in the beam-scanning section (vertical section) is set to an imaging reflector configuration, while the profile in the orthogonal non-beam-scanning section (horizontal section) is set to a ring-focus Cassegrain antenna configuration. In order to compare the proposed antenna with the conventional antenna in which multiple reflection was problematic, we designed a prototype antenna of the same size, and verified the validity of the proposed antenna. The results of the verification were that the gain in the designed central frequency increased by 0.4 dB, and the ripple of the gain frequency properties that was produced by multiple reflection was decreased by 1.1,dB. These results demonstrated the validity of the proposed antenna.

Radar Cross Section (RCS) can be obtained from near-field data by using near-field to far-field RCS transformation methods. Phase errors in near-field data cause the degradation of the prediction accuracy. In order to overcome the difficulty, we propose the far-field RCS prediction method from one-dimensional intensity data in near-field. The proposed method is derived by extending the phase retrieval method based on the Gerchberg-Saxton algorithm with the use of the relational expression between near-fields and scattering coefficients. The far-field RCS can be predicted from the intensity data of scattered fields measured at two different ranges. The far-field RCS predicted by the proposed method approximately coincides with the computed one. The proposed method also has significant advantages of simple and efficient algorithm. The proposed method is valuable from a practical point of view.