Hiroshi Shigesawa

Conductor-backed slot line and coplanar waveguide: dangers and full-wave analyses

Despite the many attractive features of conductor-backed slotline and coplanar waveguide, there are potential problems that are introduced by the use of conductor backing. These potential problems include leakage of power into surface waves or into the dielectric region between the plates, unexpected crosstalk, significant alteration of the guide wavelength, and unexpected or unwanted coupling to neighboring lines. Two theoretical approaches are presented, one purely numerical and the other in network form and analytical in nature, that agree well with each other and with measurements in a special case. They provide a quantitative description of these potential surprises when the line conductors are either infinite or finite in width.<<ETX>>

Multiband single-layer frequency selective surface designed by combination of genetic algorithm and geometry-refinement technique

This paper proposes an optimization-design method for the frequency selective surface (FSS) based on the genetic algorithm (GA) incorporated with a geometry-refinement technique. The present method takes the connectivity condition of the elements into consideration, thereby resulting in an easy fabrication. As an example, we design the multiband single-layer FSS for transmitting L band (1.5 GHz band) and S band (2.5 GHz band) and also reflecting Ka band (20/30 GHz band). The designed FSS has bandwidths broader than previous FSSs. Finally, the validity of the present method is proved by the agreement between the calculated and the measured transmission responses for the designed FSS.

Dominant mode power leakage from printed-circuit waveguides

This study is concerned with a new type of dominant mode power leakage from uniform lengths of printed-circuit waveguides, such as microstrip line, slot line, and coplanar waveguide. These leakage effects, which are presently not recognized and can therefore be unexpected as well as undesired, occur at higher frequencies, and are therefore important for millimeter-wave integrated circuits and high-speed circuits. The cross talk and coupling that result can ruin the performance of a complex, high-density circuit unless the leakage effects are understood and controlled. The leakage effects reported here occur on uniform lengths of printed-circuit waveguide on which the dominant mode is purely bound at ordinary microwave frequencies but becomes leaky at higher frequencies; they appear in addition to the leakage produced by conversion into surface waves at various discontinuities. The nature of the leakage is described, and numerical results are presented for several structures as illustrations of the variety of ways this type of leakage can arise.

The nature of the spectral gap between bound and leaky solutions when dielectric loss is present in printed‐circuit lines

For lossless printed-circuit transmission lines, it is known that a spectral gap is present at the transition in frequency between bound and leaky modal solutions, and that the solutions are nonphysical within that gap. This paper examines the corresponding modal behavior when the dielectric substrate material is lossy and reports several new interesting and important results. The new features become particularly clear when the data are displayed on the steepest-descent plane, which itself changes when loss is present and contains a new narrow spectral region that vanishes when the dielectric becomes lossless. Among the other new results we find that the leaky solution reverts to a bound solution at still higher frequencies, in contrast to the lossless case, and that the modal pole solutions are captured by the steepest-descent path at all frequencies, so that an explicit spectral gap is not found in the lossy case examined here.

Novel waveguide filters with multiple attenuation poles using dual-behavior resonance of frequency-selective surfaces

This paper develops novel waveguide filters consisting of frequency-selective surfaces (FSSs) in order to realize both the drastic size reduction and multiple attenuation poles in stopbands at both sides of passband. The FSS provides not only a passband but also attenuation-pole frequencies in stopbands since the FSS has both the aperture-element and the patch-element behaviors. In the present design method, the shape of each FSS is designed by a genetic algorithm so that the resonant curve of FSS can be fitted to that obtained from an equivalent-circuit approach. By using such FSSs and quarter-wavelength waveguides, a bandpass filter with six attenuation poles like an elliptic-function filter has been constructed. Furthermore, a technique of the size reduction by controlling adequately the resonant response of each FSS is presented. In this filter, the FSSs are closely located at the interval of the length much shorter than a quarter wavelength. It is shown from the design example that the half longitudinal length of the former example can be obtained, keeping both the passband response and attenuation poles in stopbands. The effectiveness of the waveguide filters with FSSs is validated by good agreement between the calculated and the measured results.

Simultaneous propagation of both bound and leaky dominant modes on conductor-backed coplanar strips

Contrary to expectations, the authors recently found that a new leaky, dominant mode may be present on conductor-backed coplanar strips at the same time as the conventional bound dominant mode. The frequency range over which both of these modes can propagate simultaneously increases as the strips become wider, and can be quite large even for fairly narrow strips. These new features, which could affect circuit performance significantly, have been derived theoretically and verified by measurements.<<ETX>>Contrary to expectations, we found recently that a new leaky dominant mode may be present on conductor-backed coplanar strips at the same time as the conventional bound dominant mode. The frequency range over which both these modes can propagate simultaneously increases as the strips become wider, and can be quite large even for fairly narrow strips. These new features, which could affect circuit performance significantly, have been derived theoretically and verified by measurements.

Compact low-cross-polarization horn antennas with serpentine-shaped taper

A compact horn with low cross polarization component less than -35 dB over 5% frequency bandwidth has been obtained by optimizing the taper configuration. To reduce the gain loss caused by spherical wavefront in a short length horn with a large flare angle, a serpentine-shaped taper successfully generates five higher-order modes. Such a taper configuration is represented by varying lengths of various uniform waveguides in stepwise approximation, while keeping the dimension of each step discontinuity constant. As a result, the computational time for obtaining the proposed horn is greatly reduced because scattering matrices of the step discontinuities are pre-calculated before the optimization. The verification of its high performance has been performed numerically and experimentally.

A new mode-coupling effect on coplanar waveguides of finite width

A novel mode-coupling effect that occurs on conventional coplanar waveguides of finite width is identified and explained. The coupling occurs between the standard CPW dominant mode and a newly identified dominant mode called the CPW dominant surface-wave-like mode. This coupling is different from that which is known to occur when a printed-circuit waveguide is placed in a box or package; here, there is no box, and the finite width by itself is sufficient to produce the coupling effect. Other physical effects are included in order to place the coupling effect in perspective.<<ETX>>

A compact low-cross-polarization horn antenna with serpentine-shaped taper

We propose a method to realize a novel multiple-mode conical-horn antenna. This antenna has a serpentine-shaped taper, which is quite effective to make the horn antenna compact in size and to realize a low cross polarization. Comparison between our horn antenna and the conventional ones is presented. We discus experimentally the test antennas of the X and U bands.

Negative coupling between TE/sub 10/ and TE/sub 20/ modes for use in evanescent-mode bandpass filters and their field-theoretic CAD

A new type of evanescent-mode bandpass filter is proposed, which consists of both main and subsidiary cavities operating in two nondegenerate modes and a dominant mode, respectively. These cavities are arranged to realize a high-performance bandpass filter of small size, by applying a field-theoretic CAD. Experiments prove our approach.<<ETX>>