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Directional couplers are fundamental components for the realization of power splitting and combining networks. A basic component is the four-port coupler realized in a wide range of different electrical and geometrical configurations. Using multiport directional couplers, with six or more ports, compact power splitting and combining networks can be designed. Most of the multiport directional couplers in a rectangular waveguide found in literature use an E-plane branch-line layout. A special configuration of a six-port narrow-wall short-slot directional coupler (Riblet type) is presented here. The coupler has only one central coupling region, following the Riblet concept based on the differences among the propagation constants of the modes through the coupling region. A number of different solutions have been investigated and some examples are presented here. Moreover, a multituning realized by using input resonators permits to significantly enlarge the bandwidth. A six-port coupler equally splitting on the output ports the power injected at each input port has been designed for a 8.4% working band. The theoretical and measured responses presented here prove the effectiveness of the multituning concept. Index Terms--Couplers, power combiners, wavegnide components.

We report on a time-domain analog in microwave lines to the spatial Fraunhofer (far-field) diffraction in paraxial conditions. Microstrip lines are used to design filtering configurations acting as spectrum analyzers. They are based on linearly chirped distributed Bragg coupling between the fundamental microstrip mode and the same but counterpropagating mode. Linearly chirped continuous impedance modulation in a microstrip line with varying upper plane strip-width will be shown to yield a mode-coupling location and group delay linearly distributed in frequency. Under the condition of a temporal equivalent to the spatial Fraunhofer inequality, the energy spectral density of the input signal is directly recoverable from the average output (reflected) power. It is only necessary to take into account a linear axis-change, given by the dispersion coefficient (group-delay slope) of the structure, from time to Fourier frequency. Both pulsed and nonpulsed RF signals are studied. Sequential time-gated segments of the input have to be processed in the nonpulsed case. The maximum frequency resolution achievable in this situation is discussed. The devices developed here could have important potential applications in the field of temporal signal processing, such as filtering using time-division techniques. Other application areas could be frequency-shift estimations using time-delay measurements in reflection, as well as the translation of path differences (temporal separation of signals) into frequency changes of an oscillatory interference pattern at the output. Index Terms--Characteristic impedance modulation, chirped Bragg coupling, dispersive delay line, Fourier-transform (FT) processors, microstrip technology, quadratic-phase filter.

In this paper, we propose a novel approach for the analysis of shielded microstrip circuits, composed of a number of thin metallic areas with arbitrary shapes and finite conductivity, embedded in a multilayered lossy medium. The analysis is based on the solution of an integral equation (IE) obtained by enforcing the proper boundary condition to the electric field on the metallic areas. The IE is solved by using the method of moments with entire domain basis functions, which are numerically determined by the boundary integral-resonant-mode expansion (BI-RME) method. The use of the BI-RME method allows for the efficient calculation of the basis functions independently on the shape of the domain, thus permitting the analysis of a wide class of circuits. Two examples demonstrate the accuracy, rapidity, and flexibility of the proposed method. Index Terms--Entire domain basis functions, integral equations, microstrip filters, MMICs, moment method.

The nature of the spectral gap at forward endfire for a periodic leaky-wave structure consisting of an infinite array of metallic strips on a lossless grounded dielectric layer is studied and compared with that for a simple grounded dielectric layer. The conclusions reached are expected to be valid for a general class of open periodic structures. One of the interesting features of the periodic structure is that a different branch choice is possible for each of the infinite number of space harmonics (although most of these will be nonphysical). This leads to an infinite number of steepest descent planes (SDP's) for the modal solutions, instead of only one as for the dielectric layer. As a result, one finds some interesting differences in the spectral-gap behavior, compared with that for the dielectric layer. One basic difference is that the nature of the spectral gap depends on whether or not a second space harmonic begins radiating before the main radiating harmonic is scanned to forward endfire. The spectral gap resembles that for a lossy dielectric layer when a second space harmonic is also radiating, and resembles that for a lossless dielectric layer otherwise. In addition, for the latter of these cases, the purely bound solution (which has a real propagation wavenumber and which occurs at the high-frequency end of the spectral gap) is physical only over a small frequency range, in contrast to the dielectric layer case, where that solution is present and physical at all higher frequencies. These behavioral differences are explained in detail in this paper. Index Terms - Leaky-wave antennas, leaky waves, periodic structures.

In this paper, two contributions to the study of the elliptical waveguides are given: an efficient technique to obtain the modal spectrum for elliptical waveguides and the analysis of the junctions between rectangular, circular, or elliptical to elliptical waveguides. In addition to theoretical results, experimental results are also presented finding good agreement between prediction and measurement.

In this contribution, the groundwork is laid out for the realization of efficient integral-equation/moment-method techniques, with arbitrary types of basis functions, for the computer-aided design (CAD) of geometrically complex packaged microwave and millimeter-wave integrated circuits (MMIC's). The proposed methodology is based on an accelerated evaluation of the Green's functions in a shielded rectangular cavity. Since the acceleration procedure is introduced at the Green's function level, it becomes possible to construct efficient shielded moment method techniques with arbitrary types of basis-functions. As an example, a Method of Moments (MoM) is implemented based on the mixed potential integral equation formulation with a rectangular, but nonuniform and nonfixed, mesh. The entire procedure can be extended to multilayer substrates.

Much effort has been devoted in the past to the development of electromagnetic simulation algorithms for waveguide junctions that could at the same time include higher-order mode interactions and lead to computationally efficient codes. Even though this is indeed a classical subject, there is still a strong interest in this area as waveguide systems become increasingly complex, and ever shorter development and manufacturing times are required. In this paper a simple method is described for the evaluation of the multimode network representation of planar waveguide junctions in terms of admittance or impedance parameters. The key feature of the method is that it starts from the wanted final results, the equivalent network representation, in order to obtain an analytic expression for the evaluation of the relevant matrix elements. The procedure is based on general network theory and is equivalent to ideally measuring directly the value of the admittance or impedance elements. In this paper the evaluation procedure is fully described. Measured results of actual hardware are then compared with simulations indicating that the codes developed are indeed very accurate as well as computationally very efficient.

Transmission zeros are usually implemented in microwave filters as extracted poles, or with cross couplings between nonadjacent cavities. Recent work, however, indicates that, for inductive band-pass filters, higher order-mode excitation can be usefully exploited for the purpose of creating transmission zeros. In this paper we describe a new cavity configuration that can be used to introduce transmission zeros in the electrical performance of microwave filters based on thick inductive windows in rectangular waveguides using the higher order-mode interactions. One transmission zero per filter cavity can be introduced and its frequency location can be easily controlled adjusting suitable geometrical parameters. The basic principle is discussed in detail and a computer aided design procedure is also presented. Finally, several application examples are included indicating how the new cavity design can indeed be used to improve the performance of this class of filters.

Several active bandpass filters for 1.8 and 5.5 GHz have been developed with the coplanar line technique using negative resistance circuits. For example, a first-order active bandpass filter at 5.5 GHz and two second-order bandpass filters at 1.8 GHz are presented, where one of the 1.8 GHz filters is a fixed center frequency filter and the other a tunable one. These 1.8 GHz filters have a 3 dB bandwidth of 110 MHz. The tuning range of the center frequency is 200 MHz.

The design of microwave filters and multiplexers requires a common step before actual hardware implementation, namely, the identification of a number of specific ideal electrical components with actual waveguide discontinuities. In the past, this step was accomplished semi-analytically using approximate single-mode discontinuity models. The complete filter or multiplexer could then be assembled, but additional final tuning or optimization with a large number of parameters was generally required. In this paper, we propose an alternative technique for the design of microwave filters or multiplexers that directly uses a full-wave electromagnetic simulator and that effectively decomposes the complex design task into a series of simple subtasks with clearly identified objectives. The procedure described only involves a limited number of actual physical parameters at each step so that it is computationally very efficient and rapidly convergent.

In this paper the multimode network formulation is used to characterize a multiple obstacle or aperture discontinuity in parallel plate waveguide. The procedure leads to a frequency-independent integral equation. This equation is solved numerically by using the Method of Moments, allowing for the implementation of a very general and efficient code. The frequency-independent integral equation formulation and its numerical solution are first outlined. Then, we discuss the convergency properties of the procedure, and compare its results with other theoretical results available.

Capacitive steps discontinuities are the basic components of many waveguide devices of common use, and several models have been developed for their characterizations. The multimode equivalent network formulation presented in this paper provides a new, simpler and more flexible representation of the step. The method leads to a frequency independent integral equation that is solved numerically by using the Method of Moments. From the solution, a frequency independent multimode impedance coupling matrix is computed. The coupling matrices of a number of cascaded steps can then be combined in a global matrix, so that a wide class of waveguide devices can be analyzed with only one inversion per frequency point. The results obtained from the method presented here are compared with measured data, showing very good agreement. A comparison of the run-times of the code developed with a typical mode-matching solution is performed, showing that the code based on the multimode network formulation is substantially more efficient.

- Discontinuities such as inductive steps are very frequently used in waveguide components and several descriptions can be found for them in the technical literature. These descriptions can be broadly divided into two groups. The first group can only take into account fundamental-mode interactions and is generally very efficient from a computational point of view. Representations of the second group can account for higher-mode interactions but are in general much less efficient numerically. In this paper we derive a novel, rigorous, multimode equivalent network representation for arbitrary inductive step discontinuities in rectangular waveguide that is computationally very efficient The coupling between the modal voltages and currents for all higher order modes explicitly included in the calculations is directly expressed in terms of an impedance coupling matrix that is essentially frequency independent The expression for the generic coupling matrix element is found in terms of the numerical solution of an integral equation. The novel contribution of this paper is in that it combines two powerful techniques, namely the Multimode Network Formulation of boundary-value problems and the Method of Moments for the solution of integral equations. The result is a very powerful tool that can be applied to the analysis of a wide class of discontinuities. In addition to the theoretical derivation, numerical results are also presented indicating that the theory developed is indeed very accurate and computationally efficient.

In this paper we present novel, rigorous, multimode equivalent network representations for a variety of zero-thickness capacitive windows and obstacles in a parallel plate waveguide. A key feature of these representations is that the coupling between all of the modes excited is described by a matrix whose elements do not depend on frequency or absolute dimensions. The value of the results presented is that the networks developed can be used to analyze rigorously a large variety of single and coupled planar transmission line structures.

A review of some of the activities of the European Space Agency in the area of Microwave Devices and Components for Space Application is presented. The activities discussed are in the areas of Solid State Devices and Technology, Travelling Wave Amplifiers, Microwave Filters and Time References. Each area is discussed in detail illustrating the achievements of on-going research and development programs involving European Industries and Research Centers. The growing importance of Microwave Technology for space applications is discussed and the future challenges outlined indicating how the European Space Industry is preparing for the future through the many R&D activities sponsored by the European Space Agency.

In this paper, we present a new parallel-coupled-line microstrip bandpass filter with suppressed spurious passband. Using a continuous perturbation of the width of the coupled lines following a sinusoidal law, the wave impedance is modulated so that the harmonic passband of the filter is rejected while the desired passband response is maintained virtually unaltered. This strip-width perturbation does not require the filter parameters to be recalculated and, this way, the classical design methodology for coupled-line microstrip filters can still be used. At the same time, the fabrication of the resulting filter layout does not involve more difficulties than those for typical coupled-line microstrip filters. To test this novel technique, order-3 Butterworth bandpass filters have been designed at 2.5 GHz with a 10% fractional bandwidth and different values of the perturbation amplitude. It is shown that for a 47.5% sinusoidal variation of the nominal strip width, a harmonic rejection of more than 40 dB is achieved in measurement while the passband at 2.5 GHz is almost unaltered. Index Terms--Bandpass filter, coupled-line filter, harmonic suppression, microstrip.

Coaxial waveguides are very commonly used in many microwave subsystems for the connection of various components. Significant size reduction could, therefore, be achieved by integrating the design of the coaxial transition in the computer-aided design (CAD) of microwave filters. In this context, we first discuss in this paper a coaxial transition, which can be efficiently designed interactively using an accurate and efficient CAD procedure. The key elements of the CAD tool developed are then described and a specific example of a microwave filter is discussed. In addition to theory, measured results are also presented, thereby fully validating both the CAD tool and the structures proposed. Index Terms--Coaxial waveguides, electromagnetic launching, waveguide excitation, waveguide filters.

Safety issues must be seriously considered in the practical implementation of microwave industrial systems with open ports. To preserve the radiation of these open-ended waveguide systems into permissible levels, bandstop microwave filters are widely used. In this paper, the accurate analysis, design, and experimental verification procedure of such filters are extensively studied. A singly and a doubly corrugated filter for continuous flow microwave industrial systems are designed. Two prototypes of such devices have been manufactured and experimentally verified. Index Terms--Admittance matrix representation, bandstop filters, filter design, microwave measurements, safety.

To meet the increasing demand for high-performance filters in gigahertz radio-communication systems, we have improved the design techniques and fabrication processes for surface-acoustic wave (SAW) devices. The standard optical projection printing technique based on Mine lithography used for mass-production was optimized, thus attaining a linewidth of 0.3 [micro]m. As a first SAW device prototype, we designed and fabricated a ladder-type bandpass filter from LiTa[O.sub.3] substrates, at 3.15 GHz having a 3 dB-bandwidth of 128 MHz and a minimum insertion loss of 1.7 dB. Index Terms--Ladder-type filter, SAW process technology, surface-acoustic wave filters.

In this paper, we describe a new family of dual-mode filters that is based on the use of simple inductive discontinuities in a rectangular waveguide environment. The proposed filter structure can be analyzed and optimized very efficiently using multimode equivalent network representations, thus leading to a simple and rapid development procedure. In addition to theory, the measured performance of a number of filter structures is also presented, thereby fully validating the proposed filter concept. Index Terms--Dual-mode filters, EM simulation, inductive filters.

In this paper, we describe a very accurate and computationally efficient computer-aided design (CAD) tool for the analysis and design of a wide class of boxed microwave circuits composed of arbitrary rectangular elements printed on dielectric layers. The theoretical derivations are based on an integral equation formulation, and call for the evaluation of the boxed multilayer Green's functions, thus leading to a tool that is valid for an arbitrary number of circuits and dielectric layers. In addition to theory, comparisons with measured results are presented, and several practical filter structures are also investigated, thus clearly demonstrating that the CAD tool developed can indeed be used very effectively for the design of a large variety of microwave circuits. Index Terms - Broadside coupled, integral equation, microwave filters, modal expansions, shielded printed circuits, transmission zeros.

A flexible and efficient numerical method is presented by which the multimode network representation (MMNR) of the abrupt transition between a standard waveguide and a waveguide of arbitrary cross section can be established without reference to the cross-sectional fields. The approach combines the boundary integral-equation method (BIEM) with contour integral expressions for the coupling coefficients, and a novel highly efficient scheme to express modal normalization constants in terms of coupling coefficients and eigenvalues. Application is demonstrated for a variety of multiridge circular waveguide (MRCW) configurations and transitions between MRCW and circular waveguide (CW). Comparison is made against most published results for this problem. Index Terms--Boundary integral equations, circular waveguides, dual-mode waveguide filters, mode-matching methods, waveguide discontinuities.