To meet the rapidly increasing demand for advanced microwave components and antennas in wireless terrestrial and satellite-based communications, accurate and efficient CAD tools are critical. Due to efforts made during the 1990s - the decade of global 3D electromagnetic (EM) field solvers - the task of accurate analysis of components can be considered as being largely solved.

Therefore, current challenges for CAD software relate to a new quality of efficiency and speed. Communication industries demand a drastic reduction in time-to-market, where only highly optimized microwave components/systems will meet the tight requirements of current and next generation applications. In this context, fast EM-based CAD tools are desired, yielding accurate, optimized designs within extremely short response times. To achieve this requires flexible hybrid solutions, which typically can go beyond the possibilities of single global 3D solvers.

The WASP-NET CAD and optimization tool meets these demands by the implementation of fast hybrid techniques. Based on the hybrid mode-matching (MM)/finite element (FE)/ method-of-moment (MoM)/finite difference (FD) method, it combines the advantages of all four field solvers in one single CAD tool, namely the efficiency of MM with the flexibility of the FE, MoM and FD techniques.

Hybrid Methods

Due to its efficiency, the MM method has been widely employed for designing waveguide components, where modal expansions of electromagnetic fields can be derived analytically. For structures that are not separable in Cartesian or cylindrical coordinate systems, FE, MoM and FD techniques employ space- or surface-discretization techniques, which provide the desired flexibility. However, because of rather high requirements concerning central processing unit (CPU) time for the CAD of components using pure FE, MoM, or FD methods, for reasonably fast optimizations of typical industrial components, which often require a high number of iterations to meet given specifications, more advanced techniques are desirable.

The most effective approach for solving this problem is utilizing hybrid methods based on all four field simulators directly within the applied CAD technique, hence retaining the specific advantages of all proven EM methods while largely avoiding their disadvantages. WASP-NET fully exploits this efficiency and flexibility potential.

In spite of its sophisticated computational methods, using the tool is an easy task. An intuitive Windows- or Linux-based graphical user interface (GUI) provides the necessary information during simulation, synthesis and optimization. The user can select adequate elements (for example, waffle-iron and step elements) from an element library currently comprising over 130 elements. These include irises, junctions, steps, bends, corners, crosses, apertures and coax-feeds. The desired components are built by combining the elements concerned - also via subcircuits - using sections of (rectangular, circular, coaxial and ridged) waveguides of any length (including zero). A synthesis wizard starting simply from desired specifications automatically generates often-required components, such as filters or transformers.

The new visualization tool WNVisT shows the created structures (either single elements, subcircuits, or as a whole) immediately on the screen. Common 2D and 3D output formats (DXF, OFF, STL, SAT) are suitable for further postprocessing in existing CAD environments.

Accuracy

In particular for space applications, highly optimized performance and accurate prediction of all S-parameters (often spread across large limits) are of paramount importance. Therefore, a full-wave generalized S-matrix (GSM) combination of all individual elements taking into account all required higher order modes yields high accuracy and the necessary high dynamic range. This is demonstrated with the example of a waffle-iron filter (Figure 1 ), which has been fabricated without any tuning screws (optimizing efficiency: ca 0.12 s/fp, 209 modes, using a 2 GHz P4 PC).

Furthermore, the user-friendly full-wave filter wizard provides synthesis results for all typical cavity coupled waveguide filters with all common kinds of coupling structures, such as metal insert, single and multiple posts, all irises, and also with finite radii either in the H- or E-plane, according to given specifications. The procedure is to enter the specifications, select filter type, run the wizard and get the results (structure macro, geometries, S-parameters, output files for further postprocessing), which will typically be done within a few minutes.

Figure 2 shows the application example of a double post-coupled filter (with identical post diameters for low cost fabrication). The half filter is visualized (ca 0.4 s/fp, 27 modes). The posts are modeled by a new fast and flexible hybrid MM/TransFE method, that is, the combination of MM with a 2D FE method, which yields the solution for the complete class of planar waveguides of arbitrary contour. The created filter can be stored in subcircuits for further optimization and applications (for diplexer designs, for example).

Arbitrary Structures

Many waveguide components involve structures of arbitrary cross-section, such as waffle-iron, cross-iris/coupling structures for dual-mode filters, or septum polarizers. WASP-NET solves the cross-section eigenvalue problem numerically by a specialized 2D FE method. Matching the transverse fields at the discontinuities produces the rigorous generalized S-matrix to be combined with those of other elements of the library.

The CAD tool's methods include coaxial structures of arbitrary contour. By coax-feeds providing electric (disc, metal sheet), inductive (probe) and direct post connected couplings, and by rectangular post elements, for example, it achieves the fast CAD and optimization of rectangular combline filters/diplexers, which can be fabricated by low cost milling techniques. Combinations with typical elements such as irises, corners and bends, provide the necessary high flexibility for realistic designs.

Corrugated Horns

The hybrid MM/FE/MoM/FD technique includes rigorous electric field integral equation (EFIE) and combined source integral equation (CSIE) algorithms for accurate horn calculations, involving the outer horn contour (with aperture chokes, for example) for precise cross-polarization predictions. Complicated mode-launchers are optimized via the generalized S-matrix combining the inner and outer horn structures.

Figure 3 shows the S-parameter results of an optimized dual-band corrugated horn structure (about 140 corrugations in horn taper) together with a mode-launcher containing ring-loaded slots. The horn, together with the mode-launcher, have been optimized (overnight run, ca 5000 iterations, 12 hours, 44 modes) for two frequency bands concerning the fundamental H_11c (-40 dB) and higher order E_11s modes (-30 and -25 dB, respectively).

For 3D microwave structures, such as dielectric resonator filter elements, round partial height posts, interdigital filter circular resonators, lateral coax-feeds and waffle-iron filter sections with round posts, adequate finite difference time domain (FDTD) elements are included, which are combined via the generalized scattering matrix with other WASP-NET elements.

The FDTD method applies advanced subgrid techniques combined with a locally conformal algorithm and a modified intersection approach for the efficient inclusion of arbitrarily shaped metal and dielectric surfaces. Figure 4 shows a lateral coax-fed H-iris filter cut in E-plane with finite radii, where the degradation in return loss caused by the feed can be accurately taken into account (ca 5 min., 500 fp for feed, ca 0.3 s/fp for filter, 52 modes).

Conclusion

Current challenges for EM-based CAD software for microwave components relate to a new quality of efficiency and speed. The direction that CAD tools are taking is towards hybrid methods. The latest WASP-NET CAD and optimization tool addresses these challenges by utilizing fast hybrid methods and employing all four common solvers in one single tool, thus exploiting the benefits of using the efficiency of the mode-matching method together with the flexibility of the FE/MoM/FD techniques.

MIG, Microwave Innovation Group GmbH & Co. KG,
Bremen, Germany, + 49 (421) 220 8299,
mig@bitz-bremen.de, www.mig-germany.com.

Circle No. 301