With greater emphasis on accuracy and the ability to meet tight specifications the necessity for microwave components and antennas to be optimized is becoming increasingly mission-critical, especially for modern satellite-based and wireless terrestrial communication applications. Such challenging optimization tasks demand rigorous electromagnetic (EM) CAD tools, which are accurate, flexible and, in particular, so efficient that the high number of optimization iterations that are commonly required can be carried out within reasonably short design times.

Common EM simulation using single global three-dimensional (3D) solvers – mostly based on finite-element (FE) or finite difference (FD) techniques – do yield the desired flexibility but can require long design and optimization times. On the other hand, mode-matching (MM) solvers and their extensions can provide efficiency but may not be so flexible.

With its combination of methods Microwave Innovation Group’s (MiG) established hybrid WASP-NET® EM CAD and optimization tool uses fast hybrid MM/FE/MoM/FD techniques, which typically go beyond the capabilities of single/dual methods. For antenna structures, method-of-moment (MoM) algorithms are implemented, as they do not require absorbing boundaries.

The new version 8.0 expands the tool’s capabilities even further. Particular advances include a new fast finite-element/boundary integral (FE-BI) solver and a new integral equation (IE) solver. The FE-BI solver is primarily effective for arbitrary 3D waveguide, antenna and microstrip structures (including losses and anisotropic materials). The IE solver is primarily appropriate for large-scale, open, scattering and radiating structures, even when having multiple excitations.

When using version 8.0, EM simulation times, including for general 3D structures, can typically be reduced to only a few seconds on standard low-cost dual quad core PCs, which is of great advantage for design engineers. A significant feature is that efficient EM optimizations of arbitrary 3D structures and large antennas can now be carried out directly, and thus within very short design cycles.

BACKGROUND THEORY

WASP-NET’s new FE-BI solver, which is based on the hybrid combination of the FE and the BI methods, favorably combines (like the already implemented MM/FE/MoM/FD methods) the advantages of the involved methods: flexibility and accuracy with efficiency and high dynamic range. Moreover, the FE-BI solver utilizes appropriate analytical solutions (so-called Green’s functions) wherever possible. In this way, the required discretization of the computational domain can be drastically reduced. This special FE-BI method results in significant speed improvements, particularly for general, arbitrarily shaped 3D structures.

For large-scale antenna and scattering problems, the required size of the matrix equations of IE methods can be very large. WASP-NET’s new IE solver applies advanced techniques, such as the multi-level fast multipole method (MLFMM) and the adaptive integral method (AIM). For efficient solutions of problems, where many modes (excitations) are involved, e.g. for antenna structures fed by horn antennas, the new IE solver additionally utilizes a fast iterative-free technique, which is highly efficient. This is also the case when dealing with many ‘right-hand sides’ of the involved equation system.

EXAMPLES

Microwave engineers are often concerned with design solutions for special low-cost microwave components, which mostly result in complex 3D structures, as specifically tailored fabrication techniques have to be taken into account. Combline filters fabricated in die cast techniques (see Figure 1) are typical examples: the structure requires draft angles and rounded corners for the structure to be easily extractable from the mold.


Fig 1. WASP-NET’s new fast 3D FE-BI solver applied for the design and optimization of combline filters with draft angles. Calculation time: 0.6 seconds per frequency point (2QC Xeon, 3 GHz, PC)

With the new FE-BI solver, the direct full-wave rigorous EM based optimization of such general 3D structures is now possible within an extremely short design time. For instance, WASP-NET’s calculation speed for the structure shown in Figure 1 is ca. 0.6 seconds per frequency point (on a 2QC Xeon 3 GHz PC). The calculation time can even be further reduced by a factor of about 30 when applying WASP-NET’s powerful ultra-adaptive sweep technique.

The next example is a compact, low-insertion loss filter with stop-band poles (Figure 2), for which measurements are available courtesy of A1microwave.com. In order to demonstrate the flexibility and speed features of WASP-NET’s global 3D FE-BI solver for common typical global 3D solver applications, the filter 3D CAD data file (which can be given in, STEP, IGES or SAT formats, for example) has been imported by using version 8.0’s new 3D data import feature, and the structure has been calculated as a whole. The calculation speed for this structure is ca. 0.5 seconds per frequency point (on a 2QC Xeon 3 GHz PC), finite conductor losses included. The accuracy of the method is demonstrated by excellent agreement between theory and measurements. This also holds true for the insertion-losses at the silver-plated structure (cf. e.g. 7.25 GHz: measured ca. 0.17 dB, theory ca. 0.16 dB).


Fig 2. WASP-NET’s new fast 3D FE-BI solver – 3D structure data import as STEP file. Theory compared with measurements. Calculation time: 0.5 seconds per frequency point (2QC Xeon, 3 GHz, PC), loss calculation included. Design, structure data, photograph, measured results courtesy of www.A1Microwave.com

A band-pass microstrip filter printed on anisotropic substrate [epsr = diag (3.45, 5.12, 3.45)] is shown in Figure 3. The results obtained by WASP-NET’s fast 3D FE-BI solver in comparison with results reported in literature (Drake, et al., T-MTT, Aug. 2000), are presented in Figure 3c, and shows good agreement. The high efficiency of the FE-BI solver for this kind of inhomogeneous structures is demonstrated by requiring only ca. 0.3 seconds/frequency point for the approximately 130,000 unknowns used.


Fig 3: WASP-NET’s new fast 3D FE-BI solver – Microstrip filter printed on anisotropic substrate. h = 0.6 mm, b = 11 x h mm, w = 1.36 mm, s = 0.34, d = 30.8. Calculation time: 0.3 seconds/frequency point (2QC Xeon, 3 GHz, PC), 130,000 unknowns.

The next example, Figure 4, a shaped asymmetrically displaced elliptical (ADE) dual-reflector antenna with dielectric support, von Karman radome, and septum polarizer in the feed region for achieving circular polarization, demonstrates the efficiency of version 8.0’s new IE solver. Based on the high computational speed, such structures can now be completely optimized as a whole (i.e. polarizer and the complete shaped antenna structure including radome). This conveniently results in broadband designs with low cross-polarizations for any desired application.


Fig 4: WASP-NET’s new fast IE solver – shaped asymmetrically displaced elliptical (ADE) dual-reflector antenna with dielectric support, von Karman radome and septum polarizer. Calculation time: ca. 20 seconds per frequency point (2QC Xeon, 3 GHz, PC).

The last example, a helix resonator combline filter, Figure 5, further illustrates the practically unlimited flexibility for creating user-defined structures. The new 3D structure editor conveniently models the arbitrarily shaped helix-resonators; the new flexible, fast 3D FE-BI solver then carries out the calculation. The calculation speed of only ca. 0.2 seconds/frequency point demonstrates the WASP-NET typical CAD efficiency for such general, user-defined 3D structures.


Fig 5: WASP-NET’s new convenient 3D user-defined structure editor, and fast 3D FE-BI solver – compact helix-resonator combline filter. Calculation time: 0.2 seconds/frequency point (2QC Xeon, 3 GHz, PC)

CONCLUSION

WASP-NET´s version 8.0, with its new 3D structure editor, convenient 3D data file import option, and fast 3D FE-BI and IE solvers, will support microwave design engineers by offering 3D structure CAD flexibility. These new features combine with the CAD tool´s typical high computational speeds for direct, fast EM based optimizations that is capable of solving all kinds of challenging waveguide, coaxial, microstrip, and antenna design problems.