Multiple multipole (MMP) computations of guided waves and waveguide discontinuities

Abstract

SUMMARYThe MMP (multiple multipole) code, that is based on the generalized multipole technique (GMT), offers several features to overcome problems occurring in computations of guided waves and waveguide discontinuities. The most important problems and the corresponding MMP solutions are outlined. The procedure is illustrated with different examples which demonstrate the wide range of applications of the code. GENERAL REMARKSThe MMP code1x2 is the most elaborated code based on the generalized multipole technique (GMT)3 which has been developed independently by different groups essentially in the last ten years.4 One of its origins is obviously the MOM (method of moments), but behind the GMT one can find a completely different b a c k g r o~n d .~ The GMT has been recognized as one of the most powerful techniques for solving scattering problems but the MMP code allows the solving of many other problems of computational electromagnetics, which is not yet very well known. Two classes of such problems are considered in this paper: guided waves and waveguide discontinuities. It is well known that the former can be solved by a 2-D formalism whereas the latter require a 3-D formalism in general. For reasons of simplicity, only special waveguide discontinuities that can be solved with the 2-D MMP code are considered here. It should be mentioned that the 3-D MMP formalism and code are very similar to the 2-D formalism outlined in the following. An example of a waveguide discontinuity computed with the 3-D MMP code is discussed in.2The MMP code is available on many different computers. User-friendly graphic input editors and plot programs on personal computers and SUN workstations reduce the time required for modelling and model validation. All examples shown in this paper have been computed on a portable Toshiba 5200. An add-on board with 4 IMS T8OO transputers in parallel has been used for the more time-consuming examples. The computation times are indicated in order to give an impression of the speed of the code, but it should be pointed out that overdiscretization has been applied throughout. This technique is reasonable although relatively long computation times are obtained, because the time required for modelling and model validation can be considerably reduced: all examples have been modelled within a few minutes.The examples have been selected from a tutorial point of view. For most of them, no comparisons with measurements or other computations are available. Nonetheless, the results are considered to be accurate. This can be seen from the field plots and from an error estimation provided by the code. Moreover, comparisons with analytic results of simple examples like the rectangular waveguide show that a very high accuracy can be achieved with the MMP code.