High-power analysis of passive microwave components using FEST3D

J. Armendáriz(1), S. Anza(1), T. Pinheiro-Ortega(1), M. Rodríguez(1), F. J. Pérez(1), C. Gahete(1), C. Mus(1),J. Gil(1), M. Brumos(2), C. Carceller(2), C. Vicente(1), B. Gimeno(3), V. Boria(2)

(1)Aurora Software and Testing S. L.Edificio de Desarrollo Empresarial 9B – UPVCamino de Vera s/n, 46022, Valencia, Spain

Email: carlos.vicente@aurorasat.es

(2)Departamento de Comunicaciones Universidad Politecnica de Valencia, 46022 Valencia, Spain

Email: vboria@dcom.upv.es

(3)Departamento de Fisica Aplicada y Electromagnetismo - ICMUV, Universitat de Valencia, 46100 Valencia, Spain

Email: benito.gimeno@uv.es

ABSTRACT

In this paper we present the recent advances in FEST3D in terms of analysis, design but especially related to the prediction of high power effects such as multipactor and gas discharge. Examples of passive components that can be analysed and designed using FEST3D are presented. Particular emphasis is given to those new FEST3D features related to high power applications such as the analysis of multipactor under multicarrier excitation or the analysis of power breakdown level in coaxial cavity filters.

1. INTRODUCTION

The requirements in terms of microwave breakdown (multipactor and corona discharge) of satellite components are becoming more restrictive due to the generalized increase of power used in satellite missions. As a consequence, the design of devices free of microwave breakdown becomes, in many occasions, an extremely challenging task. In order to ease this design, it is desired to have at one's disposal a software tool capable to analyse not only the electromagnetic response of microwave components but also to determine the breakdown power of such structures with a reasonable accuracy.

In particular, microwave filters for space systems have been historically implemented in waveguide technology, essentially due to low losses and high power handling capability issues. In the last four decades, the area of microwave filters for space applications has experienced significant improvements in theoretical, technological and performance subjects. For instance, during the 70s and 80s, many efforts were devoted to produce new filter topologies with reduced mass and volume, improved use of the frequency spectrum and enhanced behaviour in terms of high power handling. During the last 20 years, advanced industrial applications have been demanding increasingly tighter requirements in terms of electrical specifications, as well as drastic reductions of manufacturing costs and development times. These challenges have required the extensive generation and use of flexible Computer-Aided Design (CAD) tools based on accurate and efficient electromagnetic simulators.

In this paper, we present a powerful software tool (FEST3D[1]) capable to analyse at low and high power most of the waveguide components commonly used in space and ground applications. This tool combines several advanced electromagnetic algorithms able to analyse complicated components in extremely short computational times. Additionally, it provides the possibility to perform high power analysis for multipactor and gas discharge in such devices. This provides to the user a single platform in which the full design process can be carried out.

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2. FEATURES

From the FEST3D internal kernel point of view, there are three main groups of features that are relevant: electromagnetic analysis, synthesis and high power analysis. In the following section we provide a brief description of these features.

2.1 Electromagnetic Analysis

FEST3D is capable to analyze a wide range of elements. Among them, the most important are:• Rectangular and Circular waveguides.• Ridged, Elliptical, Cross, Coaxial waveguides.• Circular with metal insert waveguides.• T and C- Junctions.• Arbitrary metallic 2D (H-plane or E-plane) elements such as waveguides with screws or metal inserts.• Cavities with metallic screws.• Coaxial excitation for combline/interdigital filters.• Arbitrary elements with constant cross section along the direction of propagation.• Bends.• Infinite phased array.

The analysis of such elements is done by means of the integral equation technique combined with method of moments and network theory. Such an approach has several advantages over standard modal techniques such as mode matching: first, the frequency part of the Green’s function is extracted allowing a faster computational time per frequency point. Secondly, the relative convergence problems which are critical in the case of mode matching are much less important in this case.

The capability to analyze such elements allows the analysis of complicated structures such as:• Band-pass filters.• Comb-line/Interdigital filters.• Band-stop filters.• Evanescent filters.• Waffle-iron filters.• Dual mode filters.• Couplers.• Multiplexers.• Power dividers.• Polarizers.

Therefore, FEST3D is able to analyze electromagnetically (s-parameters, group delay…) the most commonly used passive waveguide components. Examples of these structures are shown in the last section of this paper.

2.2 Design & Synthesis

FEST3D also has specific tools for the automatic design of several type of components. In particular, low-pass, band-pass, transformers and dual-mode filters can be generated automatically designed just by providing the electrical requirements. These synthesis tools are very advanced and allow, for instance, designing bandpass filters up to 20 or 25% of bandwidth without post-optimization. Moreover, dual-mode filters for space communications up to 12 poles can be automatically synthesized with high accuracy in short computational times.

In addition to this, FEST3D has optimizers in order to enhance the design procedure of components. Both, local and global optimizers are available. Once the design is ready, tolerance analysis can be also performed based on a gaussian distribution.

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2.3 High Power Analysis

FEST3D software tool has many features related to high power applications. In particular, FEST3D is capable to predict the multipactor breakdown and gas discharge in a wide range of geometries and devices:

• Rectangular/Circular/Coaxial/Bar-line.• Ridge/Waffle/Elliptical.• Circular posts including coaxial excitation (for combline/interdigital filters)

Moreover, FEST3D can analyse Multipactor in the structures described above in case of multicarrier excitation. In addition to this, the last version of the software allows performing gas discharge analysis at ambient pressure. This can be useful, of course, for ground applications but also for space applications in which testing at ground wants to be performed.Finally, FEST3D incorporates a new module (SPARK3D), capable to perform high power breakdown analysis through the EM fields from common commercial tools such as CST[2] and HFSS[3]. This extends the high power capabilities to components different to waveguide structures.

3. EXAMPLES

In this section, several examples of the FEST3D capabilities described in the previous sections are provided.

3.1 EM Analysis

In Fig. (1), a band-stop filter extracted from [4] is presented. Such an example shows the ability of the software to cope with complicated large structures. In this case, the tuning screws have been varied in the simulation tool in order to center the bandstop response at 10 GHz.

Fig 1: Band-stop filter with tuning screws and slot irises. Static part: 2 s. Dynamic part: 0.05 s/fp.

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In Fig. (2), a coaxial filter is shown. It is a four cavity filter with hollow posts and tuning posts. The coaxial excitation is electrical (no contact between the coaxial probe and the post). The 3D cavities are solved by means of the Boundary Integral Resonant Method (BI-RME) method and using the Green's function of the cavity allowing a fast computation since only the perturbation of the cavity (in this case the posts) is meshed in surface.

Fig 2: Coaxial filter with hollow posts and tunning screws. Static part: 65 s. Dynamic part: 0.5 s/fp.

3.2 Synthesis

FEST3D synthesis tools can be used to design several types of components from the user specifications. As an example, Fig. (3) shows a bandpass filter with a 17% of bandwidth which has been automatically obtained in just ten seconds without the need of post-optimizing.

Fig 3: 5-pole bandpass filter with rounded corners (17% bandwidth) automatically synthesized with FEST3D.

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Another example of synthesis module integrated into FEST3D is the dual-mode synthesis tool. Fig. (4) shows an example of a narrowband (27 MHz) dual-mode filter of 4 poles with two transmission zeros. The result shown has not required post-optimization and has been obtained in just 7 minutes in a standard PC.

Fig 4: 4-pole dual-mode filter automally synthesized with FEST3D.

3.3 High Power

As mentioned above, FEST3D has specific modules capable to analyze high power effects in complex passive microwave components. In particular, its multipactor module is able to compute the multipactor breakdown onset on the waveguide sections of many type of components.

Recent advances on this area include the high power analysis in complex waveguide elements such as ridge, coaxial or multi-ridge shapes. Moreover, now it is also possible to compute the high power breakdown level in 3D cavities such as those used in combline or interdigital filters. In Fig. (5) we show an example of an interdigital filter for which the S-parameters calculated with FEST3D and HFSS are compared. The EM fields are also shown. In Fig (6) we present the corona results for this filter in which the breakdown occurs on the top of the resonator posts. The Multipactor breakdown was also computed resulting in 40 W @ 1.75 GHz.

The software is also capable to cope with multicarrier excitation and is able to predict the multicarrier multipactor breakdown for a particular configuration of frequencies and phases. Moreover, the ECSS Multipactor Tool [5] integrated in FEST3D can be used to compute the phase configuration for a user defined number of electron gap crossings. This phase configuration can be loaded afterwards to FEST3D in order to perform the full analysis.

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(a)

(b)Fig 5. Interdigital filter: (a) 3D Model and comparison between FEST3D and HFSS S-parameters, (b) Electric Field at the centre

frequency of the filter, showing maximum EM field between the posts and the tuning screws.

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Fig 6. Corona Breakdown in the filter shown in Fig. 5. at 1.75 GHz.

CONCLUSIONS

In this paper, we have presented the new features of FEST3D software. In particular, we have presented the new high power features such as the analysis of coaxial cavities. FEST3D is the first software of its class (fast modal technique) capable to include high power effects in the design cycle of a component, which makes it very suitable for such applications.

REFERENCES

[1] www.fest3d.com[2] www.cst.com[3] www.ansoft.com/products/hf/hfss/[4] G. Matthaei, L. Young, E. Jones, Microwave Filters, Impedance-Matching Networks and Coupling Structures, Artech House.[5] http://www.aurorasat.es/multipactortool.php

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