A Comparison of the Microwave Oven and Reverberation Chamber

Transcription

1 A Comparison of the Microwave Oven and Reverberation Chamber Yi Huang, Xu Zhu and Binoy Nair The University of Liverpool, L69 3GJ, UK Abstract The reverberation chamber is a facility mainly for electromagnetic compatibility (EMC) tests whilst the microwave oven is used for heating applications. They both generate a timeaveraged uniform field in the area of interest. In this paper, these two facilities are compared in terms of their applications, operation principles, design philosophy and especially the field uniformity with the aid of electromagnetic simulation techniques, where a dyadic Green s function is employed. It is revealed that a well-designed reverberation chamber can outperform a conventional microwave oven in terms of the field uniformity. A modified microwave oven design is suggested accordingly. 1. Introduction The microwave oven is a well-known domestic electronic product and has been used for heating/cooking applications for more than 30 years, whilst the reverberation chamber is much less popular and mainly employed in the electromagnetic compatibility (EMC) community for less than 30 years. However, there are a lot of similarities between them and a proper comparison is not available in the literature to the knowledge of the authors. The objective of the paper is therefore to conduct a comparison between these two facilities in terms of their applications, operational principles, design philosophy and especially the field uniformity. It is also expected to deduce certain new ideas for improving the microwave oven as well the reverberation chamber design through this comparison exercise. 2. Microwave Oven The development of microwave heating applications has followed the earlier application of lower RF frequencies to induction and dielectric heating. Serious activity began after World War II, directed towards a microwave oven for commercial and residential use. The broadening of interest to include scientific and industrial applications followed in the early sixties as new markets for microwave power sources were sought. The marketing of a countertop microwave oven for consumers gave birth to the economically important oven business in the 1960s. A brief history of microwave ovens can be found in such as [1]. The growth of this field was slowed down by a series of events questioning the safety of microwave exposure near highpower microwave systems. Although some of this has receded, the safety and interference issue remains a problem of wide interests. A lot of publications about this issue can be found from the public domain, and the latest discussion focused on the interference of microwave oven to Bluetooth and Ultra-Wide-Band (UWB) systems [2]. The microwave oven is basically a rectangular conducting cavity with a turntable and microwave generator. The basic operation principle behind it is that the generator generates electromagnetic waves (normally at 2.45 GHz, which is a license free ISM band) which make molecules/particles (such as electrons) move. This motion leads to friction, and friction leads to heating. The turntable is employed to generate a time-averaged uniform microwave field in order to achieve a good cooking uniformity. The field distribution inside a microwave oven is one of the most important issues and has been investigated using various methods, most notably some numerical methods /05/$ IEEE. 856

2 (such as FEM and FDTD) [3, 4]. Various experiments and measurements have been conducted to identify the field distribution and unevenness [5]. Reasonable field uniformity, which depends on the specific oven and the food placed inside the oven, was reported. 3. Reverberation Chamber The reverberation chamber, also known as mode-stirred chamber, was introduced in the late 1970s [6-7] for EMC measurements. Just like a microwave oven, it is also a conducting chamber with a signal generator. A stirrer is used to stir the field/modes inside the chamber so as to achieve an averaged uniform field. This stirrer can be considered as the replacement of the turntable in an oven, which occupies a relatively large space. The reverberation chamber has been used for the following applications: Radiated emissions tests Radiated immunity tests Enclosure shielding measurements Cable shielding measurements Absorber characterization Certain antenna measurements (e.g., antenna efficiency and diversity measurements) The major advantages of the reverberation chamber include: A screened environment Ease of generating a large field strength and high power for immunity tests No rotation is required since the fields are from all directions and polarisations Ease of measuring the total radiated power for emission tests Wide frequency range 4. Comparison of the Microwave Oven and Reverberation Chamber It is evident that the microwave oven and reverberation chamber were invented for different reasons and applications (one for heating, and the other mainly for EMC testing). The oven operates at a single frequency (2.45 GHz) whilst the reverberation chamber can be used for a wide frequency band (above a Lowest Usable Frequency). To generate an averaged-uniform field, they both need to operate at an over-mode environment, which means that there are at least 60 possible modes inside the cavity/chamber [8]. For a given chamber as shown in Fig. 1, the resonant wavelength λ mnp for the resonant (m n p) mode in an empty conducting rectangular cavity satisfies the following condition: 2 mπ 2 nπ 2 pπ 2 kmnp = ( ) +( ) +( ) (1) a b c where k mnp =2π/ λ mnp with m, n, and p denoting integers; a, b, and c are the dimensions of the chamber. The field inside a chamber with a source J can be expressed as 1 E = (x, y,z ) d v j G J (2) ωε source where G is the dyadic Green's function and defined by [9]. The selection of chamber dimensions (a, b and c) should ensure that enough propagating modes (at least 60 as a guideline) are generated inside the chamber for a given frequency of operation. A typical domestic microwave oven is of dimension 342 (W) x 195 (H) x 357 (D) mm 3 and the turntable diameter is about 325 mm. Thus the total possible number of modes at 2.45 GHz is [9]: N = π abc( f ) 3 which is greater than 60. If a waveguide is assumed as the excitation of the oven, then the static field distribution within the cavity is obtained as illustrated by Fig. 2, here the observation plan is at 100 mm above the cavity floor. The standing wave pattern (the dominant mode is (5 5 3)) is apparent and the field is not uniformly distributed inside the oven. The difference from one point to another can be more than 30 db. However, when the microwave oven is turned on and the turntable is in operation, any observation /05/$ IEEE. 857

3 point at the food will move around a circle. Now the time-averaged field becomes much more even, as shown in Fig. 3. The major problem for this oven seems to be around the centre. The difference between the maximum and minimum is well over 10 db. This is a common problem for microwave ovens in the market. Now let s have a look into the field inside a reverberation chamber. The dyadic Green s function method used for microwave oven field analysis can still be employed for this case, but it becomes very difficult and time-consuming. We have therefore used a numerical package, MicroStripes, based on TLM method (transmission line method). There are many different designs. The simplest one is just a single paddle as shown in Fig. 4, where the field generated within the chamber is also given in the figure. The chamber dimensions are the same as the ovens. When the paddle is moved over half a wavelength (5 cm), the averaged field variations over the most part of the chamber (including the centre region) are within the -3dB to +3dB. Similar results have been obtained for the fields at other observation planes in this microwave oven/chamber, which are in good agreement with the results obtained for reverberation chambers at frequencies with over 100 possible modes by many others [10]. The field uniformity has certainly improved with the trade-off of a reduced working space. We can therefore conclude that the uniformity inside a microwave oven is not as good as that in a well designed reverberation chamber. This seems to be an inherent problem with the current oven design: field around the turntable centre is not stirred. The uniformity can be improved by introducing a stirrer. The drawback is that we may have to sacrifice the working space and also increase the complexity of the overall oven design. The detailed design improvements and its analysis is being carried out and the results will soon be reported /05/$ IEEE Conclusions We have attempted to compare the operational characteristics of two very useful facilities: microwave oven and reverberation chamber, by focusing on the averaged field uniformity inside the cavity/chamber. A dyadic Green s function approach and a commercial package have been employed to aid the analysis. It was demonstrated that the field around the turntable centre is not adequately disturbed, thus the averaged field uniformity is not as good as that inside a reverberation chamber. It was suggested that the performance of a microwave oven be improved by introducing a stirrer. Acknowledgment: The financial support from the EPSRC for the project is gratefully acknowledged. References 1. ch/history.html 2. Matsumoto, Y.; Takeuchi, M.; Fujii, K.; Sugiura, A.; Yamanaka, Y, A time-domain microwave oven noise model for the 2.4 GHz band, IEEE Trans. on EMC, vol. 45, pp , Aug. 2003, 3. Sekkak, A.; Pichon, L.; Razek, A., 3-D FEM magneto-thermal analysis in microwave ovens, IEEE Transa. on Magnetics,, vol. 30, Sep Hanafusa, S.; Iwasaki, T.; Nishimura, N., Electromagnetic field analysis of a microwave oven by the FD-TD method-a consideration on steady state analysis, Antennas and Propagation Society International Symposium, AP-S. Digest, Volume: 3, June 1994, Pages , vol Kharkovsky, S.N.; Hasar, U.C., Measurement of mode patterns in a high-power microwave cavity, IEEE Trans. on

4 Instrumentation and Measurement, vol. 52, pp , Dec P. Corona, G. Latmiral, E. Paolini, and L. Piccioli, "Use of reverberating chamber for measurements of radiated power in the microwave range", IEEE Trans. on EMC, vol.18, pp , P. Corona, J. Ladbury, and G. Latmiral, "Reverberation- Chamber Research - Then and Now: A Review of Early Work and Comparison with Current Understanding," IEEE Trans on Electromagnetic Compatibility, vol. 44, pp , M. L. Crawford and G. H. Koepke, "Design, Evaluation, and Use of a Reverberation Chamber for Performing Electromagnetic Susceptibility/Vulnerability Measurements," NBS Tech. Note 1092, April Y. Huang and D. J Edwards, "An investigation of electromagnetic fields inside a mode-stirred chamber", Proc. of IEE 8th Int. Conf. on EMC, pp , Edinburgh, UK, Sep IEC Standard on Reverberating Chambers, y b J(x',y',z') o a x z c Fig. 1. A chamber with an excitation J Fig. 2. The calculated total field distribution inside a microwave oven at y = 0.1 m /05/$ IEEE. 859

5 Fig. 3. The time averaged field as a function of the distance to the turntable centre at various heights y from 0.05 to 0.15m within the microwave oven. Fig. 4. The calculated total field distribution inside a reverberation chamber at y = 0.1 m /05/$ IEEE. 860