Commentary: The “states-of-the-art” in microwave heating

I recently came across a twelve-page description of microwave heating, authored by the very well-known Dr. A.C. (Ricky) Metaxas, fellow and tutor at St John's college in Cambridge UK, and president of the European microwave organisation AMPERE 1995–2005. The description is published at the website of the also well-known German industrial microwave company Püschner MicrowavePowerSystems. The description must be written no earlier than in 2006. It deals with the microwave physics of heating, applicators and cavities.
Reading the description reminded me about the wide information gaps that still exists in the area of microwave heating and processing systems.

Sources of knowledge and information
There are in principle five sources of knowledge and information:

I have not included courses, since these normally provide only introductions and information on well-known technologies and processes. 

Textbooks are typically characterised by descriptions of the governing theories and equations, boundary problems such as in waveguides and at dielectric load surfaces, and then a series of descriptions of more or less practical heating systems and aspects. – Also see Conclusions and recommendations below.
Symposia/conferences offer opportunities of information on the state in academia and about some few reasonably new microwave systems and processes.
Articles in scientific journals can be highly variable with regard to the state-of-the-art. There are typically many papers of the "we-too" type (i.e. known subjects investigated by new authors). Most papers are on analysis of phenomena, more than on synthesis of new methods or systems.
The patent literature is very large, but often difficult to assess since the goal is not to sell or inform but to protect. Patent applications are published 18 months after the fist filing date, i.e. almost always before they become patents. 
Internal information is just that, and is publicly disseminated only when there is a reason such as marketing (of anything from algorithms, to heating systems to applications to products). Common channels are via the trade press and – in recent years – websites, rather than at symposia/conferences or in scientific articles.  However, patent applications may determine the timing.   

How to find the "state of the art" in the subject area
There are only two textbooks (see list)  which have had some intention to provide descriptions of the current state-of-the-art – those by Metaxas & Meredith in 1983 and by Chan and Reader in 2000. As mentioned on the Commentary page, the latter can be criticised for significant incompleteness.
Symposia/conferences may provide the best opportunity to see what is new in a easy and direct way. Internal information may be made public for the first time, and informal discussions may provide further information. However, the patent literature has repeatedly been shown to be the most important  source of information on new technologies and processes, to me and others. But there are many difficulties, due to the large number of voluminous patent applications that are intended to just protect against direct copying, and also due to that they are written in a very different way compared with scientific articles. These factors have led to – in my opinion – too few patents being used as reference in scientific papers and symposia/conference summaries. As a matter of fact, a recent investigation in Sweden showed that almost 80 % of primary information about new technologies and processes is given by patent applications! And it should be noted that all patent applications are in most countries freely accessible on the internet – as opposed to textbooks, conference summaries and scientific articles.

Is there a coherent state-of-the art? 
The twelve-page description mentioned in the introduction is used as an example. As the following list will indicate, the correlation between the present state-of-the-art and what is said in this fairly new description is quite low.
  1. Table 1 is outdated (which is also admitted by the author). For fresh information, See this link. It may also be noted that there are still allowed special bands about 896 MHz in the UK and 918 MHz in Australia.
  2. The section on Numerical modelling ignores to-day's very wide-spread use in industry and academia of commercially available software packages such as the multiphysics systems by QWED and CST. FEM methods have since several years turned out to be less competitive than FDTD methods for fields and heating pattern studies. The literature on these software packages and their uses is now comprehensive.
  3. In the section Industrial systems multi-magnetron installations are not mentioned. They are increasingly used in 2450 MHz tunnel ovens. Rather than using circulators, efforts should be on avoidance of mutual coupling (so-called crosstalk) between magnetrons, and control systems prohibiting empty operation.
  4. In the section Microwave applicators it is said that industrial multimode systems are an extension of  the systems in domestic microwave ovens.  – This is no longer true, as indicated on page 2 and 3 in the AMPERE Newsletter, April 2005 (with A.C. Metaxas as editor). Probably at least 50 % of domestic microwave ovens produced to-day can not be characterised as multimode.
  5. In the section Multimode Systems, Coupling systems it is said that magnetrons may be mounted directly into the cavity. However, this inevitably results in a very poor impedance matching, reducing the efficiency by at least 15 %. Cavity wall losses also increase, due to this antenna geometry not producing any radiation perpendicularly outwards. – I have myself investigated a system made by the German company MWT, built with directly mounted magnetrons as described by the company (click “The principle”).The description is not in line with scientific concepts. The functionality and efficiency is inferior to modern systems with directed radiators.
  6. None of the illustrations in Figure 7 to 9 seems to represent technologies from the last 10 years or so. Neither the Hephaistos nor HERA nor matchplate systems are mentioned.
  7. In the section on single-mode resonant applicators, it is said that the versatility of rectangular TE10-mode waveguide applicators is unquestioned.  It is also said that the sample/load should be positioned at the electric field maximum for optimum energy transfer. – Neither of these statements are correct. Examples of more versatile adjustment means are given in this patent, and a resonant applicator with load-independent frequency is described in this patent application
  8. At the end of the single-mode resonant applicator section, it is said that a problem is small sizes of these, resulting in only small-size loads being possible to treat. However, large resonant single (hybrid evanescent) resonant mode applicators of the HERA type were described in presentations, at the 2003 AMPERE conference (see the downloadable summaries from 2003, at the page Internal R&D), and at the 2004 and 2006 IMPI symposia. A modelling image is shown on the startpage of this website, of an almost half a meter wide applicator at 2450 MHz. 

Conclusions and recommendations
  1. The patent literature is important for getting an improved understanding of the state-of-the-art. Free searches can be made at USPO and EPO. Using the classification system and/or inventor names simplifies searches.
  2. Symposia and conference proceedings often give more new information than articles in scientific journals. This is due to the (normally much) shorter publishing times, and less strict review processing.
  3. Textbooks may be good for the physics and processing basics, but are of very variable usefulness for engineering purposes. Many authors focus on their own areas of research and interest, at the expense of what is needed more generally. Much of the basic theory can instead be obtained from old standard microwave engineering or physics books with no focus on heating/processing.
  4. More has actually happened in the subject area during the last 15 years than during any earlier period, so older literature should be considered incomplete. But also modern engineering literature should be studied with caution, due to the incoherent state-of-the-art.

Updated 2007 June 28             Back