Commentary: The “states-of-the-art” in microwave heating
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
is published at the website of the also well-known German industrial
microwave company Püschner MicrowavePowerSystems. The
must be written no earlier than in 2006. It deals with the microwave
physics of heating, applicators and cavities.
Reading the description
about the wide information gaps that still exists in the area of
microwave heating and processing systems.
knowledge and information
There are in principle five sources of knowledge and
I have not included
courses, since these normally provide only introductions and information on well-known
technologies and processes.
dealing with microwave heating – there are at least
five such books published during the last 20 years; see list.
- Symposia and
conferences on microwave heating related issues – at least
one every year – see
- Articles in
scientific journals – more than 30 articles each year
– see list.
- The patent
information, often under secrecy, in companies and among consultants.
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.
offer opportunities of information on the state in academia and about
some few reasonably new microwave systems and processes.
Articles in scientific
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
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
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,
applications may determine the timing.
How to find the "state of
the art" in the subject
There are only two textbooks (see list)
which have had some intention to
provide descriptions of the current state-of-the-art – those
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
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
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.
- 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
- The section
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.
- 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.
- In the
applicators it is said that industrial multimode systems
extension of the systems in domestic microwave
– This is no longer true, as indicated on page 2 and 3 in the
Newsletter, April 2005 (with A.C. Metaxas as editor).
at least 50 % of domestic microwave ovens produced to-day can not be
characterised as multimode.
- In the
Systems, Coupling systems
it is said that magnetrons may
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
- None of the
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.
- 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
that the sample/load should be positioned at the electric field maximum
for optimum energy transfer. – Neither of these statements
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
- 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.
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.
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.
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.
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