Note: Descriptions are shown in the official language in which they were submitted.
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Title
Microwave applicator system
Background of the invention
The present invention relates to a microwave applicator, to a system of
microwave
applicators and also to a method of using the applicator and the system.
Furthermore, the field of microwave applicators to which to present invention
belongs
include those types having a load continuously transiting the heating chamber
or
chambers of the system The present invention is an improvement of heating
systems
consisting of mainly multiple single mode applicator assemblies in which the
load to be
heated has a constant cross section.
i5
Description of the prior art
Many different kinds of microwave systems for loads fulfilling the above
characteristics
exist- The simplest such applicator is a large multimode cavity, which may
have holes in
its walls (then preferably with attached metal tubes confining the microwaves
to the
cavity). For very small loads, the short circular single mode TM0,0 cavity is
well known,
but has the drawback that it can only take loads up to about 10 Ham in
diameter under
favourable conditions, at the common microwave frequency of 2450 MHz. Better
efficiency may be obtained with a longer circular TM01p applicator.
Only single mode systems are of concern in this context, so the question is
what
significant other modes than the simplest TM mode (TMo1) may be useful and
known. It
is then of interest which mode types are created inside a load which can for
this purpose
be of a circular cross section.
Using the load axis as reference, there are then transverse electric (TE) and
transverse
magnetic (TM) modes. Any TE modes used for the excitation of the load field
have
inherently a high impedance, and the typical loads of primary
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concern herein have a rather high permittivity, mainly between 10 and 70, and
will therefore have a low impedance. Furthermore, the lossiness of dielectric
loads is by an equivalent electrical conductivity, but since TE modes lack an
axial electric field component there is neither any efficient coupling for
small
loads nor any possibility to avoid a minimum axial length of the applicator of
about half a free space wavelength. TE modes are thus inferior to TM for the
purpose here: namely allowing variations of the load permittivity, and using
an
axially short applicator, while maintaining high microwave efficiency.
The lowest order TM mode in the load is of the TM0 type. This has a
rotationally
symmetric field and provides maximum heating at the load axis. The most
advanced version is described in the patent DE-2345706, where the load
diameter is chosen so large that the heating intensity at the load periphery
is
very low; the applicator is then of the TM02 type. A drawback with that system
is
that the bound wave propagating at and in the dielectric rod-shaped load is
that
a very large fraction of its field energy resides inside the rod. This results
in
difficulties to confine the heating to only the load part inside the
applicator,
which in turn makes it necessary to allow axial zones outside the applicator
with
a length comparable to about twice the penetration depth, for residual heating
and leakage protection. Good external choking by wavetraps just outside the
applicator is not possible due to the substantial field confinement inside the
rod-
shaped load. This is disadvantageous particularly when one or several axially
short applicators are used in order to achieve a high power in density in the
load. Another drawback is the need for such large applicator diameter that
excitation of the disturbing TM 1 mode is difficult to avoid.
The next higher order TM mode in the load is of the TM 1 type. The heating
pattern in the cross section of a reasonably circular load has then two
diametrically located maxima, with a diametrical zone of zero heating at 90 .
A
microwave heating applicator with this mode is described in for example the
patent US-5,834,744. The applicator disclosed in that patent is excited by two
diametrical slots fed by a common waveguide arranged in such a way that the
TM0 modes are suppressed. In order for this particular feed system to work,
the
applicator is circular or polygonal, with the load located at the central
axis, and
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the applicator mode is characterised by being of the TMI20 type. Additionally,
the
applicator design is dedicated for functioning with a longest possible axial
length of the
load of the order of one free space wavelength.
A waveguide mode transducer from rectangular Tejo to TE20 is described in for
example
the patent GB-1364734. The transducer system is used to heat a wide and flat
load
moving past the end of the TE20 waveguide. For that reason, stubs are placed
in the
waveguide to create mode impurities which would result in a heating pattern
caused by a
combination of that by the Tejo and TE20 modes, in an added external cavity
with at least
two such applicators and equipped with load rotation means.
One drawback with this known device is that the load needs to be wide and flat
which
limits the possibilities to heat larger volumes and also limits the
possibility to control
e. g. the heating rate.
The objects of the present invention are to achieve an applicator and a system
of
applicators that enable heating of load having a large cross section, that
make it possible
to more accurately control e. g. the heating rate and that better confine the
heating in the
load.
Summary of the invention
According to an embodiment of the present disclosure, there is provided a
microwave
applicator for heating loads being a waveguide transition between the
rectangular TEIo
and TE2O modes comprising a TEIO mode section and a TE20 mode section. The
load is
inside the TE2o mode section and is located with its major axis
perpendicularly to the
major propagation direction of the TE20 mode, close to a shorting wall of the
TE2o anode
section and also close to the centreline of the propagation direction.
According to another embodiment of the present disclosure, there is provided a
system
consisting of at least two microwave applicators as described herein, wherein
the
applicators have a common load axis, and wherein adjacent applicators are
rotated by
approximately 900 around the load axis.
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According to another embodiment of the present disclosure, there is provided a
method
for designing an applicator or a system as described herein. The method
comprises:
using an essentially complete mode transducing function between rectangular
TE1o and
TE2o of the 90 H knee type, shorting the TE20 end and locating the load with
its major
axis perpendicularly to the major propagation direction of the Tea mode, close
to a
shorting wall of the section and close to the centreline of the propagation
direction,
introducing a tuning means between opposite major walls of the waveguide near
the
load, and establishing a TM1 type field in the load by performing experiments
or
microwave modelling using the diameter and positions of the tuning means as
variables.
The system of microwave applicators according to the present invention
consists mainly
of multiple air-filled single mode applicators in which the load to be heated
has a
constant cross section.
A characteristic feature of the present invention is that the TMt type field
in the load is
created by using an applicator in which the basic second order electrical
mode, in the
terminology of the theory for multipole fields, is created. This is
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characterised by two maxima of the electrical field at opposite sides of the
axis of
the load; in its pure form this occurs in a closed circular TE110 or TE120
cavity.
The simplest rectangular waveguide or resonator in which this electric mode
exists carries the TE20 mode.
The microwave applicator is for applying microwave power to a load that
preferably has a constant cross section. The applicator is a mode transducer
from rectangular TE 10 at the generator end to TE20 at the application end and
the load is located approximately centred and near a shorting wall of the
latter
section. In a system using at least two applicators the mutually 90 displaced
applicators in multi-applicator stacked assemblies have two additional
functions: to confine the heating to take place mainly inside each applicator
by
choking action, and to act as a filter which reduces the crosstalk between
adjacent applicators. The field in the load is of the cylindrical TM1 type and
the
pattern is improved by adding for example tuning rods between the opposite
waveguide walls near the load.
In cases where a high power density in the load is desired, the height of the
applicator is made low; if this height is less than a half free space
wavelength
there can then be no mode with higher middle index than 0, i.e. the applicator
fields are in principle the same at all levels. By then using a TE10 waveguide
feed the advantages addressed in the present application is utilised, such as
stacking several applicators with a common load axis and then displacing
adjacent applicators by 90 , so that not only an improved overall heating
pattern
in a flowing load is obtained, but also a choking action between adjacent
applicators so that the microwave propagation between them through the load is
strongly reduced.
The present invention is not limited to using a TE 10 waveguide with
approximately half the width of the TE20 part of the applicator, as shown in
Fig. 1 - but also a generalised feed where a portion includes a dielectric-
filled
waveguide carrying an equivalent mode to the rectangular TE10, which is also
equivalent to the circular TE11 mode.
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The invention also includes applicators with larger heights, up to more than a
full free space wavelength. The uses of such applicators are typically not for
continuously flowing loads but instead for stationary liquid loads in a round
cylindrical microwave transparent container. Such loads may be stirred by
5 additional mechanical means such as a rotating beating device or a magnetic
stirring system utilising small, magnetised bodies in the liquid. The uneven
heating pattern with two maxima in the circular cross section is then
overcome.
In order for the axial evenness of the heating pattern to be maintained, also
under conditions where the filling height and dielectric properties of the
liquid
vary, additional means are introduced according to the present invention.
Brief description of the drawings
Figure 1 shows, in perspective, an applicator according to the invention, with
a
rod-shaped load extending through it.
Figure 2 shows, in perspective, a system consisting of a second applicator
placed
directly on a first applicator, with a rod-shaped load extending through both
applicators.
Figure 3 shows the heating pattern in the central horizontal plane of an
applicator according to Figure 1, as a thermal plot obtained by microwave
modelling.
Figure 4 shows the load heating pattern in a vertical plane containing the
load
axis and the angular location of the heating maxima of a lower applicator with
a
very small height, with only the lower applicator energised, in a system
consisting of two equal 90 displaced applicators according to Figure 2, as a
thermal plot obtained by microwave modelling.
Figure 5 shows an alternative embodiment of the applicator where the part with
the load has been made significantly axially smaller than the generator feed
TE10 end.
Figure 6 shows a further alternative embodiment of the applicator in a system
where the load is a square cross section load.
Figure 7 shows an example of heating pattern in the central cross section
plane
of an applicator according to the present invention.
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Figure 8 shows a cross-sectional view of an alternative embodiment of the
applicator where the part with the load has been made significantly axially
larger
than the generator feed TE 10 end.
Figure 9 shows a view from above schematically illustrating the embodiment
shown in figure 8
Figure 10 shows a cross-sectional view of a sixth embodiment of the present
invention.
Figure 11 shows a view from above schematically illustrating the embodiment
shown in figure 10.
Detailed description of the invention
The desired excitation type is the circular TM 1 field in a load, which is
considered to have a small diameter for the purpose of this reasoning. In a
circularly cylindrical cavity with a centred axial load and where the feed is
ignored for the moment, the mode is then TM 110. The simplest rectangular
mode type in an empty waveguide that can excite the same load field type is
the
TE20 waveguide mode. The field along the centreline of propagation is then
only
magnetic, in the direction of propagation along the waveguide.
Even if, in principle, waveguides and cavities of any shape allowing the load
to
be excited by this field type are within the scope if the invention, certain
excitation methods and means as well as constraints in mechanical design
result
in practical limitations. Hence, the applicators according to the invention
have
single feeds at the periphery of the waveguide-like structure, which has zero
index in the axial (height) direction of the load. The simplest such structure
is
thus a rectangular TE201 cavity, but the feedings according to the invention
and
the fact that there is a net power propagation from the feeding towards the
load
will result in the last index being somewhat undefined, and in any case this
distance to be more than half a guide wavelength in that direction.
Hence, a first example of the simplest applicator cross section perpendicular
to
the load axis is a rectangular box supporting a field which can best be
described
as rectangular TE202. For improving the mode purity, and compensating against
the field modifications caused by the feed, a part of the rectangular shaped
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applicator wall opposing and across from the feeding has a triangular cut.
This
is schematically illustrated in figure 1.
Referring now to the figures, and most particularly to figure 1, the first
embodiment of the present invention relates to a rectangular TE10/TE20 mode
applicator (or transducer) 1 with the generator 2 connected at the TE10
section.
The TE20 section being closed by a shorting metal wall 3, and a cylindrical
load
4 is located approximately at the centreline of the TE20 section. A tuning
means
5 (here in the form of a rod) extends the whole way between the top and bottom
surfaces in the TE20 section.
The applicator is air-filled and made up from metal walls according to well-
established manufacturing technique for microwave applicators.
In the case of a pure TE20 mode, the load location at the centreline provides
the
desired cylindrical TM l field in the load. The rod 5 (preferably made from a
metal) may then not be needed to obtain a symmetrical heating pattern in the
load. However, it is of interest to provide a compact design, so in particular
the
TE20 section is quite short. The rod is then very convenient for adjusting the
heating pattern; in addition, the rod 5 may also act to stabilise the heating
pattern under conditions of different permittivity and dimensional changes of
the
load, as well as for improving the impedance matching.
The location of the load axis in relation to the shorting wall 3 should in
accordance to the first order theory be a quarter mode wavelength away.
However, it is normally determined by experiment or by microwave modelling.
Since the applicator is primarily intended for loads having a radius exceeding
half a wavelength in the load substance, there may be considerable deviations
from this first order theory, resulting in the optimum position of the load
being
closer to the shorting wall. Experiment or microwave modelling is also used
for
the determination of the diameter and location of the rod 5.
The second preferred embodiment of the present invention as shown in figure 2
relates to a system comprising two applicators 1,1' where the applicators have
a
common load axis, and that the applicators being rotated by approximately 90
around the load axis in relation to each other. It is naturally possible to
arrange
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additional applicators where each applicator being rotated approximately 90
around the load axis with regard to an adjacent applicator.
As seen in Figure 3, the heating pattern has two diametrical maxima (each
maximum is indicated by a "+"), one on each side of the TE20 waveguide
centreline 6; its angular variation can be described by a cost function,
according
to known mode theory. By the 90 displacement, a second applicator will give a
sine variation, so that the summed angular variation will be 1, i.e. not vary
at all.
According to a first aspect of the second embodiment of the invention the
energy
coupling between adjacent 90 displaced applicators by the load field may be
made very small, so that the so-called crosstalk between such applicators will
be
very small, even if the associated generators are simultaneously excited.
According to a second aspect of the second embodiment the applicator 1 is
designed so that it also works as a choke for the propagating fields from a
first
applicator through the load to a second applicator. An example of this is
shown
in Figure 4, where only the lower applicator 1 is energised, and there is a
second
applicator 1' just above but none below the first applicator. Actually, this
feature
is closely related to the first aspect of the second embodiment mentioned
above.
For efficient choking to be possible, it is necessary that a significant part
of the
microwave energy is bound to the load 4 is outside it. This may be the case
for
the TM 1 mode type, but is not for the TM0 type mode. In figure 4 the heating
pattern is schematically illustrated in the same way as in figure 3.
For the optimisation of choking, it is firstly to be considered that what
needs to
be choked in the second, "passive" applicator is a 90 rotated load field from
that
produced by this second applicator. Hence, the mode type to be choked is TE10.
The choking action is to be of the source (meaning excited load in this case)
firstly being mismatched by the shorting wall 3, secondly by a field mismatch
to
this TE10 mode in the TE20 section, and thirdly another field mismatching when
the TE10 mode in it encounters the transducer section to the TE10 section. The
third phenomenon has typically the strongest effect, and the procedure for
choking optimisation is then by variation of the length of the TE20 section,
which is arbitrary with regard to the proper function of the applicator in
heating
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mode, since the transition section as such is matched for that primary power
flow.
The second parameter, for fine-tuning of the two functions of the applicator,
is to
vary the location of the load axis in relation to the shorting wall 3, in
combination with the use of one or several metal rods 5. Rather than
performing
this co-optimisation of heating and choking functions by hardware experiments,
microwave modelling may be employed and will also allow studies of the various
field patterns and intensities to assist in the work.
A third embodiment of the present invention relates to the design and use of
multiple, low and closely stacked applicators to achieve high power densities
in
elongated or moving loads. The TE20 mode can in theory exist in a waveguide
with arbitrarily small height, but there are of course practical limitations
by the
fact that the waveguide (integrated) impedance is proportional to its height,
requiring a very large transformation ratio from the typically standard height
of
between a quarter and a half free space wavelength at magnetron generator
transition to the TE 10 portion.
There are, however, generally no problems when the height is changed in one
short step 7 as shown in figure 5, by a factor of up to 3. This is then
normally in
the TE20 section as shown in the same figure. The step can also be used to
improve the choking function, as described for the overall length of the TE20
section for the second embodiment of the present invention.
An important aspect of the present invention in conjunction with the use of
very
low applicator heights is that the load location is where the electrical field
of the
TE20 mode (there is in essence only a vertical such field) is minimum. Hence,
the risk of arcing when high power is used is very much less than with
rectangular TE 10 applicators (or, equivalently, cylindrical TMOnO
applicators).
By the combined use of multiple 90 displaced applicators with mutual choking
function, extremely high heating intensities can quite easily be achieved also
with typical magnetron powers, without any risk of arcing.
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As an example when using 2450 MHz, a TE20 section height of 12 mm with a
load diameter of 30 mm and 3 kW microwave generators in a 6-applicator
system (plus two non-energised end-choking applicators) will result in 18 kW
over a total length of 8 x 14 mm = 112 mm, i.e. 80 mL. With a specific heat
5 capacity of the load of half of that of water, the heating rate then becomes
over
100 K/ second. Such heating rates may be desirable in pharmaceutical
microwave chemistry applications, where polar liquids with reactants are very
rapidly heated under high pressure to over 200 C. Of course, larger systems
using the other common microwave heating frequency band using a frequency
10 around 915 MHz can achieve the same heating rate with commercially
available
magnetrons of 30 kW and higher. Such applications may include very rapid
expansion causing cell wall rupturing in some types of hardwood, where a
slower
heating rate would result in energy waste by loss of pressure by diffusion
thus
requiring prolonged heating time; or malfunction of the process by rupturing
not
occurring at all.
An example of the choking function also confining the heating pattern to only
the energised applicator is shown in Figure 4 where an upper and a lower
applicator are indicated.
The two stacked waveguide applicators (as illustrated in figure 2) are 25 mm
high (b dimension) and the TE10 and TE20 sections are 86 and 172 mm wide (a
dimension), respectively. The load diameter is 40 mm, its permittivity is 25-
j6,
the load is contained in a 5 mm material thickness glass tube with
permittivity 4
and the operating frequency is 2450 MHz. The distance from the TE20 shorting
wall to the centrally located load axis is 28 mm; the metal rod has a diameter
of
17 mm and is located 10 mm to the left (in the direction of the TE10 H knee
inner corner) and 80 mm from the TE20 shorting wall. There is a protective
metal tube below and above the load, outside the applicators (indicated as 4
in
figure 2). Only the lower applicator is energised. With a mode transducer
optimised triangular cut in the outer H knee corner of 29 mm at the TE 10 side
and 86 mm at the TE20 side (as indicated in for example Figure 1) and an
optimised distance between the TE20 shorting wall to the opposite side wall of
210 mm, the transmission factor between the two TE 10 ports of the applicators
becomes 0,03 (which is the same as -30 dB crosstalk power).
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In a fourth embodiment of the present invention additional metal rods 8 are
used as shown in Figure 6, with loads of such cross sectional size or shape
that
some deviations from the sin 2 angular variation occurs. Such variations are
primarily caused by internal resonance effects in the load, or by non-resonant
edge diffraction if the load has axial edges. The method for determining the
locations and sizes of these rods is again primarily by microwave modelling.
It is
then generally preferred to arrange four rods in a square pattern if the load
cross
section is also square (as in figure 6), to maintain the capability for
choking by
adjacent applicators. The rod pattern can then be varied by both side length
and
angular position in relation to the TE20 waveguide axis direction.
An example of heating pattern in the central cross section plane of a 100 x
100
mm square, long load with permittivity 30J3 at 915 MHz in an applicator with
60 mm height and 500 mm TE20 section width is shown in Figure 7. The
heating pattern is illustrated by using "++" for the warmest part, "+" for the
next
warmest parts and so on to the coldest part that is indicated with a "-". In
this
case there are no rods or other devices, and the load axis is 126 mm from the
shorting wall and displaced by 18 mm from the applicator centreline. It is
seen
that the heating pattern becomes quite even with two, and even more so with
four 90 displaced applicators.
According to a fifth embodiment of the present invention the applicator is
substantially thicker at least in the part of the TE20 mode section where the
load is arranged than in the TE10 mode section, in a direction perpendicular
to
the major wave propagation. This fifth embodiment is illustrated in figures 8
and
9.
Thus, the present invention also includes applicators with larger heights, up
to
more than a full free space wavelength.
Even if it may be possible to successfully just increase the applicator height
(7'
in figure 8) by making either a step or a slope 7 as shown in figure 5 (but
now to
a larger instead of a smaller height) to fit a load higher than about a half
free
space wavelength, and then obtain a reasonably even heating in the axial
direction, typical variations in load permittivity and load filling height
will almost
inevitably result in heating concentrations at either load end.
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A refinement of this embodiment of the invention is to then use metal plates
parallell to the broad sides (floor and ceiling) of the applicator. One metal
plate 8
is seen in figures 8 and 9. These plates may be in continuous galvanic contact
with the side (vertical) applicator walls, but that is not necessary for
proper
function. A plate acts as a mode filter, prohibiting propagation of other than
TE20p modes, provided the (vertical) distance between any plate(s) and the
applicator floor or ceiling does not exceed about a half free space
wavelength.
Several plates may thus be used.
An extension of this embodiment is to firstly employ an upwards slope 7' from
a
part of the applicator near or in its feed by a TE10 waveguide, or near the
dielectric rod feed, being the transducer means according to the sixth
embodiment described below, and secondly use a metal plate which extends to a
position rather close to the slope. This is illustrated in figure 8 where the
metal
plate 8 extends close to the waveguide slope 7' and the opposite applicator
side
wall in one cross section, and from the side wall of the TE10 waveguide almost
all the way to the load in the perpendicular cross section.
Figure 9 schematically illustrates the fifth embodiment from above where is
shown the TE20 mode section 12 provided with a metal plate 8, a load 4 and a
tuning means 5.
It is also possible to use plates, which are bent up-, or downwards in the
feed
region, to achieve the same goal which is to split the incoming power in a
controlled way, to achieve an improved heating evenness in the axial direction
of
the load.
By using one or two metal plates as just described, it is possible to use
applicator and load heights up to and exceeding a free space wavelength of the
microwaves, while maintaining a reasonably even heating in the axial
direction,
for limited intervals of liquid column height but for wide variations of the
dielectric properties of is as a load.
According to a sixth embodiment of the present invention a generalised
transducer means is arranged between the waveguide transition between the
TE10 mode section and TE20 mode section. This generalised transducer means
will be described with references to figures 10 and 11. The transducer means
is
applicable to all embodiments of the present invention described herein.
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Figure 10 shows a cross-sectional view of the sixth embodiment of the present
invention and figure 11 shows a view from above schematically illustrating the
same embodiment.
Figure 10 a schematic illustration showing the TE 10 mode section 14, a
transducer means 10 and the TE20 mode section 12. The same features are
shown in figure 11 that in addition show the load 4 and the tuning means 5.
The transducer means 10 includes a dielectric-filled waveguide carrying the
same mode as the rectangular TE10, which is equivalent to the circular TE 11
mode.
There is often a need for separating the generator and applicator parts of the
system, so that for example noxious gases or load spillage cannot escape out
from the applicator towards the generator and other ancillary equipment. There
may also be a need to heat the liquid load to temperatures above its boiling
temperature under atmospheric pressure. Such pressurised windows are just
variable thickness, microwave transparent plates under mechanical pressure
between two TE10 waveguide flanges. The impedance mismatching due to the
plate is commonly so small (since the plate is relatively thin) that
compensation
is made by simple discrete components such as metal posts in the waveguide.
For thicker windows, the fact that a half wavelength thick plate (of the
window
material) may minimise reflections may be employed. Conical tapering into both
the mating waveguides using low permittivity plastic material bodies is
another
possibility.
According to this sixth embodiment of the present invention a mode transition
between the TE 10 airfilled waveguide and a circular TE 11 or rectangular TE
10
mode in the form of the transducer means 10 being a dielectric filled metal
tube
or bore. Such a transducer means is fed from a symmetrically located hole in
the
shorted end of the TE10 waveguide and is impedance matched without any
additional means. The length of the dielectric-filled waveguide portion can
therefore be arbitrarily long. This design is inherently different to prior
art
windows by the intermediate dielectric-filled waveguide section being
impedance
matched to the airfilled waveguide.
A preferred design of the transducer means is shown in figure 10, where a
rectangular TE10 waveguide 14 has a lower height (commonly labelled b
dimension) than the other similar waveguide 12. A circularly cylindrical
ceramic
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body 10 protrudes certain but different distances into the waveguide ends, and
is surrounded by metal between the waveguides. There are no additional
matching components.
This type of matched transducer means requires certain dielectric data and
diameters of the body, in relation to the rectangular waveguide dimensions and
operating frequencies, in order for a sufficiently broadband impedance
matching
to be achieved. As a first example, with the standard WG340 (43x86 mm)
waveguide in the 2450 MHz ISM band, an alumina rod with permittivity 9 must
be about 29 mm in diameter and protrude about 25,5 mm into the waveguide.
As a second example, with a 60x86 mm waveguide and a rod with permittivity
6,8, its diameter must be about 38 mm and the protrusion must be about 28
mm.
Establishing optimum dimensions for waveguides and rods with other data can
be made by experiment or numerical microwave modelling, using the start data
above. This also applies when the rod has a square or rectangular cross
section.
If one of the waveguides is subjected to pressure, for example by the
applicator
being a direct continuation of the waveguide 12, the protruding part of the
rod
10 can be made slightly wider than the rest, so that the rod cannot slide
away.
The protrusion length of the wider part must than be made somewhat shorter.
Other deviations from the cylindrical shape can also be employed for the
purpose, and are all within the scope of the invention as defined by the }
appended claims.
When using a rod feed of the type just described, it is not necessary to feed
the
applicator via a TE10 waveguide. Instead, the rod may be protruding directly
into the TE20p applicator. This is shown in figure 11 where the applicator 12
with a load 4 and a tuning means 5 is disclosed.
According to an additional improvement of the present invention in particular
with regard to the insensitivity to liquid column height variations is to
employ
rod-shaped dielectric bodies with rather high permittivity, parallell to the
metal
rod 5. The rods must then have a permittivity comparable with that of the
liquid
load, and also a comparable cross section area. As an example, two rods with
permittivity 20 and diameter 30 mm are located close to the load, on each side
of
the TE20 centreline. The sensitivity to liquid column height variations, as
well as
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to load permittivity variations, is then reduced. Also the impedance matching
variations for variations of these load parameters is reduced.
A typical applicator for 2450 MHz will have horizontal dimensions about 170 x
5 210 mm, plus the prolongation by a TE 10 feed waveguide. With a diameter of
the
load container of about 55 mm, the filling factor (load volume divided by
applicator volume) becomes quite small. There may be instances when it is
desirable to reduce the applicator dimensions. This can then be made by three
methods:
1.Folding down or up the outer parts of the TE20 part (i.e parallel to the
power flow direction) so that an inverted U shape is created. The applicator
feed is then from below or above. However, this method is not efficient if the
waveguide applicator height is large.
2.Inserting metal ridges in the TE20 part, in the same way as in standard
ridged waveguides. This means that two ridges, ending on each side of the
load, are introduced.
3.Inserting partial dielectric filling in the TE20 part. As an example, using
PTFE with about 50 % filling factor, the l70x210 mm dimensions can be
reduced to about 125x155 mm.
As a further alternative, in particular with regard to the above-mentioned
second
method related to the ridged waveguide, the waveguide (the TE20 mode section)
is filled (or partly filled) with a dielectric material, e. g. PTFE or a
ceramic
material. This is mainly in order to decrease the size of the TE20 mode
section.
The present invention also relates to the use of the applicator, the system or
the
method for performing organic chemical synthesis reactions, and also for very
rapid heating of wood, for cell wall disruption or similar.
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16
Within the scope of the invention as it is defined by the appended claims also
the
following exemplary structural alternatives are included:
= The metal rods must not go the whole way between the major planes of the
waveguides
= Instead of using rods, metal plates may be used.
= The metal plates may be replaced by dielectric inserts or tubing, for
example
alumina ceramic.
= In order to achieve an improved heating at the load axis, the load may be
displaced somewhat from the position which gives a symmetrical heating
pattern.
= The load may be in a microwave transparent tube or holder.
= The load may be short and entirely located inside a single applicator.
= The TE 10 section may be bent and extended so that there is sufficient space
for the generators also when multiple, low stacked applicators are used
= Systems may be designed for any microwave frequency, depending on the
load dimensions, dielectric properties and required capacity of the system.
For
reasons of availability of generators, and since the systems are primarily
foreseen for high power density applications, the standard frequencies about
2450 and 915 MHz are preferred.
