Note: Descriptions are shown in the official language in which they were submitted.
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PCT/DE97/01463
Apparatus for Tuning the Frequency of a Magnetron
There are times when it is necessary to periodically tune
the frequency of a microwave system incrementally. Using the
tuning elements that have been referred to, the penetration depth
of which can be varied, this is made possible only by using a
very costly drive system. Moreover, this means that no high
adjustment speeds can be achieved. A microwave system with which
frequency tuning is meant to be carried out at a very small
period (approximately 50 ms) is a magnetron, which is used to
heat substances in a microwave oven. The lossy dielectric
properties of the substances that are to be heated convert the
microwave energy into heat. Since such a microwave oven is a
resonator in which many wave modes are excitable, an electrical
field configuration with locally fixed maxima and minima is
formed when the resonator is excited with a specific frequency.
The resulting thermal energy conforms to this field image and
forms locally fixed hot and cold areas. Thus, it is impossibl~
to achieve even heating of the substance in the microwave oven.
This disadvantage can be avoided in that the frequency of the
magnetron is varied. As the frequency changes, so does the
configuration of the field within the oven, i.e., the hot and
cold areas shift. This makes more even heat distribution within
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the oven possible. The frequency of the magnetron is best tuned
periodically, with a period duration of the frequency tuning of
approximately 50 ms being desirable.
EP O 344 438 describes a microwave oven to which the
electromagnetic field generated by a magnetron is connected
through a hollow waveguide. In order to arrive at the most even
possible distribution of energy density within the oven, a so-
called field agitator is arranged within the hollow waveguide.
This agitator is arranged relative to the connection opening in
the hollow waveguide and in the oven so that it affects the field
that is introduced into the oven in such a way that even
distribution of the energy density results.
GB 681 801 A and DE 10 02 053 C describe hollow waveguide
tuning devices in the form of disks that incorporate an eccentric
1() stub and are supported within the wall of the hollow waveguide so
as to be rotatable.
It is the objective of the present invention to describe an
apparatus that requires the least costly mechanical drive syst~m
in order to complete incremental frequency tuning of a magnetron
l~ that changes very rapidly on a periodic basis.
According to the present invention, this objective has been
achieved by the features set out in Patent Claim 1. According to
this, the penetration depth of one or more tuning elements is not
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changed for incremental frequency tuning; rather, the location of
the tuning element(s) is changed by rotating them about an axis
that is perpendicular to the longitudinal axis of a hollow
waveguide that is connected to the magnetron. Such rotation can
be executed are very high speed, with very little wear, by a
simple motor drive.
As set out in the secondary claims, it is advantageous to
arrange the tuning elements on one or a plurality of rotatable
disks. In order to avoid interruption of the hollow waveguide
wall currents to the rotating disks, it is expedient to arrange a
choke connection between the disks and the walls of the hollow
waveguide.
The present invention will be described in greater detail on
the basis of several exemplary embodiments shown in the drawings
l~ appended hereto. These drawings show the following:
Figure 1: a longitudinal cross section through a hollow waveguide
with a rotatable tuning device;
Figure 2: a plan view of this hollow waveguide, in direction A;
~~ Figure 3: a longitudinal cross section through a hollow waveguide
with a rotatable inductive loop;
Figure 4: A Rike diagram.
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Figure 1 shows a longitudinal cross section through a hollow
waveguide 1 that is connected, for example, to a magnetron 11 in
order to tune the frequency of this periodically. To this end,
within the hollow waveguide 1 there are two stubs 2 and 3 as
tuning elements, and these are supported so as to be able to
rotate about a axis 4 that is oriented so as to be perpendicular
to the longitudinal axis of the hollow waveguide. By rotating
the stubs 2 and 3, which are spaced apart from the axis of
rotation by a distance that is approximately one-quarter the
lo wavelength of the hollow waveguide, their phase effect is changed
from capacitive to inductive. In addition, as well as the phase,
the amount of the reflection factor also changes, since the
distance of the stubs 2 and 3 from the centre of the waveguide
also changes.
1~ Unlike the embodiment shown in Figure 1, instead of the
two stubs 2 and 3 it is possible to rotate either a single stub
or more than two stubs. In the same way, as is shown in Figure
3, in place of capacitive stubs, one or a plurality of inductive
loops 5 can be rotated about the axis 4 within the hollow
2~ waveguide 1. The number of tuning elements, the type of these
(stubs or loops) and their distance from the axis of rotation
will be determined according to which variation of the phase and
the amount of the reflection factor that is to be achieved.
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One or more fixed tuning elements 6 can also be
installed in the hollow waveguide in addition to the rotatable
tuning elements. Because of the interaction of the rotating
tuning element(s) 2,3,5 relative to the fixed tuning element(s)
6, the total reflection factor traces a curve in the Smith
diagram that can be approximated to a desired curve by suitable
dimensioning of the tuning elements and by their spacing relative
to each other.
As can be seen from the longitudinal cross sections
through the hollow waveguide l shown in Figures 1 and 3 and the
plan view of the hollow waveguide, in direction A, shown in
Figure 2, the tuning elements 2,3,5 are arranged on a circular
disk 7 that is supported so as to be able to rotate within the
hollow waveguide wall. The disk 7 is mounted on a drive shaft 8
l~ by which the disk 7 can be rotated by means of a motor. The disk
7 is supported in such a way in an opening in a wall of the
hollow waveguide that it is free of contact. In order to avoid
interruption of the hollow waveguide wall currents to the
rotating disk 7, a collar 9 is installed on the wall of the
hollow waveguide at the edge of the opening, and in the same way,
a collar 10 is installed on the edge of the disk 7. Both collars
g and 10 are of a height that is approximately that of one-
quarter of the wavelength ~ and form a choke, i.e., the open
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circuit at the upper end of the gap between the two collars 9 and
10 is transformed into a short circuit at the lower end (in the
wall of the hollow waveguide).
A plurality of rotating disks with one or more tuning
S elements can also be used in the hollow waveguide.
As has already been discussed, the tuning device that
has been described can be connected to a magnetron 11 in order to
adjust its frequency on a periodic basis. In order to tune the
frequency of a magnetron it is possible to use the fact that the
frequency and the output power of a magnetron are dependent on
the value of the load resistance, which can be adjusted by means
of the tuning device. This dependency is illustrated in the Rike
diagram that is shown in Figure 4. The Rike diagram shows the
frequency and power behaviour of a magnetron that is connected to
1~ different load resistances of various reflection factors. On
connection with the characteristic impedance (mid-point of the
diagram), the maximum power (> 12kW) will be at the frequency f0.
The constant-power curves are arranged elliptically around the~
mid-point. The dashed lines are constant-frequency curves. The
2() hashed area represents the prohibited zone. If the magnetron is
operated with reflection factors within this prohibited zone,
mode shifts that could result is its destruction may occur within
the magnetron. If a magnetron is to be tuned periodically over a
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wide range of frequencies, without any notable loss of output
power, the reflection factor of the terminal resistance can-, for
example, conform to the power curve for 11.5 kW, from the left-
hand limit of the prohibited zone as far as the right-hand limit,
along the constant-power curve and back.
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