Electric heating – Microwave heating – Waveguide applicator
Reexamination Certificate
2000-06-01
2001-08-14
Leung, Philip H. (Department: 3742)
Electric heating
Microwave heating
Waveguide applicator
C219S697000, C219S746000, C219S750000, C333S249000
Reexamination Certificate
active
06274858
ABSTRACT:
BACKGROUND OF THE INVENTION
Microwave ovens, now a more or less permanent fixture in many home kitchens, increasingly find use in high volume industrial applications. For example, the tempering of large quantities of frozen meat, fish, poultry and fruit is greatly enhanced with the use of microwave ovens. Not only do microwave ovens provide for greater predictability in processing, they also eliminate an otherwise several hour wait time to thaw a frozen product prior to its availability for use, while minimizing drip loss and improving sanitation.
It has been known for some time that microwave ovens preferably include some type of structure for promoting uniformity of microwave energy distribution within the cooking cavity. This is because in a typical box shaped microwave oven, the spatial distribution of the microwave energy tends to be non-uniform. As a result, hot spots and cold spots are produced at different locations. Cooking results are therefore unsatisfactory under such conditions because some portions of the food may be completely cooked while others are barely warmed. This problem becomes more severe with foods which do not readily conduct heat from the areas which are heated by microwave energy to those areas which are not. An example of a food falling within this class is cake. However, other foods frequently cooked in microwave ovens, such as meat, also produce unsatisfactory results if the distribution of energy within the cavity is not uniform.
The conventionally accepted explanation for non-uniform cooking patterns is that electromagnetic standing wave patterns, known as modes, are set up within the cooking cavity. Within such a standing wave pattern, the intensities of the electric and magnetic fields vary greatly with position. The precise configuration of the standing wave or mode pattern is dependent upon at least the frequency of the microwave energy used to excite the cavity and upon the dimensions of the cavity itself.
There have been a great many approaches proposed for alleviating the problem of non-uniform energy distribution. A common approach, used in many domestic microwave ovens, is a device known as a mode stirrer. A mode stirrer typically resembles a fan having metal blades. The mode stirrer rotates continually to alter the mode pattern within the cooking cavity. The mode stirrer may be placed either within the cooking cavity itself (usually protected by a cover constructed of a material that is transparent to microwaves), or to conserve space within the cooking cavity, may be mounted within a recess formed in one of the walls adjacent the cavity. Another approach is to use a carousel tray within the oven cavity, which rotates the food itself.
Yet another approach to the problem of non-uniform energy distribution is to introduce a polarized energy beam into an oven cavity using a number of phased feed points, or to use an antenna including one or more planar conductive plates that are mechanically rotated.
Unfortunately, while these approaches work somewhat for power levels typical of microwave ovens intended for use in domestic kitchens, they are not particularly adaptable for use in high volume industrial applications. It is not uncommon for an industrial microwave oven, for example, to be required to process several hundred kilograms of product in a several minute time span, producing radio frequency energy levels of 50 kiloWatts (kW) or more. The known approaches have limitations in power handling due to the use of coaxial sections which intrinsically have less power capability than a waveguide. In addition, the rotating parts used to vary the energy polarization also have power handling and reliability limitations.
One solution to this problem has been described in U.S. Pat. No. 6,034,362 issued to Alton and assigned to The Ferrite Company, Inc. In that design, microwave energy is coupled to many modes of a microwave cavity by generating a circularly polarized microwave signal, whereby a polarization vector of the microwave energy continually rotates. The coupling device includes a transformer to match from an input waveguide polarization, such as provided by a rectangular waveguide, to a circular or square polarization waveguide section. The polarization waveguide section contains an asymmetrical insert element disposed within it such that in the region of the asymmetrical element, electromagnetic symmetry is about a symmetry plane only. The position and dimensions of the polarization insert are selected to introduce a difference in electrical phase of 90° for polarizations which are parallel to and perpendicular to the symmetry plane.
By introducing two linearly polarized components which are 90° out of phase with one another, the resulting sum of microwave energies produced at the output end of the polarizer is circularly polarized with constant amplitude, but with an angle of polarization that continuously rotates. Since the polarization vector continually rotates, microwave energy is coupled to many modes of the enclosure as a result.
SUMMARY OF THE INVENTION
The present invention is a technique for coupling microwave energy to many modes of a microwave oven cavity, by generating a circularly polarized microwave signal in which a polarization vector of the microwave energy continually rotates.
A device constructed according to the invention includes at least one polarization waveguide section and at least one waveguide section having a bend.
The polarization waveguide section contains an asymmetrical polarization insert element disposed within it, such that in the region of the asymmetrical element, electrical symmetry is presented about a symmetry plane only. The position and dimensions of the polarization insert are selected to introduce a difference in electrical phase for polarizations which are parallel to and perpendicular to the symmetry plane. By introducing two linearly polarized components which are out of phase with respect to one another, the resulting sum of microwave energy is circularly polarized with constant amplitude but with an angle of polarization that continually rotates.
The bent waveguide section is coupled to either the input or output of the polarization section. The bent waveguide section has a bend along a major axis of energy propagation through it. The bend is such that an inner radius of the bend is shorter than an outer radius of the bend along the direction of propagation. The bend introduces a phase difference depending upon whether the microwave energy vector is in the plane of or perpendicular to the plane of symmetry of the bend. The bent waveguide section therefore compliments the polarization section by presenting an additional electrical symmetry about a symmetry plane.
The phase difference introduced by both the polarization section and the bent section thus together provide the required overall 90° difference in electrical phase. With proper orientation and dimensioning of the bent section, the required phase shift can be provided largely by the bend. This reduces the need for the phase difference to be supplied by the polarization section, which in turn permits the polarization section to be significantly reduced in length.
The ability to make use of multiple configurations of bends in waveguide sections not only provides compactness, but also provides additional versatility in the manner of connecting from a microwave energy source to a cooking cavity. For example, certain applications require a compact, low-height feed structure, and the availability of circular bent sections greatly assists in the mechanical layout of such systems.
The invention provides circularly polarized microwave energy with a phase angle that continuously varies to energize many modes of an oven cavity. The continuously varying energization is achieved with no moving parts and with greatly increased power handling capability, since only waveguide type components are used, with no need for mechanically rotating sections, patch antennas, or coaxial lines.
The feed is of particular use in ovens which are adapted for b
Hamilton Brook Smith & Reynolds P.C.
Leung Philip H.
The Ferrite Company
LandOfFree
Bends in a compact circularly polarized microwave feed does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Bends in a compact circularly polarized microwave feed, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Bends in a compact circularly polarized microwave feed will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2486992