GB2391154A - Dielectric resonator antennas for use as microwave heating applicators - Google Patents

Abstract

There is disclosed a microwave heating device in which microwave energy from a magnetron is directed into a multi-mode cavity by way of one or more dielectric resonator antennas disposed at a periphery of the cavity. Additional microwave sensors, which may also be configured as dielectric resonator antennas, may also be provided at the periphery. By using appropriate feedback and control circuitry, food or other items may be heated in the oven cavity according to a predetermined heating profile and without the need for mode stirrers or other moving parts.

Description

i DIELECTRIC RESONATOR AtITEtINAS FOR USE AS MICROWAVE lIEAl1NG

APPLICATORS The present invention relates to dielectric resonator antennas (DRAs), and in 5 particular to DRAs used as microwave heating applicators.

Dielectric resonator antennas (DRAB) are resonant antenna devices that radiate or receive radio waves at a chosen iloquency of transmission and reception, as used in for example in mobile telecommunications. In general, a DRA consists of a volume 10 of a dielectric material disposed on or close to a grounded substrate, with energy being transferred to and from the dielectric material by way of monopole probes inserted into the dielectric material or by way of monopole aperture feeds provided in the grounded substrate (an aperture feed is a discontinuity, generally rectangular in shape, although oval, oblong, trapezoidal or butterfly/bow tie shapes and 15 combinations of these shapes may also be appropriate, provided in the grounded substrate where this is covered by the dielectric material. The aperture feed may be excited by a strip feed in the form of a microstrip transmission line, coplanar waveguide, slotline or the like which is located on a side of the grounded substrate remote from the dielectric material). Direct connection to and excitation by a 20 microstrip transmission line is also possible. Altclatively, dipole probes may be inserted into the dielectric material, in which case a grounded substrate is not ! required. By providing multiple feeds and exciting these sequentially or in various combinations, a continuously or incrementally steerable beam or beams may be formed? as discussed for exernple in the present applicant's co-pending US patent 25 application serial number US 09/431,548 and the publication by K]NGSLEY, S.P.

and O'KEEFE, S.G., "Beam steering and monopulse processing of probe-fed dielectric resonator antennas", SEE Proceedings - Radar Sonar and Navigation, 146, 3, 121 - 125? 1999, the full contents of which are hereby incorporated into the present application by reference.

! The resonant characteristics of a DRA depend, inter alla, upon the shape and size of the volume of dielectric material and also on the shape, size and position of the feeds thereto. It is to be appreciated that in a DRA, it is the dielectric material that resonates when excited by the feed. This is to be contrasted with a dielectrically 5 loaded antenna, in which a traditional conductive radiating element is encased in a dielectric material that modifies the resonance characteristics of the radiating element. DRAs may take various forms, a common form having a cylindrical shape which 10 may be fed by a metallic probe within the cylinder. Such a cylindrical resonating medium can be made from several candidate materials including ceramic dielectrics.

Since the first systematic study of dielectric resonator antennas (DRAB) in 1983 [LONG, S.A., McALLISTER, M.W., and SHEN, L.C.: "The Resonant Cylindrical 15 Dielectric Cavity Antenna", EKE Transactions on Antennas and Propagation, AP-3 1, 1983, pp 406-412], interest has grown in their radiation patterns because of their high radiation efficiency, good match to most commonly used transmission lines and small physical size [MONGIA, R.K. and BaARTIA, P.: ''Dielectric Resonator Antennas - A Review and General Design Relations for Resonant Frequency and 20 Bandwidth", International Joumal of Microwave and Millimetre-Wave Computer-

Aided Engineering, 1994, 4, (3), pp 230-247. A summary of some more recent

developments can be found in PETOSA, A., ITTIPIBOON, A., ANTAR, Y.M.M., ROSCOE, D., and CUHACI, M.: "Recent advances in Dielectric-Resonator Antenna Technology,', EKE Antennas and Propagation Magazine, 1998, 40, (3) , pp 35 - 48.

A variety of basic shapes have been found to act as good DRA resonator structures when mounted on or close to a ground plane (grounded substrate) and excited by an appropriate method. Perhaps the best known of these geometries are:

! Rectangle [McALLISTER, M.W., LONG, S.A. and CONWAY G.L.: "Rectangular Dielectric Resonator Antenna", Electronics Letters, 1983, 19, (6), pp 218219].

S Triangle [rI1IPIBOON, A., MONGIA, R.K., ANTAR, Y.M.M., BHARTIA, P. and CUHACI, M.: "Aperture Fed Rectangular and Triangular Dielectric Resonators for use as Magnetic Dipole Antermas", Electronics Letters, 1993, 29, (23), pp 2001 2002].

10 Hemisphere [LEUNG, K.W.: "Simple results for conformal-strip excited hemispherical dielectric resonator antenna", Electronics Letters, 2000, 36, (11)].

Cylinder [LONG, S.A., McALLISTER, M.W., and SHEN, L.C.: "The Resonant Cylindrical Dielectric Cavity Antenna", EKE Transactions on Antennas and 15 Propagation, AP-31, 1983, pp 406412].

Half-split cylinder (half a cylinder mounted vertically on a ground plane) [MONGIA, R.K., lTTIPIBOON, A., ANTAR, Y.M.M., BHARTL\, P. and CUHACI, M: "A Half-Split Cylindrical Dielectric Resonator Antenna Using SlotCoupling", 20 EKE Microwave and guided Wave Letters, 1993, Vol. 3, No. 2, pp 38-391.

i Some of these antenna designs have also been divided into sectors. For example, a cylindrical DRA can be halved [TAM, M.T.K. and MURCH, R.D.: '4Half volume dielectric resonator antenna designs", Electronics Letters, 1997, 33, (23), pp 1914 25 1916]. However, dividing an antenna in half, or sectorising it further, does not change the basic geometry from cylindrical, rectangular, etc. Microwave heating is a very well known technique for processing a range of materials from food to nabber. In general, there is provided a generator of microwave 30 energy (often a magnetron), a transmission system (cable or waveguide) and an energy delivery system. Sometimes the magnetron supplied microwave energy

directly to the energy delivery system without passing through a transmission system.

The energy delivery system generally comprises a component that launches the microwave energy and an enclosure into which the microwave energy is launched.

The enclosure makes two contributions to the energy delivery system: firstly, the 5 enclosure contains the microwave energy (for safety reasons); secondly, the enclosure acts as a set of walls from which the microwave energy is reflected.

It is known to provide an enclosure in the form of a multi-mode cavity, for example an interior of a microwave oven. The aim of the energy delivery system in a typical 10 multi-mode heating system is to deliver microwave energy substantially unifonnly to an item being heated (e.g. food) so that it heats evenly. Microwave energy is launched into the cavity and reflects between the walls thereof until it is incident on the item being heated at which point the microwave energy is absorbed. A small amount of energy is lost at each reflection, and this energy is dissipated at heat in the 15 walls of the cavity.

In general there are problems with conventional energy delivery systems, such as directing power where required, efficiency and minimum size.

20 The aim of producing a uniform pattern of energy is rarely achieved, since some items being heated require different amounts of energy at different parts of their f volumes. For example, when cooking a chicken in a microwave oven, the wings and legs of the chicken will require less microwave energy to be absorbed than the body of the chicken, since the wings and legs have a smaller volume and will thus heat up 25 more quickly.

Another problem is that it is generally difficult to detect when an item being heated has reached a desired temperature or state (e.g. when food has been sufficiently cooked).

According to a first aspect of the present invention, there is provided a microwave heating device comprised as a dielectric resonator antenna According to a second aspect of the present invention, there is provided a method of 5 applying microwave energy, wherein microwave energy generated by a microwave generator is supplied to a dielectric resonator antenna and transmitted therefrom.

According to a third aspect of the present invention, there is provided a heating device comprising a microwave generator connected to at least one microwave I O transmitter in the form of a dielectric resonator antenna According to a fourth aspect of the present invention, there is provided a microwave generator including at least one dielectric resonator antenna adapted to transmit microwave energy from the microwave generator.

By using one or more DRAs (in place of conventional antennas or waveguides) to launch microwave energy, the energy may be delivered more accurately to an item being heated.

20 The microwave generator may be a magnetron or the like. The microwave generator may include at least one DRA as an integral component thereof to serve as a microwave launcher.

The heating device of the third aspect may include a heating chamber or cavity in the 25 manner of a conventional microwave oven or heater, the at least one DRA being mounted at a perimetral portion of the chamber.

Preferably, more than one DRA is provided in the heating device of the third aspect, each DRA being selectively activatable, either alone or in combination with other 30 DRAB, so as to direct microwave radiation to predetermined regions within the heating chamber. The amount of energy supplied to each DRA by the microwave

l generator (e.g. a magnetron) may be varied, continuously or step-wise, by appropriate circuitry so as to provide a desired heating profile within the heating chamber.

Advantageously, at least one and preferably a plurality of additional DRAs are 5 provided, preferably also at perimetral portions of the chamber, the additional DRAs being configured as sensors to detect reflected or transmitted microwave signals within the chamber. The sensor DRAB, together with suitable control circuitry or the like, can help to determine a heating profile within the chamber and thereby assist in controlling the microwave generator and/or the at least one microwave applicator I O DRA (e.g. by varying the amount of supplied energy and/or by selectively activating/deactivating one or more microwave applicator DRAs or the microwave generator) so as to meet predetermined heating requirements.

The sensor DRAs may also serve as microwave launcher Drabs, or may be adapted 15 only to sense reflected microwave energy.

Where the heating device is a microwave oven, the at least one sensor DRA can detect when food is cooked because of detectable changes in the sensed signals due to the dielectric properties of the food changing with temperature and degree of 20 cooking. Through provision of appropriate control circuitry, it is possible to set a predetermined cooking temperature for a predetermined type of food by way of i feedback control via the at least one sensing DRA.

In addition, the at least one sensor DRA can detect if there is nothing inside the 25 heating chamber when microwave energy is applied, and can cause a control signal to be issued to switch off the microwave generator and/or the microwave applicator DRA so as to prevent damage thereto.

The at least one sensor DRA may also issue control signals to change an angle of at 30 least one microwave-reflecting surface in a vicinity of the at least one microwave applicator DRA or a power launcher of the microwave generator, thereby helping to

steer or direct microwave energy within the chamber to parts thereof where it is detected that specific heating is required. Control of the reflective surface may be achieved by way of any appropriate means, e.g. servo motors or the like.

5 Alternatively or in addition, the at least one microwave applicator DRA may be configured as a steerable DRA, i.e. one having a plurality of feeds which are activatable individually or in combination so as to cause a beam of microwave energy to be steered in azimuth and/or elevation within the heating chamber. In this way, microwave energy may be directed to where it is needed most, for example towards 10 relatively dense parts of an article being heated. Steering of the microwave applicator DRA is advantageously achieved by way of feedback control signals fiom the at least one sensor DRA. For example, a griddle or the like may be constructed with a plurality of microwave applicator DRAs (e.g. cylindrical DRAB) adapted to heat food or the like placed on the griddle.

Both of the steering mechanisms discussed above may be used as an alternative to or in addition to conventional mechanisms such as rotating turntables or mode stirrers (which are generally cumbersome and/or inefficient). In this way, it is possible to achieve substantially unifonn energy density throughout an article being heated in the 20 oven (e.g. food being cooked or rubber being vulcanized).

Microwave ovens and/or heaters of embodiments of the present invention are also advantageous in that moving parts (e.g. wavestirrers and the like) associated with conventional microwave ovens and/or heaters may be omitted, since even heating of 25 an item within the oven and/or heater may be achieved by selective activation and/or steering of one or more DRAs via solid-state control electrorucs.

For a better understanding of the present invention and to show how it may be carried into effect, reference shall now be made, by way of example only, to the 30 accompanying drawings, in which:

FIGURE 1 shows a conventional microwave oven; and FIGURE 2 shows a microwave oven of an embodiment of the present invention.

5 With reference to Figure 1, there is shown a conventional microwave oven comprising a multi-mode cavity 1 and a microwave generator in the form of a magnetron 2 having a power supply 3. The magnetron 2 is configured so that microwaves are launched into the cavity 1 via an emitter 4. A rotating metallic mode stirrer 5 is provided close to the emitter 4 so as to mix the microwave modes and to 10 help prevent standing waves and the like. There is further provided a rotating turntable 6 on which food 7 to be heated is placed. Both the mode stirrer 5 and the turntable 6 are required in order to help reduce 'Dot spots" in the cavity l and the food 7.

15 With reference to Figure 2, there is shown a microwave oven of an embodunent of the present invention. As in Figure 1, there is provided a multi-mode cavity 1 and a magnetron with a power supply (not shown in Figure 2). Food 7 is located within the cavity 1 for heating. However, instead of the magnetron emitting microwave energy directly into the cavity 1 by way of an emitter 4 and a mode stirrer 5, a plurality of 20 dielectric resonator antennas 8 each connected to the magnetron is disposed about a periphery of the cavity 1 and is configured to radiate microwave energy into the cavity 1. There is additionally provided a plurality of sensors 9 (which may also be configured as dielectric resonator antennas) disposed about the periphery of the cavity 1, in this case interspersed between the dielectric resonator antennas 8.

25 Through the use of appropriate control circuitry (not shown), the sensors 9 can detect reflected or transmitted microwave signals within the cavity I and can also determined a heating profile within the cavity 1 (for example to determine when the food 7 has been completely cooked). Using an appropriate feedback mechanism, the DRAs 8 can be controlled so as to achieve a predetermined heating profile or the like 30 by way of active control. In a particularly preferred embodiment, at least some of the DRAs 8 are configured as electronically steerable DRAs 8 adapted to direct

microwave energy at different regions within the cavity 1. this way, it is possible to achieve a relatively even heating profile without the need for moving mechanical parts such as the mode storer 5 or the tintable 6 of the conventional oven of Figure I, thereby helping to improve reliability.

The preferred features of the invention are applicable to all aspects of the invention and may be used in any possible combination.

Throughout the description and claims of this specification, the words "comprise"

10 and "contain" and variations of the words, for example "comprising" and "comprises", mean "including but not limited to", and are not intended to (and do not) exclude other components, integers, moieties, additives or steps.

Claims (12)

( CLAIMS:

1. A heating device comprising a microwave generator connected to at least one microwave transmitter in the form of a dielectric resonator antenna

2. A device as claimed in clann 1, further comprising a cavity having a periphery, wherein the at least one microwave transmitter is disposed on the periphery. 10

3. A device as claimed in claim 2, further comprising at least one microwave sensor disposed on the periphery.

4. A device as claimed in claim 3, wherein the sensor comprises a dielectric resonator anterma

5. A device as claimed in claim 3 or 4, further provided with electronic control circuitry adapted to provide feedback control to the at least one microwave transmitter from the at least one microwave sensor.

20

6. A microwave generator including at least one dielectric resonator antenna adapted to transmit microwave energy from the microwave generator.

7. A microwave heating device comprised as a dielectric resonator antenna.

25

8. A method of applying microwave energy, wherein microwave energy generated by a microwave generator is supplied to a dielectric resonator antenna and transmitted therefrom.

9. A heating device substantially as hereinbefore described with reference to 30 Figure 2 of the accompanying drawings.

10. A microwave generator substantially as hereinbefore described.

11. A microwave heating device substantially as hereinbefore described.

5

12. A method of applying microwave energy substantially as hereinbefore described.