WO2004010740A1 - Dielectric antennas for use in microwave heating applications - Google Patents

Abstract

There is disclosed a microwave heating device (1) in which microwave energy from a magnetron (2) is directed into a multi-­mode cavity by way of one or more dielectric resonator antennas (8) disposed at a periphery of the cavity. Additional microwave sensors (9), which may also be configured as dielectric resonator antennas (8), may also be provided at the periphery. By using appropriate feedback and control circuitry, food (7) or other items may be heated in the cavity according to a predetermined heating profile and without the need for mode stirrers or other moving parts.

Description

DIELECTRIC ANTENNAS FOR USE IN MICROWAVE HEATING

APPLICATIONS

The present invention relates to dielectric antennas, such as dielectric resontator antennas (DRAs), high dielectric antennas (HDAs) and dielectrically loaded antennas (DLAs) used in heating applications, and in particular to dielectric antennas used as microwave heatin applicators and as induction heating applicators.

Dielectric resonator antennas (DRAs) are resonant antenna devices that radiate or receive radio waves at a chosen frequency of transmission and reception, as used in for example in mobile telecommunications. In general, a DRA consists of a volume 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 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 f om the dielectric material). Direct connection to and excitation by a microstrip transmission line is also possible. Alternatively, 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 example in the present applicant's co-pending US patent application serial number US 09/431,548 and the publication by KINGSLEY, S.P. and O'KEEFE, S.G., "Beam steering and monopulse processing of probe-fed dielectric resonator antennas", IEE Proceedings - Radar Sonar and Navigation, 146, 3, 121 - 125, 999, the full contents of which are hereby incorporated into the present application by reference. The resonant characteristics of a DRA depend, ter alia, 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 loaded antenna, in which a traditional conductive radiating element is encased in a dielectric material that modifies the resonance characteristics of the radiating element.

High dielectric antennas (HDAs) are similar to DRAs, but instead of having a full ground plane located under the dielectric pellet, HDAs have a smaller ground plane or no ground plane at all. Removal of the ground plane underneath gives a less well- defined resonance and consequently a very much broader bandwidth. HDAs generally radiate as much power in a backward direction as they do in a forward direction.

In both DRAs and HDAs, the primary radiator is the dielectric pellet. In DLAs, the primary radiator is a conductive component (e.g. a metal wire or printed strip or the like), and a dielectric component then just modifies the medium in which the DLA operates and generally allows the antenna as a whole to be made smaller or more compact.

A DLA may also be excited or formed by a direct microstrip feedline. In particular, the present applicant has found that a pellet of dielectric material may be placed on or otherwise associated with a microstrip feedline or the like so as to modify radiation properties of the feedline when operating as an antenna.

DRAs may take various forms, a common form having a cylindrical shape which 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 (DRAs) in 1983 [LONG, S.A., McALLISTER, M.W., and SHEN, L.C.: "The Resonant Cylindrical Dielectric Cavity Antenna", IEEE Transactions on Antennas and Propagation, AP-31, 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 BHARTIA, P.: "Dielectric Resonator Antennas - A Review and General Design Relations for Resonant Frequency and Bandwidth", International Journal 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", IEEE 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 218-219].

Triangle [ITTIPIBOON, 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 Antennas", Electronics Letters, 1993, 29, (23), pp 2001- 2002].

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", JJEEE Transactions on Antennas and Propagation, AP-31, 1983, pp 406-412].

Half-split cylinder (half a cylinder mounted vertically on a ground plane) [MONGIA, R.K., ITTIPΓBOON, A., ANTAR, Y.M.M., BHARTIA, P. and CUHACI, M: "A Half-Split Cylindrical Dielectric Resonator Antenna Using Slot-Coupling", LEEE Microwave and guided Wave Letters, 1993, Vol. 3, No. 2, pp 38-39].

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.: "Half volume dielectric resonator antenna designs", Electronics Letters, 1997, 33, (23), pp 1914 - 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 rubber. In general, there is provided a generator of microwave 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 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 multi-mode heating system is to deliver microwave energy substantially uniformly 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 walls of the cavity.

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

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 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 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 antenna.

According to a second aspect of the present invention, there is provided a method of applying microwave energy, wherein microwave energy generated by a microwave generator is supplied to a dielectric 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 transmitter in the form of a dielectric antenna. According to a fourth aspect of the present invention, there is provided a microwave generator including at least one dielectric antenna adapted to transmit microwave energy from the microwave generator.

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

The microwave generator may be a magnetron or the like. The microwave generator may include at least one dielectric antenna 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 manner of a conventional microwave oven or heater, the at least one dielectric antenna being mounted at a perimetral portion of the chamber.

Preferably, more than one dielectric antenna is provided in the heating device of the third aspect, each dielectric antenna being selectively activatable, either alone or in combination with other dielectric antennas, so as to direct microwave radiation to predetermined regions within the heating chamber. The amount of energy supplied to each dielectric antenna by the microwave 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 dielectric antennas are provided, preferably also at perimetral portions of the chamber, the additional dielectric antennas being configured as sensors to detect reflected or transmitted microwave signals within the chamber. The sensor dielectric antennas, 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 dielectric antenna (e.g. by varying the amount of supplied energy and/or by selectively activating/deactivating one or more microwave applicator dielectric antennas or the microwave generator) so as to meet predetermined heating requirements.

The sensor dielectric antennas may also serve as microwave launcher dielectric antennas, or may be adapted only to sense reflected microwave energy.

Where the heating device is a microwave oven, the at least one sensor dielectric antenna 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 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 feedback control via the at least one sensing dielectric antenna.

In addition, the at least one sensor dielectric antenna can detect if there is nothing inside the 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 dielectric antenna so as to prevent damage thereto.

The at least one sensor dielectric antenna may also issue control signals to change an angle of at ^"least one microwave-reflecting surface in a vicinity of the at least one microwave applicator dielectric antenna 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.

Alternatively or in addition, the at least one microwave applicator dielectric antenna may be configured as a steerable dielectric antenna, 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 relatively dense parts of an article being heated. Steering of the microwave applicator dielectric antenna is advantageously achieved by way of feedback control signals from the at least one sensor dielectric antenna. For example, a griddle or the like may be constructed with a plurality of microwave applicator dielectric antennas (e.g. cylindrical DRAs) 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 uniform energy density throughout an article being heated in the oven (e.g. food being cooked or rubber being vulcanised).

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 an item within the oven and/or heater may be achieved by selective activation and/or steering of one or more dielectric antennas via solid-state control electronics.

Microwave radiation, like all other types of electromagnetic radiation, comprises a component of magnetic field and a component of electric field. In the case of microwave heating of foodstuffs and the like in a microwave oven, it is generally the electric field component that interacts with water molecules in the food to cause heating.

It is also possible to use the magnetic field component of the microwave radiation to cause heating in some materials by induction. Materials which will heat in this way include electrical conductors (e.g. metals), ferromagnetic materials and ferrimagnetic materials. The reason that this is not usually done is that some form of magnetic field concentration is preferred for efficient heating. Means to concentrate magnetic fields at microwave frequencies tend to be inefficient e.g. the use of microwave ferrites.

Induction heating is traditionally carried out at frequencies between a few kHz and 10MHz because of equipment cost and availability, and skin depth effects. However, for some applications (i.e. those where the skin effect is not important), induction heating at microwave frequencies is feasible. At 2.45 GHz (the frequency of operation of a domestic microwave oven), equipment is available at a relatively low cost.

Induction heating at lower frequencies is also generally only applicable to parts exceeding a certain size (about 1 to 1.5cm square). This is because at lower frequencies, the traditional method of introducing the magnetic field to the object to be heated is to use a coil called the workcoil. Workcoils need to be cooled with a supply of water to prevent excessive heating by self-induction due to the high currents being carried in the workcoil. Because part of the power supplied to the workcoil is taken away by the cooling water, the process in this form is inherently inefficient. This approach also means that there is a minimum size that the workcoil can be in order to allow a sufficient flow of water to keep the coil cool (about 1 to 1.5 cm square).

The present applicant has found that a DRA or other dielectric antenna can be used as an induction heating element. This solution is both small and efficient and does not require cooling. The efficiency comes from the fact that the losses in the material are low because of its purity. This means that cooling water is not required. The size is dictated by the shape of the required magnetic field concentration which can be predicted by the electric field pattern. This approach allows heating zones of less than 1 to 1.5cm square to be achieved.

The intensification of the magnetic field by the direct intensification of the electric field is as a result of the relationship between the two field strengths. The relationship in the far field is E/H=377 ohms in free space. A simple DRA has been simulated to give the result that 1W corresponds to about H=100A/m in the region of close proximity to the surface of the DRA. Using the permeability of free space, this means that B is about 120 Gauss for 100W. This is sufficient to cause heating at this frequency.

It is to be appreciated that preferred embodiments of the present invention employ DRAs as the dielectric antennas, although HDAs and DLAs may be used as an alternative to or in combination with DRAs.

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 accompanying drawings, in which:

FIGURE 1 shows a conventional microwave oven;

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

FIGURE 3 shows a DRA being used as an induction heater.

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 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 "hot spots" in the cavity 1 and the food 7. With reference to Figure 2, there is shown a microwave oven of an embodiment 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 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. Through the use of appropriate control circuitry (not shown), the sensors 9 can detect reflected or transmitted microwave signals within the cavity 1 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 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. In this way, it is possible to achieve a relatively even heating profile without the need for moving mechanical parts such as the mode stirrer 5 or the turntable 6 of the conventional oven of Figure 1, thereby helping to improve reliability.

Figure 3 shows a DRA 30 comprising a quarter-split cylindrical dielectric ceramics pellet 31 mounted on a substrate 32 incorporating an appropriate feed mechanism (not shown). The DRA 30 is being used to heat a ferro- or ferrimagnetic article 33 by induction, using the magnetic component of a microwave signal radiated by the DRA 30 in response to an appropriate signal being supplied by the feed mechanism.

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" 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

CLAIMS:

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

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

3. A device as claimed in claim 1 or 2, wherein the dielectric antenna is a dielectric resonator antenna.

4. A device as claimed in claim 1 or 2, wherein the dielectric antenna is a high dielectric antenna.

5. A device as claimed in claim 1 or 2, wherein the dielectric antenna is a dielectrically loaded antenna.

6. A device as claimed in claim 2 or any claim depending therefrom, further comprising at least one microwave sensor disposed on the periphery.

7. A device as claimed in claim 6, wherein the sensor comprises a dielectric antenna.

8. A device as claimed in claim 7, wherein the sensor comprises a dielectric resonator antenna.

9. A device as claimed in claim 7, wherein the sensor comprises a high dielectric antenna.

10. A device as claimed in claim 7, wherein the sensor comprises a dielectrically loaded antenna.

11. A device as claimed in any one of claims 6 to 10, 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.

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

13. A microwave heating device comprised as a dielectric antenna.

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

15. A method according to claim 14, wherein an electric component of the microwave energy is used for heating by way of excitation of water molecules.

16. A method according to claim 14, wherein a magnetic component of the microwave energy is used for heating by way of induction.

17. A heating device substantially as hereinbefore described with reference to Figures 2 and 3 of the accompanying drawings.

18. A microwave generator substantially as hereinbefore described.

19. A microwave heating device substantially as hereinbefore described.

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