GB2615765A - Dual-frequency microwave antenna - Google Patents

Abstract

A dual frequency antenna emits microwave radiation with first frequency range 400–500 MHz and second frequency range 2.4–2.5 GHz. The antenna has a cylindrical portion 101, with length 119mm and outer diameter 60mm, and a frustoconical portion 102, with length 80mm and outer diameter tapering from 60mm to 20mm. Each length and diameter may be within a range of -8% and +14%. Antenna portions may be surrounded by a dielectric and metal enclosure. The antenna may be connected to a microwave power source such as a solid state power generator SSPG and have a connector for connecting to diplexer or coaxial cable, such as DIN connector. A microwave apparatus, such as a microwave oven, may have the dual frequency antenna and a waveguide separated by an odd multiple of half wavelengths of 2.45GHz, for delivering microwaves into a chamber with efficient small cavity space utilisation.

Description

DUAL-FREQUENCY MICROWAVE ANTENNA

Technical field

This invention relates to dual-frequency microwave antenna primarily but not exclusively for use in microwave apparatus for drying materials or cooking foodstuffs.

Background to the Invention

Microwave heating apparatuses (commonly referred to as 'microwave ovens') are regularly used for the processing of materials, for example, foodstuffs. Microwave ovens are used in large scale factories and smaller commercial premises such as restaurants, as well as households A common use for microwave ovens is in the heating and drying of foods and other materials.

Drying and dehydrating is of particular importance to the food industry as dried food typically has a longer storage life than undried food. By removing water from food samples, microbial growth is inhibited, allowing for food to be stored for longer periods. Drying may be advantageous for certain foodstuffs where it is not practical or desirable to use other processing techniques such as freezing or canning.

Existing microwave heating apparatuses generally use waveguides to emit radiation into a heating chamber. US 2019/0059133 Al discloses an electronic oven that comprises a control system to adjust the distribution of the application of energy in the heating chamber and uses 2.45 GHz and 915 MHz microwave radiation, emitted via 'injection ports and waveguides. Though the penetration depth of 915 MHz is greater than 2.45 GHz, it is still only approximately 100 mm. This limits the efficiency of the cooking and drying process, particularly for larger product samples.

The present invention aims to eliminate, or at least mitigate, the drawbacks of existing microwave heating apparatuses

Summary of the Invention

According to a first aspect of the invention, there is provided a monopole antenna for use in a microwave processing apparatus, wherein the monopole antenna is configured to radiate microwaves at a first frequency range of 400 -500 MHz and a second frequency range of 2.4 -2.5 GHz. The first frequency range may optionally be 420 -450 MHz, and may be 433.05-434.792 MHz.

Such a dual-frequency antenna has applications in drying and cooking. It can take the place of much bulkier waveguides, making equipment for microwave drying and cooking realistic in size for commercial, restaurants, and household applications.

The most common form of microwave generation -magnetrons -are limited to a narrow bandwidth and, consequently, fixed-frequency operation. This both limits the frequencies that can be used and the control of the electromagnetic field that can be achieved, with the single frequency microwaves typically resulting in either an uncontrolled multimode field or a fixed standing wave field. Nonetheless, magnetrons are still widely used due to their low production cost.

The present invention utilises solid state microwave sources, which are controlled by an Al (artificial intelligent) based algorithm to manage the electric field and applied power. This feature is not available in magnetron-based systems.

Power couplers used in most contemporary microwaves are general waveguide type couplers. Waveguides are commonly used as they limit the wave propagation to one dimension, so that, under ideal conditions, power does not decrease with distance from the source, increasing the efficiency of communication. A feature of waveguides is the range of frequencies that the waveguide can operate over, which is defined by the waveguide's boundary conditions. The lowest couplable frequency (the cut-off frequency) is equal to the fundamental mode of the waveguide. The widely used WR340 waveguide operates at 2.1 -3 GHz for example, making it suitable for the higher frequency of 2.4 -2.5 GHz. However, waveguides for 400 -500 MHz are often not feasible. The WR2300 waveguide (which operates at 350 -500 MHz) has dimensions of 532 x 590 x 298 mm (L x W x H). Such a waveguide is impractical for use in a drying or cooking system due to its large size and high cost.

The antenna of the invention is a dual-frequency (dual-band) antenna that is capable of emitting microwaves in the 400 -500 MHz and the 2.4 -2.5 MHz bands. This makes the antenna suitable for microwave processing applications that make use of a high and low microwave frequency, for example, processing of foods to heat the surface and interior of the food samples.

Regulations specify that radiation outside ISM bands should be at a level that it does not harm radio or naval communication. Although the ISM band of 433.05 -434.79 was used in the simulations discussed below, use of frequencies outside the ISM bands are possible because the microwave ovens (i.e. the processing cavity/chamber) are vacuum and EM sealed. As such, radiation cannot escape the chamber.

The antenna comprises a cylindrical portion, wherein the length of the cylindrical portion may be within a range of -8% and +14% of 119mm, and wherein the outer diameter of the cylindrical portion may be within a range of -8% and +14% of 60mm The antenna also comprises a frustoconical portion contiguous with the cylindrical portion, wherein the length of the frustoconical portion may be within a range of -8% and +14% of 80mm, wherein the outer diameter of the frustoconical portion may taper from a diameter within a range of -8% and +14% of 60mm to a diameter within a range of -8% and +14% of 20mm. A further cylindrical portion having a length within a range of -8% and +14% of 20mm and outer diameter within a range of -8% and +14% of 26mm connects the antenna to the coaxial connector.

The antenna may comprise a connector configured for connecting the antenna to a microwave supply source. The connector may be a coaxial connector such as a DIN connector, for example, a 7/16 inch (11.1 mm) screw-on connector.

The cylindrical connecting portion may be a separate component to the frustoconical portion. By having a limited number of separate components, the antenna is easier to manufacture and lower cost than other designs.

According to a second aspect of the invention, there is provided a microwave processing apparatus comprising: a processing chamber, and an antenna according to the first aspect of the invention, configured to radiate microwaves into the chamber.

The microwave processing apparatus may further comprise another microwave radiator for radiating microwaves into the chamber, wherein the other radiator is separated from the antenna by an odd multiple of half wavelengths (-6.12cm) of 20 2.45 GHz.

The other microwave radiator may be a waveguide. The microwave processing apparatus may further comprise a second waveguide or multiple waveguides for radiating microwave energy into the chamber. The waveguides used in the apparatus may be commercially available such as WR340 waveguides.

The processing chamber defines a cavity, and wherein the length, width and height of the cavity are preferably approximately 70 cm x 70 cm x 80 cm.

The antenna may be matched with the chamber to be resonant in two frequency bands of 400 -500 MHz and 2400 -2500 MHz.

The antenna may be configured so that the chamber behaves as a single-mode cavity when the antenna radiates microwaves in the 400 -500 MHz frequency range.

The antenna and two waveguides may be configured so that the chamber behaves as a broadband, multimode cavity with a uniform field distribution when the antenna and two waveguides radiate microwaves in the 2400 -2500 MHz frequency range.

Preferably, the antenna is separated from a first vertical side wall of a processing chamber by at least 15cm to a maximum of 21cm, whereas from a second side wall adjacent to the first side wall by at least 17cm to a maximum of 25cm. The antenna is preferably separated from the first and second side walls by approximately one quarter of a wavelength (-17cm) of 400 -500 MHz, to achieve greater than 90% of radiation efficiency.

A further aspect of the invention is the use of a customised dual-band monopole antenna instead of a WR2300 waveguide in a microwave heating apparatus. Two different types of radiators are preferably used -a monopole antenna and one or more waveguides to excite the cavity. Preferably, the antenna is customised, low profile, cost-efficient and dual-band to excite the cavity for two ISM band frequencies (433.05-434.790 MHz and 2400-2500 MHz). Furthermore, the excitation frequencies are more than two octaves apart. One octave happens at the second harmonic (2fo) of the fundamental frequency, whereas the second octave happens at 2x2fo e.g. if fo is 433 MHz then the first octave will happen at 866 MHz and the second octave would be 1732 MHz.

According to a third aspect of the invention, there is provided a microwave processing apparatus comprising a cavity and a dual band antenna configured to 30 emit microwave radiation at 400 -500 MHz or 2.4 -2.5 GHz, and at least one waveguide configured to emit microwave radiation at 2.4 -2.5 GHz, wherein the antenna and the at least one waveguide are separated by an odd multiple of half wavelengths of 245 GHz microwave radiation A further aspect of the invention is irradiating water contents within a load in a cavity with three solid-state driven radiators (2xWR340, lx antenna) at the same time (2.4 -2.5 GHz).

Cavity excitation within 400 -500 MHz band is by an antenna. The cavity is designed as a single mode cavity TE101 at 400 -500 MHz and multimode from 2.4 -2.5 GHz without manually changing the solid-state sources. A diplexer has been used at the common antenna port to connect 400 -500 MHz as well as 2.4 -2.5 GHz radiators to the antenna, allowing the cavity to switch state from single mode (TE101 at 400-500 MHz) to multimode (2.4-2.5 GHz).

Moreover, the cavity is scalable in the sense that increasing the distance between the WR340 waveguides in an odd multiple of the half wavelength (-6.12cm) at design frequency (2.4-2.5 GHz) will keep the cavity as a broadband resonating chamber.

Although discussed here in relation to the drying and cooking of food, these apparatuses and techniques are also useful for the processing of materials in other industries.

Brief description of the drawings

Embodiments of the invention will be described by way of example, with reference to the accompanying drawings, in which: Figures la to le show perspective views, side view and bottom view of an optimised antenna according to an embodiment of the invention; Figure 2 shows an S11 simulation in respect of an antenna according to the present invention; Figure 3 is a diagram of a microwave processing apparatus according to an embodiment of the present invention; Figure 4 is a three-dimensional diagram of a microwave processing apparatus according to an embodiment of the invention, Figure 5 is a top-down view of the microwave processing apparatus of Figure 4; Figures 6a and 6b are representations of a simulation of the electric field applied to a load in a microwave processing apparatus when operated in the 400 -500 MHz frequency band; Figures 7a to 7c are representations of a simulation of the electric field applied to a load in the microwave processing apparatus when operated in the 2.4 -2.5 GHz frequency band; Figure 8 is a graph of the Sll response of an antenna and w-aveguides when the microwave processing apparatus is operated in the 2.4-2.5 MHz frequency band.

Detailed description

Aspects of the present invention concern means for processing organic matter to different frequencies of microwave radiation. In particular, aspects of the present invention provide means for processing organic matter with 400-500 MHz radiation as well as 2.45GHz radiation. Since 400-500 MHz radiation has a penetration depth greater than the penetration depth of 2.45GHz radiation, the provision of 400-500 MHz radiation as well as 2.450Hz radiation allows for more efficient and flexible processing of organic matter.

It is known to use waveguides for radiating 2.4-2.5GHz and 915MHz microwaves. Although a waveguide can provide 400-500 MHz radiation, waveguides for this frequency band are large, bulky and expensive and as such are generally not suitable for smaller microwave processing ovens or other apparatus for this band of frequencies. For example, when used in a microwave processing apparatus, and arranged to emit radiation into a processing cavity, the return loss performance of a WR2300 waveguide is very poor which means it needs to be moved around to find a better location. However, the large size of the waveguide adaptor required to transform a coaxial output of an (solid state power generator) SSPG source limits its manoeuvrability.

Accordingly, use of an antenna for emitting 400 -500 MHz is deemed more practical. A simple monopole ('rod') antenna (196mm in length and extending into cavity by one quarter wavelength at 433MHz, screw thread 30mm, inner screw thread diameter 4.8mm, outer diameter 12mm) connected directly to DIN 7/16 coaxial connector and located in the centre (i.e. extending centrally from the top horizontal wall of a processing cavity of a microwave processing apparatus) was tested to provide 400 -500 MHz. However, the EM field produced in the cavity is uneven, and provides a poor SI I (reflected power) response. When the position of the antenna is moved to be one quarter of the wavelength of 433NIHz away from a vertical wall, the S11 parameter is even worse. Thus it is clear that moving antenna away from centre has made radiation and return loss 511 performance worse. Nevertheless, having the antenna (extending down from the top horizontal wall) laterally positioned closer to one or two vertical walls of a processing cavity, rather extending centrally from the top horizontal wall is desirable to improve the cavity space utilisation.

Following these tests, the geometry of a simple monopole antenna was varied and the 511 response determined. An antenna with a frustoconical section, centrally located (laterally), extending into cavity from the top horizontal wall of a processing cavity, with loading at radiating end with 7mm connecting part (7mm diameter to match connector pin diameter) to DIN 7/16 was tested. This produced an improved S11 response. The same antenna placed one quarter wavelength of 433MHz from two adjacent vertical cavity walls was also tested, but this produced a poorer Sll response than when in the centre. Additionally, the same antenna but with a 12mm diameter connecting part was tested at a central position (which produced a good Sll response) and one quarter wavelength from two adjacent vertical walls (which produced a very poor Sll response) The same 12mm diameter antenna placed 10cm away from two adjacent vertical walls provided an even poorer SI 1 response The inventors of the present invention developed a customised antenna 100 which provides improved SI 1 and radiation performance whilst allowing for efficient cavity space utilisation The antenna provided is a dual-frequency antenna that can be operated efficiently in both the 400 -500 MHz band and the 2.4 -2.5 GHz band. This makes the antenna 100 suitable for use in microwave processing apparatuses that are designed to use a high and low frequency to process food samples. The use of a higher and lower frequency is beneficial as it allows for penetration of microwave radiation to different depths in the sample, which may result in more desirable or uniform heating of the sample.

Referring to Figures la to 1 e, antenna 100 comprises a lower antenna region 101 The lower antenna region 101 is cylindrical with a diameter of approximately 6 cm and a length of 119.33mm (approximately 12.3cm). The antenna 100 comprises an upper antenna region 103. The upper antenna region 103 is cylindrical in shape and detachable from frustoconical section 102 which is contiguous with the lower antenna region 101, tapering from a diameter of approximately 6 cm to approximately 2cm. As shown in Figure 1d, upper antenna region has an outer diameter of approximately 26mm, a height of 20mm, an internal diameter of approximately 7mm defining an internal cavity which extends approximately 6mm into the main body of upper antenna region 103. The geometry of the antenna transforms, or matches, the coaxial impedance to loaded cavity impedance Antenna 100 also comprises a coaxial-type connector such as a DIN 7/16 inch (11.1 mm) screw-on connector adapted to attach directly onto a microwave radiation source and diplexer comprising a counterpart mating connector. Alternatively, the antenna 100 may be connected to the microwave radiation source via a coaxial cable. The connector has an inner cylinder (such as the centre pin or connector of a coaxial type connector) that may or may not protrude from the connector. The centre cylinder has a diameter of approximately 7 mm.

Figure la, shows antenna 100 surrounded by dielectric shielding. The dielectric shielding is required for impedance transformation in order to improve the return loss Sll The antenna 100 has a lower portion of dielectric material that surrounds a portion of the lower antenna region 101. A portion of the lower antenna region 101 is not surrounded by a dielectric material, so is left exposed. The exposed portion has a length of approximately 4.2 cm. The lower dielectric surround has a length of approximately 8 cm. The lower dielectric surround is in this example cubic in shape, with side lengths of approximately 8 cm (8 x 8 x 8 cm cube).

The antenna has an upper portion of dielectric material that surrounds the upper antenna region 102. The upper dielectric surround is cuboidal in shape, with a length of approximately 10 cm and a width and depth of approximately 8 cm (8 x 8 x 10 cm cuboid). The upper dielectric surround comprises a metal skin that encloses or shields the dielectric material In some embodiments, the lower portion and upper portion of dielectric material 30 may be separate components In other embodiments, the lower portion and upper portion may be parts of a common dielectric body.

The antenna has a transition region of dielectric material that surrounds the region directly above the upper antenna region 102. The transition dielectric surrounds the antenna upper portion 103. The transition dielectric surround is cuboidal in shape, with a length of approximately 3 cm and a width and depth of approximately 5 cm (5 x 5 x 3 cm cuboid). The transition dielectric surround has a metal skin that encloses or shields the dielectric material.

Lower portion 101 and frustoconical portion 102 are manufactured as a single piece. The antenna 100 is advantageous over existing antennas due to its design and use of segmented parts. The use of a lower region 101 frustoconical middle region 102, separate upper region 103 and separate shielding allows for manufacture and machining of the antenna 100 more easily than designs that are one single body, which reduces the associated manufacturing cost. The use of commercial parts and materials (such as Delrin thermoplastic) also reduces cost.

The present design reduces manufacturing complexity but also uses fewer parts than other designs, improving the ease of assembly. The chosen components keep the overall size of the antenna small, while still being suitable for high power RF applications.

To ascertain the geometrical tolerances of the antenna, antenna elements were uniformly increased or decreased by a scaling factor to understand the maximum and minimum size limitation before antenna performance starts degrading. A standard performance measure for an antenna is its Sll (return loss) value, which should be at least -9.5dB for 90% through and 10% reflected power. Based on this performance factor it was found that antenna size can be increased up to 14% (with some band limitation) of the size described with reference to figures la to le and still maintain Sll performance. Up to 8% size reduction can be applied to the value of the antenna described with reference to figures la to le and still maintain satisfactory Sll performance. Therefore, +14% to -8% tolerance for the antenna design described above has been found to be acceptable. Tolerances for 2.4-2.5GHz are greater than for 400 -500 MHz due to the shorter wavelength.

In a further test, when the antenna was placed 17cm/one quarter wavelength from one vertical wall (and directly adjacent to another vertical cavity wall), the Si! response was found to be slightly poorer. When the antenna was moved 2cm towards one wall, and the 17cm distance was maintained from another wall, the S1 1 response improved. When the antenna was placed 17cm-2cm from one vertical wall and 17cm + 8cm from another wall, the Sll response was very good. Thus, it has been found that, for optimal cavity excitation at 400 -500 MHz, a customised antenna positioned one quarter wavelength (at 433N1Hz) away from a vertical wall is the best starting point. The position of the antenna can then be adjusted to match the cavity loading.

When the antenna is placed 17cm (X/4) away from two walls (away from the corner) it presents -11.2dB of S11, which means greater than 90% of power is delivered to the cavity. Furthermore, when antenna is moved further 2cm towards one wall and 8cm away from another, it presents -19.2dB of Sll which means 98% power is delivered to the cavity. Accordingly, the antenna positioning maxima and minima is 17-2 = 15cm and 17+8 = 25cm. In other words, the antenna can be placed anywhere between 15cm to 17cm from one wall and between 17cm to 25cm from the other wall and its SI 1 value will be anywhere between -11.2dB to -19.2dB (90 to 98% power delivery). Antenna positioning also depends on cavity size and loading conditions but the limits established here are good reference points. Antenna positioning for 2.4-2.5GHz band has wider tolerance due to the shorter wavelength.

The tests described above were conducted under 500W, 400 -500 MHz, fully loaded cavity, where the reflected power is in the region of 4-10% of the forward power. Even though the simulated S1 1 performance improves with increasing frequency, it was found that the antenna performs better at lower side of the band.

Referring to Figure 3, an example microwave processing apparatus 200 is shown. The apparatus 200 comprises a processing chamber 201 and an antenna 100 that may be similar to the antenna described previously in Figures la to le. The processing chamber has a size of about 70 cm x 70 cm x 80 cm. Other preferred embodiments may be larger by increments of quarter wavelength of the 433MHz frequency any dimension (or any 2 or 3 dimensions), or smaller -by quarter wavelength of the 433MHz frequency. The dimensions of the chamber may be scaled to meet the physical needs of the user (e.g. how much space is available, the size of the food samples to be processed), and also to ensure that the chamber and power couplers act as a resonating cavity as desired. For example, a different arrangement or number of antenna or waveguides may necessitate a chamber with different dimensions to produce the desired electric field, within the restrains of the size constraints mentioned above in terms of quarter wavelength of the 433MHz.

The size of the chamber for 2.4-2.5GHz band does not have limitations (as long the chamber is suitable for 400 -500 MHz), however, when changing the size of 15 the chamber, the position of the WR340 waveguides may have to be reestablished by the use of EM simulations.

In this example, apparatus 200 further comprises two waveguides 202a 202b. The waveguides 202a, 202b may be W R340 or WR430 waveguides configured to operate with microwave radiation with a frequency of 2.4 -2.5 GHz. The waveguides 202a 202b are separately connected to first microwave radiation sources 203a, 203b configured to generate microwave radiation with frequencies of 2.4 -2.5 GHz, The antenna 100 is connected to a first microwave radiation source 203c configured to generate microwave radiation with a frequency of 2.4 -2.5 GHz, and also to a second microwave radiation source 204 configured to generate microwave radiation with a frequency of 400 -500 MHz. The first microwave radiation sources 203a-c and second microwave radiation source 204 may be any microwave generator but are preferably solid-state power generators (SSPGs).

The microwave generators and waveguides/antenna in this example are connected with coaxial cables.

The apparatus 200 comprises a diplexer 205 that is operable to select one of the microwave inputs from the first microwave radiation source 203c and second microwave power source 204 and supply this to the antenna 100. The diplexer 205 is connected to the antenna 100 with sufficient isolation (-20 dB) so that the antenna 100 is capable of radiating at both 400 -500 MHz and 2.4 -2.5 GHz. When one of the microwave radiation bands is selected, the other microwave radiation generator(s) may be switched off e.g. when the antenna 100 is operated to radiate at 400 -500 MHz, the first microwave radiation sources 203a-c will be switched off Likewise, when the antenna 100 and waveguides 202a-b are operated to radiate at 2.4 -2.5 GHz, the second microwave radiation source 204 will be switched off The apparatus 200 includes a controller 206 that is configured to control the power outputs of the microwave power generators 203a-c and 204. Additional electronic connections may be included as required, such as to an earth point 207.

Referring to Figure 4, the microwave processing apparatus 200 is similar to that described with reference to Figure 3. The apparatus 200 comprises a processing chamber 201, two waveguides 202a, 202b and an antenna 100 similar to that described previously in Figures 1 and 2 As will be described, the location and arrangement of the waveguides 202a, 202b and the antenna 100 have been discovered to be especially well positioned and have been tested and are preferable for chamber 201 of this example.

The waveguides 202a 202b and the antenna 100 are separated by odd multiples of half wavelengths of 2.45 GHz microwaves. The wavelength of 2.45 GHz microwave radiation is 12.24 cm. Using separations of odd multiples of half wavelength ensures that the interference between the electromagnetic waves radiating from the waveguides 202a 202b and antenna 100 is null such that it does not make the overall structure a reflector and the broadband nature of the resonating structure is preserved (discussed later in more detail).

In this example, the separation AB of the centres of the two waveguides 202a, 202b is five half wavelengths; the separation AC of the centre of the antenna 100 and centre of the first waveguide 202a is five half wavelengths; and the separation BC of the centre of the antenna 100 and the second waveguide 202b is also five half wavelengths. This is based on a wavelength of 2.45 GHz microwave radiation (12.24 cm).

These separations were tested and found to be preferable for chamber 201, shown in the example, with a width of approximately 70 cm, depth of 70 cm and height of 80 cm. Testing has shown that the broadband resonant behaviour and electromagnetic field uniformity is preserved when the separations are altered to different odd multiples of half wavelength. As such the separations may be increased (to say 11, 13, 15, etc half wavelengths) without affecting the resonant properties of the cavity, thereby allowing apparatus 200 to be scaled to larger units which may be suitable for industrial uses.

Chamber 201 is designed and scaled so that it acts as a well-defined single-mode cavity in the 400 -500 MHz band and a multimode cavity in the 2.4 -2.5 GHz band, thus allowing both frequencies to be used efficiently in the same apparatus.

As discussed previously, the use of both frequencies may be beneficial in the processing of foods as both the surface and interior of the food can be heated efficiently.

The use of both an antenna 100 and at least one waveguide 202a, 202b is advantageous in creating a uniform and controllable electromagnetic field inside chamber 201 Antennas and waveguides have very different propagation patterns (being a free-space emitter and a one-dimensional propagator respectively) A combination of these two different emitters with phase and frequency controlled by an Al algorithm allows for greater control of the electromagnetic field as compared to apparatuses which only use antennas or waveguides, or a single excitation frequency. As discussed below, apparatus 200 comprising an antenna and waveguides 202a, 202b provides a substantially uniform field, which allows for more even and controllable heating of materials In other embodiments of the invention, the greater control enabled by the use of an antenna and waveguides may be used to generate a non-uniform field, wherein different regions of the chamber experience different field densities. For example, an upper region of the chamber may have a high electric field, whereas a lower region of the chamber may have a low electric field. This may be used to process two different materials simultaneously. This phenomenon is supported by the phase and frequency sweep controlled by the Al algorithm, as mentioned previously.

In some embodiments, chamber 201 may comprise multiple shelves or guide slots wherein racks, grids, and a variety of gastronome-type containers may be secured. These shelves can be used to hold food samples during processing. The shelves may be arranged so that they experience different electromagnetic fields, enabling foodstuffs placed on different shelves to be processed differently.

Figure 5 shows a top-down view of apparatus 200 with the antenna 100 and waveguides 202a, 202b as described previously in Figure 4. The waveguides used in this example are WR340 or WR430 waveguides, with a spatial footprint on the surface of the chamber of 4.318 cm by 8.636 cm. The measurements shown are in centimeters.

Referring to Figure 2a, simulations indicate that the Sll response of the antenna shows less than -10 dB return loss for a substantial portion of the 400-500 MHz frequency band when delivering the microwave energy to the chamber as described previously. This corresponds to better than 90% efficiency for the antenna 100 delivering microwave power of this frequency to the chamber. The efficiency of microwave power delivery may allow for more efficient microwave processing of products and therefore less energy usage and faster processing times.

Referring to Figure 2b, simulations show that the microwave processing apparatus 200 described previously produces a uniform single-mode electromagnetic field. Resonating under TE101 mode (the dominant mode), the apparatus exhibits no hot spots when the antenna 100 is operated at 400 -500 s MHz.

Referring to Figures 6a -6b simulations show that apparatus 200 exhibits a substantially similar electric field across the 400 -500 MHz frequency band. Figure Ga shows the electric field experienced by a load at 433.05 MHz, while Figure 61) shows the electric field experienced by the same load at 434.792 MHz.

The field is substantially the same for the two frequencies, due in part to the narrow bandwidth of just 1.742 MHz. This allows for consistent microwave processing across frequencies in this particular band.

Referring to Figures 7a -7c, simulations show that the microwave processing apparatus 200 described previously produces a multimode excitation that resonates over the 100 MHz bandwidth of the 2.4 -2.5 GHz frequency band, unlike the single-mode resonance of the 400 -500 MHz band. Figures 7a -7c show simulations for 2.4 GHz, 2.45 GHz and 2.5 GHz respectively. The simulations show that the field is substantially uniform and similar across the frequency band. This may be beneficial as performing a frequency sweep across the 2.4 -2.5 GHz band will guarantee full exposure of the load to the electric field during processing, and thus ensure a uniform heating profile. As the penetration depth of this frequency band is low, this may be used to add crispiness to the food sample being processed.

Referring to Figure 8, simulations show that the antenna 100 and waveguides 202a, 202b all exhibit an Sll response with a return loss of less than -10 dB across the 2.4 -2.5 GHz frequency band. This corresponds to better than 90% efficiency in delivering the microwave power to the chamber. The simulation shows that the antenna 100 becomes critically coupled at 2.45 GHz, where more than 99% of the microwave power is delivered to the load inside the chamber.

Again, this efficiency may result in lower energy usage, operating costs, and processing times Although example embodiments have been described, these are not intended to limit the scope of the invention, which should be determined with reference to the accompanying claims.

Claims (16)

2. An antenna for emitting microwave radiation into a cavity, wherein the antenna is configured to emit microwave within a first frequency range of 400-500 MHz and a second frequency range of 2.4 -2.5 GHz, wherein the antenna comprises a cylindrical portion, wherein the length of the cylindrical portion is within a range of -8% and +14% of 119mm, and wherein the outer diameter of the cylindrical portion is within a range of -8% and +14% of 60mm; and a frustoconical portion contiguous with the cylindrical portion, wherein the length of the frustoconical portion is within a range of -8% and +14% of 80mm, wherein the outer diameter of the frustoconical portion tapers from a diameter within a range of -8% and +14% of 60mm to a diameter within a range of -8% and +14% of 20mm 2. The antenna of claim 1, further comprising a connector portion configured for connecting the antenna to diplexer or coaxial cable, wherein the length of the connector portion is within a range of -8% and +14% of 20mm and an outer diameter of the connector portion is within a range of -8% and +14% of 26mm.
3. 3. The antenna of any preceding claim, wherein the first frequency range is 400 -500 MHz.
4. 4. The antenna of any preceding claim, wherein a first portion of the cylindrical portion is surrounded by a dielectric material, and second portion of the lower antenna region is exposed.
5. The antenna of claim 4, wherein the frustoconical portion is surrounded by a dielectric material, and the dielectric material is surrounded by a metal enclosure.
6. 6 The antenna of any preceding claim, further comprising a transition region of dielectric material surrounding the antenna above the frustoconical region, wherein the dielectric material of the transition region is surrounded by a metal enclosure.
7. A microwave processing apparatus comprising: a processing cavity, wherein the cavity is defined by four vertical side walls and two horizontal walls; and an antenna according to any preceding claim configured to radiate microwaves from a first microwave radiator to the chamber, wherein the antenna is separated from a first vertical side wall by betweenl5cm and 17cm and from a second side wall adjacent to the first side wall by between 17cm and 25cm.
8. 8. The apparatus of claim 7, wherein the antenna is separated from the first and second side walls by approximately one quarter of a wavelength of 400 -500 MHz radiation.
9. 9. The apparatus of claim 7 or 8, wherein the processing chamber defines a cavity, and wherein the length, width and height of the cavity are approximately 70 cm x 70 cm x 80 cm
10. 10. The microwave processing apparatus of any of claims 7 to 9, further comprising a second microwave radiator for delivering microwaves to the chamber, wherein the second radiator is separated from the antenna by an odd multiple of half wavelengths of 2.45 GI-1z.
11. 11. The microwave processing apparatus of claim 10, wherein the second microwave radiator is a waveguide.
12. 12. The microwave processing apparatus of claim 11, further comprising multiple waveguides for radiating microwaves into the chamber.
13. 13 The microwave processing apparatus of any of claims 7 to 12, wherein the antenna is resonant in two frequency bands of 400 -500 MHz and 2400 -2500 MHz when the processing chamber is loaded.
14. 14. The microwave processing apparatus of claim 13, wherein the chamber behaves as a single-mode cavity when the antenna radiates microwaves in the 400 -500 MHz frequency range.
15. 15. The microwave processing apparatus of claim 13 as it depends from claim 11 or claim 12, wherein the chamber behaves as a broadband, multimode cavity with a uniform field distribution when the antenna and the or each waveguide radiate microwaves in the 2400 -2500 MHz frequency range.
16. 16. Microwave processing apparatus comprising a cavity and a dual band antenna configured to emit microwave radiation at 400 -500 MHz or 2.4 -2.5 GHz, further comprising at least one waveguide configured to emit microwave radiation at 2.4 -2.5 GHz, wherein the antenna and the at least one waveguide are separated by an odd multiple of half wavelengths of 2.45 GHz microwave radiation.