GB2127259A - High-frequency heating appliance - Google Patents

Abstract

A high-frequency heating appliance comprises a heating chamber (2, Fig. 2) waveguide (14, Fig. 2) for feeding high-frequency energy from a high-frequency oscillator (8, Fig. 2) to the chamber, and a rotary antenna 15 for microwave-wise coupling of the waveguide with the chamber. The antenna 15 comprises a vertical portion 17 substantially perpendicular to a top wall (16, Fig. 2) of the chamber and a horizontal portion 18 which is shaped as a substantially triangular plane and is connected to the vertical portion 17 in the region of an apex of the triangle. The sum of the length of a perpendicular line drawn from the connection between the vertical and horizontal portions to the side of the triangle opposite the connection and the length of a part of the vertical portion projecting into the chamber is an integral multiple or approximately one-half of the wavelength used. <IMAGE>

Description

SPECIFICATION High-frequency heating appliance The present invention relates to a high-frequency heating appliance.

In the microwave oven, which is one of the conventional high-frequency heating appliances, the uniformity of distribution of the high-frequency ener- gyfed into the heating chamber has been improved by installing a stirrerfan for stirring the high-frequency waves in the heating chamber, by turning a platform on which a heating load is positioned, or by installing a rotatable electric wave radiator, which is an antennae.

Of these methods, the method using a rotary antenna has been adopted in many microwave ovens for household use because there can be provided a large effective free space in the heating chamber. However, when such a rotary antenna is installed in the upper portion of the heating chamber, the heating characteristic of heating a heating load in the chamberwith the electromagneticwave radiatedfromtheantenna is not solely dependent on the antenna but dependent on the combination ofthe antenna with the size and shape ofthe heating chamber. Thus, there is no antenna that can be used commonlyforvarious heating chamber conditions. It is thus necessaryto design an optimum antenna for each particularsetof heating chamber conditions.Heretofore, a number of propositions have been made with respectto the shape of rotary antennae and the energyfeeding method using a rotary antenna, but as far as rotary antennae in use upto now are concerned, they have been found to have a number of disadvantages. Thus, the heating in the central zone of the heating chamber is not uniform and if, for example, the high-frequency waves are radiated to a plurality of chilled shao-mai in the central zone of the heating chamber, the tempera- ture ofthe shao-mai in the centre is far lowerthan the temperature ofthose located around it. In the case of a fried egg, the peripheral portion of the egg is overheated while the central portion is still raw.

It would thus be desirable to provide a highfrequency heating appliance which overcome the above-mentioned disadvantages of the prior art appliances and is capable of performing dielectric heating of a heating load placed in a heating chamber.

According to the present invention there is provided a high-frequency heating appliance comprising a heating chamber, a waveguideforfeeding highfrequency energy of a given wavelength from a high4requency oscillator to the chamber, and a rotary antenna operable to radiate such energy in the chamber and comprising a vertical portion extending substantially perpendicularly to an adjacent region of a top wall of the chamber and projecting into the chamber and a substantially triangular horizontal portion connected at one cornerto the vertical portion, the sum ofthe length of the shortest distance between said corner connection and the side of the horizontal portion opposite thereto and the length of the part of the vertical portion projecting into the chamber being substantially one half, or an integral multiple of one half, of said wavelength.

In a preferred embodimentthe rotary antenna coupling the waveguide to the heating chamber microwave-wise comprises a vertical portion and a horizontal portion, the horizontal portion being shaped as a substantially triangular plane and connected to the vertical portion art a position near an optional apex thereof. The sum ofthe length of a line perpendicularto the connection between the horizontal and vertical portions and the length ofthe part of the vertical portion which projects into the heating chamber is approximately one halfofthewavelength used or an integral multiple thereof, with the result that a heating load in the heating chamber is subjectto dielectric heating with substantial uniformity and good efficiency.

Embodiments of the present invention will now be more particularly described byway of example with reference to the accompanying drawings, in which: Fig. lisa perspective view offirst high-frequency heating appliance embodying the invention; Fig. 2 is a front sectional view ofthe appliance of Fig.

1; Fig. 3 is a perspective view of a rotary antenna of the appliance of Figs. 1 and 2; Fig. 4a is a perspective view of a rotary antenna of a second appliance embodying the invention; Fig. 4b is a plan view of a horizontal portion of the antenna shown in Fig. 4a; and Fig. 5 is a plan view of a horizontal portion of a rotary antenna of a third appliance embodying the invention.

Referring nowto the drawings and the embodiment of Figs. 1 to 3, in Fig. 1 the reference numeral 1 denotes a high-frequency heating appliance body which includes a heating chamber 2 with a door 3 installed at an opening thereof so that access to the chamber may be freely made available or unavailable. Disposed atop the body 1 are heater plates 4 and for cooking on these plates, operation knobsA5installedonthefront face ofthe body can be manipulated to control the power settings of corresponding plate heaters 6 shown in Fig. 2. Indicated at7 are operating knobs B for controlling the output of a heater installed inside the heating chamber 2 and the output of a magnetron 8, which is used as an example of high-frequency oscillator.

Referring now to Fig. 2, numeral 9 indicates a plate made of heat-resisting dielectric material which is disposed overthe plate heaters 6. Indicated at 10 is a heater disposed in an overhead position within the heating chamber 2, while a heater 11 is disposed externally and at the bottom ofthe chamber 2. By meansofthese heaters 10,1 1,thechamber2can be heated with good efficiency. Numeral 12 denotes a heat insulation disposed around a wall 13 of the chamber. Indicated by 14 is a tapered waveguide which propagates the high-frequency output of the magnetron into the chamber 2.The numeral 15 denotes a rotary antenna extending through the waveguide 14 and a top wall 1 6 of the chamber 2 and in contiguity with the chamber, and consisting of a vertical portion 17 substantially perpendicularto the wall 16 and a horizontal portion 18 which is substantially horizontal. The antenna 15 is driven by a motor 23 through a drive shaft 19, a belt 20 and pulleys 21 and 22.

Fig. 3 is a perspective view of the rotary antenna 15 by itself.

The portion 18 of the rotary antenna 15 is so configured as to have substantially the shape of a sector with an optional angle within the range of 60 to 1800. Moreover, the sum ofthe radius ofthefan- shaped portion 18 and the length of the vertical portion 17 extending into the chamber 2 is approximately one half of the wavelength used or an integral multiple thereof, and the dimensional ratio ofthe radius ofthe portion 1 8to the length ofthe portion 17 projecting into the chamber 2 is 2 to 1. The two ends of a fan-constituting arc ofthe portion 18 are each provided with a projection 24. The free end of each projection 24 is substantially at right angles to the radial edge ofthe fan-shaped portion and the width of the projection is approximately 1/6 ofthe wavelength.

The length of each projection 24 is notgreaterthan approximately 1115 ofthe wavelength.

The function and effect of the above-described construction are explained below.

In order than the rotary antenna 15may display its effect of rotation, it is important to provide an increased proportion of energy radiation from the horizontal portion 18. the sectorial angle ofthe portion 18 is less than 600, the amount of energy radiated in the horizontal direction is too small. On the other hand, if the angle is greaterthan 1 80", there will be generated areas where electromagnetic waves are cancelled by each other to cause a reverse effect.

However, asthesectorial angleofthe horizontal segment 18 is set within the range of 600 to 1800, even a single rotary antenna can generate radiant, equivalent horizontal electric fields in a plurality of directions, so thatthe intensity of the horizontal electricfield is increased and the amount of energy radiation from the horizontal portion is increased.

As already mentioned, the sum ofthe length ofthe portion 17 and the dimension of the portion 18 inside the heating chamber 2 is set at ofthewavelength used oran integral multiple thereof. Because ofthis, the elctromagnetic waves of half wavelength and longer can be excited without distortion from the waveguide 14to the antenna 15 and, moreover, the distribution of electromagnetic energyfromthe antenna 15 in the chamber can be effected with improved efficiency. Furthermore, because the dimensional ratio ofthe portion 17tothe portion 18 within the chamber 2 is set at approximately one to two, the proportion of energy radiation from the portion 18 can be further increased, this increase being additional to the above-mentioned effect attributableto the fan-like configuration.It was ex perimentally confirmed that the heating in the central region ofthe heating chamber can be substantially intensified.

The provision of projections 24 atthe ends ofthe arc constituting the fan-shaped portion provides a certain degree of local concentration of the electric field and as this strong electric field area is rotated, the effect of the antenna 15 is further improved and the uniformity of heating in the central zone of the heating chamber is enhanced. Sincethe electromagnetic radiation is absorbed from the external region into the core region of the food, a uniform heating is ensured when the electric field distribution in the central zone is greater than in the surroundingzone. However, if the projections 24 are too long, the electric field distribution in the central zone will tend to be somewhat attenuated.

It ways experimentallyfoundgthatthe bet result is obtained when the length of each of the projections 24 is not greaterthan 1/15 ofthe wavelength used and the width thereof is approximately one sixth of the wavelength.

Fig. 4a is a perspective view of another form of the rotary antenna 15. The horizontal portion 1 of this antenna 15 has the configuration of a triangular plane and is connected to the vertical portion 17 of the antenna in the vicinity of one of its apices. The sum of the length L of a line perpendicular to the side thereof opposite to the particular apex and the part of the vertical portion 17 projecting into the chamber2 is equal to approximately one half of the wavelength used or an integral multiple thereof, and the dimensional ratio ofthe line Lto the length of said part of the portion 17 is 2 to 1. Indicated at 24 are projections disposed at ends of the side of the triangle opposite to the apex atwhich the portion 17 is connected.The width of each projection 24 is notgreaterthan about one fifteenth ofthe wavelength used.

Thefunction and effect of the above construction will now be explained below. In orderthatthe rotary antenna 15 radiating high-frequency energy may display its effect, it is important to ensure an increased proportion of energy radiation from the horizontal portion 18. Since the portion 18 is shaped as a triangle in this embodiment, the antenna, despitethefactthat it is a single one, is capable of generating radiant, unhomogeneous horizontal electricfields in a plural ity of directions so as to intensifythe horizontal electricfieldand increase the amount of energy radiation from the portion 18.

Fig. 5 shows yet anotherform of the horizontal portion 18 ofthe antenna 15, the portion 18 being shaped as a non-equilateral triangle. This construction provides effects similarto those obtained in the preceding embodiments.

It will be apparent from the above description that with a high-frequency heating appliance embodying this invention,the high-frequency energy supplied from the oscillator through the waveguide can be radiated in good balancefromthevertical and horizontal portions of a rotary antenna into the heating chamber so that the elctricfieldcharacteristic in the central zone ofthe heating chamber, where uneven heating would otherwise occur, is improved and the heating load can be substantially uniformly heated.

Claims (11)

1. A high-frequencyheating appliance comprising a heating chamber, a waveguideforfeeding highfrequency energy of a given wavelength from a high-frequency oscillatorto the chamber, and a rotary antenna operable to radiate such energy in the chamber and comprising a vertical portion extending substantially perpendicularlyto an adjacent region of a top wall of the chamber and projecting into the chamber and a substantially triangular horizontal portion connected at one cornerthereofto the vertical portion,the sum of the length of the shortest distance between said corner connection and the side of the horizontal portion opposite thereto and the length of the part of the vertical portion projecting into the chamber being substantially one half, or an integral multiple of one half, of said wavelength.

2. An appliance as claimed in claim 1, wherein the ratio of the length of said distance to the length of said partis2to 1.

3. An appliance as claimed in claim 1, wherein the horizontal portion is substantiallyfan-shaped and has a sectorial angle in the range of 60to 180 .

4. An appliance as claimed in claim 3, wherein the ratio of the length ofthe radius ofthefan-shape and the length of said part is 2 to 1.

5. An appliance as claimed in any one ofthe preceding claims, wherein the horizontal portion is provided with a plurality of projections at said side thereof.

6. An appliance as claimed in claim 5, wherein the projections are provided one at each end of said side.

7. An appliance as claimed in eitherclaim 5 or claim 6, wherein each ofthe projections has a width of substantially 1/6 of said wavelength.

8. An appliance as claimed in any one of claims 5 to 7,wherein each ofthe projections projects by an amount of at most 1/15 of said wavelength.

9. A high-frequency heating appliance substantially as hereinbefore described with reference to Fig. 3 of the accompanying drawings.

10. A high-frequency heating appliance substan tiallyas hereinbefore described with referenceto Figs.

4a and 4b of the accompanying drawings.

11. A high-frequency heating appliance substantially as hereinbefore described with reference to Fig.

5 of the accompanying drawings.