US3754111A

US3754111A - Access tunnel and attenuator for microwave ovens

Info

Publication number: US3754111A
Application number: US00241238A
Authority: US
Inventors: P Jurgensen
Original assignee: Gerling Moore Inc
Current assignee: Gerling Moore Inc
Priority date: 1972-04-05
Filing date: 1972-04-05
Publication date: 1973-08-21
Anticipated expiration: 1990-08-21
Also published as: JPS498834A

Abstract

This invention relates to the treatment of materials by the application of microwave electromagnetic fields, and more particularly, to the development of a microwave absorbent tunnel which permits continuous free access into a microwave oven while limiting the escape of microwave energy therefrom.

Description

United States Patent [191 J urgensen [451 Aug. 21, 1973 ACCESS TUNNEL AND ATTENUATOR FOR MICROWAVE OVENS [75] Inventor: Peter-D. Jurgensen, San Carlos,

[21] Appl. No.: 241,238

[52] U.S. Cl. 2l9/l0.55

3,478,900 ll/l969 Jeppson ..2l9/l0.55 3,643,054 2/1972 Forster ..219/l0.55

Primary Examiner-J. VITruhe 4 wfixqmirlr:liy hP; le s?! Attorney-Paul D. Flehr, Robert B. Block et al.

[57] ABSTRACT This invention relates to the treatment of materials by the application of microwave electromagnetic fields, E2 and more particularly, to the development of a micro. wave absorbent tunnel which permits continuous free 56] References Cited access into a microwave oven while limiting the escape of microwave energy therefrom. UNITED STATES PATENTS 3,050,606 8/1962 Tibbs 219/1055 9 Claims, 4 Drawing Figures Coolant In Patented Aug. 21, 1973 2 Sheets-Sheet 1 Patented Aug. 21, 1973 2 Sheets-Sheet 2 ACCESS TUNNEL AND ATTENUATOR FOR MICROWAVE OVENS Microwave heating as applied to many materials can be conveniently and economically carried out with a microwave oven. For continuous processes, some means of access must be provided to the oven and should be compatible with the use of a conveyor system for carrying the material through the access and oven. A physical opening of adequate size in the oven wall presents a leakage hazard to operating personnel due to potential bialogical defects of microwave radiation. Safety regulations are presently proposed which would require the field strength of any measured leakage from an oven to be less than milliwatts per square cm.

Previous solutions to this problem have not been completely satisfactory. For frequencies most used, leakage control using a waveguide beyond cutoff requires that the opening be too small to be practical for many applications. Microwave absrobent linings used in larger tunnels are difficult to couple to the microwave fields and also require a substantial length to be effective and are also difficult to properly and adequately cool.

Access tunnels using microwave traps and reflectors have been proposed, such as cavities, capacitive structures, or slot reflectors, either stationary or moving .with the conveyed materials; Such traps and reflectors tend to be bandwidth limited and are not adequately coupled to the possible modes of propagation. In addition, they can become detuned or uncoupled by the presence of the material being treated which renders their effectiveness somewhat uncertain.

Another form ofprior device proposes the utilization of tunnel having inner and outer walls, the space between which is filled with a lossy dielectric liquid, the inner wall being made of a dielectric material through which the microwave energy passes and is dissipated in the liquid. However, the degree of mismatch between a dielectric liquid, such as water, is such that such a de- SUMMARY OF THE INVENTION AND OBJECTS In general, it is an object of the present invention to provide an access tunnel and attenuator microwave oven system which will overcome the above limitations and disadvantages.

A further object of the present invention is to provide a microwave access tunnel for a microwave oven which is characterized by being physically short and compact which has a higher attenuation than previously available in a given size together with a high-power handling capability.

Another object of the invention is to provide a microwave access tunnel which can utilize ordinary tap water as a coolant.

In general, the foregoing objects are achieved by providing a tunnel structure having an interior conductive wall which is in communication with an access port to an associated microwave oven. The wall is constructed to form a broad-band directional coupler having slot discontinuities which serve to concentrate and transfer microwave energy through that wall of the access tunnel and also at least partially provide an impedance match between the interior and exterior sides of the coupler. Adjacent the coupler are channels of lossy dielectric material preferably constructed of lossy dielectric tubing having a dielectric consonant intermediate that of a coolant which is circulated through the tubing and free space. By providing structure of the foregoing type, electrical discontinuities in the tunnel wall are created which serve to transmit energy through the wall and to concentrate it within the adjacent lossy material and coolant to obtain a maximum absorption and a high rate of attenuation per unit length of the tunnel.

1n the preferred form of the invention, a plurality of spaced parallel slots positioned one-fourth wavelength apart and at an angle of approximately 45 to the direc tion of theoretical propagation of energy in the tunnel is found satisfactory, together with a traverse coil spacing having elements spaced approximately one-fourth wavelength apart.

These and other objects of the invention will become apparent from the following detailed description thereof when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an isometric view partly broken away illustrating a microwave access tunnel and oven constructed in accordance with thepresent invention.

FIG. 2 is a cross-sectional view of the microwave tunnel construction of FIG. 1 taken along the lines 2-2 thereof. i

FIG. 3 is a cross-sectional view taken along the lines 33 of FIG. 2. 1

FIG. 4 is a cross-sectional view taken along the lines 4--4 of FIG. 2. I

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIGS. 1 through 4, there is shown one preferred form construction implying the present invention and consists generally of a microwave oven having a large access port in one wall thereof to which a microwave access tunnel 14 is coupled and supported in close fitting and fixed position therewith by suitable means (not shown). The tunnel provides access for a conveyor belt 16 which passes through the tunnel and slides over an interior lower wall thereof.

The microwave access tunnel l4 of the present invention is constructed of a plurality of conductive walls which may, for example, be made of a suitable sheet metal and comprise outer, top, bottom, and

side walls

18, 20, 22, and 24; end face walls 26; and inner top, bottom, and side

conductive walls

28, 30, 32, and 34. As shown particularly in FIGS. 1 and 2, the outer top and

bottom walls

18, 20 are formed by a single sheets of metal having bent ends 36, to which are fastened the upper and lower inner conductive walls by screws 38. The outer side walls 22, 24 are formed as a cap-like enclosure and are secured to extensions 40 of the outer top and bottom walls by screws 42, shown in FIG. 2.

The

interior side walls

32, 34 are formed by suitable sheet metal strips having top and bottom portions 44 fastened to the top and bottom

outer walls

18, 20 by screws 46. The foregoing construction has been found satisfactory for providing a rigid self-contained unit which may be detachably removable from the associated microwave oven should it be desired to convert the same to batch operation by closing off the access ports 12.

Taken together, the various walls of the tunnel form an interior waveguide structure having a uniform rectangular cross section with its axis of propagation in line with the normal surface vector defining the access port. This waveguide structure is surrounded by an annular chamber which is completely closed within a conductive box or shield.

Means is provided for forming a broad band directional coupler or matching plate for transferring microwave energy from the interior of the tunnel to the annular region between the exterior and interior walls. The coupling means is distributed throughout one broad wall interior of the tunnel, as for example, bottom wall 30 and consists of a plurality of slots 50a 50k arranged at an angle of about 45 to the direction of propagation of material and energy in said tunnel (its axis). It has been found that the slots 50 are particularly effective as a means for concentrating energy in their vicinity and thereby permitting the same to be coupled through the slots when the inter slot spacing is approximately one-quarter wavelength apart from each other as measured by the closest distance oriented at about 45 to the propagation dimension of the tunnel. The slotted plate 30 may be arranged in either one of the top or

bottom walls

28, 30 or both, it having been found that substantially all the requisite attenuation available from the present construction is obtained by providing slots solely of

walls

28, 30, as for example, the lower wall 30 as shown in FIG. 1. It should be realized, however, that it may be advantageous to provide solely the upper wall with slots so as to avoid debris falling through the slots under gravity during operation of the device.

While a detailed theoretical analysis is not available, it is believed that the coupler can be characterized as serving the function of matching plate which reorganizes the field directions of the otherwise random microwave fields entering the tunnel through port 12 so that as they travel down the tunnel, they are realigned in a direction so that their continued encounter with the slot structure of the plate 30 excites the slots and causes significant transference of energy into the annular chamber surrounding the interior walls of the tunnel.

In practice the slot orientation and spacing has proven exceedingly effective in obtaining a coupling to all propagating modes and it is not unduly frequency sensitive.

Means is provided for absorbing microwave energy transmitted through the coupling plate 30 and includes suitable microwave absorber positioned in the annular region outside the interior conductive tunnel walls to receive microwave energy from a coupler. In order to satisfactorily meet relatively high power handling demands, it is preferred that the microwave absorber take the form of a plurality of tubes 52 positioned transversely of the axis of transmission of the tunnel forming channels and designed to carry a dielectric coolant 53 such as water. Each tube is preferably constructed in part of a high loss, dielectric material having a dielectric consonant intermediate that of the coolant liquid carried through the tube and that of free space so as to more effectively handle microwave energy. While the tubes could be constructed and arranged to connect to a suitable manifold structure at each side of the tunnel, it is found to be more convenient to construct them in a connected array resembling a coil having an inlet 54 and a discharge 56 so that fluid coolant passing through them enters through the inlet and passes sequentially through each turn of the coil, finally to be discharged at the opposite end of the device. Since the greatest power handling capability is demanded at the end of the device which is nearest to the microwave oven, it is preferred that that end be'connected to the input or source of coolant fluid.

Referring now particularly to FIGS. 2 through 4 wherein the construction of each tube 52 is shown in detail, each channel is formed of an interior tubing section 58 of high loss material. An example of a suitable material is silicon rubber, heavily loaded with dispersed particulate carbon black. In view of the strength limitations of such material, it is found desirable to surround the same with a microwave transparent tube 60, which may, for example, be made of polytetrafluoroethylene and which will support the carbon-loaded silicon rubber tube against undue deformation upon application of internal fluid pressure. The end of each tube 58 is sealed into contact with a U-shaped turnaround 62 which consists of silicon rubber tubing which extends into tube 52 a length sufficient to be within the outer support tube and is sealed to the former with a suitable adhesive sealant. Support positioning of the assembled is obtained by providing a plurality of holes 64 appropriately spaced and aligned in the upper and lower portions of the

interior side walls

32, 34, such portions being of low el ecyrical consequence and provided solely for general support of the unit of the

tubes

52, 60. As shown, each turnaround shifts one-half a tubing width so that each complete turn of the coil advances or shifts the coolant from one adjacent tube to the next. The size of the tubing and the inter spacing between holes 64 is such that the spacing between turns of adjacent channels is approximately one-quarter wavelength and such that essentially all available space to locate absorber is utilized.

After an assembly and provision for suitable structional support of the operation of the access tunnel of the present invention requires only the existence of a suitable coolant fluid to the input line. In one application which was designed for use at 2,450 MHz at a microwave power rating up to 5,000 watts required 2 gallons per minute of tap water at a pressure of 45 pounds per square inch, well within usual municipal supply line capacity, it was found that a device constructed in accordance with the present invention permitted no load operation continuously in the: oven itself at full power in a conveyor mode. The microwave leakage was less than l0 milliwatts per square cm at 2 inches from any part of the system. The overall length of the microwave tunnel capable of the foregoing specifications was 18 inches.

While there has been disclosed the use of a slotted conductive plate as a directional coupler, it will be realized that resonant slots, or orifices of many kinds may be capable of serving the purposes required in the present invention. These requirements include impedance matching from the interior of the tunnel through a wall,

ability to concentrate field energy with the adjacent absorber by directing a component of the total energy flow normal to the plate and through its wall. Also, while certain absorber materials have been disclosed as preferred from those now available, they may be replaced by others, particularly as higher strength materials become available.

Accordingly, the scope of the invention should not be limited to the specific structure shown as the preferred embodiment herein by way of example, but should be interpreted with reference to the accompanying claims.

I claim:

1. In a microwave system for the treatment of materials by the application of microwave electromagnetic fields, a microwave oven, means forming at least one access port to said oven including, an access tunnel and attenuator for use therewith comprising means forming a plurality of conductive walls constructed and arranged to couple to said port and to provide an access tunnel thereto, means for forming a directional coupler for transferring microwave energy from the interior of said tunnel to the exterior thereof, said coupler means being distributed throughout at least said one wall of said tunnel, a microwave absorber positioned outside of said conductive tunnel walls to receive microwave energy from said coupler.

2. A system as in claim 1 in which said microwave absorber includes means forming a plurality of channels, tubes for carrying a dielectric coolant therethrough, portions of said channels being formed of a lossy dielectric material having a dielectric constant intermediate that of said lossy liquid and free space, said lossy dielectric material portions of said coil being arranged immediately adjacent that side of said directional coupler on the outside of said conductive walls.

3. A system as in claim 2, in which said channels are spaced one-quarter wavelength apart.

4. A system as in claim 2, in which the portion of said channel adjacent said directional coupler includes a tubing made of dielectric material having high loss at microwave frequencies.

5. A system as in claim 4, wherein said lossy material is a silicon rubber loaded with carbon particles.

6. A system as in claim 5, wherein said channel further includes a tubular section made of material transparent to microwaves and surrounding said lossy tubing to support the same.

7. In a microwave system for the treatment of materials by the application of microwave electromagnetic fields, a microwave oven, means forming access port to said oven including, an access tunnel and attenuator for use therewith comprising means forming a plurality of conductive walls constructed and arranged to couple to said port and to provide an access tunnel thereto, means associated with one of the walls of said tunnel for forming a multi mode directional coupler for transferring microwave energy from the interior of said tunnel to the exterior thereof, said coupler means being formed by a plurality of slots formed in at least one wall of said tunnel, and arranged at an angle of about 45 to axis of said tunnel, a microwave absorber positioned outside of said conductive tunnel walls to receive microwave energy from said coupler.

8. A microwave system as in claim 7, in which said slots are spaced apart approximately one-quarter wavelength.

- 9. In a microwave system for the treatment of materials by application of microwave electromagnetic field, a microwave oven, means forming access port to said oven, and access tunnel and attenuator for use therewith comprising means forming a plurality of conductive walls constructed and arranged to couple to said said coupler.

Claims

7. In a microwave system for the treatment of materials by the application of microwave electromagnetic fields, a microwave oven, means forming access port to said oven including, an access tunnel and attenuator for use therewith comprising means forming a plurality of conductive walls constructed and arranged to couple to said port and to provide an access tunnel thereto, means associated with one of the walls of said tunnel for forming a multi mode directional coupler for transferring microwave energy from the interior of said tunnel to the exterior thereof, said coupler means being formed by a plurality of slots formed in at least one wall of said tunnel, and arranged at an angle of about 45* to axis of said tunnel, a microwave absorber positioned outside of said conductive tunnel walls to receive microwave energy from said coupler.

9. In a microwave system for the treatment of materials by application of microwave electromagnetic field, a microwave oven, means forming access port to said oven, and access tunnel and attenuator for use therewith comprising means forming a plurality of conductive walls constructed and arranged to couple to said port and to provide an access tunnel thereto, a conveyor disposed through said tunnel and oven for passing material to be treated through said oven, means associated with one of the walls of said tunnel for forming a multi mode directional coupler for transferring microwave energy from the interior of said tunnel to the exterior thereof, said coupler means being located throughout at least said one wall of said tunnel, microwave absorber means positioned outside of said conductive tunnel walls to receive microwave energy from said coupler.