Twisted Microwave Applicator

Abstract

A twisted microwave applicator is provided to avoid the heating non-uniformities inherently resulting from non-twisted microwave applicators. The invention comprises a section of waveguide twisted about its axis, preferably twisted 90.degree. or more. The applicator is coupled to a microwave generator by conventional techniques. The objects to be treated are introduced into one end of the applicator, transported through the applicator and removed at the other end. The twisted waveguide provides for rotation of the field pattern, which tends to subject all portions of the objects to the same overall heating effect.

Parent Case Text

This is a continuation of application Ser. No. 208,768 filed Dec. 16, l971 now abandoned.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention is a further deveopment in the field of microwave treatment applicators, particularly in the technique of providing a uniform heating effect on all parts of an object passing through the applicator.

2. Description of the Prior Art

In conventional microwave applicators, the electric field varies in intensity over any given cross section of the applicator at any given instant of time depending upon the distance from the conductive walls of the applicator to the point at which the electric intensity is measured. In addition, further variations in field intensity will be introduced over the given cross-section because of field distortions caused by the dielectric properties of the objects to be treated. An object passing through a conventional microwave applicator will experience different heating effects on its various surfaces and internal portions because of these non-uniformities in the electric field intensity which are inherent in conventional applicators.

SUMMARY OF THE INVENTION

It is an object of this invention to provide for the exposure of all surfaces and internal portions of material treated in a microwave applicator to a heating effect of increased uniformity within the applicator.

It is a further object of this invention to achieve a heating effect of increased uniformity within a microwave applicator by providing for relative rotation between the field pattern in the applicator and the material to be treated.

It is a further object of this invention to provide for rotation of the field pattern within a microwave applicator by means of a twisted waveguide section.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a twisted microwave applicator illustrating the invention embodied in a resonant cavity applicator;

FIG. 2 is a cross-section of the applicator taken on line 2--2 of FIG. 1;

FIG. 3 illustrates the non-uniformity of the electric field intensity over a cross-section of a microwave applicator;

FIG. 4 illustrates the distortion of the electric field intensity in a microwave cavity caused by the dielectric properties of an object being treated; and

FIG. 5 is a view similar to FIG. 1 illustrating the invention embodied in a traveling wave applicator.

DESCRIPTION OF THE INVENTION

FIG. 1 shows a perspective view of a twisted microwave applicator 1 which effectively embodies the concept of this invention. The applicator comprises a waveguide section 2 twisted about its longitudinal axis. In the drawing a 90.degree. twist is shown. Waveguide section 2 is fitted with conductive end walls 3 and 4 to form a resonant cavity. Except for the twist, waveguide section 2 is made like conventional straight waveguide sections; namely section 2 is made of metal walls to form a container for microwave energy. Waveguide section 2 is rectangular in cross section at every point along its length as shown for example in FIG. 2. Although a rectangular cross-section is preferred, the principle of the invention applies to any cross-section shape which tends to increase uniformity of heating when it is twisted. The objects to be treated within the cavity are introduced and removed through openings in the end walls. In one approach the objects are introduced through a tube 5 and removed through a tube 6. Tubes 5 and 6 are made of conductive material and are designed in conventional manner to be non-propagating at the frequency of operation of the applicator so that microwave energy will not escape from the cavity. Tubes 5 and 6 are secured to the end walls but do not extend inside the waveguide 2. The objects to be treated are preferably enclosed inside the cavity by a dielectric tube 7 extending between the end walls and coaxial with tubes 5 and 6. In this way the objects to be treated can be inserted into tube 5 and push one another through the tube 7 and out tube 6. Also, the waveguide section 2 can be oriented to slope downward from the input to the output end to provide a gravity transport. Another alternative is to run a conventional conveyer belt (not shown) through the waveguide section 2 to carry the objects. If tube 7 is employed, the belt would go through the tube. If the action of the belt transport and the shape of the objects would tend to cause random rotation of the objects on top of the belt, the objects should be clipped or otherwise fastened to the belt to avoid the possibility that some objects might rotate in a way which would tend to offset the twisting effect of the twisted waveguide. Waveguide section 2 is coupled to a microwave generator (not shown) by a conventional inlet waveguide section 10 coupled to waveguide 2 through an iris opening 11. Alternatively, coaxial line and coupling loop type of coupling can be employed although such a coupling would be made in a wide wall of waveguide section 2, rather than in a narrow wall.

Waveguide section 2 is twisted to achieve rotation of the electromagnetic field pattern from one end of the microwave treatment cavity to the other. This rotation of the field pattern will cause the surfaces and internal portions of objects passing through the treatment cavity to experience a uniform heating effect. The heating effect at any point within the cavity is proportional to the square of the electric field strength at that point. If there are local variations in the field strength within the cavity, then objects having different shapes, objects having different paths through the waveguide, and different portions of a specific object will experience non-uniform heating effects unless all portions of each object are exposed to the same net field integrated over the path of the object through the cavity. In a conventional straight waveguide section the non-uniformity remains constant along the section, so for example, a portion of an object which is exposed to a low energy field, is exposed to the same low field throughout its travel through the waveguide. By rotating the field pattern about an object as it passes through the cavity, this invention provides the desired integration so that all objects and all parts of the same object tend to experience the same net exposure to the microwave energy within the cavity as they pass through the cavity.

FIG. 3 illustrates the variation in electric field intensity over the cross-section of a rectangular microwave applicator (such as any cross-section of waveguide section 2) at any given instant of time. The field variation is shown by individual field vector lines E and the line 12 which depicts the envelope of the changing field strength across the waveguide section. This variation results from the fact that the components of the electric field parallel to the narrow conducting walls decrease in magnitude as the distance from the narrow walls decrease, until E = 0 at the narrow walls. FIG. 4 illustrates the distortion of the electric field intensity over a given cross-section caused by the non-uniform cross section or non-uniform dielectric properties of an object 14 to be treated. The non-uniformities in the electric field intensity such as illustrated in FIG. 3 and FIG. 4 cause non-uniform heating effects upon objects passing through a microwave applicator in which the field pattern remains constant. However, in accordance with this invention, rotating the field pattern about the objects as they pass through the applicator, causes the objects to experience changing fields which tend to provide a uniform heating effect on the objects in the course of their travel through the applicator. As indicated by the E arrows in FIG. 1 the applicator formed by the 90.degree. twisted waveguide section 2 provides an E field which changes from vertical at the inlet end of section 2 to horizontal at the outlet end.

Use of a straight waveguide section in which the material to be treated is rotated in controlled manner as it passes through the waveguide section can be used to simulate the result achieved with a twisted waveguide. However, this requires a complex mechanical system for rotating the material. The complexity of such a system involves inherently poorer reliability and greater maintenance problems than the twisted waveguide.

Although the 90.degree. twist of waveguide section 2 is shown in FIG. 1, it will be understood that twisting of more or less than 90.degree. can be employed to achieve heating uniformity which is better than with a straight waveguide. However, at least 90.degree. twist is required to cause the electric field lines to pass through the objects parallel to the vertical axis of the objects and also parallel to the horizontal axis of the objects. Where more than 90.degree. is used, the twist is preferably in multiples of 90.degree., and preferably the number of multiples is selected so that the object to be treated is not exposed to electrical field more in one direction than in another direction. As is known by those skilled in the art, the electrical field axially along the resonant cavity applicator 1 is in form of a sine wave with zero intensity at the end walls. Thus if the twist starts immediately at the end walls 3 and 4, the full effect of the twist is not realized. Accordingly it is preferred to have straight waveguide lengths L at the inlet and outlet ends, particularly where a twist of only 90.degree. is used. Although any length L is beneficial, the straight end portions preferably have a length L on the order of 1/4 of the wavelength of the waveguide wavelength, as distinguished from free space wavelength.

It is also possible to use a twisted waveguide section in a traveling wave applicator as shown at 1' in FIG. 5. The differences between the resonant cavity applicator 1 of FIG. 1 and the traveling wave applicator 1' in FIG. 2 are that the traveling wave applicator has a waveguide output 16 which goes to a dummy load (not shown), and the coupling from the inlet waveguide 10 opens completely into the waveguide section 2 so that microwave energy passes into section 2 through the fully open end of section 10 rather than through the small iris hole 11. In the traveling wave embodiment, the field strengths lengthwise along section 2 are not held to zero at the end walls 3 and 4 so the straight end sections L are not required for the reason discussed in connection with FIG. 1. However, as a practical matter straight sections L are preferred in the traveling wave case to simplify connection of the input and output sections 10.

Claims

What is claimed is:

1. An electromagnetic energy applicator comprising a waveguide section elongate about an axis and having electromagnetic energy input means connected thereto, said waveguide section having two rectangular end walls with openings therein for passage of an object along a path through said waveguide section, said end walls being substantially opague to said electromagnetic energy, said waveguide section being configured so that the longer rectangular dimension of one of said end walls is nonparallel to the longer rectangular dimension of the other of said end walls, said waveguide section being further configured so that as said object passes along said path from said one end wall to said other end wall said object is exposed for the greater portion of said path to a continuously varying orientation of the electric field of said electromagnetic energy.

2. The applicator of claim 1 wherein said waveguide section forms a resonant cavity; and wherein said waveguide section comprises a twisted portion interposed between two nontwisted portions; said end walls being disposed to permit passage of said object sequentially through one of said non-twisted portions, then through said twisted portion, and then through the other of said non-twisted portions along said path; each of said non-twisted portions having a length along said path sufficient to provide a non-zero electric field intensity at the boundary between said non-twisted portion and said twisted portion; said twisted portion having a length along said path sufficient to provide that said object is exposed to more of said energy in said twisted portion than in said non-twisted portions.

3. The applicator of claim 2 wherein said twisted portion of said waveguide section is twisted by at least 90.degree..

4. The applicator of claim 2 wherein the length of each of said non-twisted portions of said waveguide section is on the order of one-quarter of the waveguide wavelength of said electromagnetic energy.

5. The applicator of claim 1 wherein said waveguide section comprises a twisted portion interposed between two non-twisted portions; said end walls being disposed to permit passage of said object sequentially through one of said non-twisted portions, then through said twisted portion, and then through the other of said non-twisted portions along said path; the length of each of said non-twisted portions along said path being sufficient to provide maximum electric field intensity of said energy at the boundary between said non-twisted portion and said twisted portion.

6. The applicator of claim 5 wherein the length of each of said non-twisted portion along said path is substantially no longer than one-quarter of the waveguide wavelength of said energy.

7. The applicator of claim 1 wherein said applicator further comprises electromagnetic energy output means connected to said waveguide section; said waveguide section comprising a twisted portion interposed between two non-twisted portions; said end walls being disposed to permit passage of said object sequentially through one of said nontwisted portions, then through said twisted portion, and then through the other of said non-twisted portions along said path; one of said non-twisted portions having a length sufficient to accommodate thereon the connection of said electromagnetic energy input means to said waveguide section and the other of said non-twisted portions having a length sufficient to accommodate thereon the connection of said electromagnetic energy output means to said waveguide section; said twisted portion having a length along said path sufficient to provide that said object is exposed to more of said energy in said twisted portion than in said nontwisted portions.

8. The applicator of claim 7 wherein said twisted portion of said waveguide section is twisted by at least 90.degree..

9. The applicator of claim 1 wherein said waveguide section comprises a twisted portion interposed between two non-twisted portions; said end walls being disposed to permit passage of said object sequentially through one of said non-twisted portions, then through said twisted portion, and then through the other of said non-twisted portions along said path; the length of each of said non-twisted portions along said path being sufficiently small to provide that said object is exposed to more of said energy in said twisted portion than in said non-twisted portions.

10. The applicator of claim 1 wherein said waveguide section forms a resonant cavity.

11. The applicator of claim 1 wherein said rectangular end walls are perpendicular to said axis of said waveguide section.

12. The applicator of claim 1 wherein said electromagnetic energy input means comprises an energy input waveguide which is electromagnetically coupled to said object-treating waveguide section through an iris opening.

13. The applicator of claim 1 wherein said electromagnetic energy input means comprises an energy input waveguide which is elongate about an axis, said energy input waveguide axis being perpendicular to the axis of said object-treating waveguide section.

14. The applicator of claim 13 wherein the interface between said energy input waveguide and said object-treating waveguide section lies in a plane which is perpendicular to said axis of said energy input waveguide.