US3555232A

US3555232A - Waveguides

Info

Publication number: US3555232A
Application number: US769090A
Authority: US
Inventors: William J Bleackley
Original assignee: Canadian Patents and Development Ltd
Current assignee: Canadian Patents and Development Ltd
Priority date: 1968-10-21
Filing date: 1968-10-21
Publication date: 1971-01-12
Anticipated expiration: 1988-01-12
Also published as: DE1952987C3; DE1952987A1; GB1256339A; GB1255690A; FR2021167A1; DE1952987B2; SE345786B

Abstract

A rectangular waveguide has two pairs of inwardly projecting ridges. Intensification of the electric field occurs between the ridges of each pair, thus causing an area of relative field rarefaction in a central region of the waveguide between the pairs of ridges. The ridges are tapered longitudinally to vary the ratio of intensification to rarefaction. Workpieces to be heated (dried or cooked or otherwise processed) are passed through the central area to receive controlled quantities of microwave energy, thus improving the uniformity of heating. matchbox can also be used in a slotted waveguide antenna. Variants include use of a region of field intensification for energy extraction instead of a region of field rarefaction, in which case the taper will usually be negative, i.e. will take the form of enlargement. Alternatively, the ridges can be caused to converge to vary the electric field, or a combination of converging and tapering ridges can be used. Alternatively, two spaced areas of field intensification can be used simultaneously, for example to dry the match striking compound on the sides of a matchbox cover.

Description

United States Patent [72] Inventor [21] Appl. No.

[22] Filed [45] Patented [73] Assignee [54] WAVEGUIDES 32 Claims, 29 Drawing Figs.

Primary Examiner-.l. V. Truhe Assistant Examiner-L. H. Bender Attorney-Stevens, Davis, Miller & Mosher ABSTRACT: A rectangular waveguide has two pairs of inwardly projecting ridges. lntensification of the electric field occurs between the ridges of each pair, thus causing an area of relative field rarefaction in a central region of the waveguide between the pairs of ridges. The ridges are tapered longitudinally to vary the ratio of intensification to rarefaction. workpieces to be heated (dried or cooked or otherwise [52] U.S. Cl 219/1055, processed) are passed through the central area to receive com 343/771- 219/1061 trolled quantities of microwave energy, thus improving the [51] lnLCl 05b 9/06, uniformity of heating The Structure can also be used in a 5/00 slotted waveguide antenna. [50] FIG! ofSearch 343/771; variants include use of a region of field intensification for 219/1055 energy extraction instead of a region of field rarefaction, in which case the taper will usually be negative, i.e. will take the [56] References cued form of enlargement. Alternatively, the ridges can be caused UNITED STATES PATENTS to converge to vary the electric field, or a combination of con- 3,050,606 8/1962 Tibbs 219/1055 verging and tapering ridges can be used. Alternatively, two 3,193,830 1965 r n hel' 343/771 spaced areas of field intensification can be used simultane- 3,197,601 7/1965 Wayne et al. 219/ 10.61 ously, for example to dry the match striking compound on the 3,353,968 1 H1967 Krajewski 219/10.55X sides ofa matchbox cover.

/ l| A D /7 s \\\\I w Q /3 b I B L PATENTEU JAN 1 2m:

SHEET 2 BF 7 PRIOR 6R7 R OR 9R7 2 Eff.

PATENTED JAN 121971 SHEET 3 [IF "I PATENTED JAN] 21% SHEET 8 [IF 7 PRIOR ART PRI R ART PATENTED JAN 1 2 l97i SHEET 7 BF 7 WAVEGUIDES This invention relates to improvements in waveguides and their'use.

In one application, the invention is related to an improved waveguide structure that is useful in apparatus for heating materials by microwave energy. Such heating has many industrial applications, including, for example, the cooking of food products, and the drying of materials containing excessive quantities of moisture. Another application is the curing of.

glue. More specific examples will be given below.

The present invention also provides a waveguide structure that, in another application, is suitable for use as an improved antenna for radiating microwave energy into space.

One of the important features of a preferred form of the present invention resides in the provision of an ability to vary the proportion of the totalmicrowave energy that is available at any given location along the waveguide. As above indicated, such energy may be used for heating a workpiece, or it may be removed from the waveguide to serve another purpose, e.g. to feed to one or more other waveguides, or to radiate into space.

Since energy isused or removed either continuously or at discrete locations along the waveguide, there will be attenuation of the total energy along the waveguide from upstream to downstream in the direction of energy flow. By providing an ability to vary the proportionof such total energy that is made available, and by making such variation substantially the in- I verse of the attenuation which determines the total energy at any given location along the waveguide, these two factors can uniformity of "available" energy. i's'the condition most often sought, and this is therefore the aspect of the control of energy on which principal'emphasis will be placed below.

Such controlof available energy is achieved in accordance with one feature of the invention by forming the waveguide with a cross section having an internal ridgestructure which in any given transverse plane provides a measure of control of the distribution of the electric field intensity across that plane. This ridge structure can be formed with a taper in thelongitudinal direction of the waveguide to vary the ratio between the more-intensified fieldor fields, on the one hand, and the more rarefied field or fields, on the other. As is more fully explained below, one or other of these field regions can be used for COnr trolling the amount of microwave energy available, whether for heating a workpiece or otherwise.

ln one specific form of thepresent invention the waveguide has four internal ridges arranged in two pairs spaced apart from each other in a manner calculated to achieve an area of electric field rarefaction between such pairs. Compensation for attenuation in the longitudinal direction may be incorporated into this latter waveguide by tapering the ridges to achieve the effect already discussed and decrease the extent of is formed with two pairs of inwardly projecting ridges, with the I pairs arranged on respective sides of a first one of these planes and with the ridges of each pair formed with surfaces that are spaced from each other, with one ridge of each pair extending towards the other ridge of that pair from a pair of walls of the waveguide arranged on opposite sides of the second of such planes. The result, upon energization of the waveguide, is the formation of regions of high electric field intensity between the approaching surfaces. The ridge pairs are spaced apart sufficiently from each other to provide a central region of low electric field intensity (i.e. relative rarefaction) between them, i.e. in the vicinity of the axis of the waveguide. It is this central region that is exploited for the energy extraction operation in this embodiment of the invention.

In another version of the invention the workpiece or at least one or more portions of it are moved along one or more respective regions of intensified field, instead of along a region of rarefied field. In this alternative, control over the amount of energy available, to compensate for attenuation, can be achieved by sloping the ridge structure upwardly, that is to say gradually enlarging it to increase its degree of prominence and'hence increase the ratio of field intensification to field rarefaction in the downstream direction. v

In yet another construction, the desired longitudinal variation of field intensity can be achieved by arranging for a pair of ridges located side by side to converge (or diverge), that is to say to vary their mutual spacing, rather thantheir degree of prominence.

Other features of the invention will become apparent from the various embodiments thereof that are illustrated diagrammatically in the accompanying drawings. It is to be understood that such illustration is provided by way of example only, and

not by way of limitation of the invention, the broad scope of which is defined in the appended claims.

DESCRIPTION OF FIGS. OF DRAWINGS In the drawings:

FIG. 1 is a general side view of a first form of apparatus according to the invention, which is useful for the microwave heating of cylindrical workpieces;

FIG. 2 is a section on the line II-II FIG. I;

FIGS. 2a to c are cross-sectional diagrams contrasting the performance of the prior art and the improved waveguide of FIGS. 1 and 2;

FIG. 3 is a section on the line III-III in FIG. 2, while constituting at the same time a larger scale, broken away view of essentially the same apparatus as is seen in side view in FIG. 1;

FIG. 4 is a cross-sectional view of a portion of an alternative form of waveguide that may be used in the apparatus of FIG.

' 1, illustrating a modification;

FIG. 5 is a cross section of another alternative form of waveguide that may be used in the apparatus of FIGS. 1 to 3;

. FIG. 6 is yet another alternative waveguide cross section for use in such apparatus;

FIG. 6 a is a fragmentary section on Vla-Vla in FIG. 6;

FIG. 7 is a cross section of a laterally extended form of waveguide for use with multiple workpieces;

FIG. 7a is an alternative to FIG. '7;

FIG. 8 is a plan view of another form of apparatus according to the invention, being a waveguide adapted for the microwave heating of sheet material;

FIG. 9 is a view taken on the section line IX-IX in FIG. 8, but showing the parts in perspective, and with part of a sidewall cut away to disclose inner features;

FIG. 10 is a generally similar type of cross section and perspective view to that of FIG. 9, but in this case showing the application of the invention to a waveguide constructed to act as an antenna for propagating microwave energy into space;

FIG. 11 is a longitudinal section taken along a pair of ridges of the waveguide of FIG. 10;

FIG. 11a is a diagram illustrating the performance of the waveguide of FIGS. 10 & 11;

FIG. 12 is a cross-sectional perspective of another form of ridged waveguide;

FIG. 12a is a larger scale fragment of FIG. 12;

FIG. 13 is a cutaway perspective view of a portion of the type of waveguide shown in FIG. 12 embodying a feature of the present invention;

FIG. 14 is a perspective view of a fragment of a microwave heater demonstrating use of the waveguide section of FIG. 13;

FIG. 15 is a cross section of a modification of FIG. 13;

FIG. 16 is a cutaway perspective view of a portion of yet another form of waveguide according to the invention;

FIG. 17 is a cross section of a further modified form of ridged waveguide;

FIG. 18 is a variant of FIG. 17, as seen on the section line XVIILXVIII in FIG. 19;

FIG. 19 is a fragmentary side section of a lead-in portion of a waveguide such as shown in FIG. 18, and as seen on the section line XIX-XIX in FIG. 18 but with the workpiece omitted;

FIG. 19a is a fragment of FIG. 19 illustrating a modification;

FIG. 20 is a section on XX-XX in FIG. 19; and

FIG. 21 is a section on XXI-XXI in FIG. 19.

FIRST MAIN EMBODIMENT FIGS. 1 TO 3 LONGITUDINAL WORKPIECE TRAVEL This apparatus comprises a main elongated waveguide section 10 made of a typical waveguide metal, e.g. copper or brass, to an input end 18 of which a similar metal input waveguide section 11 is connected, while at its output end 19 the waveguide section 10 extends into a similar metal output waveguide section 12. This apparatus is suitable for heating cylindrical workpieces, a prime example being the cooking of wieners or similar sausagelike products, but the apparatus may also be used for workpieces having rectangular, irregular,- elliptical or indeed any cross-sectional shape that may be accommodated in the waveguide processing region. A removable Teflon (Trade Mark) tube 13 extends through the full length of the main waveguide section 10 to provide a path through which the workpieces can be conveyed. The microwave properties of Teflon are such that the tube 13 does not absorb appreciable microwave energy. Other materials having similar properties, i.e. polystyrene, Rexolite (Trade Mark) or acrylic resins may be used. To avoid a need to bend the tube 13, to lead it into and out of the main waveguide section 10, with the consequent possibility of interruption of smooth flow of the workpieces along the tube, the input and output waveguide sections 11 and 12 are, as shown, slightly inclined in relation to the main section 10. This inclination achieves the necessary separation between the waveguide and the workpiece tube at the two ends of the apparatus. Beyond the waveguide structure, the Teflon tube 13 is supported by outer metal tube portions 9 secured to such structure. The diameter of the tube portions 9 is such as to cause them to constitute waveguides beyond cutoff, thus preventing radiation out of the waveguide along such tube portions.

As shown in FIG. 2, the main or working section 10 is rectangular in overall cross section, in this example with the typical 2:1 ratio of width to height of rectangular waveguides. FIG. 2 also demonstrates how the workpiece tube 13 extends along and is symmetrical about the central longitudinal axis of the section 10. This section is, however, modified by the provision of two pairs of opposed metal ridges 14, I and 16, 17, a first pair of ridges 14, I5 projecting towards each other on one side of the workpiece tube 13, while the other pair of ridges I6, 17 is similarly arranged to extend towards each other on the other side of the tube 13. Each pair of ridges can thus be said to be arranged on a respective side of a first plane A, while the individual ridges of each pair project towards each other from walls of the waveguide that are arranged on opposite sides of a second plane B, these planes A and B being mutually perpendicular and intersecting to define the longitudinal axis of this waveguide section 10.

As illustrated by the arrows in FIG. 2, which represent the electric field, the effect of these ridges, when the waveguide is energized with microwave energy in the T13 mode (using the conventional United States nomenclature), is for an intensification of the electric field to appear between the approaching surfaces of each ridge pair. Thus, as a result of the ridges, the field intensity in the region of the workpiece tube 13 between the pairs of ridges suffers a substantial rarefaction from that which it would normally be at the center of an unridged rectangular waveguide similarly energized in the TE mode. It is to be emphasized that the electric field shown in FIG. 2 isessentially diagrammatic, and that this FIG. is intended to represent the conditions that will prevail in the absence of a workpiece. As previously stated, the workpiece tube 13, being of Teflon, will not absorbain'fappreb iable'amount of energy. However, the workpieces themselves ('fonexample sausage products to be cooked while passing'along-the tube 13) will have a relatively high dielectric constant and loss tangent. This will cause them to absorb relatively largequantities of the available energy and also to distort the field. In other words they will be the cause of an intensification of the electric field in the vicinity of the center of the waveguide and thus in the workpieces themselves, in comparison with the field in the same region when the workpiece tube is empty.

To demonstrate this point more graphically FIGS. 2a to 0 have been provided. FIG. 2a shows a simple, unridged rectangular waveguide 8 excited in the TE mode. The electric field is distributed sinusoidally with the maximum concentration at the center. When a lossy workpiece 7 is placed in a workpiece tube 13 in this waveguide 8 (FIG. 2b), the concentration of the field down the center, is much increased, with the result that substantially all the field passes through the central portion of the workpiece and its lateral areas 7a receive relatively little heating. This phenomenon has been observed using prior art microwave heaters.

FIG. 20 demonstrates the electricfield in a lossy workpiece 7 in the tube 13 in the ridged waveguide section 10 of FIG. 2,

the field intensifying effect of the presence of the workpiece being offset against the field rarefying effect (at the waveguide center of the ridges. The result is a much greater uniformity of heating of the workpiece throughout its cross section.

By means of the ridges 14 to 17 such a degree of intensification of the electric field in the regions between the ridges can be achieved that, even with the counteracting distortion produced by very high dielectric constant workpieces, the amount of energy available for absorption by such workpieces can be kept to only a relatively small proportion of the total microwave energy in the waveguide. The nearer the ridge surfaces approach one another, the more this effect is enhanced. As a practical matter the limit of this approach is determined by the fact that the space between the ridges must be sufficiently large to ensure that electrical breakdown does not occur between them. The provision of the ridges 14 to 17 thus enables the degree of energy extraction to be controlled as well as distributing the energy more uniformly across the workpiece. While the total microwave energy transmitted along the waveguide may be comparatively high, the amount of such energy that is available for the workpieces to absorb can be severally restricted at any given point, for example at the energy input end 18 of the main waveguide section 10.

Another feature of the apparatus of FIGS. 1 to 3 is that the ridges 14 to 17 are tapered in the longitudinal direction of the waveguide. This feature is best appreciated from FIG. 3 which shows the main waveguide section 10 with the

ridges

16 and 17 at the input end 18 of the section 10 projecting well into the center of the waveguide and towards one another. Moving along the section 10 from its input end 18 towards its output end 19, i.e. from an upstream toa downstream location in terms of energy flow, the ridges I6 and 17 are seen to taper gradually until finally they disappear altogether. The

ridges

14 and 15 taper in a like manner, so that at the output end 19 the cross section of the section 10 is essentially that of an unmodified rectangular waveguide. The proportion of the total energy that the workpiece can absorb is now unimpeded by the existence of any ridges.

The effect of this taper is that at the input end 18 where the full input energy is available absorption of such energy by the workpieces is reduced by the ridges to only a small proportion of that which it would be in an ordinary unridged rectangular waveguide. On the other hand, at the output end 19, where there is much less energy available due to the attenuation that will have taken place along the waveguide, there are no ridges to inhibit the ability of the workpieces to absorb energy. Between the two ends, intermediate conditions will. prevail, the total amount of energy'decreasing from the input end to the output end, while the proportion of such energy that the workpieces are permitted to absorb is conversely increased by the gradual tapering away of the ridges.

The shape with which the ridges taper can be made complementary to the otherwise exponential attenuation curve for the waveguide (assuming workpieces of known dielectric characteristics). The opposing factors would then balance one another exactly and the energy absorbed by the workpieces would remain essentially uniform throughout the length of the main waveguide section 10. In. some instances in practice there is no need for very exact longitudinal uniformity of heating. ln sucha case it will represent an acceptable approximation to uniform heating, if the ridges taper linearly from one end to the other in the working section of the waveguide, as FIG. 3 assumes. Nevertheless, it will be apparent that the concept of having ridges that can be tapered in virtually any curve desired, provides a versatility of control over the energy absorption characteristics that is entirely unavailable'in a standard waveguide, and provides for achievement of very exact longitudinal uniformity of heating in a case where this is required.

As variants of the foregoing itis within the. scope of the present invention that the ridges should taper away to nothing before the end of the working section of the waveguide is reached, which accelerated taper would provide enhanced heating towards the output end; or alternatively, the ridges may taper more gradually, so as never to disappear altogether; or, finally, they need not taper at all. The latter two altematives, and especially the entirely untapered construction, would tend to-have the effectof diminishing the heating effect as the workpieces move towards the output end ofthe waveguide. Thereare somepractical applications where this effect is desirable, more particularly in drying rather than cooking operations. In drying methods it is often desirable-to apply fairly large quantities of energy to the wet workpieces as they enter waveguide, in order'to-drive ofi excess water uickly, but to finish off the 'drying'operation more gently by the application of reduced quantities of energy in order to avoid the risk of overdrying and of rendering the workpieces brittle or otherwise unsuitable.

The latter comment assumes that the workpieces flow along the waveguide in the same direction as the energy, but this is by no means essential; In the untap'ered example, counterflow would produce an increase of energy transfer, as workpieces move towards the energy input end. In the case illustrated in FIGS. 1 to 3, where the energy absorbed is approximately uniform alongthe length of the waveguide, it makes no real difference whether the'workpiece and energy flows are concurrent or countercurrent. j.

Another factor that may enter into the shaping of the. ridges is the electrical. loss characteristic of the workpieces. While the main example given has been the cooking of sausage products, there are many other heating functions to which the present apparatus can be applied. For example, the tube 13 may be used to convey workpieces in the form of granular materials, such as rice or flour, from which some of the moisture is required to be driven off, or in the fonn of liquids, such as milk or fruit juices, requiring pasturization or sterilization. The apparatus can be used for blanching vegetables prior to canning or for killing bacteria in baked products to extend their shelf life. I Thus the term workpiece will be seen to require interpretation in a very broad sense.

The tube 13 may be used as a path for air or for other gas,

"such as a treating smoke or inert gas, which may be heated,

and which may be used either to influence the processing of the workpieces or to remove unwanted byproducts such as steam.

While the workpiece tube 13 will normally be essential ble for all applications where food productsare involved, since it will be much easier to clean the tube than the waveguide itself, it is theoretically unnecessary in those cases where the workpiece is sufficiently rigid to travel unaided along the waveguide between suitable supports at each end. In this connection it should be mentioned that the waveguide may be mounted to extend vertically when in operation, in which case there will be no long span of workpiece to be supported in a horizontal orientation. Examples of workpieces of the type that can dispense with the workpiece tube are filamental materials generally, rubber coating on wire to be cured, licorice rope, and composite cables or rods incorporating resins or glues requiring curing by heat.

The principal advantages of microwave heating are speed, product quality and the small size of the heating chamber or chambers required. The cooking of weiners in conventional steam ovens requires about an hour. In a known microwave oven using 2.5 kilowatts, weiners can be cooked in 55 to 60 seconds. But in the apparatus of FlGS. 1 to 3 the transit time required for the same products is only about 7 to 10 seconds, and yet fully effective cooking has been achieved with a total power input to the apparatus of only 1.2 kilowatts. The main reason for this improvement is believed to be the increased uniformity of application of the energy to the products, such uniformity in the longitudinal direction of the waveguide resulting from the initial restriction on the energy available with the subsequent tapering off of this restriction, while such uniformity across the cross section'of the workpiece results from the presence of the ridges and has to be sacrificed to I because of the absence of any provision similar to that made available for the first time by the present apparatus whereby the degree of energy absorption by the various portions of the product can be closely controlled throughout the full cooking time to make the most efficient use of such time.

Apart from the obvious advantage of increased production per machine that increased speed of processing achieves,

there'is the further fact that it can be shown that flash-heating at higher temperatures is generally less detrimental to food products than slower heating at lower temperatures. There is a general industrial trend towards flash heating methods, so that the present apparatus is especially timely in this regard.

Returning to a consideration of the apparatus of FIGS. 1 to 3, the input to the main waveguide section 10 comprises a conventional source 20 of microwave energy and a transformer in the form of the input section 11, this section being a rectangular waveguide in which input ridges are provided (represented by the ridges 16d and 1711 in FIG. 3), tapering up from zero height at the beginning of the section 11 to merge into the ridges 14 to 17 where the section 11 joins the input end 18 of the main section 10, atwhich location the workpieces are introduced into the electric field by the tube 137 The wavelength of the microwave energy will conveniently be somewhere in the range of .5 to 30 cms., although other frequencies may be chosen, as desired, having regard to the dimensions for the waveguide that will be convenient in the light of the specific function it will be called upon to perform. A frequency of 2,45 Gl-lz. (12.23 cm. wavelength) is convenient, as standard sources of power are readily available at this frequency.

At the output end of the main section 10 the output section 12 is formed as a simple rectangular waveguide without ridges, and it serves to transmit any remaining energy to an absorbing load 21, which may for example be a conventional water load. Normally, when the apparatus is in operation, the energy provided and the rate of travel of the workpieces will be so adjusted that they absorb substantially all the energy coming from the source 20. There will then be little remaining for the when dealing with liquids and granular materials, and desiraload 21 to absorb. However, it is necessary to have this load, in

case the source 20 should be turned on when there are no workpieces travelling along the tube 13.

, waveguide section 10 may have, here designated section Ilia,

and having the difference that the ridges 14a and lla are formed with rounded corners. Breakdown tends to occur much more readily at sharp corners, because of the local high intensity of the electric field that is inherent in sharp corners. By rounding the corners the chances of electrical breakdown are considerably reduced. In FIG. 4 the right-hand side is symmetrical with the left.

FIG. 5 shows a construction in which the rectangular shape of the waveguide has been modified to become generally elliptical, as demonstrated by the waveguide b. This waveguide nevertheless retains the basic symmetry about the planes A and B, being provided with four ridges 14b to 17b symmetrically arranged in pairs on each side of the central workpiece tube 13. It will be apparent that the portion of the energy in this waveguide that can be absorbed by workpieces in the tube 13 will be controlled by the ridges 14b to 17b in a like manner to that of FIG. 2.

FIG. 6 shows yet another modified waveguide section 100 having ridges 14 to 17, as in FIG. 2, but further provided with masses of

material

22 and 23 arranged along each side of the waveguide. Such material might for example be Teflon, or other material of like characteristics, that will tend to have the effect of distorting the field in such a way that there is an intensification of the field in the region of the dielectric and consequently a further rarefaction of the field in the region of the workpieces. Such material must, however, have a low loss tangent so as to absorb little energy.

The various features of FIGS. 2, 4, 5 and 6 maybe interchanged. For example, the ridges of FIG. 5 may have flat outer surfaces; or those of FIG. 6 may have rounded outer surfaces; or the

material

22, 23 may be provided in elliptical waveguide such as that of FIG. 5. Moreover, in each case the ridges may be tapered or untapered; and, in FIG. 6, the masses of

material

22, 23 can be tapered in thickness, as demonstrated by FIG. 6a, or may remain untapered, as the particular circumstances require.

FIG. 7 shows a portion of another modification of a waveguide section which has been expanded sideways to accommodate a plurality of workpiece tubes 13, I3, 13'. It will be noted that each such tube is situated between two pairs of ridges which are here designated at I4, 16, I7; 24, and 26, 27 and which function in essentially the same manner as has been described in relation to FIG. 2. This waveguide will preferably be excited in the TE mode, as in the case of the other waveguides illustrated. However, to compensate for the normal nonuniform distribution of energy across the full width of the waveguide section, when it is so excited, i.e. witha relative concentration of electric field centrally and a relative rarefaction laterally, the height of the ridges can be graded as shown in FIG. 7a, the

central ridges

24 and 25 projecting least; the next outwardly arranged ridges I6, 17 and I6, 27 projecting rather more towards each other; and the

lateral ridges

14, 15' and 28, 29 projecting the furthest towards each other, in order to achieve the maximum field intensification effect. As before, this feature can be combined with any of the other features described, more especially with a longitudinal taper. The workpiece tubes are shown at I3, I3, I3" and 13".

While the positioning of the ridges within any one of the waveguide structuresillustrated is not an especially critical consideration, it will be evident that any two pairs of ridges (I4, 15,- and 16, 17, in FIG. 2 for example) must be spaced apart sufficiently from each other, and hence from the adjacent workpiece tube or tubes to ensure a region of electric field rarefaction in the vicinity of each such tube. It has been found that the centerlines of each pair of ridges, shown at C .sheet material 30 or curing a substance o rio and D respectively in FIG. 2, should preferably each be ap proximately midway between thecentral plane A and the lateral inner surfaces bfthefwaveguidefThis assuriies, of course, that thewidth of the ridges themselves is generally of the order of comparative magnitude shown in FIG. 2, so that there isstill a significant 'gap'"between thetube I3and the nearest ridge surface on eachsi'deof he t be SECOND MAIN EwinopiM-E I TRANSYERSELY TRAVELINQ OR-KPIECE FIGS. 3 and9 showanother manner the principles or the present invention maybe employed. waveguide, is employed-for heat treating, eg

a web of H I the material, whicl'rmaterial may for examplebegleathen p ywood ,jthick paper, uncured rubber or a synthetic plastic. In contrast .to the embodiment of the invention so far described, the direction of travel of the workpiece 30 is now transverse to the direction of propagation of the microwave energy along the working section of the waveguide, which is here designated 10d. It will be seen that the waveguide section 10d is basically the same as the waveguide section 10 shown in FIG., 2, in that it is generally rectangularand has been provided with the four ridges 14 to 17 which taper down from a maximumprojection towards each other at the energy input or upstrearn end to zero projection at the energy output or downstream end. As before, a conventional source 20 ofmicrowave energy is provided at the input end and a conventional loador absorber of microwave energy 21 is provided at the output end.

The manner of operation is basically the same as before, the more extensive projection of the ridges at the input end acting to reduce the proportion-of the relatively large amount of total energy that is absorbed by the near edge of the workpiece 30, while, at the output end, no such restriction is placedonabsorption of energy by the remote workpiece edge. Since the total amount of energy at the remote end will have been reduced by attenuation, substantially uniform heatingcanbe achieved in this way across the width of the workpiece, ie the dimension transverse to its direction of travel. If desired, a high degree of uniformity can be achieved by exact shaping of the taper of the ridges, provided the dielectric characteristics of the workpiece are known and remain substantially constant. However, as before, linearly tapering ridges will normally provide adequate uniformity in practice. On the other band, should the workpiece be such as to require differential heating across its width for some reason, e.g. to cure a glue line extending along one edge, a controlled amount of intentional disuniformity can readily be built into the apparatus by suitable shaping of the taper, if any, with which the ridges are provided.

TI-IIRD MAIN EMBODIMENT FIGS. 10 to I la SLOTTED ANTENNA I tapering ridges Me to 17s. Slots'31, 32, 33 etc. are arranged along the length of the upper wall of the waveguide and are alternately offset from the central plane of the waveguide, which central plane is represented in FIG. It) by the broken line 34-. As is well known in slotted antennas, the power that is radiated through each slot depends on the extent of the offsetting of that slot from the central plane. It is the central plane of the electric field that is controlling, not the physical central plane, but the two are usually coincident in a symmetrical arrangement.

In prior antennas of the slotted-type control of the am plitude of radiation at various locations distributed along the longitudinal surface of the broad face of the waveguide has been achieved by varying the degree of offsetting of the slots from the centerline For example, the slots SI and 32 near the input end of the waveguide where maximum energy is available wouldusually be offset relatively little from the central plane 34, while the amount of offset of the slot 33 and subsequent slots further away from the input end would usually be progressively increased and subsequently progressively decreased beyond a central point, to provide the desired radiation pattern and to compensate for attenuation along the waveguide. However, the rate of subsequent decrease will generally be less than the rate of initial increase, because of the attenuation. While this prior arrangement can achieve a reasonable measure of control of the radiated energy, it creates the manufacturing disadvantage that each slot, or each pair of slots, has to be offset from the centerline by a different amount. There is also the disadvantage that the tolerances have to be quite fine for slots close to the centerline. Moreover, the absence of longitudinal alignment of the slots tends to produce undesirable secondary lobes in the pattern of radiated energy.

By applying the feature of tapering ridges to the art of slotted antennas, it becomes possible to maintain uniformity of slot offset, by reducing the proportion of the total energy made available to the slots near the input end. A uniform and relatively large slot offset can then be applied throughout the entire length of the waveguide, with the obvious attendant manufacturing advantages and also improved performance.

Assuming that the desired amplitude curve of radiated energy is that shown as E in FIG. 11a, i.e. a symmetrical curve with most of the energy being radiated centrally of the longitudinal extent of the waveguide, the four ridges will be shaped as exemplified by the ridge 16 in FIG. 11, i.e. with a first prominent portion 16c a central withdrawn portion l6e" and finally a second prominent portion l6e', in which the latter portion 16e' is not so prominent as the first portion 162' to compensate for attenuation, it being assumed that the energy flow is from left to right. The curve F in FIG. 11a represents symbolically the effect of the varying prominence of the portions of the ridge 16c on the field intensity centrally of the waveguide, i.e. between the slots where the radiating slots are located. That is, the curve F represents the energy that would be radiated if there were no attenuation, the continuously increasing spacing betweenthe curves E and F represents diagrammatically the actual effect of attenuation. The ridge 17c and the othertw'o' ridges are correspondingly shaped. Once again the versatility of the concept of tapering the ridges, both .up as well as down, and the infinite variety of control over the field intensity along the waveguide that it furnishes, has been demonstrated.

- FOURTH EMBODIMENT FIGS. 12 TO CLOSELY SPACED RIDGES A prior proposal for a waveguide structure for applying intense heat to a glue line extending along the edge of a stack of paper sheets is disclosed by W. J. Bleackley in an article entitled A Microwave Glue-Line Dryer" published in the Bulletin of the Radio and Electrical Engineering Division, National Research Council of Canada Vol. 17, No. l, Jan-Mar. I967 (also disclosed in W. A. Cumming and W. .I. Bleackley U.S. Pat. application Ser. No. 685,192 filed Nov. 22, 1967) U.S. Pat. No. 3,456,355. This structure employs ridges for the purpose of obtaining an intensification of the electric field, but, in this instance, two rather than four ridges are employed. In fact four ridges can be used, but they are not necessary in this construction, and hence the simpler two ridge structure is preferably adopted. The more fundamental difference between this prior structure and the waveguide sections that have so far been described in the present specification resides in the fact that the ridges in the prior construction are placed much more closely together. FIGS. 12 and 12a show a cross section of this prior construction comprising a rectangular waveguide 40 formed with a pair of

ridges

41, 42 and, opposite these ridges in the broad face of the waveguide, a slot 43 through which the edges of a pair of

paper sheets

44, 45 can project to locate a line 46 of glue to be cured in the region of high intensity field that the

ridges

41, 42 produce when the waveguide is excited in the TE mode, as illustrated in FIG. 12a.

While this apparatus operates satisfactorily, the very intense field to which the glue is subjected at the energy input end of the waveguide where the full power is available may be undesirably strong in some instances. By applying the tapering concept of the present invention to this apparatus, it becomes possible to taper the ridges in the direction of energy flow and thus apply the power more gradually to the workpiece. Thus, while the sense of the taper is upward (i.e. increased projection towards the opposite waveguide face) in the downstream direction of energy flow, which is the reverse of that employed in the embodiments of FIGS. 1 to 3, and 8 and 9, for example, the effect is essentially the same, because this time the workpiece is located in a region of field intensification, ratherthan in a region of field rarefaction.

This embodiment of the present invention is shown in FIG. 13 by a waveguide section 50 having a pair of closely spaced

ridges

51, 52 which include upwardly tapering

portions

51 and 52. In this example the ridges have been shown as tapering up to their full height before the far end of the waveguide is reached, but it will be understood that a more gradual taper occupying the full length of the waveguide can be adopted, if preferred. FIG. 14 demonstrates a manner in which the waveguide section 50 of FIG. 13 may be used at the energy input end of a microwave heater for applying heat to a glue line 46. A source of microwave energy is shown symbolically at 54; in practice this source will include a microwave generator and a transformer section to couple the energy into the ridged waveguide, although at the input end the ridge portions 51' and 52 will project relatively slightly.

An alternative cross section 59 that can be adopted in this latter form of the invention, i.e. employing tapered, closely spaced ridges, is shown in FIG. 15. Here two ridges 60, 61 extend from opposite faces of the waveguide, with one ridge 60 being slotted to receive the workpiece edge 62. Alternatively, this construction can be considered as the equivalent of a four ridged arrangement, the two portions 60a and 60b forming one pair of closely spaced ridges, while the other ridge 61 represents two ridges that are so close together that for both mechanical and electrical purposes they have merged essentially into a single ridge. In a case where the shape of the workpiece required, i.e. where a glue line extended down the center of the workpiece, the ridge 61 could also be separated into two closely spaced ridges by a second slot in which the workpiece also travelled. As in FIG. 13, the ridges 60, 61 will be tapered upwardly in the downstream direction of the waveguide.

FIFTH EMBODIMENT FIG. 16 CONVERGING RIDGES As has been explained, the embodiments of FIGS. 13 and 15 employ the concept of tapered ridges that are closely spaced in order to achieve a longitudinal variation of intensity of a concentrated field in which the workpiece (or its operative portion) is located. In the earlier described embodiments essentially the same effect was achieved, but in those cases with the workpiece located in a region of field rarefaction. Such arrangement called for relatively wide spacing of the ridges in order to provide the necessary space between the ridge pairs for a region of field rarefaction.

Another version of the invention, which combines these concepts while achieving basically the same end result as the other embodiments, is illustrated in FIG. 16. In this waveguide section 64, the

ridges

65 and 66 are caused to converge towards one another from a widely spaced orientation at 69 (upstream in terms of energy flow) to a closely spaced orientation at 70 (downstream). As will be apparent this convergence will have the effect of gradually intensifying the field in the workpiece region between the ridges to compensate for attenuation. This type of construction is especially suited to operating with flat workpieces, because such shape of workpiece will normally allow the ridges to come closer together, but the principle is applicable to workpieces generally. While the

ridges

65 and 66 have not been shown as having any taper, in order not to complicate the drawing, it will be evident that any desired pattern of taper may be combined with any desired pattern of convergence or divergence in a waveguide having two, four, or more ridges, and used either as a heater or as a radiating antenna. in other words, all the various features disclosed above may be simultaneously adopted in any practical combination.

SIXTH EMBODIMENT FlGS. l7 to 21 DOUBLE HEATING Returning to the spaced ridged structure of FIGS. l to 3, but utilizing the regions of intense field rather than the central re' gion of rarefied field, the waveguide 80 of FIG. l7 is especially adapted for working with workpieces having two spaced-apart portions to be heated, for example a U-shaped member 81 with a pair of surfaces 82 that require drying. A succession of such members is moved through the waveguide supported on Teflon rails 83 in such a location that the surfaces 82 are positioned in the intense fields between the ridges 14, and l6, 17. As before, these ridges may be tapered in the longitudinal direction, if desired.

FIGS. 18 to 21 illustrate a variant of this construction, in which the ridges also serve as the supporting rails for match box covers 81'. Thus in a waveguide 85, the ridges 14 to 17 are formed with grooves 86 in which the corners of the workpiece 81 can be guided. The match striking surfaces 82' do not extend fully to the top or bottom of the sides to which they are fixed and are thus clear of the ridges. The surfaces 82 which are to be dried are located in the regions of intense field. FIG. 19 demonstrates one manner in which workpieces of this type may conveniently be introduced into the waveguide 85 from a metal tube 87 that is of such a size as to constitute a waveguide beyond cutoff and thus prevent radiation out of the main waveguide section 85. The tube 87 has a grooved underside at 88 (FIG. 2d) and the workpieces then travel along a Teflon member 89 also grooved at 96) (FIG. 21)

until they enter the grooves 86 formed in the ridges (FlG. l8).

If it is desired to taper the ridges longitudinally in this con struction, in order, as before, to vary the ratio of field intensife cation to field rarefaction, this effect can be achieved by means of Teflon inserts 91 as shown in combination with tapering ridges l6 and 17 in the modified waveguide 85' shown in FIG. 19a. In this way the combination of each ridge and its insert 91 remains structurally uniform throughout the length of the waveguide for smoothly guiding the workpieces, while the electrical characteristics vary. On the assumption that the tapering of the ridges is designed to render the degree of heating approximately uniform along the waveguide, the structure shown in FIG. 19a is suited to an arrangement in which the microwave energy is fed through the waveguide from right to left as seen in this HG. Obviously, if preferred, the taper can be reversed to extend in the other direction.

While the forms of the invention shown in FIGS. 17 to El are especially well adapted to workpieces of the type having two opposed portions for heating differentially, that is to the exclusion of the remainder of the workpiece, such for example as matchbox covers or small packages requiring the curing of clue along both edges, this particular construction of waveguide can also be used with workpieces having only one such portion. For example, if matchbox covers were to be formed with a striking surface on one side only, two rows of such workpieces could be caused to travel along the waveguide, side by side, and with their sides to be heated outwardly directed.

As in other embodiments of the invention, the workpieces may push one another along the waveguide, they may faii under gravity if the waveguide is mounted vertically, or a movable conveyor of suitable low loss material such as Teflon can extend along the waveguide.

ill

GENERAL DEFINITIONS From the foregoing, it will be apparent that the term converge" is to be understood throughout this document as including both positive convergence as well as negative convergence, i.e. divergence. 1 j

By the same token, the termfta'per is intended, both tapering down (reduced ridge prominence) aswell as tapering in the opposite sense (increased ridge prominence), and combinations thereof. j

Also, the term workpiece is to be understood as covering all or part of any material to be dried, cooked, cured or other wise heat treated, whether such material is rigid, flexible, particulate, granular or liquid, and regardless of the shape or size thereof.

in making reference to removing" microwave energy from the waveguide, this term is intended to cover the two concepts of a. converting the microwave energy to heat energy, or b. radiating the microwave energy into space or into some other structure, such as a further waveguide.

I claim:

1. An elongated waveguide having an interior cross-sectional shape symmetrical about each of a pair of mutually perpendicular planes intersecting each other to define the longitudinal axis of the waveguide, said waveguide being formed with two pairs of conducting ridges projecting inwardly of the waveguide, said pairs being arranged each on a respective side of a first one of said planes with the respective ridges of each pair having surfaces approaching each other from a pair of walls of the waveguide arranged on opposite sides of the second of said planes, to form regions of electric field intensification between said approaching surfacesupon energization of the waveguide, said pairs being spaced apart from each other to provide a region of electric field rarefaction in the vicinity of said axis; and means for propagating microwave energy along the waveguide in the direction of extent of said ridges. I

2. A waveguide according to claim l, for use as a microwave heater, including means for conveying a workpiece along the waveguide in the direction of extent of said ridges.

35. A waveguide according to claim 2, wherein said conveying means comprise means for conveying the workpiece along said region of electric field rarefaction.

4. A waveguide according to claim l for use as a microwave heater and including means for conveying a generally sheetlike workpiece in a direction transverse to the longitudinal extent of the waveguide and substantially along said first plane to pass through said region of electric field rarefaction.

5. A waveguide according to claim 1, wherein said ridges are tapered in the longitudinal direction of the waveguide to vary the spacing between said approaching surfaces and hence the ratio between the degree of said intensification and the degree of said rarefaction of the electric field.

6. A waveguide according to claim 5 for use as a microwave heater and including means for conveying a workpiece in the longitudinal direction of the waveguide along said region of electric field rarefaction. i

7. A waveguide according to claim 5 for use as a microwave heater and including means for conveying a generally sheetlike workpiece in a direction transverse to the longitudinal extend of the waveguide and substantially along said first plane to pass through said region of electric field rarefaction.

A waveguide according to claim 5 for use as a slotted antenna and including a series of radiating slots formed in one of said pair of walls, said series of slots extending in the longitudinal direction of the waveguide with alternate slots offset to respective sides of said first plane by substantially equal amounts.

9. A waveguide according to claim 8, wherein each of said ridges is tapered from a maximum degree of projection at one end of the waveguide down to a minimum degree of projection at a location intermediate the ends of the waveguide and is tapered out again to a degree of projection at the other end of the waveguide intermediate in degree between said maximum and minimum degrees of projection.

piece in the longitudinal direction of the waveguide with a portion of said workpiece to be heated located in a said region of electric field intensification.

11. A waveguide according to claim 10, including means defining a direction of microwave energy flow along the waveguide, wherein said ridges are tapered in the longitudinal direction of the waveguide from a maximum degree of projection at an energy downstream location in the waveguide to a minimum degree of projection at an energy upstream location in the waveguide to vary the ratio between the field intensification and rarefaction to compensate in heating effect on the workpiece for attenuation in the energy downstream direction.

12. A waveguide according to claim 6, wherein said ridges are tapered from a maximum degree of projection at an energy upstream location in the waveguide to a minimum degree of projection at an energy downstream location in the waveguide to vary the ratio between the field intensification and rarefaction to compensate in heating effect on the workpiece for attenuation in the energy downstream direction.

13. A waveguide according to claim vl, including a mass of dielectric material provided in the waveguide at a location on the side of each pair of ridges remote from said axis, said material comprising means for further rarefying the electric field in the vicinity of said axis;

14. A waveguide according to claim 13, wherein each said mass of dielectric, material is tapered in the longitudinal direction of the waveguide to vary the effect thereof on such further rarefaction of the electric field.

15. A waveguide according to claim 1, laterally extended to include at least one further pair of similar projecting ridges arranged to form a further region of electric field intensification between approaching surfaces thereof, each such further pair of ridges being spaced from another of said pairs of ridges to provide a further region of electric field rarefaction.

16. A waveguide according to claim 15, wherein at a selected location in the longitudinal direction of the waveguide the degree of closeness of the pairs of approaching ridge surfaces is greater nearer the lateral edges of the waveguide and becomes progressively less towards the center of the waveguide. I p 7 17. In a microwave heater, an elongated waveguide having two inwardly projecting, elongated,'conducting ridge structures, each extending longitudinally along said waveguide, means for propagating microwave energy along the waveguide in the direction of extent of said ridge structures, each of said structures forming an area of electric field intensification upon energization of the waveguide, said ridge structures being spaced apart from each other to provide between them a region of electric field rarefaction, and means for conveying a workpiece through said region of electric field rarefaction.

18. The structure of claim 17, wherein said ridge structures are tapered in the longitudinal direction of the waveguide to vary the ratio between the degree of said intensification and the degree of said rarefaction.

19L An elongated waveguide for use as a microwave heater and having a cross section includingan internal conducting ridge structure comprising at least one ridge projecting from one wall of the wave guide towards an opposite wall of the waveguide and to a location spaced from said opposite wall to form opposed surfaces defining at least one region in which, upon energization of the waveguide, the electric field is more intensified than in at least one other region of the waveguide where the electric field is more rarefied, and including means defining a direction of microwave energy flow along the waveguide, and means for conveying a workpiece in the longitudinal direction of the waveguide along said region of field intensification, said ridge structure being tapered in the longitudinal direction of the waveguide froma maximum degree of projection at an energy downstream location to a minimum degree of projection at an energy upstream location to vary the ratio between such field intensification and rarefaction to 14 compensate in heating effect on the workpiece for attenuation in the energy downstream direction.

20. A waveguide according to claim 19, wherein said ridge structure comprises a pair of ridges projecting from said wall of the waveguide towards said opposite wall, said pair of ridges lying closely adjacent each other to form a substantially uninterrupted region of field intensification between such ridges and said opposite wall, and a slot in said opposite wall for receiving a flat workpiece projecting therethrough to locate a portion of said workpiece to be heated in said last-mentioned region.

21. An elongated waveguide for use as a microwave heater and having a cross section including an internal conducting ridge structure comprising at least two ridges projecting from one wall of the waveguide towards an opposite wall of the waveguide and each to a location spaced from said opposite wall to form opposed surfaces defining a region of electric field intensification between said opposed surfaces upon energization of the waveguide, said ridges converging in the longitudinal direction of the waveguide between a first condition in which said ridges are spaced apart from each other to provide a region of electric field rarefaction between them and a second condition in which said ridges lie closely adjacent each other with their respective said regions of electric field intensification merging to form a substantially uninterrupted combined region of field intensification.

22. A waveguide according to claim 21, including means defining a direction of microwave energy flow along the waveguide, said first condition of theridges being upstream of said second condition in relation to said direction of energy flow, and including means for conveying a workpiece in the longitudinal direction of the waveguide between said region of field rarefaction and said combined region of field intensification.

23. The structure of claim 18, said taper being from a maximum degree of projection and hence a maximum value of said ratio at an energy upstream location to a minimum degree of projection and hence a minimum value of said ratio at an energy downstream location to compensate in heating effect on the workpiece for attenuation in the energy downstream direction.

24. An elongated waveguide for use as a slotted antenna and including a series of radiating slots formed in one wall thereof, said series of slots extending in the longitudinal direction of the waveguide with alternate slots offset to respective sides of a central electric plane of the waveguide, said waveguide having an inwardly projecting conducting ridge structure for forming at least one region of electric field intensification and at least one region of electric field rarefaction upon energization of the waveguide, said ridge structure being tapered in the longitudinal direction of the waveguide to vary the ratio between the degree of said intensification and the degree of said rarefaction to control the level of energy loss from the respective slots.

25. A waveguide according to claim 22, wherein said slots are offset by substantially equal amounts to respective sides of said central plane, and wherein said ridge structure is tapered from a maximum degree of projection at one end of the waveguide down to a minimum degree of projection at a location intermediate the ends of the waveguide and is tapered out again to a degree of projection at the other end of the waveguide intermediate in degree between said maximum and minimum degrees of projection toenable said waveguide to radiate a symmetrical energy lobe while compensating for attenuation along the waveguide.

26. An elongated waveguide for use as a microwave heater and having a cross section including an internal conducting ridge structure comprising at least one ridge projecting from one wall of the waveguide towards an opposite wall of the waveguide to form opposed surfaces defining a region of electric field intensification between said surfaces upon energization of the waveguide, said ridge structure further constituting means for supporting workpieces for travel in the longitudinal direction of the waveguide along said region of field intensification.

27. A waveguide according to claim 26, wherein said ridge structure comprises two pairs of ridges arranged with said pairs each on a respective side of a first longitudinal plane, with the respective ridges of each pair having surfaces approaching each other from a pair of walls of the waveguide arranged on opposite sides of a second longitudinal plane extending perpendicularly to said first plane to form regions of electric field intensification between said approaching surfaces upon energization of the waveguide, said surfaces being formed as rail means for supporting workpieces for travel in the longitudinal direction of the waveguide with portions of said workpieces to be heated located in said regions of field intensification.

28. The structure according to claim 32, wherein said energy is removed as heat by a workpiece travelling in the longitudinal direction of the waveguide in a said region of field rarefaction, said modification of the ridge structure being such as to reduce the ratio between field intensification and field rarefaction in the energy downstream direction to cause the level of heating of the workpiece to be substantially uniform along at least a portion of the waveguide.

29. The structure according to claim 32, wherein said energy is removed as heat by a workpiece travelling in the longitudinal direction of the waveguide in a said region of field intensification, said modification of the ridge structure being such as to increase the ratio between field intensification and field rarefaction in the energy downstream direction to cause the level of heating of the workpiece to be substantially uniform along at least a portion of the waveguide. v

30. The structure according to claim 32, wherein said eneriii gy is removed as heat by a sheetlike workpiece travelling transversely of the longitudinal direction of the waveguide essentially through a said region of field rarefaction, said modification of the ridge structure being such as to reduce the ratio between field intensification and field rarefaction in the energy downstream direction to cause the level of heating of the workpiece to be substantially uniform across the width thereof.

3E. The structure according to'claim 32, wherein said energy is removed by slots formed in a wall of the waveguide and arranged substantially equally offset alternately from a central electrical plane of the waveguide.

32. in an elongated waveguide having means forsupplying microwave energy at an input'end thereof for propagation therealong, means for controlling the level of removal of said microwave energy at various locations along the waveguide, said control means comprising:

a. a conducting ridge structure extending along the waveguide from said input end in the direction of propagation of microwave energy therealong and projecting inwardly of the cross section of the waveguide for distorting the electric field to provide at least one region of field intensification and at least one region of field rarefaction;

b. means for removing microwave energy from the waveguide along the length thereof from one of said regions; and

c. means modifying said ridge structure longitudinally of the waveguide to vary the degree of said distortion to compensate for attenuation along the waveguide and control the level of energy removal along the waveguide.

Claims

1. An elongated waveguide having an interior cross-sectional shape symmetrical about each of a pair of mutually perpendicular planes intersecting each other to define the longitudinal axis of the waveguide, said waveguide being formed with two pairs of conducting ridges projecting inwardly of the waveguide, said pairs being arranged each on a respective side of a first one of said planes with the respective ridges of each pair having surfaces approaching each other from a pair of walls of the waveguide arranged on opposite sides of the second of said planes, to form regions of electric field intensification between said approaching surfaces upon energization of the waveguide, said pairs being spaced apart from each other to provide a region of electric field rarefaction in the vicinity of said axis; and means for propagating microwave energy along the waveguide in the direction of extent of said ridges.

2. A waveguide according to claim 1, for use as a microwave heater, including means for conveying a workpiece along the waveguide in the direction of extent of said ridges.

3. A waveguide according to claim 2, wherein said conveying means comprise means for conveying the workpiece along said region of electric field rarefaction.

4. A waveguide according to claim 1 for use as a microwave heater and including means for conveying a generally sheetlike workpiece in a direction transverse to the longitudinal extent of the waveguide and substantially along said first plane to pass through said region of electric field rarefaction.

6. A waveguide according to claim 5 for use as a microwave heater and including means for conveying a workpiece in the longitudinal direction of the waveguide along said region of electric field rarefaction.

8. A waveguide according to claim 5 for use as a slotted antenna and including a series of radiating slots formed in one of said pair of walls, said series of slots extending in the longitudinal direction of the waveguide with alternate slots offset to respective sides of said first plane by substantially equal amounts.

10. A waveguide according to claim 1, for use as a microwave heater and including means for conveying a workpiece in the longitudinal direction of the waveguide with a portion of said workpiece to be heated located in a said region of electric field intensification.

13. A waveguide according to claim 1, including a mass of dielectric material provided in the waveguide at a location on the side of each pair of ridges remote from said axis, said material comprising means for further rarefying the electric field in the vicinity of said axis.

14. A waveguide according to claim 13, wherein each said mass of dielectric material is tapered in the longitudinal direction of the waveguide to vary the effect thereof on such further rarefaction of the electric field.

16. A waveguide according to claim 15, wherein at a selected location in the longitudinal direction of the waveguide the degree of closeness of the pairs of approaching ridge surfaces is greater nearer the lateral edges of the waveguide and becomes progressively less towards the center of the waveguide.

17. In a microwave heater, an elongated waveguide having two inwardly projecting, elongated, conducting ridge structures, each extending longitudinally along said waveguide, means for propagating microwave energy along the waveguide in the direction of extent of said ridge structures, each of said structures forming an area of electric field intensification upon energization of the waveguide, said ridge structures being spaced apart from each other to provide between them a region of electric field rarefaction, and means for conveying a workpiece through said region of electric field rarefaction.

19. An elongated waveguide for use as a microwave heater and havIng a cross section including an internal conducting ridge structure comprising at least one ridge projecting from one wall of the wave guide towards an opposite wall of the waveguide and to a location spaced from said opposite wall to form opposed surfaces defining at least one region in which, upon energization of the waveguide, the electric field is more intensified than in at least one other region of the waveguide where the electric field is more rarefied, and including means defining a direction of microwave energy flow along the waveguide, and means for conveying a workpiece in the longitudinal direction of the waveguide along said region of field intensification, said ridge structure being tapered in the longitudinal direction of the waveguide from a maximum degree of projection at an energy downstream location to a minimum degree of projection at an energy upstream location to vary the ratio between such field intensification and rarefaction to compensate in heating effect on the workpiece for attenuation in the energy downstream direction.

22. A waveguide according to claim 21, including means defining a direction of microwave energy flow along the waveguide, said first condition of the ridges being upstream of said second condition in relation to said direction of energy flow, and including means for conveying a workpiece in the longitudinal direction of the waveguide between said region of field rarefaction and said combined region of field intensification.

25. A waveguide accOrding to claim 22, wherein said slots are offset by substantially equal amounts to respective sides of said central plane, and wherein said ridge structure is tapered from a maximum degree of projection at one end of the waveguide down to a minimum degree of projection at a location intermediate the ends of the waveguide and is tapered out again to a degree of projection at the other end of the waveguide intermediate in degree between said maximum and minimum degrees of projection to enable said waveguide to radiate a symmetrical energy lobe while compensating for attenuation along the waveguide.

29. The structure according to claim 32, wherein said energy is removed as heat by a workpiece travelling in the longitudinal direction of the waveguide in a said region of field intensification, said modification of the ridge structure being such as to increase the ratio between field intensification and field rarefaction in the energy downstream direction to cause the level of heating of the workpiece to be substantially uniform along at least a portion of the waveguide.

30. The structure according to claim 32, wherein said energy is removed as heat by a sheetlike workpiece travelling transversely of the longitudinal direction of the waveguide essentially through a said region of field rarefaction, said modification of the ridge structure being such as to reduce the ratio between field intensification and field rarefaction in the energy downstream direction to cause the level of heating of the workpiece to be substantially uniform across the width thereof.

31. The structure according to claim 32, wherein said energy is removed by slots formed in a wall of the waveguide and arranged substantially equally offset alternately from a central electrical plane of the waveguide.

32. In an elongated waveguide having means for supplying microwave energy at an input end thereof for propagation therealong, means for controlling the level of removal of said microwave energy at various locations along the waveguide, said control means comprising: a. a conducting ridge structure extending along the waveguide from said input end in the direction of propagation of microwave energy therealong and projecting inwardly of the cross section of tHe waveguide for distorting the electric field to provide at least one region of field intensification and at least one region of field rarefaction; b. means for removing microwave energy from the waveguide along the length thereof from one of said regions; and c. means modifying said ridge structure longitudinally of the waveguide to vary the degree of said distortion to compensate for attenuation along the waveguide and control the level of energy removal along the waveguide.