Branched waveguide transitions with mode filters

Feb. 16, 1971 w. a. gumming 3,564,458

BRANCHED WAVEGUIDE TRANSITIONS WITH MODE FILTERS Filed Oct. 28, 1969 2 Sheets-Sheet 1

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Feb. 16, 1971

w. a. cumming 3,564,458

BRANCHED WAVEGUIDE TRANSITIONS WITH MODE FILTERS

Filed Oct. 2β, 1969

Sheets-Sheet 2

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United States Patent Office

3,564,458

Patented Feb. 16, 1971

3,564,458 BRANCHED WAVEGUIDE TRANSITIONS WITH MODE FILTERS William A. Cumming, Ottawa, Ontario, Canada, assignor to Canadian Patents and Development Limited, Ottawa, Ontario, Canada, a corporation of Canada Filed Oct. 28, 1969, Ser. No. 871,848 Int. Cl. HOlp 1/16; H05b 9/06

U.S. Cl. 333—21 i Claim

ABSTRACT OF THE DISCLOSURE

An input waveguide for feeding microwave energy in the TE₀₁ mode substantially uncontaminated by higher order modes into a rectangular waveguide that is used for 15 heating relatively wide webs and thus requires a width in which the higher order modes could propagate, is composed of a plurality of divergent waveguide sections each diverging to a width too small to propagate the higher modes but each nevertheless containing incipient such 20 modes by reason of the effect of the divergence, such divergent sections being followed by parallel-sided filter sections for damping out the incipient higher modes before the plurality of energy paths thus formed are recombined to form a single ΤΕθι mode in the wide wave- 25 guide.

This invention relates to improvements in waveguides for use in apparatus for dielectric heating, that is to say the heating of materials by microwave energy. Such apparatus is particularly concerned with the drying or curing of workpieces in the form of web-shaped articles, such as films, webs of paper or other material.

For this purpose it has already been proposed in U.S. Pat. No. 3,457,385 of W. A. Cumming issued July 22, 1969 to use a waveguide of rectangular cross-section in which the web of material is moved along its own longitudinal axis which extends along the longitudinal central plane of the waveguide. The heating effect is achieved by microwave energy which is intended to be mainly in the transverse electric mode known as the TE_Oi mode (employing the usual United States nomenclature). A wave of this operating mode has its electric field intensity concentrated in .the centre plane of the waveguide, i.e. the plane extending across the greater dimension of the waveguide equidistant from the upper and lower plates. Since the web is arranged to move in this centre plane the result is obtained that the maximum intensity of the electric field lies within the web.

An improvement to this structure is disclosed in U.S. 50 Pat. No. 3,466,415 of W. J. Bleackley issued Sept. 9, 1969, whereby electrically conductive vanes extend along the inside edges of the waveguide in the plane occupied by the web. In this manner the electric field providing the energy transfer is prevented from fanning out and losing 55 concentration at the edges of the web.

Maximum utilization of the micro-wave energy calls for its propagation as far as possible solely in the basic TE01 mode. However, the substantial width of webs commonly encountered in many industrial processes sets a 60 practical requirement on the width of the waveguide, requiring such dimension to be many times greater than the wavelengths most convenient from the viewpoint of energy absorption by water in the workpiece and from the viewpoint of commercial availability. For example the ⁶⁵frequency of 2450 mHz. (12.25 cm.) is commonly available for commercial uses of the present type. In the ex30 ample that follows it will be assumed that a free space wavelength λ of 12.25 cm. is used. When microwave energy of this wavelength is used in a relatively wide waveguide, say 40 cm. in width many of the higher modes can propagate. Their appearance in the waveguide, however, represents a loss of effectiveness.

One of the main problems encountered in apparatus of this kind is that of introducing the micro-wave energy into the waveguide in the TE_Oi mode without providing the type of conditions, e.g. deviations from uniformity and symmetry, that are typically conducive to the appearance of oscillation in the higher order modes. If such irregularities can be avoided the higher modes will be substantially absent even though the dimensions of the waveguide are large enough to support the higher modes.

It is the object of the present invention to provide an improved input waveguide construction for feeding micro-wave energy into the main waveguide containing the workpiece substantially solely in the ΤΕθι mode and in such a manner as substantially to avoid the subsequent appearance of higher mode oscillations, such main waveguide having a height A and a width B, where the value of B is large enough to propagate higher order modes of the microwave energy at the wavelength chosen.

According to the invention, this object can be achieved by an input waveguide comprising:

. (a) . An input section having dimensions for propagating said energy in the TE₀₁ mode but below cut-off for higher order modes, (b) Means branching from said input section to form a plurality of N of diverging waveguide sections each expanding to a cross-section of height A and width B', where the values of A and B' are too small to propagate said energy in modes higher than the TE₀₁ mode, (c) A mode filter section having parallel side walls for receiving energy from each of said diverging sections with incipient higher order modes therein and for substantially damping out said higher order modes, the length of each such mode filter section being at least as great as half a wavelength, _(d) And means for combining the energy output from said mode filter sections into a waveguide cross-section of height A and width B, where B equals NB', to propagate energy substantially solely in the TE_M mode therein.

As will be explained in connection with the illustrated examples, values for N of 2 or 4, or even higher, may be chosen. For symmetry and convenience N will usually be an even number, but there is no theoretical reason why this must be so.

Two forms of the invention are illustrated by way of example only in the accompanying drawings, in which:

FIG. 1 is a perspective view of an entire microwave heating apparatus embodying an input waveguide according to the invention;

FIG. 2 is a vertical section on the line II—II in FIG.

FIG. 3 is a horizontal section on the line III—HI in FIG. 1;

FIG. 4 is a vertical section through the input waveguide taken on the line IV—IV in FIG. 1; and

FIG. 5 is a view similar to FIG. 4 but showing a modification.

As shown in FIGS. 1 and 2, a workpiece in the form of a web 10 of material, for example paper, is fed in either direction between rolls 11 and 12 by suitable conventional driving means (not shown). The web 10 travels along the centre transverse plane of a rectangular waveguide 13 consisting of top and bottom walls 14 and

3,564,458 side walls 15 with vanes 16 projecting inwardly therefrom. The electric lines of force have been omitted from FIG. 2 for simplicity, but, assuming energization in the TEqi mode, they will be highly concentrated in the web 10. The reasons for this are:

(a) A natural concentration of lines at the centre of the field in the TE_Oi mode;

(b) The location of the web 10 along this field centre, the presence of the workpiece itself further accentuating the concentration of the electric field; and (c) The vanes 16 which avoid the tendency that would otherwise exist for the field to fan out at its ends adjacent the side walls 15.

FIG. 3 shows a cross-section through a portion 20 of an input waveguide 21, the arrows illustrating the normal electric field distribution for excitation in the ΤΕ₀, mode. In the presence of the web 10 in the operating waveguide 13 the concentration of the field across the centre of the waveguide equi-distant between the top and bottom walls will be significantly greater than in FIG. 3..

No particular difficulty is encountered in reflecting the energy by an inclined surface 23, where the input waveguide portion 20 feeds into the operating waveguide 13, without introducing distortions and higher order modes into the oscillations. The oscillation conditions (e.g. degree of purity of the basic TEqi mode) in the portion 20 will be reproduced in the waveguide 13. Thus the need is to set up oscillations in the input portion 20 that are substantially free of contamination by higher order modes, and this object can be achieved by use of the form of input waveguide 21 seen in FIG. 4 in crosssection.

The input waveguide 21 has a bifurcated input section 24 with a width W that is small enough to be below cutoff for all the higher order modes. Calculation of this dimension will be explained more fully below. Thus microwave energy introduced from a conventional probe (not shown) to propagate in the basic ΤΕ₀, mode will be unable to form higher order oscillations in the section 24, and these same conditions will continue down waveguide branches 25 that are formed between the outside diverging side walls 26 of the two portions of the waveguide 21 and diverging walls 27 of inner diamond shaped structures 28. The walls 27 lead into converging walls 29 which together with the side walls 26 form diverging waveguide sections 30. Where the walls 29 and 26 come together they continue for some distance as extensions 31 projecting into the waveguide portion 20 parallel to the side walls 32 thereof.

When a wave front travels along a diverging waveguide with the electric lines of force extending across the waveguide, they become curved with the central portion bowed forward to lead the edge portions. In the present construction, if the arrangement were not divided and the inner structures 28 were not present so that the wave front were free to expand between a simple pair of diverging side walls, such as the two outer side walls 26, the electric field would achieve a shape as shown by the arrow 33. At a point 34, for example, this field would have a forward component 35 as well as its main transverse component 36. Such a forward component is equivalent to and appears as a higher mode oscillation which tends to continue on for the full length of the waveguide system, producing either non-uniform heating of the workpiece or loss of efficiency, the latter resulting from field components perpendicular to the plane of the workpiece. The foregoing assumes that the width B of the waveguide is sufficient to support the higher modes, for the reasons indicated above, namely the necessary width of the web 10 in relation to the wavelength that it is practicable to use. The higher modes could be the TE,,, TM_lb TEis or TM,3 modes, for example.

However, with the construction shown in FIG. 4 in which the inner structure is divided into four separate sections 30,. each of width B', it is possible to make this dimension sufficiently small to prevent higher order modes appearing. The four separate wave fronts will each be curved, as shown for example by the arrow 37 representing one of the electric fields. This form of energy inherently contains the higher mode oscillations, but they are ⁵ suppressed by the fact that the waveguide portions 40 are beyond cut-off for such higher modes by reason of its dimensions B' and A. Since these higher mode oscillations cannot propagate they are damped out, some of ,θ the energy being dissipated in the walls of the waveguide ^υ and some being reflected back. The waveguide sections 40 in the portion 20 thus serve as mode filters, the electric lines of force by the time they reach the ends of the sections 40 being substantially straight as shown by ₁₅ the arrow 38. The damping of the higher modes is exponential, so that perfect eradication of the high modes is never achieved. In practice, if the length L of the filter sections 40 is given a value from 16 λ to λ, sufficient filtering is achieved to eliminate the majority of the higher 2o mode energy. In any case where extreme purity is required the length L can be increased accordingly. On passage beyond the downstream ends of the extensions 31 the lines of force link up to form single straight lines, as shown at 39.

For propagation in the TE_Oi mode it is necessary that the dimension A be greater than 16λ and less than λ. Below ’Λ λ would be cut off even for the primary TEqi mode; above λ would permit other higher order modes in the A direction. To eliminate the first higher order modes in 30 the B direction, i.e. the TEn and TM_n modes, the dimension B' must be below cut-off for these modes. Elimination of them automatically results in elimination of all the still higher modes. For such cut-off B' must be not greater than

A

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If A is made %λ for example, B' must be no greater than approximately 0.675λ. If A is decreased to nearly 1/2 Λ, to 0.55λ for example, then the above equation solves for B' as not being greater than approximately 1.2λ. Thus by making A close to 16 λ (but not so close as to introduce tuning difficulties), it is possible to increase the permissible value of B' and hence that of B which is equal to NB', where N is the number of divisions, i.e. 4 in the present case. With λ= 12.25 cm., such a value for A could enable B to be nearly as great as 60 cm., thus accommo50 dating a wide workpiece, e.g. 40 to 50 cm. If even wider webs were to be used as the workpiece, the input waveguide could be modified to be divided into a larger number of sections 30 than the four shown in FIG. 4, e.g. N=6 or 8. Conversely, if a narrower workpiece is con55 templated, N can equal 2, a simplification shown in FIG. 5 with an input waveguide 21' providing a single division into two sections 30' and a portion 20' having only two mode filters 40', provided that the width B' of the filters 40' is such as to be less than the value determined by the above equation.