US2921276A - Microwave circuits - Google Patents

Description

Jan. 12, 1960 E. G. FUBlNl 2,921,276

MICROWAVE CIRCUITS Filed Aug. 30, 1955 2 Sheets-Sheet 1 INVENTOR EUGENE G. FUBINI ATTORNEYS Jan. 12, 1960 E. G. FUBINI 2,921,276

MICROWAVE cmcuns Filed Aug. 30, 1955 2 Sheets-Sheet 2 FIG. IO n4 FIG. I2

INVENTOR 0 9 EUGENE e. FUBINI ATTORNEYS 7 2,921,276 MICROWAVE CIRCUITS Eugene G. Fubini, Glen Head, N.Y., assignor, by niesne assignments, to Cutler-Hammer, Inc., Milwaukee, Wis., a corporation of Delaware Application August 30, 1955, Serial No. 531,323

13 Claims. (Cl. 333-95) This invention relates to microwave transmission lines for guiding and conveying electromagnetic energy.

In the microwave region it is common to employ waveguide for transmittingelectromagnetic energy. Although perhaps the widest use is above 3000 megacycles per second, waveguide is occasionally employed down to 1000 megacycles, and even at lower frequencies for high power applications.

The conventional waveguide consists of a metallic tube through which the electromagnetic energy is propagated. Although various cross-sectional configurations are possible, the most widely used waveguide has a rectangular cross-section.

It is often desirable to make measurements on waveguide in order todetermine the losses therein, or the standing wave radio, orto facilitate tuning associated apiparatus, etc. This can be accomplished by slotting one wall of the waveguide and inserting a probe. Such slots are narrow compared with the width of the corresponding wall of the waveguide, thus limiting the size of probes than can be inserted therein. As the waveguide becomes smaller for higher frequencies, the problem becomes more severe. Also it is sometimes desirable to insert elements for loading, impedance matching, etc., and similar space difiiculties arise. r

In accordance with the present invention a transmission line, functioning generally on waveguide principles, is provided in which one wall may be left completely open so as to facilitate inserting probes for measuring or other purposes. 7 j

An additional advantage of the transmission line of the present invention is that the bandwidth for the dominant mode of operation may be made considerably greater than that of conventional rectangular waveguide, and yet the structure is mechanically simple and easy to fabricate.

The transmission line of the present invention is termed a ftrough transmission line because of its overall trough-like shape. Generally speaking, the line has sidewalls and a central member (which may be of fin-like shape) positioned therebetween with the conductive sides of the central member conductively connected to the sidewalls at the bottoms thereof, but the top wall may be left completely open. The top may be capped to keep the line free of dust and moisture, but the cap may be removed, e.g., for purposes of measurement, without substantially affecting the characteristics of the line. For certain relative dimensions, as described hereinafter, it is preferable to employ insulating caps of low dielectric constant, but with other relative dimensions it is possible to use a cap'of conductive material without appreciably affecting the transmission line characteristics.

The transmission line may be used in place of other forms of waveguide or in place of coaxial transmission lines, or it may be used in conjunction therewith. When used with conventionalrectangular waveguide provision must be made for coupling energy from the conventional waveguide to the trough transmission .line of the present invention without excessive mismatch.

In accordance 2,921,276 Patented Jan. 12, 1960 2 with a further feature of the invention, a suitable transition section is provided. Usually the transition section will be used to couple extended sections of rectangular waveguide and trough waveguide, but in particular applications it can be used by itself.

The invention will be more fully understood by reference to the following description of specific embodiments thereof, taken in conjunction with the drawings, in which:

Fig. 1 shows in perspective a portion of a trough trans mission line in accordance with the invention;

Fig. 2 is a cross-section of the transmission line of Fig. 1, showing the electric and magnetic fieldsfor the dominant mode of operation;

Fig. 3 is a longitudinal section taken alongthe line 33 of Fig. 2; f

Fig. 4 is an elevation from one end of a trough transmission line showing a suitable mechanical structure using stock materials;

Fig. 5 is an end view of a modification of the embodiment of Fig. 1;

Fig. 6 is a plan view of a rectangular waveguide-totrough transformation;

Fig. 7 is a side elevation of Fig. 6;

Figs. 8 and 9 are cross-sections taken alongvthe lines 8-8 and 99 of Fig. 6, respectively;

Fig. 10 illustrates certain constructional features employed in a practical embodiment;

Fig. 11 is a cross-section of the embodiment of Fig. 10 taken along the line 1111 thereof; and

Fig. 12 is a diagram illustrating electric and magnetic fields of a rectangular waveguide in its dominant mode of operation. I

Referring now to Fig. 1, an embodiment of the trough transmission line of the present invention is shown in one of its simplest forms. As shown, the sidewalls 10 extend transversely'from a bottom wall 11, and a central member 12 is positioned between the sidewalls and likewise extends transversely from the bottom wall 11. As specifically illustrated, the central member 12 is a'fin-like in appearance, and for convenience will sometimes be termed a fin hereinafter. However, it may be made substantially wider than shown, if desired."

The several walls and bottom extend longitudinally of the line,as indicated by'the arrow 13, and electr0- magnetic energy propagates down the line in the direction indicated by' the arrow. I

Preferably the sidewalls 10 and central member 12 are flat and parallel, as'shown, with the central member posi-, tioned midway between the sidewalls. Also, preferably the bottom 11 is flat, as shown. However, considerable departure from the configuration illustrated is possible. For example, the sidewalls might be bowed andalso the bottom given some curvature. It is possible alsoto depart.

somewhat from the flat configuration of the central member 12. In general, however, it is desirable to make the structure symmetrical about a plane passing through the central member 12', so that one side of the line is substantially a mirror image of the other, in order to avoid spurious modes.

The entire structure shown in Fig. 1 may be made of a suitable metal of high conductivity. However, since high frequency current flow is confined essentially to the inner surfaces of the sidewalls, designated 10, the upper surfaces 11' of the bottom, and the sides. 12' of the central member, if desired'only these surfaces needv be of conductive material. If the entire unit is made of relatively inexpensive metal, the designated surfaces can be silver-plated or otherwise given a highly con ductive surface coating so as to minimize losses.

Further modifications are possible which may be de-' sirable for particularapplications. Asrwill be explained 3 hereinafter, substantially no component of current exists perpendicular to the free edge 12" of the central member. Consequently it is possible to employ a sheet of insulating material coveredon both.sideswitha. conductive coatingor foil, etc., as the central; member 12. Further, although not preferred, itispossibletto use two spaced sheets of'condu'ctive materialas thecentral member 12, with a narrow opening. therebetween to allow insertion of a narrow probe or. other couplingmeans. In such case the two sheets will. be joinedto respective bottom surfaces, and the overall arrangement can be obtained by placing two U-shaped troughs besideeach other, with the inner vertical:surfaces shorter than. the

outer.

Fig. 2 shows the approximateconfiguration of theelectrio and magnetic fields for the dominant mode of operation, it being understood that the dominant mode is-that mode of operation which hasthe lowest .entoffirequency, and is the mode commonly employed in; practiceIIn Fig. 2 the full lines.14.represent.the field lines or. lines of flux of the electric vector, and the dash lines 15. rep.- resent the field lines or lines of flux of: the mag netic. vector. It will be noted that the. field lines of thezelectric vector extend from the conductive sides12l of'the central member to the inner surfacesof the; respective sidewalls 10. The spacing between adjacentflfield'lines 14 is an indication of the relative intensity of ithexelectric field. At the bottom of. the trough, near'surfa'ces 11", the intensity of the electric field is small. The field becomes progressively more intense in,the=upwards direction along the central member and;;is armaximumxa't the free edge of the central member. On the other hand, transverse currents on the sides of the central member 12 vary from a. minimum at the free edge to a maximum at its base. The field lines of the electric vector. are generally parallel to the bottom surfaces inthe. regions where they central member 12 lies directly opposite the sidewalls. In-the regions above the free edge of the central member, the field lines extendupwards along curved paths to the respective sidewalls. Itwill berec ognized that the field lines of the electric vector are everywhere substantially transverse to the direction. of.

propagation down the line.

In Fig. 3 the field lines of the electric vector arezagain shown by full lines .14 extendingbetweenthe central member 12 and the sidewalls 10, and. theintensity' of the field is indicated by the spacing of .the lines. Since 3. is a longitudinal section, the field lines of' the magnetic vector are seen end-on, solid dots. 15! repre senting the magnetic vector directed. toward theobserver and open dots 15" indicating the magnetic'vector directed away from the observer. The configuration of the fields in Fig. 3 is to be visualized asmoving longitudinally (that is, upwards in the plane of the paper).at.a. speed equal to the phase velocity of. the. electromagnetic wave.

Referring back to Fig. 2, the cutolfawavelength" (h is primarily determined by the electrical height ofthe central member 12, and in general the cutoff wavelength is approximately that at which. the electricaliheight of member 12 is a quater-wavelength. Thus, .the .electrical height of member 12 should ingeneral. notexceed a quarter-wavelength of the lowest frequency to be-transmitted. As will be understood by thosein the.art, the electrical height of member 12. will ordinarily be somewhat greater than the physical .height-due to .fringinga of the electrical field and spreadingof the magnetic field around its .upper edge.

Explainingv thissomewhatmore fully, the electric field shown. by lines 14 has approximately a half-sine. wave.

distribution which is substantiallyzero at-the bottomsurface 11 of one side of central member, increases to.a.

maximumnear the free edge of member12, and 'xdecreases to-substantially :zero .at the bottom surface on the :o'therside. A path existsdnthespace about'mmber 12 whose length is approximately one-half that of the longest wave that is capable of being transmitted by the waveguide.

The effective length of this path is subject to calculation for a given configuration by methods known in the art. It has been found that for most practical purposes the etfective length L is closely approximated by the following equation:

. L=2D+0.4-4W 1 where D is the height of the central member and W is the separation of the sidewalls as illustrated in Fig. 2. This equation is given as a general guide which has been found valuable in practice and is believed to be reasonably accurate. However, it-is not insisted upon.

Considering the functioning of the central member 12 more generally, it possesses the property of transverse resonance at frequencies for which its height makes the effective path length L an odd integral multiple of a quarter of the cutolf wavelength. Due to the considerable fringing of the electric field, or s'o-c'alled end" effect, at the. free e'dgeof member'l-Z', the cutoff wavelengthis increased appreciablybeyond that indicated by the physical height of the member. 1

It willbe noted from Figs. 1 and 2- that the top of the transmission line is completely open. Thus radiation might be expectedto occur,with consequent loss of energy. This is prevented in the transmission line of the present invention by proper selection of the spacing W of the sidewalls and the amountiby which the height of the sidewalls exceeds that of the central member'l2.

In general, the spacing W should'b'e less than a'halfwavelength at the=operatingfretplency of the line. If the line is intended to beoperated over a range of frequencies, spacing W should not exceed=ahalf-wavelength of the highestfrequency in the desired range. The sidewalls 10-can then beextended' upwardly sufficiently to reduce the radiation, or the coupling between the region within the waveguide and the'regionoutside of it, to any desired value'. In general, it is found that making the height of the sidewalls exceed that of the central member 12- by the spacing-of the sidewalls, that is, by making H -D equal to W, sufiicientreduction of transverse radiation is obtained for most "practical purposes, and it is preferred to extend=the sidewalls by at least this amount; If W approaches a lialf-wavelength, then itis desirable to make H.--D"considerably greater than W in order to secure sufficient reduction of'transverse radiation. Furthermore, even with close spacing of the sidewalls, H''Dcan be'made considerably greater than W to furtherreduce radiation and consequentlosses, or to reduce-coupling with regionsoutside the waveguide:

The spacing of the sidewalls;- W, also has an effect on'the characteristicimpedance of'th'e line, andin general the characteristic impedanceincreasesas thespacing W- decreases.

Assuming that it is desirable to beable tooperate at frequencies down to 'the region ofcutofll; for'this condition the following relationships apply-z W \,/2 (2.) L=2D+O.44W=)\ /2 (3) By combining these equations thefollowing-relationship is obtained:

This. shows; that .the height. of "the." centralmember 12 eral dimensions is given as ant aid' to -the ready practice ofithe.-.invention,..and:is znotintended' to' limit the invention thereto,

With proper selection of dimensions, the trough waveguide of the present invention can be operated in its dominant mode over a range approximately three times the cutoff frequency before higher propagating modes of operation are supported. By way of contrast, conventional rectangular waveguide can be operated only over a range of approximately twice the cutoff frequency before higher propagating modes are supported. This is a considerable advantage of the present transmission line where broad band operation is sought, with freedom from higher propagating modes. In this event, the spacing W of the sidewalls is advantageously made less than a half-wavelength at the highest frequency in the range, and the sidewalls extended sufiiciently to reduce radi-' ation and consequent losses to a desired low value.

As will be apparent from Figs. 1 and 2, the top of the transmission line may be completely open to facilitate the insertion of probes for measuring or other purposes. However, in a practical application it may be undesirable to leave the top open at all times. In such case a removable cap can be placed over the top. In general the material used for the cap and its position should be chosen so as not to affect materially the field distribution in the trough. If material of low dielectric constant approaching that of air is employed, the spacing and height of the sidewalls may be substantially the same as that employed for physically open-top line. For material of higher dielectric constant, and for conductive materials, it is desirable to increase the height of the sidewalls for a given spacing so that the field is negligibly small in the region of the cap.

In general, if the height of the sidewalls exceeds that of the central member 12 by about twice the spacing W, and if the spacing is substantially smaller than a half-wavelength at the operating frequency, the electric field at the top is approximately 54 db lower than the field at the edge of the fin. Capping with a conductive member or with a member of high dielectric constant will therefore not appreciably affect the electrical characteristics. For any other height of sidewalls the rule that, for spacing substantially smaller than a half-wavelength, the field decreases at the rate of approximately 27 db per spacing W (by which the sidewalls exceed the height of the central member) will permit the designer to compute the effect of the cap and choose suitable dimensions accordingly.

If desired, of course, the cap may be made integral with the sidewalls, rather than removable.

The structure illustrated in Fig. 1 can be fabricated in any desired manner. To avoid tooling where only small quantities are involved, it is desirable to make the line of stock materials readily available. Sheet metal can be bent into the required shape and secured by soldering, brazing, spot welding, etc. Fig. 4 shows an alternative in which sidewalls 21 and the central mem her or fin 22 are fiat strips of sheet metal and the bottom is made of two lengths 23 of somewhat thicker metal, with the assemblage held together by suitable means such as bolts 24. In this type of construction care should be taken to insure good electrical contact between the several members. Any desired length of line can be made in this manner by using suitable lengths of material.

Referring now to Fig. 5, a modification of thestructure of Fig. 1 is shown in which the width of the bottom surfaces is markedly reduced. Central member 12 is positioned between sidewalls 45 and the conductive sides of member 12 are conductively connected to thesidewalls 45 by narrow conductive surfaces 25 of the bottom 25. Sidewalls 45 are arranged to slant inwards toward the bottoms thereof rather than being parallel as in Fig. l. The spacing W of the tops of the sidewalls 45 is advantageously less than a half-wavelength at the highest operating frequency and the height H of the sidewalls preferably exceeds the height of the central member 12 by at leastthe spacing W to prevent transverse radiation. The sidewalls may be still higherto further reduce transverse radiation, as explained hereinbefore.

In some instances the angle between'sidewalls 45 may be chosen so that the sidewalls can be flat and still provide a sufliciently narrow spacing W for a desired height H. For wider angles, the upper portions 45' may be at an angle to the remainder of the sidewalls, as specifically shown, to restrict the spacing W.

If desired, the width of the bottom surfaces 25 may be further reduced to substantially zero, in which case the conductive sides of the central member 12 join the sidewalls 45 at the bottoms thereof respectively. Also, instead of forming the sidewalls of fiat sections, they may be curved. In such case, it is preferable to keep the lateral spacing of all corresponding points of the sidewalls less than a half-wavelength. As in the case of Fig. 1, it is desirable to keep the structure of Fig. 5, and modifications thereof, symmetrical about a plane through the central member 12 so as to avoid spurious modes.

The exact distribution of the electric and magnetic fields for the configuration of Fig. 5 will differ somewhat from that shown in Figs. 2 and 3, but the general character of the distribution will be similar.

Figs. 6-11 are various views of a transformation which may be used to couple a length of conventional rectangular waveguide to a length of trough waveguide with small loss and impedance mismatch. Alternatively, the transformation can be used by itself if desired for a particular application. The transformation can be considered as composed of an end section A whose crosssection is that of the rectangular waveguide to which it is to be connected, an intermediate section B, and ,an-

other end section C whose configuration is selected to match that of the trough waveguide with which it is to be connected.

End plates 31 and 32 are provided to facilitate connection to adjacent line sections. End plate 31 has a rectangular opening 33 therein (Fig. 8) whose crosssection matches that of the waveguide with which it is to be associated. The opposite plate 32 has a rectangular opening 34 therein (Fig. 9) to match the trough Waveguide with which it is to be associated. It will be I noted that the rectangular opening 33 at the waveguide end has its longer dimension horizontal, whereas the opening 34 at the trough end has its longer dimension vertical.

A conductive bottom 35, sidewalls 36 and a slotted top 37 are provided. In section A the four walls constitute substantially a continuation of the waveguide with which it is to be attached, and the slot 38 in this region (designated 38) makes it analogous to a slotted waveguide. At the opposite end the top of section C is open to match the trough waveguide. Section C contains a central member or fin 39 to match that of the trough waveguide with which it is to be attached. Preferably member 39 is conductively attached to an associated trough waveguide, and to this end a spring clip arrangement 40 may be provided.

The fin 39 extends into the intermediate section B beneath the portion 38" of slot 38, and tapers to sub stantially zero height at the end thereof toward section A, as indicated at 39'. On the other hand, the height of the sidewalls of section B gradually increases in the direction of section C. Near the section C the slot 38 widens at 38" to match the open trough configuration of section C. The angle of taper of section 39 preferably does not exceed 45 and may be made considerably less, as shown. Also the intermediate section 39" may be given a slight taper, with advantage.

Fig. 12 shows the fields of a conventional rectangular waveguide operating in its dominant mode. The electric 7 field'is shown by'full' lines'44'and the'magnetic field by' dash lines43. By 'cornparis'on with'thefie'ld configura'tion in the trou'gliwave'guide shown in Fig. 2, 'it will be observed that the electric fields in the two sections are at approximately'right'angles to each other near the bottom.

The provision f'slot38, thetapering of fin 39, "the gradual increase in the height'of the sidewalls,and the wideningbf theslot at38""all combine 'to provide a gradual transition in the electric "and magneticfields between thetwo end sections.

When the transformation is used'with a trough'waveguide whosecross-sectionds like that "of Fig. 5, the bottom of the intermediate section B"ma be tapered to a narrower width at section C, and may approach a point'if the"sidew alls' of section C intersect the central member-thereof. "Also the sidewalls of section B maybe 'graduallyinclined or'cu'rved so as to match the selected configuration" of the trough at section C.

Various modifications ofthe" transformation illustrated m'aybe" made in practice tomeet the conditions of a particular application. As has been pointed out, with sufficiently high sidewalls of the trough section, it is possible to use a' conductive topwithoufinterfering with the'operation thereof. Hence in some cases it may be possible to omit the widening 'of the slot at'38 and graduallyr'duce' the width of theslot to zero. Also the' pointat which the central "member tapers to zero may 'be'chauged; and the point atwhich the slot 38 begins (toward section A) may be altered.

In Figs. 10 and 11 certain structural features are shown"wh'ich facilitate construction. Here the transition unitis'made of a plurality of sheet'm'etal sections united'by s'uitable'joining sections 41. Brackets 42 aid in supporting the end plates. Solderingand brazing may'be" employed"as "desired to insureel'ectrical continuity and m'echa'nicalstrength.

Although a suitable arrangement for coupling a convention'al waveguide to a trough waveguide has been described, it is possible to introduce and remove energy fromthe tr'ough'by means of probes and coupling elements known in the art"for"'cou'pling to' conventional waveguide, c'avities,"etc. Such 'feedsystems should preferably'be designed to have a'major electric'or'magnetic field"compon'ent (or"both), which 'is "in'the direction of "the corresponding "component 'in' "the trough" waveguide,"as' discussed in connection With Figs 2' and 3, for the dominant "mode"ofop'era'tion. Also, it isdes'irable to employ feed systems which are structurally symmetrical'withrespect' to theplane'bf the fin'or central member 12, soas' to avoid the possibility of setting up waves whichjwill propagate betweenthe sidewalls in' a direction perpendicular tothe'longitudi'nal axis of the'trough.

Anov'el "coaxial-to trough 'linec'o'upling is described in a'copending application of Eugene G. Fubini and Henry SIKeen, entitled'Microwave Circuits, filed concurrently herewith.

The invention has been described in connection with specific embodiments thereof. It will be understood that many variations and alterations are possible within the scopeof the invention, and'may be made as suits the designer or meets the needs of a particular applic'atlon.

In the specification and claims use has-been made of terms -such as bottom, top, sidewalls, --height, e'tc.,-"in order to define the relationships in convenient language whi'chc'an be readily understood. The employment of such terms-however, is not intended to mean that the transmissionline must be used-in practice with the top up, the bot-torn down, etc.,-'as specifically illustrated, sincethe trough may be inverted, or laid on :itsside; etc., 'as meets the requirements of aparticular application.

I claim: I

i 1. A microwave trou'gliwaveguide which comprises a pair of spaced conductive'sidewalls and a central member extending longitudinally of said waveguide, said central "m'e'rriber having conductive sides extending upwards between said"s'ide'walls with the bottom edges'of said central member conductively connected to said sidewalls at'the' bottoms thereof respectively, the height of said sidewallsbeing substantially greater than the height of said central'member, and the waveguide being substantially open between the tops of said sidewalls.

2. A microwave trough waveguide which comprises a pair of spaced conductive sidewalls and a central member extending longitudinally of said waveguide, said central member having conductive sides extending upwards between said sidewalls with the bottom-edges of said central member conductively connected to said sidewalls at the bottoms thereof respectively, the spacing of the tops of said sidewalls being less than substantially a half-wavelength atthe operatingfrequency ofthe waveguide and the height of the sidewalls exceeding the height of said central member by at least said spacing.

3. A microwave trough waveguide which comprises a pair of spaced conductivesidewalls and a central member extending longitudinally of said waveguide, said-central member having conductive sides extending upwards between said sidewalls with the bottom edges of said central member conductively connected to said sidewalls at the bottoms thereof respectively, the spacing of the tops of said sidewalls being less than substantially a half-wavelength-at the operating frequency of the waveguide and the height of the sidewalls exceeding the height of said central member by at least said spacing, and thewaveguide being substantially open between'the tops of said sidewalls.

4. A microwave trough waveguide'which comprises a pair of spaced conductive sidewalls, a central member and bottom conductive surfaces, said sidewalls extending longitudinally of said waveguide, said central member having'conductive sides positioned betweensaid sidewalls and extending longitudinally of said waveguide, and said bottom conductive surfaces connecting said sidewalls with the conductive sides of said central member respectively, said'sidewalls and central member extending transversely from said bottom surfaces and the top of the waveguide being substantially open between the tops of said sidewalls, the spacing of said sidewalls being less-than substantially a half-wavelengthvat the operating frequency of the waveguide and the height of said sidewalls exceeding the height of said central member by at least the spacing of the sidewalls.

5. A microwave trough waveguide which comprises a pair of spaced conductive sidewalls, a central member and bottom conductive surfaces, said sidewalls extending longitudinally of said waveguide, said central member having conductive sides positioned between said sidewalls and extending longitudinally of said waveguide, and said bottom conductive surfaces connecting said sidewalls with the conductive sides of said central member respectively, said sidewalls and central member extending transversely from said bottom surfaces, the spacing of said sidewalls being less than substantially a half-wavelength at the operating frequency of the waveguide and the height of said sidewalls exceeding the height of said centralmemher by at least the spacing of the sidewalls, and the height ofsaid central member lying within substantially the range-014x to 0.25% where h is the cutotf frcquency of the waveguide.

6. A'microwave trough waveguide which comprises a substantially fiat conductive bottom and substantially flat conductive sidewalls perpendicular thereto, and a sub stantially fiat central conductive member extending perpendicularlyfrom said bottom midway between said sidewalls, said bottom, sidewalls 'and central member extending longitudinally of said waveguide with the top of the waveguide substantially open for substantially the full width of the bottom thereof, the spacing of said sidewalls being less than substantially a half-wavelength at the operating frequency of the waveguide and the height of said sidewalls exceeding the height of said central member by at least the spacing of the sidewalls.

7. A microwave trough waveguide which comprises a substantially fiat conductive bottom and substantially flat conductive sidewalls perpendicular thereto, and a substantially fiat central conductive member extending perpendicularly from said bottom midway between said sidewalls, said bottom, sidewalls and central member extending longitudinally of said waveguide with the top of the waveguide substantially open for substantially the full width of the bottom thereof, the spacing of said sidewalls being less than substantially a half-wavelength at the operating frequency of the waveguide and the height of said sidewalls exceeding the height of said central member by at least the spacing of the sidewalls, and the height of said central member lying within substantially the range 0.14% to 0.25)\ where is the cutoff frequency of the waveguide.

8. A microwave trough waveguide which comprises a pair of spaced conductive sidewalls and a central member extending longitudinally of said waveguide, said central member having conductive sides extending upwards between said sidewalls with the bottom edges of said central member conductively connected to said sidewalls at the bottoms thereof respectively, the height of said sidewalls exceeding the height of the central member by at least twice the spacing of the tops of the sidewalls.

9. A microwave trough waveguide which comprises a pair of spaced conductive sidewalls, a central member and bottom conductive surfaces, said sidewalls extending longitudinally of said waveguide, said central member having conductive sides positioned between said sidewalls and extending longitudinally of said waveguide, and said bottom conductive surfaces connecting said sidewalls with the conductive sides of said central member respectively, said sidewalls and central member extending transversely from said bottom surfaces, the spacing of said sidewalls being less than substantially a half-wavelength at the operating frequency of the waveguide and the height of the sidewalls exceeding the height of said central member by at least twice the spacing of the sidewalls.

10. A microwave trough waveguide which comprises a substantially flat conductive bottom and substantially fiat conductive sidewalls perpendicular thereto, and a substantially flat central conductive member extending perpendicularly from said bottom midway between said sidewalls, said bottom, sidewalls and central member extending longitudinally of said waveguide, the spacing of said sidewalls being less than substantially a half-wavelength at the operating frequency of the waveguide and the height of the sidewalls exceeding the height of said central member by at least twice the spacing of the sidewalls.

11. A microwave trough waveguide which comprises a substantially flat conductive bottom and substantially flat conductive sidewalls perpendicular thereto, and a substantially fiat central conductive member extending perpendicularly from said bottom midway between said sidewalls, said bottom, sidewalls and central member extending longitudinally of said waveguide, the spacing of said sidewalls being less than substantially a half-wavelength at the operating frequency of the waveguide and the height of the sidewalls exceeding the height of said central member by at least twice the spacing of the sidewalls, and the height of said central member lying within substantially the range 0.14 to 0.25% where A is the cutoif frequency of the waveguide.

12. A microwave trough waveguide which comprises a pair of spaced conductive sidewalls and a central member extending longitudinally of said waveguide, said central member having conductive sides extending upwards between said sidewalls with the bottom edges of said central member conductively connected to said sidewalls at the bottoms thereof respectively, the height of said sidewalls being substantially greater than the height of said central member, substantially all the field lines of the electric vector extending from said conductive sides of the central member to the respective sidewalls for the dominant mode of operation when electromagnetic energy is propagated along the waveguide.

13. A microwave trough waveguide which comprises a pair of spaced conductive sidewalls, a central member and bottom conductive surfaces, said sidewalls extending longitudinally of said waveguide, said central member having conductive sides positioned between said sidewalls and extending longitudinally of said waveguide, and said bottom conductive surfaces connecting said sidewalls with the conductive sides of said central member respectively, said sidewalls and central member extending transversely from said bottom surfaces and the height of the sidewalls being substantially greater than the height of the central member, the field lines of the electric vector extending from said conductive sides of the central member to the respective sidewalls for the dominant mode of operation when electromagnetic energy is propagated along the waveguide.

References Cited in the file of this patent UNITED STATES PATENTS 2,155,508 Schelkunofl' Apr. 25, 1939 2,403,289 Korman July 2, 1946 2,433,368 Johnson Dec. 30, 1947 2,439,285 Clapp Apr. 6, 1948 2,477,510 Chu July 26, 1949 2,656,513 King Oct. 20, 1953 2,683,256 Kumpfer July 6, 1954 OTHER REFERENCES Ragan: Microwave Transmission Circuits, vol. 9, M.I.T. Rad. Lab., published by McGraw-Hill, 1948, pages 35861.

Packard: Machine Methods Make Strip Transmission Line, Electronics, September 1954, vol. 27, No. 9, page 148.

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