US3166723A - Variable directional coupler having a movable articulated conductor - Google Patents

Description

Jan. 19, 1965 J ocK ETAL 3,166,723

VARIABLE DIRECTIONAL COUPLER HAVING A MOVABLE ARTICULATED CONDUCTOR Filed March 6, 1961 2 Sheets-Sheet 1 COUPLER \o 28 MOVABLL COUPU NC: SECTION COUPLED OUTPUT s \GNALS MA/ev/N J Bock F/20/?/ c/ S. HOWARD INVENTORS ZOVLJHI zztm \COUDLER O 15v W Jan. 19, 1965 M. J. BOCK ETAL 3,166,723

VARIABLE DIRECTIONAL COUPLER HAVING A MOVABLE ARTICULATED CONDUCTOR Filed March 6, 1961 2 Sheets-Sheet 2 MOVABLE COUPUNG ECTlON 54) ABSORBER ABSORBER I MOVABLE. COUPLING 5ECT\ON as MARI 0v .x BOCK FREDER/ CK 5. Ho WA RD INVENTORS A 7TORNEY$ United States Patent 3,166,723 VARIABLE DRECTIQNAL C-IB'UPLER HAVHNG A MGVABLE ARTECULATED (IUNDUCTOR Marvin J. Bock, Santa Barbara, and Frederick Howard, Los Angeles, Caiii, assignors, by mesne assignments, to Micro-Radionics, l'nc., Van Nuys, Calif.

Filed Mar. 6, 19b1, Ser. No. 93,530 3 (Ilaims. (Cl. 333-119) This invention relates to devices for controlling the transmission of Waves at microwave frequencies, and more particularly to devices for providing a mechanically variable coupling, attenuation, and amplitude control for microwave systems.

Couplers, attenuators, modulators and the like are basic components in microwave systems. Often a number'of different functions may be performed by a single device, so that a coupler, for example, might be able to serve additionally as an attenuator or modulator. This may be better understood by reference to the senses in which the terms coupling, attenuation and modula tion are usually employed. The term coupling is used to indicate the transfer of a predeterminedor selected amount of microwave energy from one conductor to another. The term attenuation is used to indicate the controlled dissipation of a selected amount of microwave energy which is propagated along a conductor. Where the attenuation is varied with time in some predetermined fashion, this may of course constitute an amplitude mod ulation. It is therefore evident that certain kinds of couplers may also operate as attenuators, and if they operate variably with time in addition serve the function of modulators.

A mechanically operable coupler which can perform all these functions is widely sought for microwave systems. Such devices have not heretofore been provided because of a number of ditficulties which have been encountered in getting satisfactory electrical and mechanical performance. Many fixed couplers are known, but those which have wide band performance are usually not susceptible of. conversion to a variable operation. Variable couplers have usually had limited band width, and particularly have been subject to troublesome nonlinearities in operation. The non-linearities have required complicated mechanical motions in order to achieve accurate coupling settings, which in turn have greatly increased costs, limited accuracy and madereproducibility extremely difiicult.

A microwave variable coupler of suitably small size and low cost which can be coupled into standard guide systems and testing equipment widely used at microwave frequencies.

It is therefore an object of the present invention to provide an improved microwave variable coupler.

A further object of the present invention is to provide an improved variable couplerwhich is precisely adjustable but which'is simple and economical to construct.

Yet another object of the present invention is to pro- Tvide an improved wave coupling device for microwave applications, which device may serve as a variable coupler,- attenuator or modulator. 5 -Another'o'bject of the present invention is to provide an improved mechanically variable which operates bidirectionally.

In accordance with the invention, variable wave coupling between two conductors is provided by transverse movement of one conductor section relative to another and the employment of an articulated conductive connection between the movable section and terminal points.

In a specific example of a mechanically variable coupler section in accordance with the invention, the coupler is arranged as a four-port slab transmission line device. A principal center conductor section is fixedly coupled between two of the ports, while the remaining two ports are coupled by an articulated center conductor section including a centrally located coupling element. The coupling element is transversely movable relative to the principal center conductor by a linear motion mechanism, and may occupy different positions in a wave coupling region. The relative transverse spacing between the center conductors in the coupling region exercises an ex-' tremely precise control over the amount of coupling. Further, incident waves may be propagated on either the principal center conductor or the articulated center conductor, so that the device is fully bidirectional. The entire coupler is extremely compact, simply constructed although linearly variable with changes of the control element, and fully compatible with coaxial and other guided wave systems. With the addition of an attenuator device at a selected terminal, waves variably coupled to the selected terminal may be dissipated so that the device functions as a variable attenuator. Further, the mechanical positioning of the coupler section may be varied with time so as to provide amplitude modulation.

Another aspect of the present invention is the provision of means by which the frequency response of the coupler may be varied as desired. In such arrangements, the coupling lengths of the center conductors are made not only transversely movable relative to each other, but are also longitudinally variable. Through the longitudinal variation, the length along the center conductors by which coupling is effected may. be changed to be optimum for any given frequency. Accordingly, the already inherently wide band characteristics which the device provides may be extended to any useful rangewhich is desired.

The novel features ofthe invention maybe better understood by reference to the following description, taken in conjunction with the accompanying. drawings, in which: 7 f

FIG. 1 is a perspective view, partly broken away, of one form of guided wave coupler in accordance with the present invention;

FIG. 2 is a plan view of the arrangement of FIG. 1;

FIG. 3 is an enlarged perspective view of a fragment of the arrangement of FIG. 1;

FIG. 4 is a sectional view of the fragment of FIG. .3, taken along the line 44 in FIG. 3 and looking in the directionof the appended arrows;

FIG. 5 is a plan view, partly broken away, of a wide band variable coupler in accordance with the present invention, and

FIG. 6 is a perspective view of a fragment of the wide band variable coupler of FIG. 5.

A bidirectional variable coupler 10 (FIGS. 1-3) according to the invention employs slab transmission line construction for transmitting waves at microwavefrequencies. Slab transmission line is a widely used guided wave'deyice. Because it is essentially an open-sided coaxial line, it is often used in conjunction. with-coaxial line systems and related systems, such as st rip tra1'1smissionline units. A'specific example is provided, by way 'of illustratiohfof a coupler covering atwotoone frequencyrange and operating at a nominal frequency of Patented Jan. 19, 1965 microwave coupler sitions and impedance changes. tral coupling element is connected both electrically and 6 kilomegacycles (hereinafter kmc.) in the range 4 to 8 kmc. The main body of the coupler 1G is provided by a frame 12 of generally rectangular construction, the principal electrical elements which are used being a top plate 14 and a bottom plate 1'5, these plates serving as V the ground vplane conductors for the slab transmission lines. '.The 'remainder' of the frame 12 may be completed by eppropriateside and end elements which may or may not be conductors as desired, although conductive elements tend to reduce external radiation to a low magnitude and are preferred.

.tially filled with dielectric material between their center 'conductorsand their outer conductors.

For smooth coupling of Waves from the slab lines to the coaxial lines, and vice versa it is preferred to utilize an air transition section in those parts of the terminal conductors 26-23 which are directly joined to the associated center condoctor of, the slab lines. The various input and output terminals FIG; '1 have been labeled for convenience with differentinput and output designations, but these areinerely'. illustrative of different wa'ys'in which conplings ina'y'be' etfe'ctedl, Inithis example, the first ter minal conductor is directly coupled to a system load 24 through the second terminal conductor 21. The coupled output signals are derived at the third terminal conductor 22, and may be applied to an associated system or matched termination (not shown) depending upon the use intended for the coupler It]. The fourth terminal conductor 23 is here unused, and is properly terrninated in a matched termination 25.

The coupler ltrincludes a fixed or principal length of 'slabfline ce'ntei 'co'nduct'or26 coupled between the center conductors of the oppositely disposed first and second i terminal conductors 20 and'21. A movable coupling 1 section 28 forms a slab line center conductor which con ne'cts the center conductors of the third and fourth terininal conductors 22 and 23. At each end of the movable section' 28, a'fixed length of slab line center conductor 36," 31 is attached to the adjacent terminal condoctor 22, 23, respectively. In the approximate center of the movable coupling section 28, the slab center conductor is .i'nithe .form of. a generally U-shaped coupling element. 33 whose principal length is parallel to the fixed center" conductor '26.' The volume between the ground plane conductors 14-, 15 which encompasses the associated'lengths of the fixed and movable center conductors 26, 33 may be termed a Wave coupling region. The free. ends and the corners of the U-shaped coupling element 33 are designed in accordance with well known slab line construction techniques to minimize sharp tran- Each end of the cen mechanically to the closest fixed end or 31 of the movable coupling section by a different pair of articulated arms 35, 36 or 37, 38, respectively, having the cross-sectional configuration chosen for the slab line center conductor.

' The'articulated construction of the paired arms 35,

and .37, 38 is achieved by a tongue and groove ar 'rangernent, as shown in best detail in the enlarged fragment of FIG. 3. A tongue 40 extending from one of the "arms 37, for exa1nple, mates slidably in a matching groove 41in;thefacing end of the coupling element 33, A pivot pin 42 extending through the coupling element 33 registers with an appropriate aperture in the tongue 40 andp'ermits a pivotalfmovement of the arm 37-relative tothecouplingelement 33. The end surfaces of the various abut-ting sections are appropriately curved in mating fashion so that there is no binding between the arm sections. The relative pivotal motion which is permitted between various arms is restricted to a plane which is parallel to the plane of the ground plane conductors Id, 15. The two pairs of articulated arms 35, 36 and 37, 38 are generally similar, so that the base leg of the U-shaped central coupling element 33 may be maintained parallel to the fixed center conductor 26 despite transverse movement.

With this arrangement, the central coupling element 33 is moved transversely with respect to the length of the fixed center conductor 26, by a suitable mechanism which may be controlled externally to the frame 12. In the form shown by Way of example, the central coupling element 33 is fixed to a dielectric insulator 45 which provides mechanical support but which insulates the center conductor sections from the remainder of the assembly. A rack member 46 is also coupled to the dielectric insulator 45, and extends between the ground plane conductors i4, 15 in a direction normal to the length of the fixed center conductor 26. Teeth in the rack member 46 are engaged by the teeth of a pinion 47, which is rotatably mounted in the top ground plane conductor 14. A control knob 48 mounted outside the frame 12 may be used to adjust the position of the rack member 46 through rotation of the pinion 47.

Several mechanical alternatives which may be employed have been omitted in orderto simplify and clarify the drawings, inasmuch as these may assume a wide number of forms and are not essential to the operation of couplers in accordance with the invention. They may, however, be briefly discussed. In order to position the central coupling element 33 where desired, appropriate indicia (not shown) may be included on the top ground plane conductor 14 adjacent the controi knob 43. For vernicr control, a step-down gearing or coupling may be used for precise minute adjustment of the transverse position of the central coupling element 33. A setscrew or other lock arrangement may also be employed for retaining the movable elements in position, once a position has been selected. For firm support of the fixed center conductor 26 or the movable coupling section 28, dielectric elements having a low dielectric constant may be employed between the various center conductors and the ground planes. The central coupling element 33 and the articulated arms 35 38 of the movable coupling section 28 are slidably mounted on a support element 49 shaped to conform to the rack member 46 and aifixed to either or both of the ground plane conductors 14, 15.

Resistive elements 53, 51 or other microwave energy absorbing elements are mounted between the top and bottom ground plane conductors 14, 15 in the areas between the fixed center conductor and the different articulated arm pairs 35, 3s and 5'7, 3S,respectivcly.

In the operation of the arrangement of FIGS. 13 as a variable coupler, the transverse displacement of the central coupling element 33 from the fixed center conductor 26 in the wave coupling region determines the amount of coupling from a selected input terminal to a selected output terminal. Taking the first terminal conductor 20 as the source of input signals, a very close spacing between the central coupling element 33 and the fixed center conductor 26 would at the most cause a substantially equal split of the waves (3 db of coupling) between the second terminal conductor 21 and the third terminal conductor 22. The distribution of the electric fields in the coupling region with the center conductors 2s and 33' very closely juxtaposed would result in substantially like electric fields being associated with each of the center conductors 26 and 33. In this case, halt the microwave energy would be transmitted directly to the system load 2% at the second terminal conductor 21, and half would be coupled onto the movable coupling section 28. The waves coupled onto the central coupling element 33 are reversed from the original direc- -device.

pled output signals would appear at the third terminal.

conductor 22 and the device would be a 3 db coupler.

In the majority of instances in which a variable coupler is used, 3 db of coupling is not sought. The above illustration is merely given to demonstrate the manner in which coupling is achieved.

A significant feature of devices provided in accord ance with the present invention is that the transverse displacement of the central coupling element 33 from the fixed center conductor 26 determines the amount of coupling with a high degree of linearity. Thus, if the central coupling element 33 is moved further away from a selected spatial relationship to the fixed center conductor 26, the percentage of coupling is proportionately reduced. This direct relation between displacement and percentage of coupling which is a logarithmic factor is extremely useful, because percentage of coupling is the parameter which is controlled in system use.

FIG. 4 shows alternative transverse displacement positions of the central coupling element 33 of the movable coupling section 25 relative to the fixed center conductor 26. Dotted lines between the center conductors 26, 33 and the top and bottom plates 14, 15 represent electric field distributions in the slab transmission lines. With the present arrangement, there is a linear relationship between the percentage of coupling and the displacement of the center conductors 26, 33. -As the movable coupling element 33 is moved transversely away from the fixed center conductor 26, the extent of coupling onto the movable coupling element 33 diminishes. This diminution is at a rate ofapproximately 1 db of coupling for 0.010 inch of displacement, in a typical practical For this same practical example, an accuracy of :1 db is maintained between 5 and 70 db over a two to one frequency range. Thus the percentageof coupling, though a logarithmic variable, is achieved very simply by a linearly varying drive and indicated with commensurate ease by-equally spaced indicia about the control knob =38 or other control element.

7 Another important feature of these devices is their bidirectional operation. If input signals. are applied at the second terminal conductor 21, for example, there is direct transmission to the first terminal conductor 2%, and coupled transmission to'the fourth terminal conductor 23. The operation 'is thereforeprecisely as previously described except for the changes in the terminals at which the output signals appear. Through use of this capability the versatility of coupler systems is greatly enhanced. It will also be noted that any or all of thet'erminal conductors Zh-ZS may be shifted in the plane of the ground plane conductors by appropriate bends in the center conductors. Thus, two terminal conductors may extend normal to the others to accommodate particular system requirements. A

In accordance with conventional fixed and variable coupler operations, inwhich the bidirectional properties are employed, waves may be propagated from any adjacent pair of terminal conductors 2t 22 or 21, '23 in a selected phase relation'so as to 'be combined at a selected I terminal conductor andcancelled at the other terminal conductor.

A surprising result which is achieved by arrangements I 5 transverse displacement between the central coupling element 33 and the fixed center conductor 26.

The resistive elements 50, 51 or other energy absorbers in the spaces between the fixed center conductor 26 and the articulated arms 35-38 also contribute materially to the linearity of the device. These cards 50, '51 suppress, by dissipation, higher order modes which might be set up along the slab transmission lines.

Devices in accordance with the invention may be used as attenuators without internal modification. The difference between the variable coupler and the variable attenuator results only from the external connections to the device, because with both applications selected proportions of energy are provided to a utilization device. In the attenuator, energy which is not coupled off is dissipated in a matched termination, whereas in the coupler, as shown in FIG. 1, this energy is the output signal. With an attenuator device, therefore, input signals might be applied to the first terminal conductor ,20 while coupled wave output signals are derived at the third terminal conductor 22 and the second terminal conductor 21 is connected to a matched termination (not shown). The output signal may be. referenced to either the input signal or the percentage of attenuation.. In the coupler, the corresponding coupled wave will usually be referenced to the input signal;

The variable coupler which also has a variable frequency band is shown in FIGS. 5 and 6. There is a general correspondence between the elements of FIG. 5

and those of FIGS. 1-3, in that an articulated slab line construction is used to couple between four terminal conductors 20-23. As in the view'of FIG. 2, the plan view of FIG. 5 is shown with the top ground plane conductor removed. Various sources, utilization devices and matched loadshave been indicated but not shown in detail. It is assumed that input signals are provided to a third terminal conductor 22 which is coupled by a slab transmission line'section having fixed ends 30, 31, and an intermediate transversely movable coupling section 28 to a fourth terminal conductor 23. The fourth terminal conductor 23 is connected to a system load. This por tion of the arrangement corresponds essentially to the like numbered elements in'the arrangement of FIG. 1.

A control knob 48 (FIG. 6) turns a pinion 47 which controls the position of the transversely movable coupling section 28.

A second movable coupling section 54 couples fixed center conductors 56, 57 which are connected directly to the first and second terminal conductors 20, 21. The second movable coupling section 54 includes an L-shaped central coupling element 58 having one leg which is-main tained parallel to the principal length of the transversely movable couplingelement 33. A dielectric support member 60 including a toothed rack surface includes extending arms having tongues 61 (FIG. 6) configured to register in grooves in the bottom ground plane conductor 1.5, or ,in a support member. The grooves confine the'L- shaped coupling element 58 to longitudinal movement. That is, the leg of the -L-shaped coupling element 58 which directly opposes the principal length of the transversely movable coupling element 28, and which is parallel thereto, moves along only its longitudinal axis. The arm and support mechanism which is shown is merely one expedient which may be used.

Pairs of articulated'arms 64, 65 and 66, 57 having the cross-sectional configuration of the slab line center conductor couple each of the fixed center conductor segments 5d, 57, respectively, to a different end of the L- shaped coupling element 58. The longitudinal position of the L-shaped coupling element 58 is controlled by a pinion 68 which registers with the teeth provided in the dielectric support member 60, The pinion 68 is directly connected to an external control knob 69 by which frequency range is selected. Resistive elements '74-, or other microwave absorbing elements positioned between the top and bottom plates 14, 15 and each extending into the region between the opposed articulated arm sections dissipate higher ordcr'modes propagated within the coupler section. I

The arrangementshown in FIGS- and 6 is adjustable so as to provide variable coupling or attenuation over an extremely wid'e range" of" frequencies. When used as a variable coupler, with input signals coupled into third terminal conductor 22, output signals appear at the directly connect'ed fourth terminal conductor 23 and, in a selected proportion, at the wave coupled to the first terminal conductor 20. A matched resistive load or other termination coupled to the remaining second terminal conductor 21, together with the presence of resistive elements 74, 75 permits operation with high efficiency. As before, the percentage of coupling is controlled by the transverse displacement between the U-shaped coupling element 33 and the parallel leg of the L-shaped coupling element 58. For different frequency ranges, however, the relative longitudinal position of these two coupling elements' 33, SSmay be adjusted so that the parallel lengths of the coupling elements 33, 58 are coextensive over a quarter wavelength of the waves at the central operating frequency. If it is desired to shift the variable coupler from the 2000-4000 rnc. band to the 4000-8000 mc band, for example, the coextensive length of the two coupling elements 33, 58 need only be adjusted to be a quarter wavelength at 6000 me. instead of at 3000 me.

The percentage of coupling between the two coupling sections is affected by the relative longitudinal positions of the coupling elements 33, 58, only where frequency sensitivity becomes an important factor because of improper setting of the frequency control knob 69. With the frequency control knob 69 initially set to a selected frequency, the length of exposed coupling loop is a quarter wavelength at the operating frequency. Given this proper prior adjustment, a given transverse displacement, as determined by the control knob 48, provides a substantially constant percentage of coupling for all frequencies of interest. Both fabrication andtad justment-are maten ally simplifiedthereby. u

l. A'variablemicrowave coupler including first and second spaced apart ground plane conductors, first, seci ond, third and fourth coaxial terminal conductors positioned about the spaced apart ground plane conductors, a first fixed center conductor positioned between the ground plane conductors and connected to each of the first and second terminal conductors to provide a first slab'transrnission line electrically coupling the first and second terminal conductors, a second center conductor positioned between the ground plane conductors and providing a selected coupling length which is substantially one-quarter wavelength at the operating wavelength of waves propagated along the first slab transmission line, first and second articulated center conductor means, the first articulated center conductor means being coupled from the third terminal conductor to the adjacent end of the second center conductor and the second articulated center conductor means being coupled from the fourth terminal conductor to the adjacent end of the second center conductor, the first artic ulated center conductor means, the second center conductor and the second articulated center conductor means providing a second slab transmission line between the third and fourth terminal conductors, means coupled to the second center couductor for moving the second center conductor transversely relative to the first While maintaining the coupling length parallel to the length of the first center conductor, thus to provide variable coupling of waves between the first and second wave transmission lines, microwave energy absorbing means disposed between the first and second slab transmission lines between the first and second ground plane conductors for the suppression of higher order modes excited by waves propagated along the slab transmission lines.

2. The invention as set forth in claim 1 above, wherein the means for moving the second center conductor includes dielectric support means coupled to the second center conductor, a rack and pinion drive coupled to the dielectric support means, and a control member. containing indicia which denote the relative percentage of coupling between the first and second slab transmission lines as determined by the relative transverse displacement of the first and second center conductors.

3. A variable, adjustable frequency, coupler for microwave applications including a pair of spaced apart ground plane conductors, a first center conductor coupling element positioned between the ground plane conductors and forming a portion of a first wave transmission line therewith, the first coupling element extending along a selected axis, means coupled to the first coupling element for selectively moving the first coupling clement parallel to the selected axis, a second center conductor coupling element positioned between the ground plane conductors parallel to said first coupling element and providing a portion of a second wave transmission line therewith, said second coupling element being juxtaposed with respect to said first coupling element for a predetermined distance the magnitude of which is determined by the longitudinal position of the movable said first coupling element, means coupled to the second coupling element for moving the second coupling element relative to the first coupling element in a direction normal to the selected axis, means coupled to the first coupling element and completing a wave transmission line between first and second terminals of said coupler, and means coupled to the second coupling element and forming a wave transmission line between third and fourth terminals of said coupler.

References lined in the file of this patent UNITED STATES PATENTS 2,531,777 Marshall Nov. 28, 1950 2,615,982 Zaslavsizy Oct. 28, 1952 2,684,469 Sensiper July 20, 1954 2,794,959 Fox June 4, 1957 FOREIGN PATENTS 749,337 Great Britain May 23, 1956

Claims (1)

1. A VARIABLE MICROWAVE COUPLER INCLUDING FIRST AND SECOND SPACED APART GROUND PLANE CONDUCTORS, FIRST, SECOND, THIRD AND FOURTH COAXIAL TERMINAL CONDUCTORS POSITIONED ABOUT THE SPACED APART GROUND PLANE CONDUCTORS, A FIRST FIXED CENTER CONDUCTOR POSITIONED BETWEEN THE GROUND PLANE CONDUCTORS AND CONNECTED TO EACH OF THE FIRST AND SECOND TERMINAL CONDUCTORS TO PROVIDE A FIRST SLAB TRANSMISSION LINE ELECTRICALLY COUPLING THE FIRST AND SECOND TERMINAL CONDUCTORS A SECOND CENTER CONDUCTOR POSITIONED BETWEEN THE GROUND PLANE CONDUCTORS AND PROVIDING A SELECTED COUPLING LENGTH WHICH IS SUBSTANTIALLY ONE-QUARTER WAVELENGTH AT THE OPERATING WAVELENGTH OF WAVES PROPAGATED ALONG THE FIRST SLAB TRANSMISSION LINE, FIRST AND ARTICULATED CENTER CONDUCTOR MEANS, THE FIRST ARTICULATED CENTER CONDUCTOR MEANS BEING COUPLED FROM THE THIRD TERMINAL CONDUCTOR TO THE ADJACENT END OF THE SECOND CENTER CONDUCTOR AND THE SECOND ARTICULATED CENTER CONDUCTOR MEANS BEING COUPLED FROM THE FOURTH TERMINAL CONDUCTOR TO THE ADJACENT END OF THE SECOND CENTER CONDUCTOR, THE FIRST ARTICULATED CENTER CONDUCTOR MEANS, THE SECOND CENTER CONDUCTOR AND THE SECOND ARTICULATED CENTER CONDUCTOR MEANS PROVIDING A SECOND SLAB TRANSMISSION LINE BETWEEN THE THIRD AND FOURTH TERMINAL CONDUCTORS, MEANS COUPLED TO THE SECOND CENTER CONDUCTOR FOR MOVING THE SECOND CENTER CONDUCTOR TRANSVERSELY RELATIVE TO THE FIRST WHILE MAINTAINING THE COUPLING LENGTH PARALLEL TO THE LENGTH OF THE FIRST CENTER CONDUCTOR, THUS TO PROVIDE VARAIBLE COUPLING OF WAVES BETWEEN THE FIRST AND SECOND WAVE TRANSMISSION LINES, MICROWAVE ENERGY ABSORBING MEANS DISPOSED BETWEEN THE FIRST AND SECOND SLAB TRANSMISSION LINES BETWEEN THE FIRST AND SECOND GROUND PLANE CONDUCTORS FOR THE SUPPRESSION OF HIGHER ORDER MODES EXCITED BY WAVES PROPAGATED ALONG THE SLAB TRANSMISSION LINES.

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