US3678420A

US3678420A - Spurious mode suppressing waveguide

Info

Publication number: US3678420A
Application number: US84436A
Authority: US
Inventors: Dietrich Anselm Alsberg
Original assignee: Bell Telephone Laboratories Inc
Current assignee: AT&T Corp
Priority date: 1970-10-27
Filing date: 1970-10-27
Publication date: 1972-07-18
Anticipated expiration: 1989-07-18

Abstract

Conversion of the TE01 energy mode of electromagnetic wave transmission into spurious or unwanted modes can be reduced by utilizing a dipole array which is interposed in a dielectric lining layer located between the inner wall surface of a conductive waveguide tube and a coupling screen concentric with the tube. A further reduction in mode conversion can be achieved by applying a second dielectric lining containing a dipole pattern around the inside of the coupling screen.

Description

United States Patent Alsberg [151 3,678,420 1 July 18,1972

[54] SPURIOUS MODE SUPPRESSING WAVEGUIDE [72] Inventor: Dietrich Anselm Alsberg, Berkeley Heights, NJ.

[73] Assignee: Bell Telephone Laboratories, Incorporated,

Murray Hill, NJ.

[22] Filed: Oct. 27, 1970 [21] Appl. No.: 84,436

[52] US. Cl. ..333/95 R, 333/98 M [51] Int. Cl. Hlp l/l6,l-l0lp 3/12 [58] Field oiSear-ch ..333/98 M, 95, 98 R [56] References Cited UNITED STATES PATENTS 2,751,561 6/1956 King ..333/98 R 2,769,148 /1956 Clogston .333/ X 2,779,006 1/1957 Albersheim 333/98 M 2,848,695 8/1958 Pierce ..333/98 M 2,848,696 8/1958 Miller.... ..333/98 M 3,016,502 1/1962 Unger.... ..333/98 M 3,016,503 l/ 1962 Pierce ...333/95 R 3,018,452 l/l962 Friis et al. ..333/95 R 3,066,268 11/1962 Karbowiak ..333/95 R 3,110,001 11/1963 Unger .....333/95 R 3,275,955 9/1966 Prache ..333/95 R OTHER PUBLICATIONS Barlow, H. E. M., A Method of Changing the Dominant Mode in a Hollow Metal Waveguide & Its Application to Bends," IEE Vol. 1063 supp. 13, 1959, pp. 100-105 Karbowiak A. E., Microwave Aspects of Waveguides for Long Distance Transmission," lEE Vol. C 1958, pp. 360- 369 Primary Examiner-Eli Lieberman Assistant Examiner-Wm. H. Punter Attorney-R. J. Guenther and Edwin B. Cave [57] ABSTRACT 9 Clains, 2 Drawing Figures PAIENTEDJuuMsn 3678.420

//v l/ENTOR 0.14. ALSBERG SPURIOUS MODE SUPPRESSING WAVEGUIDE BACKGROUND OF THE INVENTION 1. Field of the Invention suppresses This invention relates to waveguide transmission systems and, more particularly, to a waveguide for such a system which suppresses generation of spurious modes and which provides a wide range of waveguide wall impedances.

2. Description of the Prior Art In the transmission of electromagnetic wave energy in a hollow pipe or other type waveguide, it is well known that energy can be supported in a number of different transmission modes depending upon the cross sectional size and shape of the guide itself and upon the operating range of the selected frequencies. Generally, it is desirable to confine the energy propagation to one particular mode which is chosen because its propagation characteristics are particularly suited to the specific application involved. However, other considerations may require the use of waveguide sizes which support not only the desired mode. but also one or more unwanted or spurious modes. This is particularly true of systems utilizing TE circular electric wave modes. Propagation in the TE mode is suited for long distance transmission of high frequency widebandsignals because of its decreasing attenuation with increasing frequency. A

In an ideal system utilizing waveguides which are perfectly straight, uniform, and conducting, propagation of the TE waves therethrough would be-undisturbed and the fact that other modes may exist in the waveguide is not a problem. However, terminal operations, curvature of the waveguide, and inherent imperfections in the waveguide tend to disturb the TE mode and cause conversion and reconversion of power between that mode and the unwanted modes. These unwanted modes are capable of being propagated through the waveguide and therefore have a deleterious effect upon transmission of information through the system. Consequently, every effort must be made to minimize conversion and reconversion between the TE mode and the spurious or unwanted modes.

Conversion between the TE mode and spurious modes is inversely proportional to the differences between the attenuation and phase constants of the TE mode and the undesired modes. Thus, one approach to solving the degradation of the transmitted signal by mode conversions is to make the differences in attenuation and phase constants between the TE mode and the spurious modes as large as possible so that mode conversions are reduced.

Various ways of increasing the differences between the phase constants of the TE mode and the unwanted mode have been proposed. One such method is the use of a dielectric lining, either isotropic or anisotropic, in the waveguide to introduce differences in the phase constants. Other methods include the use of structure such as corrugated waveguides, helical windings and spaced conductive ring structures. The differences between attenuation constants have been increased by utilizing electrically lossy jackets in the waveguide. In all of these prior art methods, he loading of the spurious modes to achieve differences in attenuation and phase constants with respect to the TE mode is optimal only for a specified frequency or limited band of frequencies within the transmitted signal. Further, in the prior art waveguide structures, the loading of the spurious modes is substantially uniform along the length of the waveguide. Thus, even though the conversion of the TE mode into spurious modes is small in magnitude, the spurious modes tend to combine and buildup periodically along the waveguide because of uniform loadmg.

Accordingly, it is an object of this invention to suppress the tendency of TE waves to convert to unwanted or spurious modes in a circular electric wave mode system.

Another object is to provide a means of applying maximum loading to spurious modesin a circular electric wave mode system over a wide frequency range so that the generation of such modes is inhibited.

Another object is to provide a means for optimizing the phase differences between the TE wave mode and spurious modes at a specified frequency or band of frequencies in a circular electric wave mode system.

SUMMARY OF THE INVENTION The foregoing objects and others are achieved in accordance with the principles of this invention through the use of a dipole array interposed in or mounted on a dielectric lining between the interior of the a conductive waveguide tube and a coupling screen. In a typical embodiment a coupling screen is formed by winding a dielectric coated wire in a helical fashion on a mandrel. On the outside of this wire helix a dielectric layer is applied on which is superimposed an array of dipoles which are typically oriented in the longitudinal direction of the guide. A second layer of dielectric is applied over these dipoles for protection and to complete the dielectric lining and the whole assembly is then encased in a conductive tubing. The dipoles between the layers of the dielectric lining form impedors which are coupled through the screen to spurious modes generated in the waveguide, thereby changing the phase constants of these spurious modes with respect to the TE mode. The differences in the attenuation constants between the TE mode and spurious modes also are increased if electrically lossy dipoles are used. The size, shape and orientation of the dipoles can be designed to achieve a wide range of impedances, so that the loading of the spurious modes can be optimized at specified frequencies or bands of frequencies.

The phase separation between the spurious modes and the desired T15 mode can be further improved by applying a dielectric lining to the inside of the helix screen. Still further improvement can be obtained by applying to the inner dielectric lining a dipole pattern in which the dipoles are oriented in a longitudinal direction to minimize coupling to the electrical field component of the TE mode and to maximize coupling to the longitudinal electrical field components of the spurious modes. These dipoles can either be of a length which is substantially less than that of a quarter wavelength to produce what is essentially an anisotropic artificial dielectric, or they can be designed as artificial dielectric, or they can be designed as tuned dipoles to enhance the effect of the dipoles at specific selected frequencies.

DESCRIPTION OF THE DRAWING The invention will be more fully comprehended from the following detailed description and accompanying drawing in which:

FIG. 1 is a perspective view, partly broken away, of a waveguide according to this invention; and

FIG. 2 is a perspective view of the waveguide of FIG. I further including a dielectric lining and array of dipoles around the inside of the coupling screen.

DETAILED DESCRIPTION FIG. 1 is a partly broken away, detailed view of a waveguide I01 in accordance with this invention which may be used in a long distance waveguide transmission system. Waveguide 101 comprises a coupling screen 2 which, in the illustrative embodiment, is formed by helix wire 4 having turns 6 substantially transverse to the longitudinal axis of waveguide 101. Adjacent turns 6 must be insulated from each other by spacing or by utilizing a dielectric coated wire or similar method. Helix wire 4 may comprise either a solid conductor or stranded conductor as desired. Coupling screen 2 also can be formed by a series of spaced concentric rings, or by a metallic sheet in which coupling orifices have been provided by mechanical or photochemical means, or by other methods which will be ap parent to those skilled in the art.

Surrounding helix wire 4 and overlaying the outer surface thereof is a dielectric layer 7 comprising a suitable dielectric material known in the art. An array 8 of dipoles 10 is applied to the outer surface of dielectric layer 7. Array 8 is overlayed and protected by a second dielectric layer 12 to complete a dielectric lining and the entire assembly is encased in a conductive tube 14.

Dielectric layer 7 is relatively thin so that dipoles can be placed in close proximity to the gaps between adjacent turns 6 of wire 4. Wire 4 behaves similar to a multiple-slot directional coupler such that dipoles 10 are coupled to spurious wave modes generated within waveguide 101. Dipoles 10 act as impedors to these spurious modes to which they are coupled. Thus the phase constants of these spurious modes are changed with respect to the desired TE mode and conversion of the TE mode is greatly reduced. The difference in attenuation constants of the TE mode and spurious modes also is greatly increased when dipoles 10 are electrically lossy, thereby further reducing mode conversion.

The particular spurious modes to which dipoles 10 are coupled depend upon such factors as the orientation and location of these dipoles with respect to wire 4. Typically, dipoles 10 will be oriented in the longitudinal direction of waveguide 101 and thus substantially transverse to turns 6 of wire 4. As previously mentioned, dipoles 10 are placed in close proximity to the gaps between adjacent turns 6. With such orientation and location, dipoles 10 are strongly coupled to any spurious modes having electric field or current components in the longitudinal direction. Since the desired TE mode has no such components, it is not affected by dipoles 10.

The configuration, size, spacing, and characteristics such as resistivity of dipoles 10, can be designed to achieve a wide range of impedances. Thus the loading of the spurious modes to which dipoles 10 are coupled can be optimized at specified frequencies or bands of frequencies. For example, the characteristics of dipoles 10 can be designed so that dipoles 10 are resonant at specified frequencies or bands of frequencies, thereby greatly enhancing the capability of dipoles 10 to change the attenuation and phase constants at these frequencies. The design of the coupling screen 2 can also be changed to adjust the degree of coupling between the undesired modes and the dipoles 10. Such flexibility in loading is not provided in any prior art waveguide. Further, other changes, such as the randomization of loading along the waveguide 101 to prevent periodic build-up of spurious modes, or the variation of loading along the waveguide 101 to obtain desired variations in impedances can be readily achieved in the waveguide of the invention. The dimensions and material of dielectric layers 7 and 12 can also be varied to complement variations in dipole 10 of array 8.

Dipole array 8 can be formed by various methods. For example, dipoles 10 can be formed on dielectric layer 7 by printed circuit techniques. Array 8 can also be formed by suspending appropriate conductive fibers in a dielectric liquid and orienting these fibers by an electrostatic field while the dielectric solidifies.

The operation of waveguide 101 can be further improved by the use of a dielectric lining alone or in conjunction with an array of dipoles around the inside of screen 2 as shown in FIG. 2. For example, the differences in phase constants of the undesired wave modes and the desired TE mode can be increased, thereby decreasing mode conversion, by utilizing a dielectric lining comprising a single dielectric layer 16 around the inside of screen 2. Dielectric layer 16 will significantly affect wave modes which have electric field components near the inner surface of screen 2 and thus will change the phase constants of such modes. Dielectric layer 16 has substantially no effect on the desired TE since the electric field for this mode almost disappears near the inner surface of screen 2.

A greater change in phase constants of the undesired modes can be obtained by using an array of dipoles in conjunction with the dielectric lining. For example, the effectiveness of dielectric layer 16 can be greatly improved by utilizing therewith a dipole array 18 comprising a plurality of dipoles 20. Dipoles 20 may be applied to the inner wall surface of dielectric layer 16 as shown in FIG. 2 in which case a protective layer 22 of dielectric material can advantageously be placed over dipoles 20. Alternatively, dipoles 20 can be dispersed within dielectric layer 16. Dipoles 20 can be formed by techniques such as those previously mentioned as suitable for the formation of dipoles 10.

The spurious modes to which dipoles 20 couple depends upon such factors as the orientation of the dipoles. Typically, dipoles 20 will be oriented along the longitudinal direction of waveguide 101 and thus will be strongly coupled to spurious modes having longitudinal electric field components while simultaneously affecting the TE mode only slightly. Dipoles 20 effectively increase the conductivity of dielectric layer 16 in their direction of orientation and also attenuate currents in this direction because of their electrically lossy nature. Thus the dipoles and dielectric lining acting together substantially increase the difference in propagation constants of the T5 mode with respect to spurious modes by increasing the difference in both the attenuation and phase constants.

When dipoles 20 have a length substantially less than one quarter wavelength of the transmitted frequencies, these dipoles present what is essentially an anisotropic artificial dielectric to such frequencies which functions as described previously. By proper choice of size, configuration, etc., dipoles 20 may also be designed as tuned dipoles to resonate at selected frequencies or bands of frequencies so that their effects on spurious modes at these selected frequencies or bands of frequencies will be greatly enhanced.

From the foregoing it is apparent that the use of dipole arrays in accordance with this invention provides flexibility for tailoring the waveguide to specific needs which was not possible in the prior art waveguides. The loading of the waveguide along its length can be tailored to obtain desired impedances by varying the characteristics of the dipoles. This loading can be optimized at specific frequencies or bands of frequencies, thereby in essence tuning the waveguide to these frequencies, by designing the dipoles to be resonant at such frequencies. Thus, the waveguide can be readily designed to eliminate such problems as periodic build-up of spurious modes along the waveguide.

While the invention has been described with respect to a specific embodiment thereof, it is to be understood that various modifications thereto might be made by those skilled in the art without departing from the spirit and scope of the invention.

W-hat is claimed is: What 1. A waveguide for transmitting electromagnetic wave energy in the circular electric wave mode comprising, in combination:

a hollow tube of conductive material;

a first lining of dielectric material mounted around the inner wall surface of said tube;

a coupling screen comprising a helix wire winding mounted about the inner surface of said first lining;

a first array of dipoles disposed in said first lining in close proximity to said screen for coupling through said screen to unwanted wave modes generated within said waveguide and presenting substantial impedance thereto such that said unwanted modes can be reduced, said first array of dipoles having characteristics which may be preselected such that said impedance can be optimized at selected frequencies;

a second lining of dielectric material around the inner surface of said screen for changing the phase constants of said unwanted modes with respect to said circular electric wave mode; and

a second array of dipoles disposed within said second lining for further changing said phase constants, said second array of dipoles having characteristics which may be preselected such that said changing of said phase constants can be optimized at selected frequencies.

2. A waveguide for transmitting electromagnetic wave energy in the Te circular electric wave mode comprising, in combination:

a hollow tube of conductive material;

a coupling screen mounted about the inner surface of said first lining; and

an array of dipoles comprising a plurality of segments of conductive material disposed in said first lining in close proximity to said screen for coupling through said screen to unwanted wave modes generated within said waveguide, said segments presenting substantial impedance to said unwanted modes, said impedance being a function of the characteristics of said segments, said characteristics being preselected to maximize said impedance at preselected frequencies.

3. Apparatus in accordance with claim 2 including a second lining of dielectric material around the inner surface of said screen for changing the phase constants of said unwanted modes with respect to said TE mode.

4. Apparatus in accordance with claim 2 wherein said characteristics are preselected to cause said dipoles to be resonant at said preselected frequencies so that said dipoles have increased capabilities for attenuating said unwanted modes at the inner to unwanted wave modes generated within said waveguide;

a second lining of dielectric material around the inner surface of said screen; and

a second array of dipoles disposed within said second lining,

whereby said unwanted modes are reduced by increasing the attenuation of said unwanted modes and changing the phase constants of said unwanted modes with respect to said TE mode.

6. Apparatus in accordance with claim 5 wherein said dipoles of said second array are oriented in the direction of the longitudinal axis of said waveguide such that the phase constants of wave modes having electric field components in the direction of said longitudinal axis are changed.

7. Apparatus in accordance with claim 5 wherein said dipoles of said second array have a length substantially less than one quarter of the wavelength of the frequency of said energy being transmitted such that said second array of dipoles acts as an anisotropic dielectric with respect to said frequency.

8. Apparatus in accordance with claim 5 wherein portions of said second array of dipoles are resonant at selected frequencies contained within said energy being transmitted such that said second dipole array has increased capability for changing said phase constants at said selected frequencies.

9. Apparatus in accordance with claim 5 wherein said second lining comprises first and second layers, said second array of dipoles being disposed between said first and second layers.

Claims

1. A waveguide for transmitting electromagnetic wave energy in the circular electric wave mode comprising, in combination: a hollow tube of conductive material; a first lining of dielectric material mounted around the inner wall surface of said tube; a coupling screen comprising a helix wire winding mounted about the inner surface of said first lining; a first array of dipoles disposed in said first lining in close proximity to said screen for coupling through said screen to unwanted wave modes generated within said waveguide and presenting substantial impedance thereto such that said unwanted modes can be reduced, said first array of dipoles having characteristics which may be preselected such that said impedance can be optimized at selected frequencies; a second lining of dielectric material around the inner surface of said screen for changing the phase constants of said unwanted modes with respect to said circular electric wave mode; and a second array of dipoles disposed within said second lining for further changing said phase constants, said second array of dipoles having characteristics which may be preselected such that said changing of said phase constants can be optimized at selected frequencies.

2. A waveguide for transmitting electromagnetic wave energy in the Te01 circular electric wave mode comprising, in combination: a hollow tube of conductive material; a first lining of dielectric material mounted around the inner wall surface of said tube; a coupling screen mounted about the inner surface of said first lining; and an array of dipoles comprising a plurality of segments of conductive material disposed in said first lining in close proximity to said screen for coupling through said screen to unwanted wave modes generated within said waveguide, said segments presenting substantial impedance to said unwanted modes, said impedance being a function of the characteristics of said segments, said characteristics being preselected to maximize said impedance at preselected frequencies.

3. Apparatus in accordance with claim 2 including a second lining of dielectric material around the inner surface of said screen for changing the phase constants of said unwanted modes with respect to said TE01 mode.

4. Apparatus in accordance with claim 2 wherein said characteristics are preselected to cause said dipoles to be resonant at said preselected frequencies so that said dipoles have increased capabilities for attenuating said unwanted modes at said preselected frequencies.

5. A waveguide for transmitting electromagnetic wave energy in the TE01 circular electric wave mode comprising, in combination: a hollow tube of conductive material; a first lining of dielectric material mounted around the inner wall surface of said tube; a coupling screen mounted about the inner surface of said first lining; a first array of dipoles disposed in said first lining in close proximity to said screen for coupling through said screen to unwanted wave modes generated within said waveguide; a second lining of dielectric material around the inner surface of said screen; and a second array of dipoles disposed within said second lining, whereby said unwanted modes are reduced by increasing the attenuation of said unwanted modes and changing the phase constants of said unwanted modes with respect to said TE01 mode.