US3386054A - Method and apparatus for tuning waveguides - Google Patents

Description

May 28, 1968 R. W. SPIKULA METHOD AND APPARATS FOR TUNING WAVEGUIDES 'Filed oct. u, 1965 INVENTOR ATTORNEY United States Patent O 3,386,654 METHOD AND APPARATUS FOR TUNING WAVEGUIDES Russell W. Spiltula, Winston-Salem, N.C., assignor to Western Electric Company, Incorporated, New York,

NY., a corporation of New York Filed Oct. 11, 1965, Ser. No. 494,314 14 Claims. (Cl. S33-21) ABSTRACT OF THE DISCLOSURE One, or a plurality of paramagnetic slugs are inserted and magnetically positioned inside a waveguide to tune the waveguide by attenuating undesired reections and reducing the standing wave ratio. The magnetic positioning of the slug inside the waveguide represents the optimum locations for discrete compensating depressions without repetitive drilling and tapping of the waveguide. Alternatively, a pair of crescent-shaped paramagnetic slugs are placed and magnetically positioned in a circular waveguide to alter the polarization of a polarized wave being propagated in the waveguide.

This invention relates to a method and apparatus for tuning waveguides and more particularly to a method and apparatus for tuning rectangular and circular waveguides using discrete compensating elements positioned within the waveguides by external magnetic means.

Microwave energy is transmitted through a waveguide in the form of electromagnetic waves. The boundary conditions imposed by the smooth, conducting surfaces of the waveguide act to constrain the electromagnetic waves Within the confines of the waveguide. Any irregularity in the smoothness of the conducting surfaces will cause a portion of the incident electromagnetic wave to be reiiected back towards the source resulting in standing waves along the waveguide and a diminution of the power received at the terminating load. Similarly, any element connected to the waveguide will also cause reflections if it presents an irregularity or discontinuity to the incident wave. Such elements include the load device itself, as well as directional couplers, filters, attenuators, etc., connected to the waveguide. A sharp, or in some cases, a gradual bend in the waveguide will also tend to produce undesirable reflections.

In many cases it is possible to tune out the effects of these irregularities by the use of tuning slugs which are inserted into the waveguide. A tuning slug usually consists of a threaded yscrew projecting into the waveguide, parallel to the electric eld in the waveguide. The energized slug acts as a small antenna and is excited by the electric iield in the waveguide. The energized slug radiates a signal which cancels out part of the reilections caused by the waveguide or component irregularities. Frequently, two `or three slugs are used simultaneously, spaced apart 1A; wavelength or 2%; wavelength respectively. The phase of the reflected signal may be altered by increasing or decreasing the length of the protrusion into the waveguide. The microwave slug tuner is analogous to the halfwave stub used to tune coaxial transmission lines.

ln development Work on prototype waveguide components, the determination of the optimum location for the tuning slugs is accomplished by somewhat of a trial and error technique. An exact mathematical solution is 3,386,054 Patented May 28, 1968 ICC generally impossible due to th complexity of the equations involved. The trial and error method requires the repeated drilling and tapping of holes to receive the slugs, or, alternatively, the use of a slotted waveguide. The use of a slotted waveguide is undesirable, as the slot is centrally located and cannot be moved, and, further, the slot may affect the transmission characteristics of the device. Once the optimum location and depth of the slugs has been established for the prototype device, permanent slugs, dents, or dimples, are fabricated in the production version of the device at the locations previously predetermined by the trial and error technique. It is obvious that the repeated drilling and tapping of the prototype device to locate the best position for fabricating the tuning dents in the finished product is both time consuming and expensive.

Accordingly, it is an object of this invention to provide a new and improved method and apparatus for tuning a waveguide.

Another `object of the invention is to provide a new and improved method and apparatus for tuning a waveguide where no physical connection is made between a tuning element on the inside of the waveguide and a device for positioning the element from the outside..

Another object of the invention is to provide a method and apparatus for magnetically positioning shaped slugs within a waveguide to alter the polarization Iof microwaves traveling down the waveguide.

Yet another object of the invention is vto provide a new and improved method and apparatus for magnetically positioning one or more slugs to locate the optimum position for a discrete compensating depression in a prototype waveguide without repetitive drilling and tapping of the waveguide.

These and other objects are accomplished in the invention by magnetically positioning one, or a plurality, of paramagnetic slugs inserted inside the waveguide to attenuate undesired reflections and reduce the standing wave ratio; or, to change the polarization of microwave energy being impressed through the waveguide. The waveguide is terminated at one end and energized at the other.

In one embodiment of the invention, a suitable indicating device is connected at a convenient point on the waveguide, and by means of a plurality of magnets the paramagnetic slugs are positioned within the waveguide until the indicating device registers the minimum number of reflections in the waveguide. The positions of the external magnets are then marked and the production versions of the waveguide manufactured with tuning dents at the marked locations.

In another embodiment of the invention for use with circular waveguides, a pair of crescent-shaped, paramagnetic slugs are placed within the waveguide and magnetically moved relative to each other and to a reference axis to correct any change in polarization which may have occurred during transmission of the signal down the waveguide.

Other objects and advantages of the present invention will be apparent from the following detailed description when considered in conjunction with the accompanying drawings, wherein:

FIG. 1 is a perspective view of a rectangular waveguide depicting a tuning slug that is selectively positioned by a magnet in accordance with the principles of the invention;

FIG. 2 is a cross-sectional view of an alternative embodiment of the invention wherein a tuning slug is positioned by an electromagnet;

FIG. 3 is a cross-sectional view of a further alternative embodiment of the invention wherein a circular waveguide has internal crescent-shaped tuning slugs that are magnetically positioned to change the polarization of a polarized microwave that is being impressed through the waveguide;

FIG. 4 is a perspective view of the crescent-shaped tuning slugs shown in FIG. 3;

rFIG. 5 is a di-agram illustrating various types of polarization found in rectangular and circular waveguides; and

FIG. 6 is a diagram of a typical test arrangement connected in accordance with the principles of the invention to locate the optimum position for tuning rectangular and circular waveguides using discrete compensating elements.

Referring now to FIG. 1 of the drawings, there is shown a hollow, rectangular waveguide 11 having an outer surface 12 and an inner surface 13. A slug 14 of paramagnetic material is inserted within the waveguide 11 and a position-adjusting device 15 located to move a bar magnet I6 in contact with the outer surface 12 of the waveguide 11 to attract the slug 14 and hold it firmly in contact with the inner surface 13. In the alternative, the device 15 may be dispensed with and the bar magnet may be then manual- 1y manipulated. The magnet 16 may be of any suitable size and strength provided that su'icient tiux passes through the Wall of the waveguide 11 to attract and retain slug 14 against the inner usrface 13. Typically, the wall of the waveguide 11 may `be 1/16 of an inch thick, and a permanent magnet manufactured from an aluminum, nickel, cobalt alloy with a residual induction of 10,000 gausses may be used. Obviously, the use of a thicker wall in waveguide 11 will require a corresponding increase in the ystrength of magnet 16. The dimensions of slug 14 will also vary with the size of the waveguide, but are not critical. The thickness of the slug y14 will, however, atect the phase of the signal radiated by the slug.

Among the purposes of the invention is to locate the optimum position for fabricating tuning bumps in the production version of the waveguide. Once an approx- 'imate position has lbeen located by means of the slug and the positon marked, a yconventional screw-in tuning slug may be inserted into the waveguide at the marked location for precision tuning. This will establish the depth of the desired tuning bumps. However, if the apparatus of this invention is used to permanently tune a production waveguide, and not a prototype, then it will ybe necessary to have several slugs of different thicknesses and substitute them one at a time until the best tuning is obtained. Typically, the slugs may be 1/2 to 3%; inch long and 3A@ to 1A inch thick. Moving the magnet 16 along or across the outer surface 12 of waveguide l11 will cause the slug 14 to move correspondingly along or across the inner surface `13 to tune the waveguide.

T he inner walls of a waveguide are generally extremely smooth and the coeiiicient of friction `between the slug 14 and the inner surface '13 is small. However, in some applioations it is necessary to coat Ithe slug with a thin layer of friction-reducing plastic such as polytetrauoroethylene to reduce the coeicient of friction and permit the slug 1'4 to be positioned within the waveguide 11 with greater ease. Without this friction-reducing coating, there would be a tendency for the slug 14 to jump or walk along the inner surface 13 with the possible result that the exact optimum location might be missed. A similar coating might Ibe lapplied to the contacting surface `of magnet 16 if required.

It should be emphasized that, provided slug 14 is made from electroconductive, paramagnetic material, neither the characteristics of slug 14, nor the insulating characteristics of any friction-reducing coating used, will have any affect on the radiation characteristics of the slug 14 and hence its tuning behavior.

FIG. 2 shows an alternative embodiment wherein an electromagnet 17 is used in place of the permanent magnet 16 of FIG. 1. The electromagnet 17 consists of a soft iron core '18- about Iwhich is wound a coil 19. The coil 19 is connected :by leads 2'1-21 to a suitable source of D.C. power 22. A switch 23 is connected in series with one of the leads 21 and when closed, permits current to iiow from power supply 22 to the coil 19, energizing the electromagnet 17 which attracts the slug 4, as previously described.

The choice of permanent magnet 15 or electromagnet 17 in no way iniuences the tuning effect of slug 14. For example, in applications where high iiux density is required, it may be more economical to use the electromagnet 17 rather than the bar magnet 16, which may be quite bulky.

There is illustrated in FIGS. 3 and 4 a further alternative embodiment wherein a pair of crescent-shaped slugs 214-24 are inserted into a hollow, circular waveguide 26 and retained in position by a pair of horseshoe-shaped, permanent magnets 29-29- Electromagnets may be substituted for the permanent magnets 29-29 if desired. The outer arcuate surfaces 27-27 of slugs 24j-24 should preferably have the same radius of curvature as the inner surface '31 of circular waveguide 26 and may be coated with a thin layer of friction-reducing plastic if needed.

Referring to FIG. 5, the various types of polarization found in microwaves and other radio frequency transmissions, are illustrated. FIG. 5a shows linear polarization in which the component of the electric field intensity in the direction of the reference Y axis, Ey, is in time phase with the component of the electric eld intensity in the direction of the Z axis, EZ. This is the most common type of polarization, and is the type of interest with respect to rectangular waveguides. The term, vertical and horizontal polarization, is sometimes used to describe a particular type of linear polarization, and means simply that the resultant electrical field vector El. obtained by adding the vectors Ez and Ey lies in the horizontal or vertical direction.

In FIG. 5a it will `be seen that the resultant vector Er is at a 45-degree angle, and the wave is neither horizontally nor vertically polarized. With respect to rectangular waveguides, regardless of the actual orientation of the waveguide, it is conventional to define vertical polarization as occurring when the electrical eld vector is parallel to the shorter dimension of the waveguide.

Due to the nonsymmetrical structure of the rectangular waveguide and the boundary conditions imposed by the conducting walls of the waveguide, the polarization angle of a transmitted wave cannot change substantially. When circular waveguides are used, however, due to the prefect symmetry of the waveguide it is quite possible that substantial changes in the polarization may occur.

FIG. 5c illustrates a circularly polarized wave where EZ and Ey are equal in magnitude, but degrees out of phase. If in transmission through the circular waveguide, the magnitude or phase of either the EZ or Ey component is altered, then the circular polarization will be changed to elliptical polarization, the degree of ellipticity depending on the relative magnitude and phase of the EZ and Ey components.

FIG. 5b illustrates elliptical polarization where the magnitude of EZ is greater than the magnitude of Ey. This is, of course, the more general form of non-linear polarization, the perfect symmetry required to produce circular polarization seldom occurring in practice.

Referring again to FIG. 3, increasing or decreasing the radius of curvature of the inner arcuate surfaces 23 28 of slugs 24-2.r will affect the magnitude and phase of either the E7 or Ey component of the wave; thus, by proper positioning of slugs 2li- 24- within the circular waveguide 26, a circularly polarized wave-front traveling down the waveguide can be changed to an elliptically polarized wave-front. In addition, the slugs 24--24 may be used to restore the circular polarization of a wave-front which has been distorted by passage through equipment connected to the waveguide.

FIG. 6 shows a typical test set-up for locating the optimum positions for fabricating dents or dimples in the production versions of the waveguides. Prototype waveguide 11, having a plurality of slugs 14-14 inserted therein, is terminated at one end by a dummy load 32.

A plurality of permanent magnets 16--16 retain the` slugs 14-14 against the upper wall of waveguide 11. One end of a directional coupler 33, having a probe 34, is connected to the other end of `waveguide 11, and sweep frequency microwave generator 36 is connected to the other end of the directional coupler 33. The probe `34 is connected to detector 37, whose output is displayed on an oscilloscope 38. Synchronizing signals are fed over a lead 39 from the generator 36 to the oscilloscope 38 to keep the horizontal sweep of the oscilloscope 38 in step 4with the frequency sweep of generator 36. The microwave voltage sampled by the probe 34 is converted by the detector 37 and used to modulate the vertical deflection of the oscilloscope 38.

Microwave systems used for communication purposes generally require a much broader frequency band-width than those used for radar, for example. Typic-ally, a waveguide used for a microwave communications system has a band-width of from 500 megacycles per second to 1,000 megacycles per second, and the sweep frequency generator 36 must be capable of sweeping that range.

Referring again to FIG. 1, the position-adjusting device 15 comprises a nonmagnetic rod 41 attached at one end to the magnet 16 and have a handle 42 at the other. The rod passes through an aperture 43 in a movable support member 44, which rests on an outer surface 12 of the wave-guide 11. The rod 41 is freely slidable within the aperture 43, permitting the magnet 16 to be selectively positioned along the length of the Iwaveguide 11.

A second rod 46, with its axis perpendicular to the axis of rod 41, is connected at one end to support 44 by flange 47. The other end of rod 46 passes through an aperture 48 in a fixed block 49, which is securely fastened to the waveguide 11. The rod 46 is terminated in a knob 51 and is freely slidable within the aperture 48, permitting the support member 44 and, hence, magnet 16, to be selectively positioned across the width of the waveguide 11.

The method of operation is as follows: The sweep frequency generator 36 and oscilloscope 38 are switched on and allowed to come up to their operating tempera tures. The mid-band frequency and sweep frequency deviation of generator 36 are then adjusted to correspond to the system characteristics for which the waveguide is designed. Typically, the mid-band frequency may be 4,000 Imegacycles per second and the deviation plus or minus 250 megacycles per second. Any irregularities in the transmission characteristics of the waveguide 11 over the band of interest will cause portions of the incident wave to be reflected back towards the source 36. These will be sensed by the probe 34 and displayed on oscilloscope 38. While observing the pattern on oscilloscope 38, the magnets 16-16 and, hence, slugs 14-14, are moved individually or in combination along the upper surface of waveguide 11 until the oscilloscope indicates that there are substantially no reiiections remaining. The positions of the magnets 16-16 are marked `with a suitable marking device, and tuning dents or dimples manufactured at those precise locations on the production runs of the waveguide.

As previously discussed, if even greater accuracy is desired, the magnets 16-16 and slugs 14-14 may be removed and holes drilled and tapped at the marked locations to receive conventional screw-in tuning slugs and the test repeated with the depth of protrusion of the slugs being varied. In this manner not only the location, but

6 the depth of the tuning dents or dimples can be established.

The method and set-up used to correct for changes in polarization in circular waveguides is basically similar to that shown in FIG. 6 except, of course, arcuate slugs 24--24 and circular waveguide 26 are substituted yfor slugs 14--14 and waveguide 11. Detector 37 or directional coupler 34 should be made sensitive to the plane of polarization so that oscilloscope 38 will indicate when the desired degree of correction has been obtained. Generator 36 should be capable of generating circularly polarized waves within the waveguide.

In some applications directional coupler 33 and probe 34 are not used, and the polarizationsensitive detector 37 is connected at the far end of the waveguide 26 in lieu of dummy load 32. Oscilloscope 38 will thus indicate the power received by the detector rather than indicate the number of reflections in the waveguide. l

It is to be understood that the apparatus and methods of this invention are not restricted to use with waveguides, but may also be used for tuning and locating the optimum positions for fabricating tuning dents in filters, couplers, oscillator tubes, E and H bends, mitre bends, directional couplers, magic tee, hybrid junctions, etc. While the apparatus has been described as being suitable for microwave waveguides, it will be obvious that various changes and modifications may be made therein without departing from the spirit an-d scope of the invention.

What is claimed is:

1. A method of reducing the standing wave ratio of an energized waveguide, which comprises the following steps:

placing a paramagnetic slug on an inner surface of said waveguide;

placing a magnet on an outer surface of said waveguide opposite said slug; and

moving said magnet and said slug along said waveguide until said standing wave ratio is reduced to its minimum value.

2. A method of reducing reliections from surface irregularities in an energized waveguide, which comprises the following steps:

placing paramagnetic slugs on the inner surface of said waveguide near said surface irregularities;

placing a plurality of magnets on the outer surface of said waveguide opposite said slugs; and

moving said magnets along said waveguide to move said slugs to reduce the refiections in said waveguide.

3. A method of tuning a section of waveguide along which is being impressed microwave energy, which cornprises:

placing a slug of paramagnetic material against one inner `wall of said waveguide; moving a permanent magnet -along the outer wall of waveguide to advance the paramagnetic slug; and

ymeasuring and comparing the input and output energy of said microwave energy while said paramagnetic slug is being advanced to obtain an indication of maximum transmission of said energy with respect to the positioning of said slug. 4. A method of altering the polarization of a nonlinearly polarized microwave impressed through a circular Waveguide, which comprises:

inserting a pair of crescent-shaped slugs within the waveguide, the outer arcuate surfaces of said slugs conforming to the inner surface of said waveguide;

energizing one end of the waveguide with a source of nonlinearly polarized microwaves;

connecting a polarization-sensitive detecting device with a visual display to the other end of the waveguide; and

rotating a magnetic field of sufficient strength about said waveguide to support and rotate said crescent-shaped slugs about the centr-al axis of said waveguide and along 4the guide wall while observing said visual display of the alteration in polarization.

5. A method of determining the optimum position for fabricating discrete compensating elements in a waveguide, which comprises:

placing a plurality of paramagneti'c slugs on an inner surface of said waveguide;

placing a `plurality of magnets on an outer surface of said waveguide opposite said slugs;

terminating said waveguide at one end with a dummy load;

connecting a directional coupler having a probe to the other end of said waveguide;

connecting a detector including an oscilloscope to the probe of said directional coupler;

applying a source of swept frequency microwaves to said directional coupler and waveguide;

moving said mangets and said slugs while observing said oscilloscope until a minimum reflected signal is observed; and

marking the positions of said magnets on said waveguide for subsequent fabrication of permanent discrete compensating elements at said positions. 6. A method of fabricating a plurality of discrete compensating tuning dents in a production microwave component having an identical prototype microwave component, which comprises:

placing a plurality of paramagnetic slugs on an inner surface of said prototype microwave component;

placing a plurality of magnets on an outer surface of said prototype microwave component opposite said slugs;

terminating said prototype microwave component at one end with a dummy load;

connecting a directional coupler having a probe to the other end of said prototype microwave component; connecting a detector having a visual display output to the probe of said directional coupler;

applying a source of swept frequency microwaves to said directional couple-r and said prototype microwave component;

moving said magnets and said slugs while observing said visual display until a first minimum reflected signal is observed;

marking the positions of said magnets on said prototype microwave component;

removing said magnets from the outer surface of said component and said slugs from the inner surface of said prototype microwave component;

drilling and tapping holes in the wall of said prototype microwave device at said marked positions;

inserting threaded screw-in tuning slugs in said tapped holes;

adjusting said screw-in tuning slugs while observing said visual display until a second minimum reflected signal is observed; recording the depths of penetration of said conventional slugs in said prototype microwave component; and

fabricating tuning dents in the wall of said production microwave component, the location :and depths of said dents corresponding exactly to the location and depth of protrusion of said conventional slugs in said prototype component.

7. An apparatus for altering the polarization of an electromagnetic wave which comprises:

a `crescent-shaped paramagnetic radiating member excited by said electromagnetic wave; and

magnetic means, external to the field of said electromagnetic wave, for moving said radiating member about the center of curvature of its convex surface within the field of said electromagnetic wave.

8. An electrical tuning device, which comprises:

a paramagnetic radiating member;

means for impressing microwave energy on said paramagnetic member to excite said member to radiate a signal proportional to said impressed energy; and magnetic means external to the field of said microwave energy for supporting and moving said radiating member within, and adjoining the boundary of, said eld.

9. Apparatus for tuning a waveguide, which comprises:

a source of microwave power for impressing microwaves through said waveguide;

a paramagnetic slug freely placed on an inner wall of said waveguide;

an external magnet movably mounted adjacent to an outer wall and in register with said paramagnetic slug;

means for moving the magnet along the outer wall of the waveguide to advance the paramagnetic slug along the inner wall of the waveguide; and

means for indicating variations in power of the microwaves impressed through said waveguide.

lil. A device for transmitting circularly polarized microwaves, which comprises:

a circular waveguide;

a pair of crescent-shaped paramagnetic slugs freely positioned within said waveguide;

a thin layer of friction-reducing material on said pair of slugs;

a first magnetic device movably mounted on the outside of the waveguide for selectively positioning a first one of said slugs with its outer arcuate crescent surface in contact with the inner periphery of said waveguide; and

a second magnetic device movably mounted on the outside of the waveguide for selectively positioning a second one of said slugs with its outer arcuate crescent surface in contact with the inner periphery of said waveguide. 11. A device for optimizing tne transmission characteristics of a length of circular waveguide, which comprises:

a slug of paramagnetic material contacting the inner surface of said waveguide and having a crescentshaped cross-section, the greater radius of curvature of said cross-section corresponding to the radius of curvature of said inner surface;

a thin layer of friction-reducing material on said slug;

magnetic means, contacting the outer surface of said waveguide and movable to any desired position along the length of said waveguide, for positioning said slug around the circumference of said inner surface, said slug and said magnetic means defining a magnetic circuit having sufficient flux through the wall of said waveguide to maintain said slug and said magnetic means in juxtaposition; and

means responsive to the transmission characteristics of said waveguide for indicating when said slug has ben positioned to optimize said transmission characteristics.

12. An apparatus for altering the polarization of a nonlinearly polarized wave being propagated inside a cylindrical waveguide comprising:

a crescent-shaped radiating member inserted within the waveguide, said member having its convex surface contiguous with a portion of the inside surface of a discrete waveguide section and its concave surface shaped at its extremities to form a continuous boundary with the remainder of the inside surface of the discrete waveguide section, for extracting and reradiating energy from the incident wave to alter the polarization of said wave.

13. An apparatus for altering the polarization of a nonlinearly polarized wave being propagated inside a cylindrical waveguide comprising:

a crescent-shaped radiating member having a convex surface, the circle of curvature of which coincides with the circle of curvature of the inner guide wall of the waveguide, and a concave surface, the outer extremities of which coincide with the outer extremities of said convex surface; and

means to support said crescent-shaped radiating member within the waveguide with its convex surface in References Cited `contiguous relationship with the inner guide wall of UNITED STATES PATENTS the waveguide to expose said radiating member tothe ener of the incident wave for alte 'in the olarization if; Said vs g P 5 2,917,719 12/1959 Brown 333-7 14. An apparatus as defined in `claim 13 wherein: 311661725 1/1965 Enger 33:533 said crescent-shaped radiating member is made of paran. 4 magnetic material; and HERMAN KARL SAALBACH, P/mmly Examiner. said Supporting means includes means for applying a L. ALLAHUT, Assistant Examiner.

magnetic force to support said radiating member, 10

2,840,820 6/1958 Southworth.

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