US2858512A - Apparatus for varying the phase in waveguide systems - Google Patents

Description

Oqt. 28, 1958 APPARATUS FOR VARYING THE PHASE IN WAVEGUIDE SYSTEMS Filed May a, 1954 E. F. BARNETT 2,858,512

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F'IEQE faward F Barneff 9 ATTORNZE'YS Oct. 28, 1958 E. R. BARNETT 2,858,512

APPARATUS FOR VARYING THE PHASE IN WAVEGUIDE SYSTEMS Filed May 3, 1954 I 2 Sheets-Sheet 2 P1 Evll:

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INVENTOR. fawara /-T Barneff flTTORNE'YS United States Patent APPARATUS FOR VARYING THE PHASE IN WAVEGUIDE SYSTEMS Edward F. Barnett, Stanford, Califl, assignor to Hewlett- Packard Company, Palo Alto, Calif., a corporation of California This invention relates generally to high frequency systerns of the waveguide type, and more particularly to phase shifting means having broad band characteristics.

Differential phase sections and differential phase shifter have been described in literature. See for example, Principles and Application of Waveguide Transmission, by George C. Southworth, the Van Nostrand Company, Inc., pages 327-335. A serious limitation in these devices is that they can be used only over a relatively narrow frequency band. When they are used over broad frequency bands resonant modes occur and errors in phase shift are introduced.

It is an object of my invention to provide an adjustable phase shifter for waveguide systems which is operable} over a broad band of frequencies.

It is a further object of my invention to provide a phase shifter of the above character having means for suppressing unwanted modes. 7

Additional objects and features of the invention will appear from the following description in which the preferred embodiment of the invention has been set forth in detail with the accompanying drawing.

Referring to the drawings:

Figure 1 is an exploded view showing three sections of a waveguide adapted for use in the present invention.

Figure 2 is a plan view of two quarter-wave plates and a half-wave plate, made in accordance with the present invention, and applicable to the sections of Figure 1.

' Figure 3 is a side elevation of the wave plates shown in Figure 2. I

Figure 4 is an end view of the complete assembly illustrated in Figure 6, taken along the line 44 of Figure 6. 1

Figure 5 is an enlarged cross sectional view taken along the line 5-5 of Figure 6.

Figure 6 is an assembly including the waveguide sections of Figure 1 together with the wave plates of Figures 2 and 3, the same forming a differential phase shifter in accordance with the present invention.

Figure 7 is a plan view of one of the quarter-wave plates, with dotted lines indicating the TM modes superposed.

' Figure 8 is an end View of the plates shown in Figure 7.

Figure 9 is likewise an end view of the plates shown in Figure 7, but looking at the right-hand end.

Figure 10 is a cross sectional view taken along the line 10-10 of Figure 8, and illustrates a typical mode suppressor strip.

Figure 11 is a graph of differential phase shift as 'a function of frequency, for a particular 90-degree section constructed in accordance with Figure 6.

When a thin slab of dielectric material is placed parallel to the electric field of an incident electro-magnetic wave of a waveguide, the wave has a maximum velocity retardation. Least velocity retardation occurs when the incident electromagnetic wave has its electric field perpendicular to the dielectric material.

A It is permissible to regard a dominant wave approach- 7 ing a dielectric slab as made up of two components, one lying in the plane of the slab and the other perpendicular thereto. The velocity of one component with respect to the other is retarded. Consequently a phase shift between the components is introduced. This is called a dilferential phase shift.

A section which introduces a 90-degree phase differential is called a A90-degree phase section. The dielectric material is called a quarter wave plate or slab.

The present invention' is primarily concerned with the case when the incident electric field makes an angle of 45-degrees with the plane of the dielectric slab. In that case the incident wave may be represented by two equal vectors, one lying in the plane of the slab and the other perpendicular thereto. If the dielectric slab is a quarter wave plate then the two component vectors will emerge witha 90-degree differential phase shift. It can be shown that this emergent wave may be regarded as a circularly polarized electric wave. Therefore the linearly polarized electric wave is changed to a circularly polarized electric wave by traveling through a A90-degree phase shift section.

If a circularly polarized electric wave is sent through a A90-degree section it emerges with a linear polarization having an axis of polarization dependent upon the sense of rotation of the incident circularly polarized wave. A circular wave guide section whichis dimensioned so that it represents electrically two A90-degree sections in tandem, is referred to as a AISO-degree section. If a circularly polarized electric wave is introduced into such' a section, it emerges with circular polarization in an opposite sense, i. e., from a clockwise to a counterclockwise sense. Further, if the MSG-degree section is rotated through an angle 5 the emergent circular wave will have. its. phase shifted through an angle 2,8. The foregoing outline of fundamental principles involved will serve to outer race 13 and inner race 14 is telescoped on each end of the circular section. The three sections are mated as illustrated. With this arrangement the circular section 11 can be rotated for the purposes presently described. The flanges 16 and 17 on the sections 10 and 12 are connected together by the spacer and tie rods 18, whereby all of the sections are aligned on a common rectilinear axis. Sections 10 and 12 are also shown provided with flanges 19 and 20 to facilitate coupling these sections to the waveguide sections of an associated waveguide system.

In Figure l the rectangular and circular portions of the waveguide section 10 are indicated at 10a and 10b respectively, and the corresponding portions of section 12 are similarly indicated at Ma and 12b. Transition between the rectangular and circular portions is also illustrated at 10c and 120 and is in accordance with well Patented Oct. 28,- 1958 substantially perpendicular to the plane of the dielectric slabs or plates associated with the same, and in addition the axes are parallel, and they are spaced apart in a direction corresponding to the axis or length of the waveguide. 10, two of the recesses 21 are in the region of the transition 10c, and the third is in the cylindrical portion 10b. Waveguide section 12 is provided with recesses 22 which are formed in the same manner and distributed in the same way as the recesses 21. The spacing of the recesses along the length of the waveguide, with respect to each other, is approximately one quarter wavelength of the mid frequency of the frequency band of operation.

The dielectric slabs incorporated in the complete assembly of Figure 6, are illustrated in Figures 2 and 3. The slabs 23 and 25 are adapted for installation in the waveguide sections 10 and 12, while the slab 24 is adapted to be mounted within the circular waveguide section 11. Slabs 23 and 25 are constructed to function as quarterwave plates, while slab 24 functions as a half-wave plate. The slab 23 is provided with integral studs 26 which are adapted to interfit the holes 27 in waveguide section 10, to thereby mount the slab in proper operating position. Slab 24 is similarly provided with the studs 26a which are accommodated in the openings 28 provided in section 11. Slab 25 has studs 29 which engage in the openings 30 of section 12.

The slabs 23, 24 and 25 can be contoured as illustrated in Figures 2 and 3, to minimize reflections. Thus slab 23 has its one end provided with the open V-shaped slot 32, and its other end tapered to form the pointed end portion 33. As viewed edgewise in Figure 3, both end portions of this slab are tapered from the thickest middle part of the slab, towards the ends. Slab 25 is similarly provided with the open V-shaped slot 34, and the pointed end portion 35, and this contouring as viewed from the edge is the same as slab 23. Slab 24 is provided with V-shaped slots 36 and 37 whereby these end portions are complementary to the opposed pointed end portions of the slabs 23 and 25. Likewise the end portions of the slab 24 are tapered toward the ends as illustrated in Figure 3.

To facilitate installation of the slabs it is desirable to form each of the sections 10, 11 and 12 in separable halves, divided on a plane coincident with the longitudinal axis of the guide. The division of sections 10 and 12 in'this manner is illustrated particularly in Figures 4 and 5. The halves are held together in suitable means such as the screws 41.

In the complete assembly illustrated in Figure 6, the slabs 23 and 25 are located in a common plane which is coincident with the longitudinal axis of the guide, and this plane is at an angle of 45-degrees with respect to the plane of the rectangular waveguide portions 10a and 12a. This is illustrated particularly in Figures 4 and 5. Figure also shows the relationship between the positioning of the slabs and the location of the recesses 21.

Inthe complete assembly it will be evident that rotation of the middle circular waveguide section 11 serves to rotate the plane of the slab 24 relative to the plane of the slabs 23 and 25. As will be presently explained the rotation can be for the purpose of phase adjustment, or-

it may be continuous, for purposes of modulation.

In Figure 7 one of the quarter-wave plates, namely the plate 23, has been shown with dotted lines representing a TM mode which arises due to discontinuity in the system. In order to suppress this undesired mode the plate 23 is pro ided with two elements 42, which are in the form of strips, inserted in the dielectric material of the plate as illustrated. Each strip preferably comprises a plurality of parallel resistive elements which are insulated from each other. One satisfactory Way to form such elements is to employ a strip of mica, upon which a thin metal film is vaporized. The metal coating is then Referring particularly to waveguide section severed along parallel longitudinal lines, thereby providing a plurality of parallel conductor elements. In a typical instance the strip may have three such conductor elements. The construction just described is illustrated in Figure 10. The lines 4211 represent the small gaps between adjacent conductor elements 42b.

The exact positioning of the elements 42 is not critical However each element should be positioned near the maximum of the electric field of the TM mode as is indicated generally in Figure 7. The indicated values of one-half and one-quarter, in Figure 9, represents the portioning, rather than actual measurements. Thus in this specific example each element 42 is located substantially 'midway between the center line of the plate, and the corresponding edge of the plate.

The resistance elements formed by the metal film present sufiicient resistive loading of the TM mode, to suppress the same.

Mode suppression elements similar to the elements 42 just described, are incorporated as elements 43 in the plate 25, and elements 44 in the plate 24. Thus the TM mode is suppressed for each of the sections 10, 11 and 12.

Operation of the apparatus described above is as follows: It is assumed that the assembly of Figure 6 is inserted in a waveguide system, whereby a linearly polarized wave enters the left-hand end. Because of the position of the slab 23, as previously described, the plane of this slab is at an angle of 45-degrees with the incident electromagnetic wave. Therefore a linearly polarized wave entering this section emerges as a circularly polarized wave. The action of the circular section 11, with its half-wave plate, is such that the circularly polarized wave leaves this section circularly polarized in an opposite sense. In the section 12, there is a transition from circular to linear polarization, whereby the wave emerges from the right-hand end of the assembly as a linearly polarized wave, the plane of polarization being parallel to the original plane.

By changing the angular setting of the section 11, as by turning it about its axis, it is possible to vary the electric phase difference between the input and output wave by an angle of from 0 to 360-degrees. The amount of phase shift is dependent upon the position of the AlSO-degree section. A rotation of 5 degrees of section 11 results in a rotation of 2;; degrees for the circularly polarized wave. Therefore a phase shift of two times the angular displacement of the circular waveguide section 11, necessarily results. In other words by rotating this section to an angle of 180-degrees the phase difference between the input and the emergent wave is changed 360-degrees.

It will be evident that this device can be used to advantage in many instances where it is desired to effect a predetermined shift in phase in a waveguide system. The section 11 may be connected with a suitable operating gear, to enable it to be turned to any desired angular position by hand. It is also possible to turn the section 11 by suitable motor means at a predetermined rate, thus providing phase modulation.

In one particular instance three -degree phase shift sections were constructed. These phase shift sections consisted of: l) a rectangular to circular waveguide transition section with a quarter wave dielectric slab, (2) a rectangular to circular waveguide transition section with a quarter wave dielectric slab having mode suppression strips, and (3) a rectangular to circular waveguide transition section having recesses with a quarter wave dielectric slab having mode suppression strips. The components of the above section were dimensioned as follows: The rectangular to circular Waveguide transition section went from a .900 inch by .400 inch I. D. rectangular section (X band waveguide) to a 1.000 inch I diameter circular waveguide, and had a length of 4% inches. In transition section there were 6 recesses machined opposite the dielectric slab, these recesses were .400 inch in diameter, .060 inch deep and .425 inch apart (between centers) along the axis of the section; the quarter Wave dielectric slab was made of polyethylene and was 2.15 inches long and .990 inch wide. Its V-slot was 1.340 inches long and its pointed end 1.340 inches long. The thickness of the slab was .330 inch in the central portion, increasing from zero thickness at the two ends to the .330 thickness in a distance of 1.340 inches as measured from the ends. The mode suppressing resistive strips were made by vaporizing a sufiicient thickness of Carna metal on mica to give a-surface resistivity of 80S), and severing the coating along parallel longitudinal lines which were .040 inch apart making 3 resistive elements each .040 inch wide and inch long. Each was placed mid-way between the center line of the slab and the adjacent edge in a plane perpendicular to the plane of the slot.

In Figure 11 I have plotted a curve of difierential phase shift vs. frequency for the frequency band 8.2 to 12.4 megacycles for each of the three 90-degree phase shift sections listed above. Curve A shows differential phase shift vs. frequency for the A90-degree section which had no recesses in the waveguide components and no mode suppressing strip inserted in the dielectric slab. It is apparent that this was not a broad band device. Two difficulties may be ascertained from the curve: First, the curve has a rising slope indicating an increased phase shift at the higher frequencies, and secondly, resonant modes appear at 9.75, 10.7 and 11.65 kmc./s.

The resonance peaks at 9.75 kmc./s. and 10.7 kmc./s. are due to modes which are excited by asymmetrical discontinuities. The 11.5 peak is due to symmetrical discontinuities and was shown, mathematically, to be the TM mode. This mode is shown superposed on the quarter waveplate in Figure 7.

Curve B of Figure '11 shows a plot of differential phase shift vs. frequency over the same band of frequencies. This curve was obtained for a 90-degree phase shift sectio'n in which the dielectric slab had the mode suppressing strips inserted. Asymmetrical discontinuities were eliminated as much as possible. The operation was considerably improved, as can be seen from curve B. All the resonant modes are suppressed.

Figure 11, curve C, shows a plot of differential phase shift vs. frequency for a similar frequency band. This curve was obtained with the third 90-degree phase shift section in which asymmetrical discontinuities were eliminated as much as possible. This section had both the mode suppressing strips and the recesses in the waveguide components. greater phase shift at the lower frequencies than at the higher frequencies. This is indicated in curve C. The recesses in combination with the mode suppressing strips provide a constant phase shift over a broad band of frequencies.

Apparatus was constructed as shown in Figure 6 and dimensioned as follows: The rectangular to circular waveguide transition section, quarter wave plates, recess and mode suppression strips were constructed as described above. The circular waveguide section was 1.000 inch in diameter and had a length of 6% inches. The half wave dielectric slab was 4 inches long and .990 inch wide, the V-slots were 1% inches long. The thickness of the slab was .330 inch in the central portion, increasing from zero thickness at the end to the .330 inch thickness in the distance of 1.340 inches as measured from the ends.

When the apparatus was tested the results indicated that the phase shift varied linearly with the angle of rotation of the circular waveguide portion. Further, a rotation of ISO-degrees resulted in a 360-degree phase shift.

I claim:

1. In a waveguide system for operation over a broad band of frequencies, phase shifting means comprising a rectangular to circular waveguide section, a circular waveguide section and circular to rectangular waveguide sec- The addition of the recesses provide a tion, the circular section being interposed between the transition sections and aligned for the transmission of microwave energy through the same, a dielectric slab dis-. posed in said rectangular to circular waveguide section whereby the energy fed thereto is converted from linear to circular polarization, a dielectric slab disposed in the circular waveguide section whereby the wave is circularly polarized in an opposite direction, and a dielectric slab disposed in the rectangular to circular waveguide transition section whereby the incident energy is converted from circular to linear polarization, resistive strips extending a substantial distance in the direction of the waveguide axis carried by said dielectric slabs for mode suppression, and machined recesses formed in the wall portions of said waveguide sections for introducing a greater differential phase shift at the lower frequencies than at the higher frequencies of said band, said recesses having their axis perpendicular to the plane of the slabs and parallel to one another and being spaced apart in a direction corresponding to the axis of the waveguide.

2. Ina waveguide system for operation over a broad band of frequencies, phase shifting means comprising a rectangular to circular waveguide transition section, a circular waveguide section and a circular to rectangular waveguide section, the circular section being interposed between the transition sections and aligned for the transmission of microwave energy through the same, a degree phase shifting means comprising a dielectric slab disposed in the rectangular to circular waveguide transition section whereby the energy fed thereto is converted from linear to circular polarization, a ISO-degree phase shifting means comprising a dielectric slab disposed in the circular waveguide section whereby the wave is circularly polarized in an opposite direction, a 90-degree phase shifting means comprising a dielectric slab disposed in the rectangular to circular waveguide transition section whereby the incident energy is converted from circular to linear polarization, the said circular section being rotatable about its axis whereby a desired phase shift can be introduced between the incident electromagnetic wave and the emergent electromagnetic wave, means carried by said dielectric slabs for a mode of suppression, said means comprising a resistive strip displaced midway between the center lines and edges of said dielectric slabs and lying in a plane which is perpendicular to the plane passing through the center line of the slab and the center lines of the edges, and means for introducing a greater differential phase shift at the lower frequencies than at the higher frequencies of said band, said means comprising machined recesses formed on the wall portions of said rectangular to circular wave guide transition section and said circular to rectangular transition section, said recesses being located opposite the plane passing through the center lines of the edges of said slabs and spaced longitudinally one-fourth wave-length apart at the mid frequency of said band.

3. In a waveguide system for operation over a broad band of frequencies, a phase shifting means comprising a rectangular to circular waveguide transition section, a circular waveguide section and a circular to rectangular waveguide section, the circular section being interposed between the transition sections and aligned for the transmission of microwave energy through the same, a dielectric slab disposed in the rectangular to circular waveguide transition section to introduce a 90-degree differential phase shift whereby the energy fed thereto is converted from linear to circular polarization, a dielectric slab disposed in the circular waveguide section to introduce a ISO-degree differential phase shift whereby the incident wave is circularly polarized in an opposite direction, a

- posite the plane passing through the center line of the dielectric slab and the center lines of the edges of said slab and spaced longitudinally along the waveguide section.

transfer of microwave energy over a broad band of frequencies, a dielectric slab disposed in said section and serving to introduce a 90-degree differential phase shift, means carried by the dielectric slab for mode suppression, said means comprising strips .of resistive material in the form of parallel resistive elements insulated one from the other displaced substantially midway between the center line and the side edges of said slab and lying in 4. In a waveguide system, a Waveguide section for a plane which is perpendicular to a plane passing through the center lines of the edges of the slab, and means for introducing a greater differential phase shift at the lower frequencies than at the higher frequencies of said band, said means comprising machined recesses formed on the wall portions of said waveguide section that are located opposite the plane passing through the center lines of the slab and the center lines of the edges, said recesses being spaced longitudinally one fourth wave length apart at the mid-frequency of the band of operation.

5. A waveguide section as in claim 4 in which the waveguide section consists of a rectangular to a circular waveguide transition section.

References Cited in the file of this patent UNiTED STATES PATENTS

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