US 3760300 A
Method and apparatus for reducing the loss in a microwave phase shifter by the use of circular TEn1 modes where n is greater than unity. The apparatus comprises: a ferrite filled circular waveguide having a diameter sufficiently large to propagate TE21, TE31 or higher modes; a solenoid adapted to be operated so as to provide a Faraday rotation on the electromagnetic fields propagated in the ferrite filled waveguide; and a pair of circular TE mode transducers or converters respectively coupled to each side of the circular waveguide where for example a circular TE11 mode rf input signal is fed into the first mode converter. The rf is transformed into a selected higher TEn1 mode where n>1 and where it is fed through the ferrite filled circular waveguide. A predetermined Faraday rotation of the TEn1 wave takes place with the second mode converter then transforming the TEn1 signal back to the input mode which then comprises an rf output shifted in phase with respect to the rf input, the phase shifted in phase with respect to the rf input, the phase shift being effected through the higher mode rf signal propagating in the ferrite filled circular waveguide.
Claims available in
Description (OCR text may contain errors)
1 Sept. 18, I973 i 1 REDUCED LOSS PHASE SHIFTER UTILIZING FARADAY ROTATOR Henry C. Leahy, Severna Park, Md.
Westinghouse Electric Corporation, Pittsburgh, Pa.
Filed: 1 July 31,1972
Appl. No.: 276,488
US. Cl 333/31 A, 333/21 R, 333/24.l, 333/243 Int. Cl. H01p 1/18 Field of Search 333/21 R, 24.1, 24.3, 333/31 A References Cited UNITED STATES PATENTS Primary Examiner-Paul L. Gensler Attorney-F. H. Henson et al.
 ABSTRACT Method and apparatus for reducing the lossin a microwave phase shifter by the use of circular TE modes where n is greater than unity. The apparatus comprises: a ferrite filled circular waveguide having a diameter sufficiently large to propagate TE, TE or higher modes; a solenoid adapted to be operated so as to provide a Faraday rotation on the electromagnetic fields propagated in the ferrite filled waveguide; and a pair of circular TE mode transducers or converters respectively coupled to each side of the circular waveguide where for example a circular TE mode rf input signal is fed into the first mode converter. The rf is transformed into a selected higher TE, mode where n l and where it is fed through the ferrite filled circular waveguide. A predetermined Faraday rotation of the TE, wave takes place with the second mode converter then transforming the TE, signal back to the input mode which then comprises an rf output shifted in phase with respect to the rf input, the phase shifted in phase with respect to the rf input, the phase shift being effected through the higher mode rf signal propagating in the ferrite filled circular waveguide.
8 Claims, 6 Drawing Figures l0 I2 2 /l6 l8 22 2o w 24 ClRCULAR CIRCULAR n: FEREITE m1 TE NEE WEBER Q LQW II u RF OUTPUT RF INPUT Patented Sept. 18, 1973 3,760,30Q
' 2 Sheets-Sheet 1 ClRCULAR Patented Sept. 18, 1973 2 Sheets-Sheet 2 ELECTRIC MAGNETIC ELECTRIC FIELD, NETIC FIELD,
ELECTRIC FIELD E .MAGNETIC FIELD ,H
REDUCED LOSS PHASE SIIIFTER UTILIZING FARADAY ROTATOR BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates generally to electromagnetic waveguides and more particularly to a variable rf phase shifter.
2. Description of the Prior Art Microwave phase shifters are well known to those skilled in the art. Typically Faraday rotators such as disclosed in U.S. Pat. No. 3,150,334 issued to G. Petrossian provide a constant phase shift in which microwave frequency signals are Faraday rotated by gyromagnetic materials such as ferrites and the like. A variable phase shifter employing a circular waveguide propagating the TE mode is disclosed in U.S. Pat. No. 3,257,630, issued to CF. Davidson, wherein phase shifting is accomplished by a physical telescoping joint in the waveguide. The concept of one type of mode conversion in circular waveguides is taught in U.S. Pat. No. 2,762,982, issued to S.P. Morgan, Jr.
SUMMARY The present invention is directed to a method and means for reducing the loss in a microwave phase shifter by the use of higher propagating modes than the TE dominant mode in circular waveguide. The subject phase shifter comprises a section of ferrite filled circular waveguide surrounded by a solenoid to provide an axial magnetic field for providing a Faraday rotation and a circular TE mode converter at each end of the ferrite filled waveguide section. The first mode converter converts an input wave into a circularly polarized TE, wave where n 1. The TE, wave is then coupled to the ferrite filled waveguide, whereupon phase shifting is obtained by Faraday rotation in the ferrite. The shifted wave is fed from the waveguide to the second mode converter which converts the wave back to the input mode but the wave is now shifted in space and phase with respect to the input wave. Since both phase shift and the energy loss are proportional to the length of the ferrite, by utilizing the TE, modes where n is greater than unity, the Faraday rotation required for a given phase shift and the amount of loss is reduced by a factor of l/n.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a view partially in section of the preferred embodiment of the subject invention;
FIGS. 2, 3 and 4 are illustrative of field configurations of certain TE wave modes in circular waveguides; and
FIGS. 5a and 5b are illustrative of the electrical field configuration for a TE, mode undergoing a Faraday rotation in the apparatus shown in FIG. 1.
DESCRIPTION OF THE PREFERRED EMBODIMENT transmission line while the symbol TM indicates the magnetic field is everywhere transverse to the axis of the transmission line. The present invention will concern itself for purposes of illustration with TE modes for circular waveguide wherein the subscript n denotes the number of full period variations of radial component of field in the angular direction while the subscript m denotes the number of half period variations of angular component of field in the radial direction.
The TE mode is the dominant mode and is most commonly utilized for propagating microwave signals in circular waveguide. The field configuration for such a mode is shown in FIG. 2 wherein the solid lines designate the lines of force of the electric field E while the dashed lines indicate the lines of force of the magnetic field B. These lines of force lie in a plane transverse to the central axis of the waveguide. The electrical field lines E thus lie in the transverse plane while the magnetic field lines are everywhere orthogonal to the electrical field lines but form closed paths running parallel with axis of the waveguide.
Considering now the configuration shown in FIG. 1, reference numeral 10 generally designates a Ferrite rotator comprised of a section of circular waveguide 12 having a diameter sufficient to propagate modes higher than the dominant circularly polarized TE, mode. More particularly, the diameter is selectively chosen to propagate a TE, mode where n is greater than unity. The circular waveguide 12 is completely filled with a ferromagnetic ceramic body commonly known as ferrite 14. The ferrite body M is excited by means of a unidirectional axial magnetic field produced by a solenoid l6 wound around the outer surface of the circular waveguide 12. The ferrite thus excited acts to rotate the plane of polarization of an electromagnetic wave propagated axially therethrough. The amount of rotation produced by the ferrite body M is a function of the axial magnetic field strength and the length of the ferrite body. However, for a given axial magnetic field, the energy loss or attenuation of the wave propagated through the ferrite is also a function of its length. To this end it is the object of the subject invention to provide means for providing a predetermined phase shift with respect to a circularly polarized rf input and output wave propagating for example in the TE mode with reduced circuit losses.
To this end the ferrite filled waveguide 12 is coupled at one end to a circular TE mode converter 18 which is coupled to an input circular waveguide 20. The input waveguide 20 is propagating an rf input wave for example in the TE, mode. The mode converter 18 changes the TE, mode selectively to a higher TE, mode where n 1 (TE TE etc.) which is fed to the ferrite filled waveguide 12. A Faraday rotation of the 'IE, mode waves takes place in the ferrite filled waveguide 12 for example as shown by FIGS. 50 and 5b wherein an input wave converted to the TE, mode has a Faraday rotation of an angle 0 provided by the axial magnetic field and the length of the ferrite body M. A second circular TE mode'converter 22 is coupled to the opposite end of the ferrite filled waveguide 12 for converting the TB, mode back to the mode of the rf input wave, that is the TE mode. This mode is coupled from the converter 22 to an output circular waveguide M which then propagates an rf output in the TB mode shifted in phase with respect to the rf input. Reduced loss is provided by effecting a phase shift of the wave propagated in the higher TE mode rather than the TE, mode thereby requiring a reduced length of ferrite to accomplish the desired phase shift between the rf input and rf output.
The insertion loss of the device is almost entirely due to dielectric losses in the ferrite which can be expressed dielectric loss K tan 8 E,/)\ X (kg/k) db/unit length where K is a proportionality constant whose value depends on the unit system employed, 8 is the loss tangent of the dielectric, E, is the relative dielectric constant, A is the free space wavelength, and A, is the waveguide wavelength. By appropriate choice of waveguide diameter of the waveguide 12, A, will be the same for a TE, and TE device. All the other parameters are identical so the loss is proportional to length and hence n.
It should be pointed out that the foregoing explanation should not be interpreted in a limiting sense because the input and output waves need not necessarily be TE circularly polarized waves. Any convenient input and output transmission line means may be resorted to when desirable so long as appropriate mode converter means to and from the circular TE, mode are substituted. For example the widely utilized rectangular waveguide operating in the TE mode may be used. Coaxial lines and/or other two conductor lines, and waveguide of any desirable cross-section and mode can also be adapted for use. The only mode and geometry of importance is that in the vicinity of the ferrite body 14.
To explain this phenomenon reference is now made to FIGS. 2, 3 and 4 which illustrate the mutually orthogonal electric E'and magnetic H fields for the TB TE and TE, modes respectively. In FIG. 2, the electric field E and its mutually orthogonal magnetic field H is spatially related by 7r/2 radians, i.e. the spatial relationship of the two fields could be reversed by a 11/2 radian rotation which can be accomplished for example by Faraday rotation. However, when the TE, mode is considered such as shown in FIG. 3, a rotation of only rr/4 radians is required to achieve the spatial field reversal. Likewise, considering FIG. 4, wherein the TE, mode is illustrated only 1r/6 radians of rotation is required. Thus in the TE. mode, the electrical angle and spatial angle 0, are equal i.e. (9 0,. However, in V the higher modes less spatial phase is required for the corresponding electrical phase, that is, 0, 0,/n.
As an illustrative example wherein a 1r/2 phase shift is desired between the rf input and rf output, prior art apparatus utilizing only the dominate TE mode would apply the propagated wave through a ferrite rotator which would operate to rotate the plane of polarization by 1r/2 radians. In the present invention as shown in FIG. I, assume for sake of illustration that the ciarcular waveguide 12 is configured to propagate the TE, mode where n is equal to 3, the mode converter 18 would operate to change the TE rf input mode to the TE, mode and the mode converter 22 would change the TE,, wave back to the TE mode. What is significant, however, is that the Faraday rotation is performed on the TE mode propagated and therefore 1r/6 radians of Faraday rotation is all that is required in order to provide a 1r/2 phase difference between the rf input and rf output appearing in the circular waveguides and 24, respectively. Since a Faraday rotation of 1r/6 is all that is necessary, the required length of the ferrite body 14 is correspondingly less, resulting put is reduced by the factor of l/n. Stated another way,
the present invention reduces the insertion loss both conductive and dielectric per realizable amount of phase shift by exactly l/n of the TE, loss.
What has been shown and described, therefore, is a method and apparatus for converting an input wave to a circular TE, mode, were n 1 performing a Faraday rotation on the TE, mode, and then reconverting to the original input mode whereby a phase shift between the input and output waves is accomplished by a correspondingly smaller Faraday rotation performed on the wave being propagated in the TE, mode.
Having described what is at present considered to be the preferred embodiment of the subject invention,
1. A microwave phase shifter comprising in combination:
a waveguide of circular cross section having a predetermined diameter capable of propagating a circularly polarized TE, mode of electromagnetic wave propagation where n is greater than unity;
a ferrite body disposed within said circular waveguide having a length sufficient to cause a predetermined Faraday rotation of said TE mode in the presence of a unidirectional axial magnetic field;
means for producing a unidirectional axial magnetic field through said ferrite body;
input means for providing input waves of a predetermined input mode;
first electromagnetic wave mode converter means coupled between said input means and one end of said waveguide for converting said input waves to said TE, mode;
second electromagnetic wave converter means coupled to the other end of said waveguide for converting said TE, mode waves back to said predetermined input mode waves; and
output means coupled to said second wave converter means for providing electromagnetic waves thereat shifted in phase relative to said input waves.
2. The apparatus as defined by claim 1 wherein said means for producing the unidirectional axial magnetic field through said ferrite comprises a solenoid disposed on the outside of said waveguide.
3. The microwave phase shifter defined by claim 1 wherein said input means provides circularly polarized TE mode input waves;
said first wave mode converter converts said TE mode input waves to circularly polarized TE, mode waves; and
wherein said second wave mode converter converts said TE mode waves back to said TE mode waves.
4. The apparatus as defined by claim 3 wherein said TE, mode comprises the TE, mode.
5. The apparatus as defined by claim 3 wherein said TE, mode comprises the TE, mode.
6. The invention as defined by claim 1 wherein said ferrite body has a circular cross section substanti-ally equal to the internal diameter of said circular waveguide.
7. The phase shifter as defined by claim 6 wherein the waveguide and said ferrite body are of sub-stantially equal length.
the Faraday rotation back to a predetermined mode of output electromagnetic wave propagation whereby a relative phase shift is provided between said input and output wave propagation which is greater than the Faraday rotation applied to said TE, wave mode.
t IV =0 l