US3593210A

US3593210A - Waveguide junction circulator wherein all modes in each branch arm are evanescent

Info

Publication number: US3593210A
Application number: US886639A
Authority: US
Inventors: Richard Finnle Skedd
Original assignee: International Standard Electric Corp
Current assignee: STC PLC
Priority date: 1969-03-05
Filing date: 1969-12-19
Publication date: 1971-07-13
Anticipated expiration: 1988-07-13
Also published as: DE2008584A1; GB1190547A; FR2034666A1; FR2034666B1; CH515625A

Abstract

A waveguide junction circulator of the type having a resonant cavity loaded with ferrimagnetic material wherein the coupling ports consist of a length of waveguide having a cutoff frequency which is higher than the operating frequency and wherein said waveguide is tuned to the operating frequency by inserting therein, along the broad wall at predetermined lengths, screws whose capacitive reactance is equal to the conjugate of the imaginary characteristic impedance of the waveguide. In a preferred embodiment, the cutoff frequency of the rectangular waveguide is made tunable and the resulting passband variable by inserting in said waveguide, along the sidewalls, ferrimagnetic strips and subjecting said strips to a variable magnetic field.

Description

United States Patent [72] inventor Rk'llard Flnnie Sltedd Bkhops Stortlord, England [21 I Appl. No. 886,639 F 221 Film Dec. :9, 1969 [4S] Patented July 13,197] [73] Assignee International Standard Electric Corporation New York, N.Y. [32) Priority Mar. 5, I969 [33] Great Britain [31] 1 "83/69 [54] WAVEGUIDE JUNCTION CIRCULATOR WHEREIN ALL MODES IN EACH BRANCH ARM ARE EVANESCENT 5 Claims, 8 Drawing Figs.

[52] U.S.Cl. 333/].1, 333/73 [5t] Int. Cl. i. H0lp l/32, l-lOlp 5/12 [50} FleldolSearch 33311.!

[56] Relerences Cited UNITED STATES PATENTS 3,492,60l l/l970 Omori 333/Ll INDER l l l l FERRIMAGNET CYL OTHER REFERENCES Chart et a]. New Microwave Circulators," TRONl(S.Dec. [8 195933311 ELEC- ABSTRACT: A waveguide junction circulator of the type having a resonant cavity loaded with ferrimagnetic material wherein the coupling ports consist of a length of waveguide having a cutoff frequency which is higher than the operating frequency and wherein said waveguide is tuned to the operating frequency by inserting therein, along the broad wall at predetermined lengths, screws whose capacitive reactance is equal to the conjugate of the imaginary characteristic impedance of the waveguide. In a preferred embodiment, the cutolf frequency of the rectangular waveguide is made tunable and the resulting passband variable by inserting in said waveguide, along the sidewalls, ferrimagnetic strips and subjecting said strips to a variable magnetic field.

PATENTEnJuusml 3593.210

H93 Ffiequeng/ 0/12 FERRIMAGA/ r/c I 8 LOADING STRIP I lnvenlor RICHARD F. $KD0 A Home y WAVEGUIDE JUNCTION CIRCULATOR WHEREIN ALL MODES IN EACH BRANCH ARM ARE EV ANESCENT FIELD OI" THE INVENTION This invention relates to waveguide junction circulators.

SUM MARY OF THE INVENTION It is an object of the present invention to provide an improved junction circulator.

According to the invention there is provided a waveguide junction circulator of the type having a resonant cavity loaded with a ferrimagnetic material, a source of a magnetic field, and a plurality of ports coupled to said cavity wherein at least one of said ports comprises at least one section of waveguide having a cutoff frequency above the resonant frequency of said cavity and means for terminating said section length of waveguide with a reactance whose value is equal to the conjugate of the imaginary characteristic impedance of each section length of waveguide.

BRIEF DESCRIPTION OF THE DRAWINGS The above-mentioned and other objects of the invention will become apparent by reference to the following descrip tion in conjunction with the accompanying drawings, in which:

FIG. I shows a three-port waveguide junction circulator with ports of rectangular cross section,

FIG. 2 is a transmission line equivalent circuit of a section of one port of the circulator of FIG. I in respect of evanescent H waves,

FIG. 3 is a lumped circuit equivalent of the section,

FIG. 4 shows the performance of a junction circulator as shown in FIG. 1,

FIG. 5 shows a three-port waveguide junction circulator with ports of rectangular cross section loaded with ferrimagnetic material,

FIG. 6 shows effective permeability vs. angular frequency for transversely magnetized ferrite,

FIG. 7 shows a section of waveguide loaded with ferrimagnetic sidewall strips,

FIG. 8 shows insertion loss vs. frequency for a ferrite loaded section of waveguide in cutoff condition with DC magnetic field as a parameter.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. I shows a three-port junction circulator with a central resonant cavity I containing ferrimagnetic material 2. The ferrimagnetic material 2 is subjected to a DC magnetic field H m in. the direction indicated to obtain circulation in an anticloclrwise direction.

The three ports 4 of the circulator are identical in construction and operation. Each port 4 consists of a length 5 of rectangular cross section waveguide of length 21 with two adjustable capacitive screws 6 and 7 on the longitudinal centerline of the upper broad wall of the waveguide. The Iongitu' dinal spacing between the screws 6 and 7 being I (each screw being at a distance of [/2 from the center point of the length) with each screw extending into its respective waveguide.

The rectangular cross section (height and width) of each length 5 is such that at the operating frequency of the circulator, i.e., the resonant frequency of the cavity 1, the cutoff frequency of each length 5 is above the operating frequency. For example, at an operating frequency of4 GI'IL, each length 5 has typically a height of 0.4 ins. and a width of 0.9 ins.

Assuming electromagnetic energy at the required operating frequency to be present, in the dominant H mode, at the outer end of the left-hand (inlet) port 4, since the length Sis dimensioned to be beyond cutoff at this frequency. all modes in the length are evanescent.

A waveguide at frequencies below cutoff exhibits characteristics common to all nondissipative filter networks in their stopband region. The characteristic impedance which is real In the passband becomes imaginary in the stopband. The propagation constant which is imaginary in the passband becomes real in the stopband.

The transmission line analog of a section of length I of the input port 4 is shown in FIG. 2 as a line of length I having a positive imaginary characteristic impedance jZ and a real propagation constant y To evanescent H waves therefore the length 1 looks like a pure inductance. The lumped circuit equivalent of FIG. 2 is shown in FIG. 3 as a rr equivalent circuit giving the inductance reactance values as functions of 2 y, and I.

If the section of waveguide is terminated in a capacitance C such that the capacitive reactance X is the conjugate of the inductive reactance of the section, there will be full energy transfer through the section. The energy transfer is frequency sensitive and the section behaves as a bandpass filter.

The passband frequency limits (I, andfz) are given by:

The center frequency, f, occurs at the geometric mean Therefore The bandwidth is a function of y, and (in the ideal lossless case) as 7,! approaches a due to 1 then tanh ,1 coth ,1, and the bandwidth (frf reduces towards zero.

The inlet port therefore behaves as a two section band-pass filter, in which the required value for each section capacitance to provide conjugate matching with the inductance of the respective section is provided by suitably adjusting the capacitive screws 6 and 7 of the inlet port.

Thus there is full energy transfer through the inlet port to the central cavity l and subsequent anticlockwise rotation and transmission into the adjacent port. Full energy transfer through this output port occurs by the same process as that described for the inlet port.

FIG. 4 shows the performance of a waveguide junction circulator as shown in FIG. I, having ports of rectangular cross section with internal dimensions of 0.622 ins. X 0.40 ins., capacitive screws 8 B.A., and a ferrite cylinder 0.32 ins. diameter X 0.40ins.

The general functioning of the circulator is conventional, in permitting energy transfer from one port to an adjacent port only in the direction of rotation determined by the magnetic field applied to the ferrimagnetic material. Application of the circulator is also conventional such as coupling one port to an aerial, one port to a microwave radio transmitter, and one port to a microwave radio receiver.

Energy propagation external to the circulator ports may be by propagating waveguide, in which case a further capacitive matching screw will be required at each junction between the evanescent waveguide and the larger dimensioned propagating waveguide. Alternatively, the circulator may be coupled into a microwave system constructed entirely in evanescent waveguide.

FIG 5 shows another form of three-port junction circulator, with the same reference numerals as in FIG. I used to indicate like elements. As in the circulator of FIG. 1, each port behaves as a two section evanescent waveguide band-pass filter tor full energy transmission therethrough at the operating frequency.

Each port 4 is constructed of waveguide having height and. width dimensions such as to be propagating at the operating frequency. Each port contains loading strips 8 of ferrirnagnetic material, such as ferrite or garnet, symmetrically ar-- ranged one at each sidewall of the waveguide. The ferrimagnetic material strips 8 are subjected to a DC magnetic field m,-

It has been established that for a transversely magnetized ferrite in rectangular waveguide (TE mode) the cutoff frequency may be controlled by the DC magnetic field. The cutoff frequency can be made higher or lower than the empty waveguide value. This is a consequence of the fact that the ef fective permeability t, of the ferrite can be varied from positive to negative values by the DC magnetic field as shown in FIG. 6 in which m is the cutoff frequency and w, the gyromagnetic resonance for the infinite ferrite medium. Thus for O the RF field is concentrated in the ferrite and the effective width of the waveguide is increased, whereas for t, O the RF energy is excluded from the ferrite and the effective width is reduced. This latter effect is illustrated in FIG. 7.

Thus, in each port ofthe circulator of FIG. 5, the length of waveguide is in the evanescent condition brought about as explained above by the DC magnetic field. This condition is illustrated in H6. 8 and also shows how the cutoff frequency of each port is dependent on the value of DC magnetic field. The field may be made variable in value when applied by per manent magnet pole pieces by arranging for the pole pieces to be movable, and when applied by electromagnetic pole pieces by varying the current.

Each port is tunable in frequency by variation in the value of the DC magnetic field. An increase in field raises the frequency, and a reduction in field lowers the frequency.

Since the resonant frequency of the cavity 1 is also frequency variable by its magnetic field, the junction circulator is frequency variable.

The junction circulator may alternatively be constructed with ports of waveguide lengths having square or circular cross section.

In the junction circulators of FIGS. 1 and 5, each port may have one, three or more sections each containing a single capacitive screw.

In both the described junction circulators, the capacitive screw or screws in each section of the ports may be replaced by other forms of capacitive obstacle, such as adjustable capacitive diaphragms.

The described junction circulators olTer a fundamentally broader frequency band capability than conventional (dispen sive) waveguide junction circulators because of the lumped element nature of the evanescent mode ports.

I claim:

I. A junction circulator of the type having a resonant cavity loaded with a ferrimagnetic material, a source of a magnetic field, and a plurality of ports coupled to said cavity wherein at least one of said ports comprises:

at least one section of waveguide having a cutoff frequency above the resonant frequency of said cavity; and

means for terminating said section length of waveguide with a reactance whose value is equal to the conjugate of the imaginary characteristic impedance of each section length of waveguide.

2. A junction circulator, according to claim I, having three ports, each port comprising two contiguous section lengths of waveguide having a cutoff frequency above the resonant frequency of said waveguide and wherein said means for terminating each section length includes one or more capacitive screws mounted on one broad wall of each section length of waveguide.

3. A junction circulator, according to claim I, wherein said at least one section length of waveguide includes a waveguide loaded with ferrimagnetic material.

4. A junction circulator, according to claim 3, wherein said waveguide is symmetrically loaded with ferrimagnetic material on each sidewall.

5. A junction circulator, according to claim 3, having three ports, each port comprising two contiguous section lengths of waveguide and wherein said means for terminating each section length includes one or more capacitive screws mounted on one broad wall of each section length of waveguide.

Claims

1. A junction circulator of the type having a resonant cavity loaded with a ferrimagnetic material, a source of a magnetic field, and a plurality of ports coupled to said cavity wherein at least one of said ports comprises: at least one section of waveguide having a cutoff frequency above the resonant frequency of said cavity; and means for terminating said section length of waveguide with a reactance whose value is equal to the conjugate of the imaginary characteristic impedance of each section length of waveguide.

2. A junction circulator, according to claim 1, having three ports, each port comprisinG two contiguous section lengths of waveguide having a cutoff frequency above the resonant frequency of said waveguide and wherein said means for terminating each section length includes one or more capacitive screws mounted on one broad wall of each section length of waveguide.

3. A junction circulator, according to claim 1, wherein said at least one section length of waveguide includes a waveguide loaded with ferrimagnetic material.