US3103639A - Absorption modulator switch utilizing gyromagnetic rotator split by resistive sheet - Google Patents

Description

Sept. 10, 1963 F. REGGIAI ABSORPTION MODULATOR SWITCH UTILIZING GYROMAGNETIC ROTATOR SPLIT BY RESISTIVE SHEET 3 Sheets-Sheet 1 Filed May 12, 1961 INVENTOR Fran/r Reggia Sept. 10, 1963 F. REGGIA 3,103,639

ABSORPTION MODULATOR SWITCH UTILIZING GYROMAGNETIC ROTATOR SPLIT BY RESISTIVE SHEET 5 Sheets-Sheet 2 Filed May 12, 1961 momkmmmo Q EE om n n 8 S 8 ow 2 Omum v GE ISOLATION (db) WfVV Z an 6? Sept. 10, 1963 F. REGGIA 3,103,639

3 ABSORPTION MODULATOR SWITCH UTILIZING GYROMAGNETIC ROTATOR SPLIT BY RESISTIVE SHEET 3 Sheets-Sheet 3 Filed May 12, 1961 O O PERM EABILITY INVENTOR Fran/r Reggia Ji l- TETMQ/ aai2,m 1+}? MAM,

United States Patent 0 3,103,639 ABSORPTION MODULATOR SWITCH UTILIZING GYROMAGNETIC ROTATOR SPLIT BY RESIS- TIVE SHEET Frank Reggie, 3318 Jones Bridge Road,

Chevy Chase 15, Md. Filed May 12, 1961, Ser. N 109,793 11 Claims. (Cl. ass-24.2 (Granted under Title 35, US. Code (1952), sec. 266) This application is a continuation-in-part of copending application Serial No. 4,828, filed January 26, 1960, for Absorption Modulator Switch, now abandoned.

The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment to me of any royalty thereon.

This invention relates generally to a broadband absorption modulator-switch capable of providing very rapid switching of microwave energy.

Microwave devices used for switching or amplitude modulation of microwave energy preferably should have a low insertion loss in the normally On state and broad band operation. When the overall response of the device is independent of the frequency of the microwave energy which is selectively switched, the operation may be termed broadband.

Other desirable characteristics include a matched input impedance for all values of applied magnetic field, high isolation substantially independent of the magnitude of the applied magnetic field in the Off state, reciprocal switching and modulating properties, low strength magnetic control fields, and small physical size.

An object of this invention is to provide an absorption modulator switch which has all of the above-mentioned characteristics.

' Another object of this invention is to provide a modulator switch having nearly constant electrical characteristics over a large X-band frequency range A further object is to furnish a reciprocal microwave switch which has a switching time of less than one microsecond.

Still another object of the present invention is to provide an absorption modulator microwave switch in which the switching action is independent both of the microwave input frequency and of the strength of the applied magnetic field.

An additional object is to provide a microwave switch having a reciprocal response.

According to this invention, a magnetized ferrite rod which is incorporated in the waveguide section is split longitudinally and a thin resistive film is placed between the split sections of the ferrite. When the ferrite is centrally located inside a standard rectangular waveguide excited in its fundamental TE mode, with the resistive film in the ferrite oriented perpendicular to the input electric field, the resistive film highly attenuates the orthogonal TE mode generated by the longitudinally magnetized ferrite rod. The attenuation of this orthogonal TE mode makes the occurrence of Faraday rotation impossible since the latter can exist only if the TE and the TE modes are present simultaneously. By the effective suppression of Faraday rotation in the present switch, nearly constant electrical characteristics over a very large X-band frequency range are achieved. The reciprocal switching of microwaves is another result of suppressing Faraday rotation. It should be understood that the term reciprocal is intended in its usual sense. It means that the electrical characteristics of the switch are independent of the direction of propagation of microwave energy, as well as the axial direction of the applied magnetic control field.

The specific nature of the invention, as well as other objects, uses and advantages thereof, will clearly appear from-the following description and from the accompanying drawing, in which:

FIG. 1 is a sectional side view of the modulator switch of this invention.

FIG. 2 is an end view of the device of FIG. 1.

FIG. 3a is a sectional side view of one embodiment of the resistive film placed between split sections of the ferrite rod.

FIG. 3b is a sectional side view of a preferred embodiment of the resistive film.

FIG. 4 is a plot of the electrical characteristics of the absorption modulator switch of this invention.

FIG. 5 illustrates the ferrite rod with the various electric and magnetic fields shown therein.

FIG. 6 is .a plot of coupling coefficient K against the applied magnetic field H FIGS. 1 and 2 illustrate the microwave modulator switch 11 of this invention. 1 Therein, a standard rectangular waveguide section 12 is excited in its fundamental TE mode. A microwave signal to be switched is applied to one end of waveguide 12, exciting it in the aforesaid mode. A ferrite rod 13 is located on the central axis of waveguide 12. The rod 13 is split in half through itslongitudinal axis, forming the two sections 15 and 16 shown in FIG. 1. A resistive film 17, described in detgfl subsequently, is located between ferrite sections 15 and.16, and is supported by the opposed planar surfaces of sections 15 and16. A sleeve 19 of polystyrene foam orother suitable plastic supports rod 13 in a central position within waveguide 1'2 so that its longitudinal axis is concentric .with the longitudinal axis of the waveguide. Resistive film 17 is .thus placed along the waveguide axis in a plane perpendicular to the input R.-F. electric field. The sleeve .19 also serves to insulate the rod 13 from waveguide section 12. The ends of ferrite rod 13 are tapered, respectively, as indicated at 24 and 25 in FIG. 1. Tapered ends 24 and 25 provide the impedance matching necessary for broadband operation.

-A solenoid coil 14 wrapped in convolutions around the outside of waveguide '12 selectively applies a D.-C. magnetic field-H shown in FIG. 5, to \ferrite rod 13. Coil 14 is connected by a control switch 18 to any suitable source of electrical energy, such as battery 20 of FIG. 1. As set forth in more detail below, the state or condition of switch 11 is controlled by controlling the application of current from source 20 to coil 14. When current is applied to coil 14, longitudinally magnetizing ferrite rod 13, the microwave switch 11 is in the Off state. When control switch 18 is open, as shown in FIG. 1, and no current is present in coil 14, microwave switch 1 1 is in its On or tnansmit-ting est-ate.

The operation of switch 11 may be explained in the following manner. In general, the internal R.-F. magnetic field h and the associated R.-F. flux density b induced by field h in a ferrite medium are related by a tensor the subscripts referring to the three mutually perpendicular axes x, y and z. From the above expression, it has been shown that, in the case of a magnetized but unsatu- See, for example, G. T. -Rlado, Electromagnetic Chameterization of Ferromagnetic Media, Transactions of the IRE '(PGAP), vol. AP-4, No. 3, pp. 512-525, July 1956. For the latter case, a static D.-C. magnetic field H is applied along the z axis and n, K and ,u are the complex permeability components in the three mutually perpendicular directions. Expanding b: .7K 0 h:

y J' 0 i b, O 0 p, it, gives the following equations:

b h -jKh b =jKh h z=P-z x It can 'be seen from the first two equations of (4) above that an applied R.-F. magnetic field h induces a component b of the R.-F. flux density b which is proportional to ;1.. Due to the precession of spinning electrons in the ferrite medium about the z axis, the magnetic field h also induces a component b in the y-direction proportional to K. Accordingly, ferrite rod 13 when magnetized Iby coil 14 transfers portions of the applied microwave energy from the original T E mode to a tightly coupled orthogonal TE mode. The latter mode corresponds to the flux density component b generated in the magnetized tern'te 13.

The operation of absorption modulator switch 11 depends in part upon variations of complex permeability component K with the static magnetic field H With a low-loss ferrite medium at microwave frequencies where field H is well below magnetic saturation, the real portion K' of the complex quantity K to a first approximation is where M, is the magnetization of the medium in the z direction, 7 is the :gyromagnetic ratio (for electrons equalling 2.8 megacycles per second per oersted, and w is the angular frequency of the incident microwaves in radians per second. The variation of coupling coeflicient --K with the external magnetic field is plotted in FIG. 6, in accordance with experimental results. Only the real component is shown, the imaginary part being small in comparison.

As seen from the curve in FIG. 6 and Equation 5 above, coeflicient -K is zero when no external magnetic field is applied by coil 14. Since ,u'=,u.' =0.76 for this case, t and IL'Z being, respectively, the real portions of n and ferrite rod 13 exhibits a scalar permeability. This scalar permeability of rod 13 of less than unity at Xebfllld frequencies and with no applied field, as opposed to the tensor permeability [,u] of rod 13 when axially magnetized, is due to the gyromagnetic resonance of the election spins in the internal crystalline anisotropy field of the ferrite. When it is desired to place switch 11 in its On state, control switch 18 is opened so that ferrite rod 13 is not magnetized. The incident microwave energy propagated in the TE mode has its electric field E directed perpendicular to the plane of the resistive 17, in the manner illustrated by FIG. 5. The input energy thus serves to apply an R.-F. magnetic field h of FIG. 5 to the ferrite rod :13. Since there is no field h in the y direction associated with the incident energy and since com- 4 ponent K is negligible with the ferrite unmagnetized (its major portion K'=0), Equations 4 reduce to:

x=I x b =0 z==/z z mode. While waves having an electric polarization in a plane parallel to resistive film 17 are dissipated thereby, the input electric field E is perpendicular to the plane of resistive film 17. It therefore does not induce currents in film 17 and suffers substantially no loss in propagating through waveguide section 12. The switch 11 is effectively in its On state.

To turn the microwave switch 11 01f, control switch 18 is closed so that coil '14 produces the D.-C. magnetic field H within waveguide '12. H is applied in the z direction, as seen in FIG. 5. Therefore, the ferrite 13 exhibits the tensor permeability 1.] discussed supra. FIG. 6 shows that for any substantial applied field greater than 10 oersteds, the coupling coetficient -K' has an appreciable magnitude of 0.5 or more. Accordingly, permeability component K is no longer negligible, as was the case with switch 18 open. Now Equations 4 take the form because the h component of the incident energy is zero, as before. FIG. 5 illustrates the conditions present in ferrite rod 13. The incident R.-F. magnetic field h induces the flux density. b as before, and field h further induces a flux density b in the y direction within the ferrite medium 13 as shown by Equations 7. Flux density component b accounts for the microwave energy converted by ferrite rod 13 to the tightly coupled orthogonal TE mode. This energy converted by ferrite 13 has an electric polarization shown in FIG. 5 vector E 1 It is seen that electric vector E perpendicular to b is parallel to the plane of the resistive film 17. The TE wave associated with b induces currents within the resistive film 17 and is therefore immediately absorbed and dissipated within film 17. The end portion of ferrite rod '13 located at the input side of waveguide '12 receives the incident energy first and begins to generate the orthogonal TE mode. That portion of the incident microwave energy which the initially encountered section of fer-rite rod 13 converts to the TE wave is substantially attenuated or damped as just described. The balance of the incident energy, which remains in the 'IE mode corresponding to the flux density component b of FIG. 5, is continuously and progressively attenuated as the signal propagates axially along rod 13, even though its electric polarization is perpendicular to resistive film 17. This can be explained byconsidering the conditions at a point farther along rod 13 as shown in FIG. 5 by axes X and Y. While the original R.-F. field of the incident energy was h at axes X and Y, it is reduced to a smaller strength h,, as illustrated by FIG. 5. The microwave R.-F. field is reduced from h to 11,, because the b component of the original incident energy associated with I1 is dissipated in film 17 as heretofore described. But the small fraction of the original input signal which reaches axes X and Y contains the R.-F. field h The latter field again induces in the ferrite medium 13 two flux densities b and b the enengy converted to the polarization of b being dissipated in resistive film 17. Therefore, by merely employing a ferrite rod 13 of sufficient length, substantially all of the applied energy is attenuated and switch I I is Off whenever a D.-C. field H is present.

i The microwave switch 11 of this invention has electrical switching characteristics which are substantially independent of the microwave frequency and of the strength of control field H Since this switch does not rely in its operation upon the Faraday rotational effect, its insertion loss and electrical isolation do not critically depend upon the frequency of the incident energy or the control field magnitude, as has been true of prior ferrite switching devices which make use of the Faraday rotation phenomenon. With the microwave switch 11 it is merely necessary for the ferrite rod 13 to have sufiicient length and cross-sectional area to produce the desired degree of isolation at the lowest operating frequency expected. The cross-sectional area must be large enough to concentrate the incident microwave energy in the ferrite medium at the lowest frequencies. As the input frequency increases, progressively shorter lengths of the ferrite rod are required for a given degree of attenuation or damping. At higher frequencies, the ferrite rod 13 is more effective in converting the applied energy to the orthogonal TE mode. Therefore, by making the ferrite rod-13 long enough to isolate the lowest microwave frequency encountered, the rod has more than enough length at higher frequencies. In this manner, independence of the input frequency is obtained. i

The present switch is also reciprocal in nature and opends. The above switch gives an isolation well 'over 60 film is properly adjusted, nearly zero phase shift occurs inch, while films 22 and 23 are 0.0004 inch thick. Aquadag films 2 2 and '23 consist of a colloidal suspension of graphite in water painted onto the opposite sides of dielectric film 21. Film 21 may be any thin dielectric material such as Mylar.

A preferred type of resistive film '17 is shown in detail in FIG. 3b. A very thin strip of high quality mica 26 serves as a base. Mica film 26 has a thickness of 0.0005 inch and should be selected for uniform thickness. 'A very thin layer 27 of a pure metal such as chromium is deposited by vaporization techniques upon film 26. The layer or film 27 has a thickness of 0.0001 inch. Since the pure metal is in the form of a very thin film, it has a high resistance. The metal film 27 is covered by a film 28 composed of a low loss dielectric serving as a protective coating to prevent oxidation of metal film 27. Film or layer 28 also has a thickness of 0.0001 inch.

The curves of FIG. 4 illustrate the electrical characteristics of particular modulator switches constructed in accordance with this invention. The solid curves 31 and 32 illustrate, respectively, the insertion loss in the Off state and the VSWR in the On state for a microwave switch with a laminated resistive film 17 such as shown in FIG. 3a when operated at an input frequency of 9250 mc. The switch :11 producing the response shown by curves 31 and 32 contains a split magnesium manganese ferrite rod 13 having a height of 0.280'inch, a width of 0.300 inch, and a length of 4 inches including a one inch taper at each end for impedance matching (see FIGS. 1 and 2). This ferrite rod is supported by polystyrene foam insulation in a brass rectangular waveguide section having an 0.40 inch by 0.90 inch internal cross section. A switch so constructed provides nearly constant electrical characteristics over a frequency range from 7,500 megacycles to 10,500 megacycles. The zero field insertion loss in the normally On state is aproximately 0.5 decibel and the isolation for the Off state exceeds 40 decibels, as shown by curve 31. The input voltage standing wave ratio is less than 1.2 for all values of applied magnetic field, and the switching time is less than one microsecond. A magnetic decibels at 9,250 mc., as shown by curve 3 3 for control fields of30 'oersteds or more. It has been found that this switch provides isolations greater than 60' decibels over a 2500 mc. bandwidth and as much as 55 db isolation was obtained over a 3500 mc. bandwidth. 7

Finally, when the resistivity of the attenuating resistive at the desired input frequency. Thus, amplitude modulation without any phase modulation is possible with the absorption modulator switch of this invention.

It will be apparent that the embodiment shown is only exemplary and that various modifications can be made in construction and arrangement within the scope of the invention as defined in the appended claims.-

I claim as my invention: I

1. An absorption modulator switch having broadband characteristics, comprising: a section of rectangular waveguide, a coil mounted around the outside of said waveguide section for applying a longitudinal magnetic field within said waveguide section, a ferrite rod insul at-ingly supported within said waveguide section so that the longitudinal lELX S of said rod and saidwaveguide section are coaxial, said rod split longitudinally thereof forming two opposing sections, a resistive film lyingwholly in a plane which is oriented parallel to the longer side walls of said waveguide section and located between said two ferrite sections, and a control circuit for selectively applying current to said coil.

2. An absorption modulator switch as claimed in 1, wherein said resistive filmcornprises a thin film of dielectric witlra coating of a suspension of minute graphite particles.

3. An absorption modulator switch as claimed in claim 1, wherein said resistive film comprises a base layer of mica having a thin coating of metal vapor deposited thereon.

4. A reciprocal absorption modulator switch having broadband characteristics, comprising: a rectangular waveguide excited in the fundamental TE mode, a coil around said waveguide for selectively applying a longitudinal magnetic field to said waveguide, a ferrite rod insulatingly supported within said waveguide for generating microwave energy in the TE mode, the longitudinal axis of said rod being coaxial with the longitudinal axis of said waveguide, said rod being longitudinally splitso as to form two halves, and a resistive film between said two halves for attenuating said TE mode energy said resistive film lying wholly in a single plane which is oriented parallel to the longer side walls of said waveguide.

5. A reciprocal switch as claimed in claim 4, wherein said resistive film comprises athin film of dielectric with a coating of a suspension :of minute graphite particles.

6. A reciprocal switch as claimed in claim 4, wherein said resistive film comprises a base layer of mica having I a thin coating of metal vapor deposited thereon.

8. A reciprocal switch as claim in claim 7, wherein said resistive film comprises a thin film of dielectric with a coating of a suspension 0f minute graphite particles.

9. A reciprocal switch as claimed in claim 7, wherein said resistive film comprises a base layer of mica having a thin coating of metal vapor deposited thereon.

10. A reciprocal microwave switch comprising a rectangular waveguide excited in the TE mode, coil means around said waveguide for selectively applying a longitudinal magnetic field inside said waveguide, ferrite means mounted along the longitudinal axis of said waveguide for selectively switching microwave energy inde pendently of the strength of said magnetic field, said ferrite means comprising a ferrite rod split in half and resistive attenuating means positioned perpendicular to the TE mode electric field and between the halves of said ferrite rod, and control means for selectively applying current to said coil means.

11. A microwave switch comprising: a rectangular wave guide; means for selectively producing an electromagnetic field within said wave guide; and means tor reciprocally switching incident microwave energy within said wave guide; said switching means comprising a ferrite 8 rod split into two sections and a resistive film positioned perpendicular to the shorter walls of said rectangular wave guide said resistive film being positioned between said two sections of said ferrite rod.

References Cited in the file of this patent UNITED STATES PATENTS 2,802,183 Read Aug. 6, 1957 2,802,184 Fox Aug. 6, 1957 2,922,964 Turner Jan. 26, 1960 3,018,454 Sfernazza Jan. 23, 1962 3,022,475 Blasberg etlal Feb. 20, 1962 OTHER REFERENCES

Claims (1)

1. AN ABSORPTION MODULATOR SWITCH HAVING BROADBAND CHARACTERISTICS, COMPRISING: A SECTION OF RECTANGULAR WAVEGUIDE, A COIL MOUNTED AROUND THE OUTSIDE OF SAID WAVEGUIDE SECTION FOR APPLYING A LONGITUDINAL MAGNETIC FIELD WITHIN SAID WAVEGUIDE SECTION, A FERRITE ROD INSULATINGLY SUPPORTED WITHIN SAID WAVEGUIDE SECTION SO THAT THE LONGITUDINAL AXES OF SAID ROD AND SAID WAVEGUIDE SECTION ARE COAXIAL, SAID ROD SPLIT LONGITUDINALLY THEREOF FORMING TWO OPPOSING SECTIONS, A RESISTIVE FILM LYING WHOLLY IN A PLANE WHICH IS ORIENTED PARALLEL TO THE LONGER SIDE WALLS OF SAID WAVEGUIDE SECTION AND LOCATED BETWEEN SAID TWO FERRITE SECTIONS, AND A CONTROL CIRCUIT FOR SELECTIVELY APPLYING CURRENT TO SAID COIL.

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