US2930932A - Electromagnetic wave phase shifter - Google Patents

Description

March 29, 1960 R. H. GEIGER ELECTROMAGNETIC WAVE PHASE SHIFTER 2 Sheets-Sheet 1 Filed April 1, 1957 FlG.l

F ck b 50 5:85 0 a 40 w m w -i|x|o' I ExlO' 32o INVENTOR Richard H. Geiger I60 240 Tube Voltage (Vpc) L3 5 E az ta ATTORNEYS March 29, 1960 R. H. GEIGER 2,930,932

I ELECTROMAGNETIC WAVE PHASE SHIFTER Filed April 1, 1957 2 Sheets-Sheet 2 INVENTOR Richard H. Geiger 0 BY M w MM M W ATTORNEYS ELECTROMAGNETIC WAVE PHASE SHIFTER Richard H. Geiger, Emerson, N.J., assignor to Roger White Electron Devices, Inc, Haskell, N.J., a corporation of New Jersey Application April 1, 1957, Serial No. 649,855

Claims. (Cl. '3l539.3)

This invention relates to phase shifters for electromagwhich such phase shifts may be made, very rapidly, in'

electron cloud of controllable density in a portion of the space associated with that structure and through which the wave must pass in propagation down that structure. This electron cloud changes the elfective dielectric constant of that space and hence alters the velocity of propagation of the wave over that portion of the slow wave structure with which this electron cloud is coupled. This alteration in propagation velocity alters the phase change which the wave undergoes in traversing the portion of the slow wave structure in question.

Phase shifting devices have previously been proposed employing ionized gases at low pressures as a means of controlling the dielectric constant of a medium. These have, however, suffered from serious shortcomings, especially when it has been desired to introduce very quick changes of phase shift, and also in applications involving waves carrying large amounts of power.

The discrete and, in terms 'of a cycle of the oscillations involved, substantial time required to move heavy gas ions and hence to change the degree of ionization of a volume of gas even at low pressures of the order of 100 microns of mercury has severely limited the speed with which it has been possible with these prior art devices to alter the phase change imposed on a wave during its passage through such an ionized gas. In addition, when high power levels are encountered the potential gradient in the RF. wave itself may take over control of the degree of ionization from the D. C. control electrodes provided for that purpose. Moreover, gas ion bombardment of the elements of apparatus introduces problems of contamination and variability of gas pressure with age-which render these previous devices generally unsatisfactory.

According to the present invention instead, control of dielectric constant and hence of phase shift is achieved by the provision of an electron cloud of variable density which is produced in a highly evacuated space by thermionic emission. The cloud is controlled, ina preferred embodiment of the invention, by the combination of an accelerating electrostatic field and a magnetic field transverse thereto, the magnetic field being adjusted to diminate by cutofl the actual current flow between them;- celerating electrodes. This makes it' possible toialter the imposed phase shift by the application "ojfvoltage signals alone, little or no power inputfbein'g' required to alter the phase shift achieved, 'ince substantially no Patented Mar. 29, 1960 gas ions are present, control of the electron density is not subject to capture by the waveintendedjto bacontrolled in phase. v h

The invention will nowbe further described with reference to the accompanying drawings in which:

Figure 1 is a diagrammatic view illustrating the basic concept of the present invention;

Figure 2 is a diagram in axial section of a phase shifter according to the invention employing a helical line as a slow wave structure; I

Figure 3 is an axial section view through a phase shifter according to the invention employing a helical line as a slow wave structure, shown coupled to input and output waveguides; and

Figure 4 is a graph useful in explaining the operation of the invention. V In Figure 1 a transmission line is diagrammatically indicated at 2, with a section of a transmission line 4 of the so-called slow wave structure type coupled into it. By slow wave structure is meant a transmission line Whether of single wire, waveguide, coaxial line, or other type, which by folding or otherwise provides a component of the electric field of a wave applied thereto which advances from one end of the structure to the other with a velocity which is a small fraction of velocity of light.

In a region '6 comprising at least a part of the space associated with line 4, i .e. at least a part. of the space immediately adjacent the line traversed by the wave pass ing down the line, there is provided an electronic dielectrio. This consists of an electron cloud produced by thermionic emission from a cathode and concentrated in a desired region adjacent or within the physicalelectrode elements of the transmission line 4 by an accelerating voltage which may be applied between a cathode es pecially provided for the purpose and the transmission line 4, or between electrodes distinct from those of the line 4 and the cathode. A suitable envelope not shown in Fig. 1 is provided for exhaustion of the space within which the electron cloud is generated. This electron cloud alters the effective dielectric constant of the space in which it exists, and hence the phase constant of the line 4 and't'hevelocity'of propagation the of the wave therealong, with a resultant alteration in the phaseshift undergone by the'wavc between its points of entry into and emergence from the region 6 on "the line 4. The density of the electron cloud and hence the magnetic field oriented transverse to the direction of the accelerating voltage between cathode andanode. This causes the electrons to move in orbital paths, and when the field is adjusted to an appropriate value it-brings the thermionic current between cathode and accelerating e c. trode to zero. A source of magnetic field is illu'st in-Fig, 1 as a solenoid coil 8, producinga field on its ax s perpendicular to the electro-st'atic accelerating field; 18hplied to the electrons.

An embodiment of the invention is shown in further detail in-Fi'gures 2 and 3. This embodiment employs a helic'al'line as a slow wave structure. As illustrated in Fig. 2, a helical line 12 is coupled at one end to an input wave guide 14 and, at the other end, to an outputwave guide 16, the coupling being effected in each case by a between the cathode shield and the envelope.

short metallic cylinder 18 coaxial of and electrically continuous with the helix. The helix is relatively tightly wound, with a pitch preferably substantially less than its diameter.

Coaxially surrounding the helix there is disposed at thermionic cathode, made up of a metallic cylinder 20 having an oxide coating 22 on its inner surface and a heater coil 24 on its exterior surface. The oxide coating 22 has a length axially of the helix amounting preferably to at least several times the helix pitch. The coil 24 is provided with an insulating coating to prevent it from being short-circuited by the cylinder 20. A heater voltage is applied between the ends of the coil 24, and an accelerating voltage is applied between the cathode 20 and the helix, which functions as an anode. This accelera'ting voltage is thus directed radially of the axis of symmetry of the helix and produces between the cathode and helix an electron beam of cylindrical section. Conveniently one end of coil 24 should be connected to the cathode to reduce the number of external leads.

A magnetic coil 26 disposed coaxially about the helix externally of the cathode generates a field parallel to the axis of the helix. Upon proper energization of this coil .the magnetic field generated thereby sharply reduces and then cuts off the thermionic current between cathode and helix. The magnetic field generated by the coil 26 is parallel to the axis of the helix and is preferably symmetrical in that axis particularly as regards the annular region between the helix and the cathode.

Fig. 3 illustrates actual construction of a phase shifting tube according to the invention. The helix 12 is wound on a rod or tube 28 of insulating material, for example of quartz, having a short metallic cylinder or ferrule 18 at each end. The cathode with its heater coil 24 is supported within a metallic cathode shield 29 and all of these elements are enclosed within an evacuated envelope 30, about which the magnetic field coil 26 is supported coaxially of the helix by independent means, not shown. The envelope is highly evacuated, preferably to a pressure of the order of or 10- mm. of mercury.

The helix is shown as a metallic ribbon or tape, for example of molybdenum or molybdenum alloy. The cathode shield may be formed with inwardly turned end flanges 32 to which are attached, as by means of screws 34, cathode 20 and mica spacing washers 36 which fit over the helix and which help to maintain the helix and cathode in coaxial relation. Spring clips 38 are stressed Three leads are provided, one at 40, which connects to the cathode shield for the cathode side of the accelerating voltage. Another is provided at 42, which connects to the helix via one of the ferrules 18 for the anode side of the accelerating voltage, and a third lead is provided at 44 to permit the application of a heater voltage between that terminal and terminal 40.

In the tube of Fig. 3 the effective interaction space over which the electron cloud is effective is the length of the cathode over which an emitting coating is provided. This may be the full length of the cathode cylinder 20. Over the length of the helix contained within the cylindrical volume thus defined the electron cloud produced by the combined action of the cathode, the accelerating voltage applied between cathode and helix and the magnetic field is effective to alter the dielectric constant in the annular .space between helix and cathode. This alteration in di- ;.electric constant leads to an altered phase constant for this portion of the helical slow wave structure. With a helical shape for the slow wave structure this altered phase constant can be written as cot =1r times the diameter of the helix divided by its pitch, w =(2rrf) being the frequency of the applied wave, to is the natural frequency of the electron cloud or gas, which is related to the electron density N in electrons per cubic centimeter by the approximate relation w,= s.6-10 /1'\ This phase constant k,, is referred to the phase velocity of the wave down the axis of the helix rather than to its phase velocity along the helical conductor, the latter velocity being equal or nearly equal to the velocity of light. It is for this reason that the factor cot ,0 appears in the expression, cot #1 being, for small pitch angles #1, approximately equal to sin l//, which latter function represents the ratio of the helical pitch to the length of a helical turn and hence the ratio of the axial phase velocity to the helical phase velocity 0.

In general the differential phase shift A introduced into a length L of transmission line by substitution of a medium of specified dielectric in place of air or a vacuum in the space adjacent or between the transmission line conductor or conductors may be written as in which h is the phase constant with air or vacuum as a dielectric and h is, as before, the phase constant with the substituted dielectric. h like h is a phase constant which refers to the axial phase velocity of the helix. It may be approximated, for a helical line, by the expression 21 c cot 1p With increasing N and consequent increase in u the effective dielectric constant is reduced over the length of the line having interaction with the electron cloud to values successively lower below unity. This reduces the efiective phase constant h and accelerates the wave, increasing the differential phase shift and causing the wave to emerge from the phase shifting tube at earlier and earlier phases. Reduction in the electron density N from a positive value toward zero obviously has the reverse efiect.

The accelerating voltage applied in Fig. 2 between cathode and helix controls the electron density and thereby the phase shift, higher accelerating voltages producing higher electron densities even though the magnetic field strength is kept constant for all accelerating voltages at values above cutoif. It will be noted by those skilled in the art that with a helical line as a slow wave structure, the phase shifters of the invention may possess a large bandwidth.

The nature of the relationships between electron density, incremental phase shift and accelerating voltage is set forth in the two curves of Fig. 4. The full line curve represents incremental phase shift, for which the axis of ordinates is given at the left side of the figure. The dashed line curve represents electron densities, for which the axis of ordinates is given at the right.

With a helix of 5-inch diameter and an interaction space one inch long, measured axially of the helix, it is readily possible to achieve electron densities of the order of 10 to 10 electrons per cubic centimeter. With an accelerating voltage of a few hundred volts, and by varying this voltage over a hundred volts or so, a 3,000 megacycle signal applied to the helix can be shifted in phase (i.e. subjected to an incremental phase shift which changes) through an angle of the order of one complete cycle.

While the invention has been described in terms of one preferred embodiment, numerous variations upon For example, the phase shifters of the invention may be coupled between transmission lines other than the waveguides shown. Moreover, as has already been stated, other types of slow wavestructure may be employed in place of the helical line which has been illustrated. Thus, there may be employed a section of coaxial line having a succession of metallic discs afiixed to the center conductor thereof, or a waveguide may be employed with a succession of apertured discs on the inner surface thereof. These can be regarded as folded transmission lines as if in each case a long section of line had been axially compressed with the resultant production of wrinkles on one conductor thereof in the form of the discs mentioned. Folded transmission lines of the type employed in carcinotrons may also be used. It will be recognized, of course, the phase shifters according to the invention may also be constructed with permanent magnets as the source of the magnetic field employed to achieve cutoff of the thermionic current.

I claim:

1. A phase shifter comprising a cylindrical support of insulating material, a conductor wound on said support in the shape of a helix having a pitch less than its diameter, coupling electrodes electrically connected to the ends of said conductor, a cylindrical cathode supported coaxially on said conductor exteriorly of said helical shape, said cathode extending axially of said conductor over a plurality of turns thereof, an evacuated envelope enclosing said cathode and at least so much of said conductor as lies Within the cylindrical volume defined by said cathode, the said cathode emitting an electron cloud, and electrostatic and electromagnetic field generating means to control the density of the said electron cloud.

2. A microwave phase shifter comprising: a slow wave structure having first and second ends; means forming an evacuated envelope around at least a portion of'said slow wave structure; means for launching an electromagnetic wave along said slow wave structure from said first end toward said second end; cathode means positioned within said evacuated envelope and around said slow wave structure; means for generating a radial electric field between said cathode and said slow wave structure to extract electrons from said cathode; and means for generating a magnetic field transverse to said electric field to produce an electron .cloud adjacent said slow wave structure to control the propagation constant thereof.

thereof; and means for generating transverse electrostatic v I and electromagnetic fields forcontrolling the density of said electron cloud.

4. A microwave phase shifter comprising: a helical slow wave structure having first and second ends; means forming an evacuated envelope around at least a portion of said slow wave structure; means for launching an electromagnetic wave along said slow wave structure from said first end toward said second end; cathode means positioned within said evacuated envelope concentric with I said helix; and electrostatic and electromagnetic field generating means coupled to said cathode for generating an electron cloud adjacent said slow wave structure to control the propagation constant thereof.

5. A microwave phase shifter comprising: a slow wave structure having first and second ends; means forming an evacuated envelope around at least a portion of said slow wave structure; means for launching an electromagnetic wave along said slow wave structure from said first end toward said second end; cathode means positioned within said evacuated envelope and around said slow wave structure for generating a substantially cylindrical electron cloud around said structure to control the propagation constant thereof; and electrostatic and electromagnetic field generating means for controlling the density of said electron cloud.

References Cited in the tile of this patent UNITED STATES PATENTS 2,141,976 Blewett et a1 May 13, 1941 2,721,953 Rothstein Oct. 25, 1955 2,788,464 Geiger Apr. 9, 1957 2,806,974 Haeff Sept. 17, 1957 2,811,664 Kazan Oct. 29, 1957.

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