US2626316A - Microwave slot coupling - Google Patents

Description

Jan. 2o, 19534 J. E. GIBSON 2,626,316

MICROWAVE SLOT COUPLING Filed OQt. 25, 1945 2 SHEETS-SHEET l me/whew JOHN E. GIBSON Patented Jan. 20, 10953 UNITED STATES PATENT yOFFICE (Granted under Title a5, U. s. code (1952), sec. 266) Claims.

This invention relates in general to the transfer of electrical energy and in particular to an arrangement for the coupling of high frequency energy between components of a microwave system or between two such systems.

In the usual high frequency system, such as is encountered in the field of radio echo ranging and related activities, the problem of transferring energy from one component to another must frequently be solved by means which take into account the fact that even the smallest circuit element, if possessed of any power-handling ability, has physical dimensions one or more of which constitutes an appreciable fraction of a wavelength. This electrical-length property denies a solution by ordinary direct current or low-frequency methods, since circuits at very short wavelengths rarely satisfy the conditions which warrant the assumption of an infinite characteristie electromagnetic velocity. Just as the need for the satisfaction of these conditions orfor methods particularly adapted to the problems presented dictated the adoption of concentric transmission lines and waveguides, so also did they dictate the adoption of energy coupling methods which bear little physical resemblance to D. C. or low-frequency coupling methods.

It is an object of this invention to provide a means of transferring energy at frequencies in the frecquency spectrum above 1000 megacycles' per second.

Itis another object of this invention to provide an efficient, convenient means of transferring energy from one transmission line to a second transmission line.

It is another object of this invention to provide a coupling-slot means of transferring energy from one system to another which permits the efficiency of transfer to be maintained at its optimum value over a wide range of frequencies by simple mechanical tuning.

Other objects and features of this invention I will become apparent upon a careful consideration of the following detailed description when taken together with the accompanying drawings in which:

Fig. 1 is a diagram illustrating certain of the principles upon which this invention is based;

Fig. 2 is an elevational view of a high frequency generator utilizing one embodiment of this invention 3 is an end view of the embodiment of Fig. 2 partially in cross-section looking in the direc-v tion of arrows 3-3 of Fig. 2.

Fig. 4 is a detailed view of the embodiment of this invention illustrated in Fig. 2.

Reference is now had in particular to Fig. l wherein there is shown a diagram illustrating certain of the principles upon which the teachings ofv this invention are based. Surface I represents a portion of one surface of a conducting sheet serving to separate, and to act as conducting boundary surfaces for..regions in which electromagnetic fields may be caused to exist. At frequenciescorresponding to centimeter or decimeter wavelengths the depth of penetration of the electromagnetic kfields intoA the surfacesv of,

the conducting sheet isso small that essentially no mutual interaction between the electromagnetic elds is possible even though the conducting sheet serves as a common boundary conductor. Consider an electromagnetic field to be gen-V erated, by means not shown,l in the region bounded by surface I with such an orientation as to tend to produce a current ow on surface I everywhere parallel to bounding edges-2 and 3. An aperture 4 having a width wwhich is a fractional part of a quarter wavelength and a length.

l equal tol one half wavelength is cut in the conducting sheet with its long dimension lying perpendicular to the direction inwhich currentk tends to ow on surface I. Such an aperture or slot has many properties similar to those of a half-wave two-wire Lecher frame. Consideredl independently of any shunt impedances provided,- by any circuit system of which the conducting.

the midpoint and the ends in a manner, similar1 to that obtained with a Lecher frame of corre,- sponding length. Since the slot is Jcommon to both surfaces of the conducting sheet,.it represents a common impedance between any circuit.

system of which surface I is a part and any cir:

cuit system of which the oppositey surface is a part. Thus, the provision of` an electromagnetic eld tending to induce a current in surface I parallel to edges 2 and 3 will tend to cause a current flowin the opposite surface and thereby a current flow in any circuit system of which this surface is a part.k Further, the provision of anl electromagnetic field tending to induce a current in surface I not parallel to edges 2 and 3 but of, an orientation suchv that aperture 4 has atleast a component of its length dimension perpendicular to the direction of induced current now will also cause a current flow in the opposite surface. In general, the greater the component perpendicular to the direction of induced current, the more effective is the production of a current in the opposite surface. The manner in which these phenomena are utilized for the transfer of energy will be described more fully in connection With a practical embodiment of this invention.

In Fig. 2 is shown such a practical embodiment in the form of a generator of oscillations in the 2400 to 3400 megacycle per-.second band of the frequency spectrum. This generator, comprising two lighthouse-type triode vacuum tubes particularly designed for high frequency work and a plurality of concentric cylindrical members as resonant sections, operates in a manner described in my co-pending application entitled Ultra- High Frequency Oscillator, Serial No. 624,618, led October 25, 1945. The two tubes, one of which is indicated at 20, are generally disposed in face-to-face relationship` at opposite ends of the oscillator and are separated by the cylindrical members which constitute the resonant section. Furthermore, the tubes operate with a phase difference of 180 degrees so that the generator functions in a manner somewhat analogous to the manner in which a conventional push-pull tuned-plate tuned-cathode low frequency oscillator functions. While this analogy islimited by the inadequacy of ordinary circuit theory at extremely high frequencies, it does extend to the requirement for an energy transfer from the plate-grid resonant section to the cathode-grid resonant section in order toprovide the feedback coupling to support self-oscillation.

In Fig. 2 a section has been cut away to expose the disposition of the components comprising one end of the oscillator. The unexposed end is essentially identical and includes the other triode tube with its anode cap facing that ofthe visible tube. In general, telescopic cylinder represents the inner conductor and telescopic cylinderV 6 the outer conductor of a three half Wavelength resonant concentric transmission line. Thisline constitutes the plategrid resonant section of the oscillator and in the mode in which the generator isoperated-is characterizedY by the location of a voltage node and currentV loop at the generator midpoint defined by a plane 3 3` through the generator transverseA to its longitudinal axis. Likewise, and'again in general, telescopic cylinder- 6 represents the inner conductor and telescopic cylinder'l the outer conductorof a portion of a ve halfwavelength resonant concentric transmission line. This line constitutes 4the cathode-grid resonant section and is likewise characterized by the location of a voltage node and current loop at the generator midpoint. Since oscillationV requires that anode and cathode voltages with respectto grid-be essentially in phase, the difference in electrica-llength of an odd full wavelength between the two sections establishes a phase difference of essentially 180 degrees for'the currents' on the outer and inner surfaces of cylinder 6 4at the midpoint of the generator. There is no coupling between the tWo resonant sections merely by virtue of their common use of cylinder 6 since, at the frequencies under consideration, current flow is substantially a surface phenomenon and the thickness of cylinder 6 is very large in comparison to the depth of current penetration. The resonant transmission-line sections operate in the usual TEM mode of coaxial transmission lines so that the electrical field is radial, the magnetic field is circumferential, and current ows in a direction parallel to the longitudinal axis of the generator.

Telescopic cylinder 5 comprises in part a central element 5', telescopic cylinder 6 a central element 5', and telescopic cylinder 'I a central element l', all of such central elements being held in a fixed position relative to chassis 8. The movable elements (5, E, and 1) of the three cylinders provide the means by which the physical lengths of the resonant sections may be changed and thus the generator tuned to a desired frequency within its band of operation.

At the generator midpoint there is cut into centralelement 6 a circumferential slot corresponding to aperture 4 of Fig. 1. This slot, which has been given the reference numeral 9, is best illustrated in Fig. 4 which is a diagram of the mid-section of element 6'. It will be seen that this slot is disposed with its length transverse to the direction of current now along the inner andV outer surfaces of element 6 and that it has been provided with a pair of shoulders Il and II not. present in the case of aperture 4 of Fig. l.

Shoulders I0 and II comprise a circular conducting disc xed to element 6 across aperture 9 which disc has hadits midsection removed to coincide with the interception of the aperture and represents a part of the means by. which, according to the teachings of this invention, the electrical length of a slot of fixed physical length, such as slot 9, is maintained at an optimum value for the particular frequency tof which thecgenerator may bectuned In the same Wayin which a lumped capacitance across the midpoint of a half-wave Lecher frame shorted at both ends can ibe utilized to maintain the electrical length while a reduction is made in its physical length, the lumped capacitance across the midpoint of slot 9 serves to extend the equivalent electrical length,

capacitance across the slotatits midpointr con-A sists, not only of the, fixed capacitance between shoulders I0 and II, Ibut also of the seriessum of the capacitance between Ashoulder I0 andthe portion of plate I2immediatelyadjacent thereto and;

the capacitance .between shoulder I I and thepor,- tion of plate I2 adjacent to it. Thus, movement of plate I2 may be utilized to vary this total capacitance and thereby to vary the electrical length of the slot to the optimum value for the transfer of energy between the tWo resonant sections. Threaded adjusting member I3 is sup` ported from outer cylinder element I by meansl of boss I4. Plate I2 is preferably surfaced with mica to prevent the short-circuiting of shoulders.

I0 and II when plate I2 is adjusted to a position providing little separation.

Since slot Sis disposed in a transverse sense to the longitudinal flow of alternating current along the inner surface of element 6", a condition exists, such as wasA described in connection.

with Fig. l, in which slot 9 constitutes a common impedance between the plate-grid resonant section and the cathodegrid resonant section and provides for coupling by virtue of the mutual impedance which exists. Thus, electromagnetic en-A ergy existing in the Iplate-grid resonant section surface of elementA 6 at the midpoint tending to be everywhere parallel to the longitudinal axis of the oscillator, and the flow of this current in the vicinity of the slot is impeded by a total impedance consisting of the slot impedance in shunt with the effective impedance of the cathode-grid resonant section as seen at the slot looking from the plate-grid resonant section into the cathode-grid section. Therefore, if, for example, optimum tuning of the cathode-grid resonant section is represented by the resonance condition, the effective impedance of the cathode-grid section as measured from terminals at the slot is essentially resistive, and if the slot is tuned to resonance, that is, tuned to represent resistive impedance when taken by itself, the total impedance across the slot to current flowing on the inner surface of element 6 in the vicinity of the slot is resistive and has a magnitude equal to the parallel sum of the slot impedance and the cathode-grid section effective impedance.

In this case, at points corresponding to the oscillator midpoint, the directions of the current ows on the outer and inner surfaces of element 6' corresponding to excitation of the plate-grid and cathode-grid sections each as a whole are op posite at al1 times during the cycle. Also, in this case, the current now around the ends of the slot, corresponding to the excitation of the slot itself, occurs with a quarter cycle phase difference with respect to the current flow in the resonant sections as a whole. The degree of coupling provided by the slot is dependent in part upon the proportion of the circumference of element 6' occupied by the slot, and varies from negligible values for slots of innitesimal length to large values where the slot length is made to approach the circumference of element 6'. For the same tuning conditions with respect to resonance, increasing the slot length decreases the eifective impedances of the plate-grid and cathode-grid resonant sections as measured from opposite points across the slot. If the most desirable phase of excitation of the cathode-grid section with respect to the plate-grid section differs somewhat from that corresponding to direct opposition of the current fiows on the outer 'and inner surfaces of element 6', the slot may be detuned from resonance byzan amount and in a sense such as to obtain a reactive slot impedance v whichin lcombination with the effective impedances of theAplate-grid and cathode-grid sections yields the desired phase of excitation in accordance with the principles underlying the operation of the more familiar circuit networks. gThus, it is seen that ther application of the teachings of this invention permits the provision of a slot coupling means affording a desired degree of coupling, and permitting the adjustment of the phase of excitation to an optimum value at any frequency in a wide portion of the frequency spectrum.

Although a particular slot shape has been described, this particularity has been employed, not to establish limits in configuration, but to facilitate description of the principles of operation. Actually, the dimensions of the aperture and the shape of the aperture may vary according to the requirements of a particular situation. The length dimension of one-half wave length is a special case, the optimum length under other conditions may be more or less than one-half wavelength. The same basic eect is obtainable with apertures in shapes other than simple slots without exceeding the limits 'of this invention." While reference has been made herein to a simple along the direction of current flow (slot widthV being taken as negligible). The degree of coupling depends upon the angle of the slot and varies from zero for the slot parallel to current ow to a maximum value for the transverse position. In the cylindrical generator application,

this feature affords a variable coupling possibility.

separate from the variation ofcoupling possible through changes in the proportion of cylinder circumference occupied. To those versed in the art this obviously represents a convenient means of varying the coupling in certain suitable arrangements such as waveguide apparatusv Again, it should be understood that, although the coupling slot which has been described is a preferred embodiment of this invention, it by nomeans establishes the boundary application.v For example, a similar coupling slot, similarly oriented, but inserted in outside cylinder l., at the generator midpoint could be used to transfer energy from the cathode-grid resonant section to a waveguide so as to excite the waveguide in a suitable mode to act as a transmission line between the generator and an external load. Further, it will be seen that other applications, differing in physical arrangements but the same in principles, may be made to advantage. As an example of this, a resonant circuit may consist of parallel plane conducting walls which may contain an aperture or apertures which function to transfer energy in the manner explained. For the purposes of this disclosure, such van arrangement is defined, in the same sense that the plate-grid and cathode-grid sections of the generator of Fig. 2 are defined, as a wall enclosed component part.

Since certain further changes may be made in the foregoing construction and different embodi-r factured and used by or for the Government ofv the United States of America for government purposes without the payment of any. royalty thereon ortherefor.

What is claimed is:

1. In combination, a plurality of conducting wall members arranged to constitute a plurality of nested energy containing sections of which at least two have a common wall member, said two sections having said common wall member being so oriented that the directions of current flow along the surfaces of said common wall member in said two sections are parallel, a means coupling energy from one of said two sections into the other of said two sections comprising an aperture cut in said common wall member, and a lumped variable capacitance across said aperture at its physical midpoint, said aperture being, in width, a fractional part of a wavelength at the frequency'of said energy and, in length, less than a half wavelength at said frequency, said aperture. also: being 'disposed insaidY common wallV memberwith its length dimension .transverse to section removed to coincide with the interception of said. apertureand the other of which is mov- Aable.witl'rrespect`-to said xed disc, said nlovability` providing'a range of variable reactance such that theA electrical length of said aperture may be made to represent substantially a half wavelength at said frequency.

2. Inv combination, a plurality of conducting walll members arranged tov constitute a plurality of. nested energy containing sections of .which at least two have a common wall member, said two sections having said common wall member being so oriented that the directions of current flow along the surfaces of said common wallV member in said two'sections are parallel, ameans coupling,

energy from one of said. two sections into the other of said two sections comprising a slot cut in said common wall member transverse to said. directions of current flow, said slotl being, in width a fractional part of a' wavelength at they frequency of said energy and, in length less thana half Wavelength at said frequency,.a pair of conduct@V ing parallel plates, one of said plates being xed on said common wallmember at the physical midpoint` f said slot, said plates having a width greater thanthat of said slot and a' length less than that of said slot, said one plate having its midsection removed to form' an. aperture coincident With but of smaller width than said slot, and means for moving said other plateaxially toward and away fromsaid rst plate.l

3. In combination, a plurality of conducting wall members arranged to constitute a plurality of :nested energy containing sections of which at least two have a common wall member, said two sectionshaving said common wall member being so oriented that the directions of current flow along the surfaces of said common wall member in said two sections are parallel, a means coupling energy from one of said two sections into the other of said two sections comprising a slot cut in said common wall member transverse 'to said directions of current flow, said slotbeing, in Width a fractional part of a wavelength atthe frequency of said energy and, in length less than a half.

wavelength at said frequency, afpair'of conducting coaxial parallel discs', one of said discs being.

fixed on said common wall member `at the physical midpoint of said slot, said discs having a diameter. greater than the slot wid-th and less than the.

slot. length, said onev disc having lits midsectionY removed tdforman aperture coincident with but of smaller width .than that. of said slot, and means for moving said other disc axially toward and away from said one disc.

4. In a cavity type oscillator having concentric cavities forming awall member common to two cavities on which the current in each of the two cavities is parallelcoupling means between said two cavities providing the optimum feedback for supporting oscillations comprising, an-

elongated slot in said common Wall member having ay length less than. ahalf Wavelength at the frequency of said. oscillations, said slot beingl positioned transverse to the flow of said currents, apair of conducting parallel plates, one` of said plates being iixedI on saidcommon wall member at the physical midpoint of said slot,.said plates.

having a width greater than that of said slot and alength less than that vci? said slot, said oneplate having its midsection removed to form an aperture coincident with but of smaller width than said slot, and means for moving said other plate axially toward and away from said iirst plate.

5. In acavity typeoscillator having. concentric cavities forming a wall member commonto two cavities on which the current ineach of the two cavities is paralleLcoupling means between said two cavities providing the optimum feedback for supporting oscillations comprising,y an elongated slot in said common wall member having a length less than ahalf wavelength at the frequency ofv said oscillations, said slot being positioned Vtransverse to the i-loW of said currents, a pair of conducting coaxial parallel discs, oneof said discsy being xed on said common Wall member at the physical midpoint of said slot, said discs having a diameter greater than the. slot Width and lessv thanthe slot length, said one disc having its midsection removed to form an aperture coincident with but of smaller width than that of said slot, and means for moving said other disc axially toward and away from said one disc.

JOHN E. GIBSON.

REFERENCESv CITED The following references are of record in the le of this patent:

UNITED. STATES PATENTS

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