US3183458A - Radio frequency liquid dielectric load with inner conductor and tapered shell - Google Patents

Description

May M, w65 A. A. GOLDFINGER 3,183,45

RADIO FREQUENCY LIQUID DIELECTRIC LOAD WITH INNER CONDUCTOR AND TAPERED SHELL Filed Dec. s, 1960 United States Patent() 3,183,458 RADEQ FREQUENCY LIQUED DIELECTREC LAD WH'H INNER CQNDUCTQR AND TAERED Simili Arthur A. Goldfinger, Palo Alto, Calif., assigner to Eitel- McCullough, Inc., San Carlos, Calif., a corporation of Caiifornia Filed Dec. `8, 1960, Ser. No. 74,571 3 Claims. (Cl. S33-22) This invention relates to radio-frequency loads, and more particularly, to a microwave water load for the dissipation of electromagnetic energy.

In the electronics industry it has become necessary to develop devices to temporarily absorb the large amounts of power which are generated by electron tubes and apparatus of various types, such as radar units, UHF television transmitters, magnetrons, and klystrons. Because of the high power capabilities of these units it is necessary to provide dummy loads which permit generation of the required energy and tuning of apparatus while precluding detectable radiation of the energy. Thus, in a military radar installation, particularly under battle conditions, it is necessary to tune the radar equipment to an optimum power output. The energy generated must be dissipated or absorbed in some manner without radiating it into the atmosphere and thus disclosing the location of the transmitter. For this purpose, Water loads have been found to be the most ideal dummy loads heretofore provided. It has been found that when microwave energy is exposed to the pointed or tapered end of an attenuating medium, such as water, such energy is absorbed and converted into heat in the attenuating medium.

Because a rapid rise in the temperature of the attenuating body of water occurs, the container elements fluctuate in dimension due to thermal expansion and contraction, thus creating problems of sealing the liquid dielectric or attenuating medium within the container. It is therefore an important object of the invention to provide a liquid attenuator or water load designed to accommodate such thermal expansion and contraction without destructive strains.

It has been determined that in the operation of a water load, the sharper the tip or cross-section of the tapered body of liquid dielectric or attenuating medium and the longer the taper, the lower will `be the VSWR, and the more eiiiciently will the load absorb radio-frequency energy. t is therefore still another object of the invention to provide a water load, including a coaxial transmission line portion, in which the taper is minimal at the electromagnetic energy input end of the water load, and gradually increases in diameter to a maximum dimension equalling the interior diameter of the outer conductor of the coaxial transmission line.

The water load of the present invention is designed to absorb radio frequencies ranging upward from about 200 megacycles. Water loads designed to operate in this frequency range are normally required to have a very long impedance transformer to provide the desired VSWR, and because of their length such transformers tend to be fragile. lt is accordingly another object of the invention to provide a water load which incorporates a long tapered container within a coaxial transmission line, the tapered container being adapted to contain a liquid dielectric constituting an impedance transformer less fragile than transformers of conventional waterloads.

To reduce undesirable reflections of radio-frequency energy admitted to a water load, it is desirable that the transformation ratio from the coaxial transmission air line to the liquid dielectric coaxial section be reduced to a minimum. Thus, in a water load for use with a SO-Ohrn line in which the inner conductor of the load has a constant diameter, the transformation ratio would be about 9 to 1. It is therefore another object of the invention to provide the inner conductor of a water load with a configuration which will reduce the transformation ratio to about 4 to 1.

It is a still further object of the invention to provide a water load which may be constructed from standard coaxial transmission line materials adapted to demountably receive a tapered dielectric container.

In using water loads to absorb radio-frequency energy, it is desirable that the transition from the impedance of an air dielectric coaxial transmission line to the impedance of the liquid dielectric coaxial section of the line be as smooth as possible. It is therefore another important object of the invention to provide a coaxial water load in which the impedance is gradually reduced and the attenuating characteristic of the load `section of the transmission line is gradually increased.

Inasmuch as the radio-frequency load must be asse i.- bled from rigid mechanical units having different dielectric constants, it is extremely difficult to provide a smooth progressively increasing or decreasing cross section of liquid dielectric without discontinuities, both electrical and mechanical, in the linner and outer conductors which adversely aifect the VSWR. It is therefore another object of the invention to provide means for compensating the effect produced by the introduction of materials having dilferent dielectric constants to prevent such differences fr'om adversely aifecting the dissipation of energy in the load.

1n a coaxial water load constructed according to this invention and capable of dissipating over 50 kw. of radiofrequency power at frequencies ranging between 225 and 1200 megacycles, the length may reach approximately thirteen feet. ln water loads of this length it is diiiicult to support the inner conductor throughout its length in coaxial relationship with the outer conductor. It is therefore another object of the present invention to provide means for supporting the inner conductor at appropriate intervals, and compensating for the different dielectric constant introduced by the support structure.

A still further object of the invention is the provision of a coaxial water load which may be constructed in a short embodiment to absorb radio-frequency power at a high frequency limit, and which may be extended by the addition of easily attached extensions to make the load applicable to the dissipation of radio-frequency power at lower frequency limits.

The invention possesses other objects and features of advantage, some of which, with the foregoing, will be apparent from the accompanying description and drawings. lt is to be understood, however, that the invention is not limited to the embodiment described and illustrated, as other forms may be used within the scope of the appended claims.

Brieily described, the radio-frequency water load of this invention comprises a length of coaxial transmission line having inner and outer conductors. At one end the outer conductor is provided with a flange for detachably connecting the water load to a coaxial transmission line, while the associated end of the inner conductor is provided with a resilient coupling or connector for connecting the inner conductor of the water load to the inner conductor of the associated transmission line. The opposite end of the water load is provided with an end cap structure having inlet and outlet ports for a liquid dielectric, such as Water or ethylene glycol.

The inlet port connects with the interior of the hollow tapered inner conductor, and liquid introduced into the inner conductor is discharged therefrom adjacent the R-F input end of the water load into a conically tapered annular chamber surrounding the inner conductor and being contained and defined thereby and by a conical dielectric sleeve, rf`he sleeve at one end is connected to 4the inner conductor intermediate its ends in a manner to provide for the expansion and contraction of the parts due to increases and decreases in temperature. Means are associated with this end of the sleeve to compensate for its introduction into the path of the electromagnetic wave. The taper of the sleeve and the taper of the inner conductor are proportioned to provide an infinitely thin edge to the body of liquid at the R-F input end of the water load. The sleeve diverges toward the outer conductor of the water load and terminates in a feather edge which provides a smooth transition between the conical and cylindrical sections of the water load. Means are provided for clamping and sealing the large feathered end of the dielectric sleeve in liquid-tight relation with the outer conductor.

Referring to the drawings:

FIGURE 1 is a horizontal half-sectional view showing the interior construction of an extended version of the water load. A portion of the water load is broken away to decrease its length.

FIGURE 2 is a fragmentary view of the end cap arrangement in a short version of the water load.

FIGURE 3 is a transverse sectional view taken in the plane indicated by the line 3 3 in FIGURE l.

FGURE 4 is an enlarged sectional view showing an alternative method of joining two sections of the ex-` tended version of the water load.

FIGURES 1, 2, and 3 are drawn to a scale approximately one-half actual size, and FIGURE 4 is drawn to a scale approximately full size.

Electromagnetic wave energy is dissipated in a dummy load by causing the electromagnetic energy to encounter an attenuating substance, such as water, which completely attenuates the wave by transforming the electromagnetic energy into heat. To prevent undesirable refiections of the wave it is desirable that impedance change be effected gradually, with minimum attenuation occurring adjacent the R-F input of the load and gradually increasing until maximum attenuation is effected. Attenuation at maximum value is continued until the transformation of wave energy to heat is complete, which ideally occurs in advance of the terminal end of the load.

Coaxial water loads of the type illustrated are utilized to terminate coaxial transmission lines along which electromagnetic energy is propagated through the annular space between inner and outer conductors. To effectively attenuate the waves so propagated, a water load must, therefore, provide a body of an attenuating medium or liquid, preferably water, in the annular space between inner and outer conductors, arranged so that attenuation of the wave is effected gradually.

Minimum attenuation is effected where the cross-sectional area of the liquid dielectric first encountering the electromagnetic wave is minimal, and it is therefore desirable that the edge of the liquid which the electromagnetic Wave first encounters be as sharp as possible. Since the inner conductor of the water load extends for the full length of the outer conductor, the liquid must surround the inner conductor in 4the annular space between the inner and outer conductors and intermediate the ends thereof. This necessitates that the liquid be contained in a manner to provide the requisite gradually increasing cross-section by means which are also transparent to electromagnetic energy.

Through extensive experiments it has been established that a very satisfactory cross-sectional configuration for the body of liquid dielectric is one in which the sharpest j and leading edge of the body of liquid surrounds the inner conductor adjacent the R-F input end of the water load and gradually tapers away therefrom in opposite transverse directions to a maximum cross-sectional area annular in form defined by the inner diameterof the cylindrical outer conductor and the minimum outer diameter of the tapered inner conductor. The diameter of the thin leading edge of the liquid preferably approximates the mean diameter of the annular maximum cross-sectional area of the liquid. Electromagnetic energy entering the load thus passes through the electromagnetic transparent means confining the liquid and is gradually transformed into heat whichis dissipated by causing the liquid attenuator to circulate through the load.

The dim-cuit" of arranging rigid mechanical elements in a liquid-tight manner to provide the body of liquid attenuator with the requisite tapered configuration, while at the same time preventing undesirable reflections of the electromagnetic energy will be readily appreciated.

To obviate these sealing and reflection problems, which are present in most conventional water loads, the radiofrequency load of the invention comprises a cylindrical conductive container or metallic housing 2, having an R-F input end designated generally by numeral 3, and a liquid dielectric input-output end cap structure designated generally by numeral 4. The R-F input end of outer conductor 2 is provided With a flange 6 for appropriate connection to a corresponding flange on a coaxial transmission line (not shown). At its other end the cylindrical conductive container 2 is provided with an end cap structure including a body portion '7 bored to provide inlet passage S adapted to be connected by an inlet fitting 9 to a source of liquid dielectric. Egress of the liquid dielectric is permitted by outlet passage 12 in the body V7 provided with an appropriate connector 13. A radial flange 14 on body portion 7 provides means for detachably securing the end cap structure to a complementary liange 15 on the end of the container -2 or an extension thereof as shown. A rubber O-ring 16 interposed between the flanges cooperates with bolts 17 to form a liquid-tight union of the fianges.

The inner conductor includes an input connector sleeve 1S detachably supported intermediate its ends on the outer conductor 2 by an annular dieletric ring 19, and having at its forward end a plurality of resilient fingers 29 for resilient mechanical and electrical interconnection with the inner conductor of the transmission line to which the water load is connected. At its other end the connector is provided with similar resilient fingers which, in the drawing, have been broken away for the sake of clarity. Adjacent their base the fingers form cylindrically arranged tianges 21 providing a seat for one end of a tubular inner conductor extension 22 which slips over the resilient ngers in a snug sliding fit for good electrical continuity. The other end of the tubular inner conductor extension is brazed to the rabbeted periphery of contact block 23, preferably silverplated copper, which also forms a part of the inner conductor.

The contact block isdetachably connected by cap screw 26 to the closed or solid base end 27 of hollow conical inner conductor section 28. Contact block 23 is centrally bored to receive dielectric insulator bushing 29 which serves to electrically insulate cap screw 26 from the contact block. The end of the contactblock adjacent base end 27 of the hollow inner conductor section 28 is counterbored to provide a recess into which a centrally disposed boss 31 on the end of base 27 is adapted to extend. As shown in the drawing, the peripheral circular surface of the boss 31 is spaced from the complementary inner periphery of the recess in the contact block an amount to snugly receive a dielectric insulator 32, which electrically insulates boss 31 from the contact block.

The adjacent outer peripheral edges of contact block 23 and base 27 of the inner conductor 28 are provided with rabbeted recesses thereabout, the rabbet in base 27 being adapted to accommodate a deformable O-ring 33, and the rabbet in the contact block being adapted to accommodate the inner radially fianged apex end 34 of the truncated hollow dielectric cone 36, preferably of Teflon, which constitutes a radio-frequency window Vand a liquid-impervious wall interposed between inner and outer conductors. As shown in FIGURE 1, cap screw 26 interposed between contact block 23 and base 27, draws these two parts together and effectively binds the inner end of the dielectric cone Vbetween the deformable O-ring 33 and the contact block, thussealing the .union between the cone and the base 27 in a liquid-tight manner. The counterbored central recess in the contact block and the rabbeted periphery thereof dene 'betweenthem acyli-ndrically extending ange l37 which makes electrical contac with the base end 27 of the inner conductor when cap screw 26 is tightened. When the parts are assembled, it is this ange which iirst makes electrical contact with base 27 and, by appropriate means, such .as an ohm meter connected von opposite sides of the joint, indicateswhen the flange 37 `has bottomed onto the end of the inner conductor base. The input connectorl is. detachably connected to this assembly by Vmeans of capscrew 38 threaded into bore 39 in headftt) ofv cap screw 26.

As previously discussed,` abrupt variations` in the crosssectional diameter of the spacevbetween inner conductor and outer conductor are apt to create undesirable dis- -turbances in the wave being transmitted therethrough.

Thus, at the union of the contact block 23 -and the base end 27 where the two are' peripherally rabbeted, the Wave sees an abruptly larger spacing between inner and outer conductors because of the -groove formed inthe inner conductor at this point -by the juxtaposed rabbets. To compensate for this abrupt enlargement -of spacing, a di electric sleeve orrshell 4l having a tapered body is slipped over the inner conductor and is provided at `its end adjacent the groove with a radially extending liange .42 proportioned to reintroduce into the annular space an appropriate amount of reactance to compensate the abrupt change in diameter of the inner conductor. The tapered body of the compensator shell extends forwardly over the contact block Vand `terminates at its end remote from flange 42 in a feathered edge ylying intermediate the ends of extension 22,--thus eliminating the abrupt step at the end S14-of cone'36. A plurality of dielectric pins 43 driven through shell 4i and into contact block 23 secure the compensator shell in position.

From its apex end 34, the dielectric cone 36 diverges outwardly to a base end V44 having a radially extending iiange 46. As shown inFlGURES l, 2, and 4, the outer conductor 2 is provided with a radially extending integral tlange 47 having a recess 43 formed around the inner periphery lthereof -within which radially'extending flange 46 may seat. In embodiments of the water load which continue in length beyond the transformer section defined by dielectric cone 36, an outer conductor extension 49 is provided with a radially extending iiange 51 corresponding to iiange 47. Bolts 52, circumferentially spaced about and extending through the anges, bind the anges together.

To ensure water-tightness at tlt's union, a gasket 53 is provided between the anges and abutting the base end ange 46 of the dielectric cone. Since the inner diameter of the cone at its base 44 is somewhat smaller than the inner diameter of the outer conductor at this point, this abrupt differential in diameters forms a step eliminated lby a tapered dielectric shell 54, having a cylindrical outer periphery and a conical inner periphery forming a continuation of the conical inner peripheral surface of cone 36. A groove 56 in the outer periphery of shell 54 cooperates with a plurality of dimples 57 formed in the outer conductor extension 49 to secure shell S4 in position. It will thus be seen that terminal end 58 of compensator shell 54, being feathered or tapered to the same diameter as the inner diameter of extension 49, provides a smooth transition for the electromagnetic wave from the conical section to the cylindrical section. Alternatively, compensator shell 54 may be secured in outer conductor extension 49 by the construc- Y when empty.

tion shown in FIGURE 4, in which the compensator shell is provided vwith a radially extending flange 59 caught between ilanges 47 and 51. Water-tightness ofthe union is ensured by gasket 63 interposed between anges 46 and 59.

Brazed to the apex :end of the conical metallic inner conductor section 2S `and forming an extension thereof, is .a tubular extension 66, lthe remote end 67 of which isslidably supported in 4a'liquid-tight manner inrcentrally bored boss 68 extending from body 7 of end cap structure d. Suitable gasket means, such'as a deformable O-ring, interposed between the bossand end 67 of tubular extension 66, provides -a water-tight union while permittingrelative axial movement of the partsdue to thermal expansion and contraction.

it will thus be seen that a liquid dielectric admitted through passage `8- passes successively through inner conductor'extension k66 Vand'conical inner conductor section 28, andis discharged into the conically-tapered liquid container or chamber `.70 through passages or apertures 71 in base end 427 of theinner conductor. The coolest water isA therefore admitted :to Vchamber 7,0 at its thinnest cross section and circulates backward `toward `the outlet passage 12.

4It has Vbeen .found through .experiment that a water load capable of dissipating over 50 kw. ,of R-F power in the range between about 500 and 1200 megacycles is only about 351/2 inches long between its mounting flanges, and 3% inches-in diameter, andweighs only 131/2 pounds Larger diameters and lengths may, of course, be used .in ,accordance with frequencyl ,and -power .dissipation-requirements. The uid ,capacity .of a water load having the proportionsvnoted is approximately one- ,half gallon andthe tlow to dissipate ,the power `indicated is .preferably ten .gallons per minute. 1t hasbeen .found that up to .about p.s.i. pressure ,on the liquid dielectric canbe sustained bythe .water Vload of this invention, but to provide the requisite ow andA fiuid `temperature ofthe liquid dielectric `the pressure should be. correlated` to the amount of power it is desired to dissipate. For example, to dissipate approximately .50l kw., .with..a 20 C. rise in the temperature of the liquid dielectric, a riow rate of 10 g.p.m. .would require approximately a tive-pound head. Tap -water may be used .as .the liquid dielectric, or if desired, ,deionized water, `Oria lsolution of V60% ethylene glycol and 40% deionized water, may be used.

In embodiments -of Ithe .water .load having a length up to about.13 feet, itis desirable ithat` the inner conductor be supported at intervals Valong its length. Conventional supports for coaxial transmission line, ihowever, are inadequate for inclusion in a water .load of thistype because they obstruct the tlow of liquid dielectric and also because they introduce at intervals material having a dierent dielectric constant than has the liquid dielectric and therefore cause an abrupt increase in the impedance which introduces a sharp electrical discontinuity in the water load. To obviate these problems and still provide adequate support for the inner conductor, the inner conductor is supported by a dielectric spacer (FIGURE 3) having a hub 72 integral with a plurality, preferably three, radially extending arms 73 contacting the inner surface of the outer conductor. The hub of the dielectric spacer surrounds a metallic compensating ring 74 secured as by brazing to the inner conductor, the Spacer or hub being secured to the compensating ring by a dielectric set screw 76. The metallic compensating ring compensates for any abrupt discontinuity in impedance caused by introduction of the dielectric spacer. To lighten the spacer and also to remove as much material as possible to lessen the effect of the spacer as a deterrent to the ow of liquid through the water load, and also to minimize the low dielectric medium to lessen the electrical discontinuity caused thereby, the arms 73 are preferably apertured as shown.

From the foregoing it will be seen that with water ad- 7 Y mitted to the water load and passing therethrough at approximately 10 gallons per minute, and with radio-frequency energy admitted at the input end, an increase in the temperature of the water will be reflected by an increase in temperature of the mechanical elements which channel and contain it. Such increase or decrease in temperature of the mechanical elements will result in expansion and contraction of these parts. Inasmuch as the outer conductor 2 will remain at substantially constant or ambient temperature, or perhaps cooler, it is important that elements of the combination which expand and contract with increase or decrease in temperature be arranged in a manner to prevent destructive stresses. The inner conductor being metallic, and the base end of the dielectric cone 36 being anchored in position, it will be obvious that these two elements will expand and contract at different rates. It is therefore important that the input end of the inner conductor be slidably supported in the insulator 19. It is likewise important that the terminal end 67 of the inner conductor in either the short or extended embodiments be slidably supported in the boss 68. Fluctuations in temperature, and consequent expansion and contraction will then result in the apex end of the dielectric cone being caused to oat so that it assumes a position determined by its own expansion and contraction rate, but the metallic elements extending in opposite directions from the apex end ofthe cone will be slidably displaced axially relative to their respective supports. From this it is seen that no appreciable stress will be placed on the dielectric cone 36resulting in a long life therefor and liquid-tight joints for the length of such life.

In tuning equipment to which the water load is connected, it is often desirable to sample the power being admitted to the water load. For this purpose a coaxial outer conductor tting 77 is fixed to the outer conductor 2 adjacent its input end and in advance of the cone 36. The fitting permits the insertion of a convenient sampling device, such as a coupling loop (not shown), to sample the power being admitted to the water load.

I claim:

1. In a radio-frequency load for coaxial transmission lines, the combination comprising a hollow outer housing, an inner conductor located within the outer housing and slidably supported adjacent each of its ends spaced from the outer housing, dielectric wall means sealingly inter,`

posed between the inner conductor and outer housing intermediate the ends thereof and therewith dening a chamber for a liquid dielectric, and tapered shell means adjacent opposite ends of said well means forming extensions thereof merging smootlhy and respectively in contact with the associated inner conductor and outer housing.

2. A radio-frequency load for radi0-frequency transmission lines comprising a hollow outer housing, an inner conductor extending through the outer housing and including a hollow conical portion closed at one end and having a plurality of apertures adjacent the closed end thereof, a contact block detachably secured to the closed end of the hollow conical inner conductor portion and constituting an extension thereof, a connector detachably supported on the housing and conductively connected to thecontact block, dielectric wall means comprising a truncated conical sleeve sealingly interposed between the inner conductor and outer housing and therewith defining a chamber communicating with the interior of said inner conductor to admit a liquid dielectric into the chamber, an end cap structure closing one end of the chamber and having an outlet port communicating therewith, and tapered dielectric shell means adjacent opposite ends of said wall means forming extensions thereof merging smoothly with and respectively in contactV with the associated inner conductor and outer housing.

3. The combination according to claim 2, in which one of said tapered dielectric shells is secured to the inner conductor adjacent the apex end thereof and another of said shells is secured to the outer housing adjacent the other end of the conical sleeve, and means for supporting said inner conductor at spaced intervals from said outer housing, said supporting means comprising a dielectric spacer having a hub encircling said inner conductor, said spacer having a plurality of radially extending arms integral with said hub, said arms being in contact with the inner surface of said outer housing and provided with apertures for permitting passage of said liquid dielectric therethrough.

References Cited by the Examiner UNITED STATES PATENTS 2,848,683 8/58 Jones S33-22 2,881,399 4/59 Leyton 333-22 2,894,219 7/59 Federico 333-22 2,946,005 7/ 60 Watereld et al. 333-22 3,044,027 7/62 Chin et al 333-22 OTHER REFERENCES Freedman: Water Loads, Radio-Electronic Engineering,

0 pages 14, 15, and 35, May 1954.

HERMAN KARL SAALBACH, Primary Examiner.

ELI I. SAX, BENNETT G. MILLER, Examiners.

w fun-B.,

Claims (1)

1. IN A RADIO-FREQUENCY LOAD FOR COAXIAL TRANSMISSION LINES, THE COMBINATION COMPRISING A HOLLOW OUTER HOUSING, AN INNER CONDUCTOR LOCATED WITHIN THE OUTER HOUSING AND SLIDABLY SUPPORTED ADJACENT EACH OF ITS ENDS SPACED FROM THE OUTER HOUSING, DIELECTRIC WALL MEANS SEALINGLY INTERPOSED BETWEEN THE INNER CONDUCTOR AND OUTER HOUSING INTERMEDIATE THE ENDS THEREOF AND THEREWITH DEFINING A CHAMBER FOR A LIQUID DIELECTRIC, AND TAPERED SHELL MEANS ADJACENT OPPOSITE ENDS OF SAID WELL MEANS FORMING EXTENSIONS THEREOF MERGING SMOOTHLY AND RESPECTIVELY IN CONTACT WITH THE ASSOCIATED INNER CONDUCTOR AND OUTER HOUSING.

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