US 3621479 A
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United States Patent Inventors Noel C. Peterson Severna Park, Md.; Gerald I. Klein, Westbury, N.Y. Appl. No. 1,353 Filed Jan. 8. i970 Patented Nov. 16, 1971 Westinghouse Electric Corporation Pittsburgh, Pa.
Assignee APPARATUS FOR DISSIPATING WAVE ENERGY 8 Claims, 4 Drawing Figs.
US. Cl 333/22 R, 333/81 A Int. Cl HOlp 1/26 Field of Search 333/22, 81, 84
References Cited UNITED STATES PATENTS 1/1967 Maines... 333/22 10/1967 Barker....
11/1967 Steidlitz OTHER REFERENCES Blackburn, Components Handbook," McGraw-Hill, New York, 1949, TK453B5, title pg. and pp. 70 72 Primary Examiner-Herman Karl Saalbach Assistant Examiner-Marvin Nussbaum At!orneysF. H. Henson and E. P. Klipfel ABSTRACT: Described is a power-dissipating termination for wave energy transmission lines, characterized in having a low voltage standing wave ratio essentially independent of input power and operable in ambient temperatures up to 300 C, over a frequency range from zero to ultrahigh frequencies and above.
PAIENTEnuuv 16 ran FIG. 4.
IN VEN TORS. NOEL C. PETERSON 8 GERALD I. KLEIN B (Te 4 A r rorney APPARATUS FOR DISSIPATING WAVE ENERGY BACKGROUND OF THE INVENTION In certain applications, it becomes necessary to terminate a wave energy transmission line in a load which will dissipate varying amountsof energy over a wide frequency range. One of the most important requirements of such a termination is that it present a low voltage standing wave ratio over a wide temperature range, meaning that the impedance of the termination must remain essentially constant over the range of temperatures. Furthermore, the device must be capable of transferring heat generated in a resistive load to its surroundings rapidly and efiiciently. These requirements become especially difficult when the physical dimensions of the device are limited, or when it must operate at high altitudes where there is little surrounding atmosphere.
Most known devices for dissipating electrical wave energy are unsatisfactory if the foregoing conditions are to be met. For example, a hot wire (as in an incandescent bulb) will not operate as a load unless the applied power is essentially constant. At very low powers, the cold wire acts like a dead short. Consequently, such devices cannot be used where, for example, the device must dissipate power from zero up to 25 watts. Another energy dissipating device such as a long helical wire could be designed to match 50 ohms at 250 megahertz, but it is not possible to obtain the necessary attenuation for matching and still keep within reasonable dimensions. A good termination can be obtained for low power, room temperature operation by using a carbon resistor, but its value will vary too widely to provide a constant low voltage standing wave ratio over a range of temperatures.
Liquid dielectric loads, which can handle large powers, are unsuitable if the ambient temperature is high; and certain loads require the need for forced cooling, thereby complicating the device and increasing its size. Dry loads for wave energy transmission lines are generally rugged and can mount at any position. They usually consist of either a cylindrical film resistor on a dielectric rod as a section of the center conductor of a coaxial transmission line, or a disc-shaped resistor connecting the inner and outer conductors of the transmission line. The power absorbed by such devices is limited by the poor heat flow path through small cross sections of the dielectric materials and the ability of the surrounding air to take heat from the outer conductor. Other loads utilizing materials which depend upon magnetic loss mechanisms are ineffective in the 200 to 400 megahertz range. Most of these materials are also temperature sensitive and/or use organic binders which are unserviceable at higher temperatures.
SUMMARY OF THE INVENTION As an overall object, the present invention seeks to provide a new and improved radiofrequency termination for electromagnetic wave energy transmission lines.
More specifically, an object of the invention is to provide a new and improved wave guide termination that offers a low voltage standing wave ratio independent of input power up to tens of watts and ambient temperatures up to 300 C. over a frequency range from zero to ultrahigh frequencies and above.
In accordance with the invention, apparatus for dissipating wave energy passing through a transmission line is provided including means for causing the wave energy to propagate in the space between a pair of electrically insulated conductors. Preferably, this means comprises a conventional coaxial transmission line. A resistive element is connected between the center and outer conductors of the coaxial line and is formed from a material whose resistivity remains essentially constant over a wide temperature range. Preferably, this material comprises Nichrome (Trademark) comprising an alloy of 80 percent nickel and percent chromium. I-IOwever, as a substitute, Constantan (Trademark), an alloy of 55 percent copper and 45 percent nickel, or Manganin (Trademark), an alloy of 13 percent manganese and 87 percent copper can be used.
The resistive element, preferably vapor deposited on an aluminum oxide substrate in the form of a film, is sandwiched between dielectric slabs which are usually formed from boron nitride but may also be formed from beryllium oxide. The main requirements of the dielectric material are that it have high thermal conductivity and low electrical conductivity. Finally, the resistive element between the dielectric slabs is surrounded by a heat sink formed from upper and lower aluminum slabs separated by a spacer formed from a metal whose coefficient of thermal expansion closely matches that of the dielectric slabs. Means are provided to maintain the upper and lower aluminum slabs in close abutting and good thermal contact with the dielectric slabs, whereby minimized thermal barriers occur at the dielectric-to-metal interface.
The above and other objects and features of the invention will become apparent from the following detailed description taken in connection with the accompanying drawings which form a part of this specification, and in which:
FIG. 1 is an exploded view of the resistive termination of the invention;
FIG. 2 is a cross-sectional view of the termination of the invention in its assembled form;
FIG. 3 is an illustration of an alternative embodiment of the film resistor of the invention wherein the resistor assumes a serpentine configuration; and
FIG. 4 is an illustration of still another embodiment of the invention employing stacked layers of films of resistive materia].
With reference now to the drawings, and particularly to FIG. 1, an end connector 10 for a coaxial line is shown which receives a center conductor 12 having a flattened area 14 at one end thereof. The flattened end 14,. in turn, engages a contact 15 of conductive material deposited on an aluminum oxide substrate 16, perhaps best shown in FIG. 2. Also deposited on the aluminum oxide substrate 26, by vapor deposition techniques, is a film resistor 18, the opposite end of the resistor 18 being engaged by a second vapor-deposited contact 20.
The aluminum oxide substrate 16 with the film resistor 18 deposited thereon rests on a lower dielectric slab 22; while above the film resistor 18 is a second dielectric slab 24. The slabs 22 and 24 with the aluminum oxide substrate 15 and film resistor 18 sandwiched therebetween are received within an opening 26 in a spacer 28. On top of the spacer 28 is a first aluminum block 30 and beneath the spacer 28 is a second aluminum block 32, perhaps best shown in FIG. 2. The entire assembly is held together by means of a plurality of screws 34. The screws 34 have heads 36 which fit into countersinks 38 formed in the upper aluminum block 30 and are provided with shank portions which pass through openings in the spacer 28 and are threaded into the lower aluminum block 32.
In order to effectively dissipate wave energy over a wide range of temperatures, the film 18 must have a resistivity which remains essentially constant over that temperature range. For this purpose, Nichrome (Trademark), an alloy of percent nickel and 20 percent chromium, is desired. It has a high resistivity, low change in resistance with temperature, good adherence to dielectric materials, and ease of achieving a stable surface, protected by a layer of chromium oxide after deposition. Although an alloy film of this type is somewhat more difficult to deposit than a pure metal, no pure metal offers all of these advantages. Other alloys whose resistivity remains constant over a wide temperature range can be used in place of Nichrome. These are Constantan (Trademark), an alloy of55 percent copper and 45 percent nickel or Manganin (Trademark), an alloy of l3 percent manganese and the remainder copper. Nichrome is desired, however, since its adherence and stability are better that the latter two materials.
The dielectric slabs 22 and 24 should have high thermal conductivity, low thermal expansion, a low dielectric constant, and good machinability. For this purpose, boron nitride is preferred; however in certain cases beryllium oxide can be used in its place.
The spacer 28 is preferably formed from Kovar (Trademark) comprising an alloy of 20 percent nickel, 17 percent cobalt, 0.1 percent manganese and the balance iron. This material, like the dielectric slabs 22 and 24, has a very low coefficient of expansion. The aluminum slabs 30 and 32, while having excellent thermal conductivity characteristics for dissipating the heat generated by the resistor 18, have high 'coefficients of thermal expansion. AS shown in FIG. 2, the upper aluminum slab 20 is provided with a stepped portion 40 which protrudes down into the opening 26 formed in the spacer 28. The expansion of this step is exactly the same as that of the spacers 28, so that the net expansion of the cavity occupied by the boron nitride spacers 22 and 24 is zero over the range from C. to 400 C. This technique of compensation for thermal expansion is also used in fastening the device together. The expansion of the the spacer 28 and that portion of the upper aluminum slab 30 beneath the screw head 36 will exactly equal the expansion of the screws 23 which are formed from cold rolled steel. Thus, the joints of the device will remain tight, without stress, up to 400 C. while maintaining minimized thermal barriers at the dielectric-to-metal interfaces.
In the manufacture of the device, the film resistor 18 and its contacts are vacuum deposited. A high temperature silver paint is then applied to the contacts to insure broad area, low resistance joints to the input center conductor 14 and to the Kovar spacer 28, which is grounded. The boron nitride element is then vacuum fired to drive the volatile materials from V the paint. The joints between the outer conductor and spacer, outer conductor and connector, and center conductor to spacer are all hard brazed for good conductivity and mechanical strength. A device such as that shown in FIG. 2 has a voltage standing ratio of less than 1.20 throughout a wide range of operating conditions. In FIG. 3, a configuration is shown wherein a resistor 18A is again vacuum deposited on an aluminum oxide substrate 16A. However in this case, the resistor 18A assumes a serpentine configuration to increase its length for a given amount of area. Contacts 15A and 20A are deposited at the ends. This assembly would then be substituted for elements l5, 18, 20 and 22 of FIGS. 1 and 2.
In FIG. 4, still another embodiment of the invention is shown wherein serpentine resistors 42A-42E are each disposed between slabs of dielectric material 48 and 50, the ends of the serpentine conductors between the respective layers being interconnected by means of electrical strip conductors 52. The entire assembly is encased within a housing 54 which may, for example, be formed from aluminum.
Although the invention has been shown in' connection with certain specific embodiments, it will be readily apparent to those skilled in the art that various changes in form and arrangement of parts may be made to suit requirements without departing from the spirit and scope of the invention.
We claim as our invention:
1. Apparatus for dissipating wave energy passing through a wave energy transmission line, comprising means for causing said wave energy to propagate in the space between a pair of electrically insulated conductors, a resistive element connected between said conductors, said resistive element being sandwiched between slabs of dielectric material having a low coefficient of expansion, and a heat sink surrounding said dielectric slabs and in contact therewith, said heat sink being formed from a spacer of material having an opening therein which receives said resistive element sandwiched between said dielectric slabs, and blocks of metal secured to said spacer on opposite sides of said opening and in thermal contact with said dielectric slabs.
2. The apparatus of claim 1 wherein said resistive element comprises an alloy comprising percent nickel and 20 percent chromium.
3. The apparatus of claim 1 wherein said resistive element is formed from an alloy comprising 55 percent copper and 45 percent nickel.
4. The apparatus of claim 1 wherein said dielectric material com rises boron nitride. I
5. he apparatus of claim 1 wherein said dielectric material comprises beryllium oxide.
6. The apparatus of claim 1 wherein said spacer is formed from a metal having essentially the same coefficient of thermal expansion as said dielectric slabs and said blocks are formed from aluminum.
7. The apparatus of claim 6 wherein said spacer and said blocks are secured together by screws which pass through one of said blocks and said spacer and are threaded into the other of said blocks, one of said blocks having a portion which extends into the opening in said spacer and abuts one of said dielectric slabs, the expansion of said portion upon heating along the depth of said opening being essentially equal to the expansion of the spacer itself along the depth of said opening.
8. The apparatus of claim 7 wherein said screws are provided with heads which abut said one block, the axial expansion of said screws upon heating being essentially equal to the expansion of said spacer and the material of said one block beneath said heads along the axes of said screws.