US3790904A

US3790904A - Rf termination

Info

Publication number: US3790904A
Application number: US00342784A
Authority: US
Inventors: L Lesyk; D Kaltenborn
Original assignee: Bird Electronic Corp
Current assignee: Bird Electronic Corp
Priority date: 1973-03-19
Filing date: 1973-03-19
Publication date: 1974-02-05
Anticipated expiration: 1991-02-05
Also published as: CA984923A; FR2222765A1; DE2402025C2; GB1456423A; FR2222765B1; DE2402025A1

Abstract

A conduction-cooled load or termination for an RF coaxial transmission line. A flat film resistor coated onto a dielectric substrate of high thermal conductivity is series-connected between the coaxial conductors to convert electrical energy into heat energy. The heat energy is rapidly conducted through the substrate to the body of the termination which is in turn, attached in heat transfer relationship to an appropriate heat sink. A shaker with a composite surface of generally exponential form relative to the path of conduction across the film is located over the exposed surface of the film to establish for the termination a low VSWR characteristic over a wide frequency range.

Description

Patent [1 1 Feb. 5, 1974 RF TERMINATION Inventors: Leo Lesyk, Walton Hills; David A.

Kaltenbom, Ravenna, both of Ohio Assignee: Bird Electronics Corporation, Salon,

Ohio

Filed: Mar. 19, 1973 Appl. No.: 342,784

US. Cl. 333/22 R, 333/34 Int. Cl. H011) 1/26 Field of Search 333/22 R, 22 F, 81 R, 81 A,

333/81 B, 84 R, 84 M References Cited UNITED STATES PATENTS 2/1971 ll/l97l SIGNAL GENE RATOR Primary Examiner-James W. Lawrence Assistant Examiner-Marvin Nussbaum Attorney, Agent, or FirmCarl A. Rankin 7] ABSTRACT A conduction-cooled load or termination for an RF coaxial transmission line. A flat tilm resistor coated onto a dielectric substrate of high thermal conductivity is series-connected between the coaxial conductors to convert electrical energy into heat energy. The heat energy is rapidly conducted through the substrate to the body of the termination which is in turn, attached in heat transfer relationship to an appropriate heat sink. A shaker with a composite surface of generally exponential form relative to the path of conduction across the film is located over the exposed surface of the film to establish for the termination a low VSWR characteristic over a wide frequency range.

6 Claims, 5 Drawing Figures PATENTED 5W 3. YSLLQM SHEET 2 BF 2 FliG. 4

SIGNAL GENERATOR RF TERMINATION BACKGROUND OF THE INVENTION This invention relates to dummy loads for use as reflectionless terminations for coaxial transmission lines. More particularly, the invention relates to dry-type terminations which rely on conduction rather than convection to dissipate heat energy. Dry-type terminations differ from the wet type in that with the wet type, a liquid cooling medium is circulated through the device to carry off the heat energy generated by the dissipation of electrical energy across the load.

Frequently in testing transmitter apparatus or in measuring radio frequency power, a substantially reflectionless termination or dummy load is used to termimate a coaxial transmission line. The termination must be capable of absorbing and dissipating the power in the form of heat. Also, the termination must be matched to the electrical characteristics of the coaxial line as determined by the physical dimensions of the line in order to avoid reflection of radio frequency waves (e.g., from DC to 3000 MHz) from the termination. In order to minimize reflection and maximize power transfer from a transmission line to a termination the load should have a characteristic impedance that is matched to the characteristic impedance of the line. For coaxial lines the characteristic impedence is defined by:

Z, V L/C where:

2,, Characteristic impedence L 2 Distributive inductance C Distributive capacitance More conventional types of terminations employed for coaxial lines utilize a tapered horn principle to minimize reflections. The microwave signals are passed along a resistive layer defining the tapered surface, and the signal is thus gradually attenuated in an advantageous manner to minimize reflection. These devices are typically of the wet type described above. Typical examples of such line terminations are disclosed in US. Pat. Nos. 3,300,737; 2,556,642; 2,752,572; and 3,634,784.

The present device, however, is a dry-type termination which relies on conduction or more specifically, transfer of energy to a heat sink in order to achieve the necessary heat dissipation. The heat sink may be an equipment cabinet, an bulkhead or air frame, or a massive, specially designed metal unit. Dry-type terminations have particular utility in connection with space vehicles, missiles and other applications that require high-power dissipation in a limited space or where the power dissipation element must be cooled by conduction rather than convection.

One specific technique used in connection with this type of termination resides in the use of flat film resistors. The film is conventionally deposited on a dielectric substrate with a high thermal conductivity, preferably on the order of that of aluminum in order to rapidly transfer the heat generated in the film. A particularly advantageous material for the substrate is beryllium oxide, a ceramic material having approximately the same thermal conductivity as aluminum. The substrate is usually mounted in heat transfer relationship to the housing for the device which is usually formed of aluminum, in order to rapidly dissipate the power to the housing.

One such device is identified by the model number 460A manufactured by Sierra Electronics Division of Philco Corporation. This device utilizes two resistive films supported on opposite sides of a beryllium oxide substrate. The dry'type devices of this conventional construction achieve adequate heat transfer, however, they do not afford efficient attenuation of RF waves across the resistor, at least not of the order achieved in tapered horn-type terminations, for example, of the type described in US. Pat. Nos. 3,300,737 and 3,634,784. This disadvantage limits the effectiveness of dry-type coaxial line terminations presently obtainable in the art.

The device of the present invention, however, reduces the disadvantages described above and affords other features and advantages heretofore not obtainable.

SUMMARY OF THE INVENTION It is among the objects of the invention to minimize reflection and maximize power transfer from a coaxial transmission line to a dry-type line termination.

These and other objects and advantages are achieved by the dry-type line termination of the present invention which includes a conventional connector adapted to receive the mating connector from the coaxial transmission line. The body of the termination defines a closed interior space and is formed of material of high thermal conductivity such as aluminum so that when the device is connected in heat transfer relation to a suitable mass which serves as a heat sink, the desired energy dissipation is afforded. Within the body of the termination is a solid substrate formed of dielectric material of high thermal conductivity such as beryllium oxide, and mounted in heat transfer relationship with the metal body. The substrate has a flat, thin resistive film deposited thereon facing the interior space. The resistive film is series-connected between the contacts for the inner and outer conductors and is adapted to convert the electrical energy being transmitted by the transmission line into heat energy that is transferred to the substrate.

Facing the surface of the resistive film within the space is a shaker that defines asurface form spaced from the surface of the film and so generated that it defines, when viewed in a plane perpendicular to the film surface and parallel to the path of conduction, a generally exponential curve whereby the shaker surface serves to establish a low VSWR characteristic for the termination over a wide frequency range.

The surface of the shaker is advantageously formed of a composite of angularly disposed planar surfaces which roughly define in cross section the desired exponential curve. The curve is predetermined according to the dimensions of the film and the physical characteristics of the substrate as will be apparent to those skilled in the art. A shaker having a composite planar portion that approximates the exponential curve is easier to machine and affords essentially comparable advantageous results.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a elevational view of a heat sink unit with the front plate broken away for the purpose of illustra tion, and having a coaxial RF line termination embodying the invention mounted in heat transfer relation therein;

FIG. 2 is a cross sectional view of the unit of FIG. 1 on an enlarged scale and taken on the line 22 of FIG.

FIG. 3 is a cross sectional view on an enlarged scale of the coaxial RF line termination of FIGS. 1 and 2 with parts broken away for the purpose of illustration and taken from below the line 3--3 of FIG. 2;

FIG. 4 is a cross sectional view taken on the line 44 of FIG. 3; and

FIG. 5 is a transverse sectional view taken on the line 5-5 of FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring more particularly to the drawings and initially to FIGS. 1 and 2, there is shown an RF termination A for a coaxial transmission line, mounted in a heat sink unit B designed with cooling fins, etc., to provide adequate exposed surface area to dissipate the heat energy generated by the termination A.

The heat sink B is shown to illustrate one particular means for providing the necessary mass to dissipate heat energy. However, it will be understood that other types of heat sink mass are more convenient in many circumstances, such as housings for electrical equipment, air frames and the like. The heat sink B comprises an I-I shaped metal body having longitudinally extending radial convection plates 11 extending outwardly therefrom to provide a maximum surface area. The body 10 has a top plate 12, a bottom plate 13 and front and

rear end plates

14 and 15 respectively. A handle 16, suitably insulated from the rest of the heat sink unit B by insulators 17, is provided for conventience. The line termination A is securely anchored to the central web portion of the metal body 10 by means of machine screws 19 that extend through openings in the housing 20 for the termination A. As illustrated in FIGS. 1 and 2, the termination A is mounted face down but will be illustrated in an upright position in FIGS. 3 through 5.

The housing 20 is formed of material that is both electrically conductive and has a high thermal conductivity. Aluminum is preferred and in the embodiment shown three machined aluminum parts are assembled to form the housing. These include a main body 21 generally channel-shaped as best illustrated in FIG. 5, a front member 22 and a rear member 23. The three members define therewithin a closed resistor chamber 25. The form member 22 is secured to the main body 21 by means of machine screws 26 while the rear member 23 is attached at the opposite end of the main body by machine screws 27. The machine screws 26 and 27 have their heads recessed in counterbores 28 that are plugged with suitable elements 29.

An N-type female connector assembly 30 is attached to the front member 22 by machine screws 31, in coaxial relation with a counterbore 32. The center conductor of the connector assembly 30 is insulated from the housing front member 22 by means of a cylindrical insulator 33. The center conductor of the assembly 30 contacts a center conductor element 35 mounted within the front member 22 coaxial with the connector assembly 30.

The element 35 has a contact spring 36 on its inner end that extends into the resistor chamber 25. The element 35 is insulated from the housing front element 22 by an annular insulator 37 that matches the configuration of the counterbore 32 and a bore 28 that extends between counterbore 32 and the resistor chamber 25.

Located within the resistor chamber 25 and tightly secured between the housing rear member 23 and a mating shoulder 39 on the main body 21 is a substrate 40 formed of a dielectric material having a high thermal conductivity. In the present instance the substrate 40 is formed of beryllium oxide (also called beryllia) which has a thermal conductivity closely approaching that of aluminum. The dimensions of the substrate in the embodiment shown are 1.8 inches X 1 inch X 0.0275 inch. A film of heat transfer compound such as the compound identified by the trade designation TC-4 manufactured by Emerson Cummings Corporation is applied to one or both of the mating surfaces between the substrate 40 and the rear member 23 to optimize heat transfer from the substrate 40 to the housing 20.

Located on the flat surface of the substrate 40 that faces the resistor chamber 25 is a flat, thin resistive metal film 41 deposited on the surface by techniques well known to those skilled in the art, such'as, for example, by the process utilized for such purposes by Pyrofilm Resistor Co. Inc. The film 41 extends endwise along the substrate in a wide band that stops short of the ends of the substrate at both ends.

Contacts

42 and 43 are provided at the opposite ends of the film 41 in intimate contact therewith. The contact 42 is engaged by the contact spring 36 of the center conductor assembly 35 while the contact 43 is engaged by a contact spring 45 connected to the housing 20. Thus, the resistive film 41 is electrically connected between the center conductor element 35 and the aluminum housing 20 which connects to the outer conductor of the connector assembly 30.

The dimensions of the resistive film 41 are selected to afford the desired resistance (e.g., 50 ohms) depending upon the particular rating of the device. In the present instance the resistance is adapted to accommodate a load of 300 watts or 1,025 BTUs per hour. The film dimensions selected to achieve this rating in the embodiment shown are a length of 1.550 inches and a width of 0.375 inches. The dimensions, of course, depend upon the physical characteristics of the conductive material used for the resistive film.

The dimensions of the substrate 40 are selected to be compatible with the physical characteristics of the film and to provide the desired terminating impedance. In this instance, the ratio of the width of the film to the depth of the substrate is selected to be about 1.35. Accordingly, since the width of the film in this instance is 0.375 inches, the height of the substrate is 0.275 inches. The length of the substrate is dependent upon the length of the film which is selected to provide the desired area for energy dissipation. Since the length of the film is 1.550 inches, the length of the substrate has been selected as 1.8 inches to provide the desired space at the ends of the film for the

contacts

42 and 43. The width of the substrate 40 is 1 inch.

A shaker 50 is anchored to the main body 21by machine screws 51 as best illustrated in FIG. 4. The element is preferably formed of the same material as the housing, or in this case, of aluminum. The shaker has a composite surface portion that faces the resistor chamber 25 and which is spaced above the resistive film 41. The spacing between the resistive film and the exposed composite surface portion of the shaker 50 is calculated to provide a gradual attenuation of the RF signals in a predetermined manner to allow the fields of 5 force to vary progressively with very little distortion. This arrangement inhibits reflections of the electric and magnetic fields of the microwave signals and results in a minimum VSWR.

The curve defined by a plane perpendicular to the resistive film 41 in the space between the film and the exposed surface of the shaker 50 approximates an exponential curve calculated, depending on the impedence of the transmission line and the thickness of the substrate 40, according to procedures well known to those skilled in the art. While the surface is ideally formed by a generatrix running perpendicular to the exponential curve, in the present instance, to afford ease of manufacture, the surface is composed of three angularly disposed

flat surface portions

54, 55 and 56. The surface portion 54 defines an angle of 1 with the resistive film 41, the surface portion 55 an angle of 26, and the surface portion 56 an angle of 54.30". This provides an approximation of an exponential curve.

The high dielectric characteristic of the substrate 40 considered in conjunction with the shaker 50, contributes to the provision of the required terminating impedence and further minimizes reflection.

The result of this unique arrangement and these geometrical relationships is that the RF signal applied to the termination is dissipated with a minimum of reflection or, in other words, with a minimum VSWR. The termination itself is of minimum size and can be conveniently mounted on structures available assuming that the structure is of sufficient mass and exposed surface area to dissipate the heat energy generated under operating conditions.

The specific line termination A of the invention shown and described herein is nominally rated at 300 watts over a frequency range of up to 3000 MH, however, the power capacity can be greatly increased by using a more efficient heat sink mass such as the heat sink unit B of FIGS. 1 and 2. For example, the termination A can handle from 500 to 600 watts when used with the unit B.

As indicated above, the line termination of the invention has particular utility in connection with space vehicles, missiles and other applications that require highpower dissipation in a limited space. Generally stated, the invention has application primarily where the power dissipation element must be cooled by conduction rather than convection.

While the invention has been shown and described with respect to a specific embodiment thereof, this is intended for the purpose of illustration rather than limitation and other modifications and variations of the specific machine herein shown and described will be apparent to those skilled in the art all within the intended scope and spirit of the invention. Accordingly,

the patent is not to be limited, to the specific embodiment herein shown and described nor in any other way that is inconsistent with the extent to which the progress in the art has been advanced. by the invention.

We claim:

1. A dry termination for a high frequency coaxial transmission line, the termination having a connector with first and second contact means for the inner and outer co-axial conductors respectively and a body defining an enclosed interior space, said body being formed of material of high electrical and thermal conductivity and adapted to be mounted in heat transfer relation with a heat sink mass, comprising:

a solid substrate formed of dielectric material of high thermal conductivity mounted in heat transfer relationship with said body,

a flat, thin resistive film deposited on a generally flat surface of said substrate and series connected between said first and second contact means, said film being adapted to convert electrical energy being transmitted by said transmission line to heat energy that is transferred to said substrate, and

a conductive element mounted to said body for minimizing the reflected energy from said termination, said element being located in said space and defining a surface form facing and spaced from the surface of said film, said surface form defining, when viewed in a cross sectional plane perpendicular to said film surface and parallel to the path of conduction, an approximation of an exponential curve comprising a plurality of adjacent flat surface portions angularly disposed relative to said resistive film whereby said surface form serves to establish a low VSWR characteristic for said termination over a wide frequency range.

2. A termination as defined in claim 1 wherein said substrate is formed of beryllium oxide.

3. A termination as defined in claim 1 wherein said surface form comprises three flat surface portions including a first surface portion adjacent the end of said film connected to said second contact means and disposed at an angle of approximately 1 relative to said film, a second surface portion joining said first surface portion and disposed at an angle of approximately 26 relative to said film, and a third surface portion joining said second surface portion and disposed at an angle of approximately 54.30 relative to said film.

4. A termination as defined in claim 1 wherein the surface dimensions of said film comprise a width of about 0.375 inch and a length of about 1.55 inches.

5. A termination as defined in claim 4 wherein the dimensions of said substrate comprise a width of about one inch, a length of about 1.8 inches and a depth of about 0.275 inch.

6. A termination as defined in claim 1 wherein the dimensions of said substrate are calculated to cooperate with said surface form to afford the desired terminating impedence and minimize reflection.

Claims

1. A dry termination for a high frequency coaxial transmission line, the termination having a connector with first and second contact means for the inner and outer co-axial conductors respectively and a body defining an enclosed interior space, said body being formed of material of high electrical and thermal conductivity and adapted to be mounted in heat transfer relation with a heat sink mass, comprising: a solid substrate formed of dielectric material of high thermal conductivity mounted in heat transfer relationship with said body, a flat, thin resistive film deposited on a generally flat surface of said substrate and series connected between said first and second contact means, said film being adapted to convert electrical energy being transmitted by said transmission line to heat energy that is transferred to said substrate, and a conductive element mounted to said body for minimizing the reflected energy from said termination, said element being located in said space and defining a surface form facing and spaced from the surface of said film, said surface form defining, when viewed in a cross sectional plane perpendicular to said film surface and parallel to the path of conduction, an approximation of an exponential curve comprising a plurality of adjacent flat surface portions angularly disposed relative to said resistive film whereby said surface form serves to establish a low VSWR characteristic for said termination over a wide frequency range.

3. A termination as defined in claim 1 wherein said surface form comprises three flat surface portions including a first surface portion adjacent the end of said film connected to said second contact means and disposed at an angle of approximately 1* relative to said film, a second surface portion joining said first surface portion and disposed at an angle of approximately 26* relative to said film, and a third surface portion joining said second surface portion and disposed at an angle of approximately 54.30* relative to said film.