US2994049A - High-frequency radial coaxial attenuator - Google Patents

Description

July 25, 1961 B. O. WEINSCHEL HIGH-FREQUENCY RADIAL COAXIAL ATTENUATOR Filgd March 14, 1960 i 23 3g i'rl '1' IN VEN TOR.

Bruno O. We/hsche/ Un d State Pm- 2,994,049 HIGH-FREQUENCY RADIAL COAXIAL ATTENUATOR Bruno 0. Weinschel, Bethesda, Md., assignor to Weinschel Engineering Co., Inc., Kensington, Md., a corporation of Delaware Filed Mar. 14, 1960, Ser. No. 14,855 9 Claims. (Cl. 333-81) This invention relates to an attenuator for highfrequency-coaxial conductor systems and has for its primary object the provision of a precision attenuator of relatively high current-carrying capacity, electricallyefiicient design and great mechanical strength and shockresistance.

In the construction (fabrication) of 1rand T-padsfor use with coaxial high-frequency lines, it has been known to use a central resistor assembly in series with the central conductor of the coaxial system, with one or more disctype shunt resistors extending between the central resistor assembly and the outer conductor of the coaxial line. For very high-frequency use, to which the present invention is particularly directed, the current flow is essentially at the surface of the respective conductors, and the resistor elements are therefore usually of the surface-coated type, generally applied on an insulating base, e.g., ceramic or the like. Where a solid disc has been used, in accordance with the prior art practice, since the current fiow must be from the center of the disc toward its circumference, it is clear that the powerhandling capability of the attenuator unit is determined by the smallest cross-sectional area of the disc resistor, i.e., at its area of contact with the central conductor or central resistor, if one is used, while the area available to carry the current near the perimeter is much larger,

-'but is not effective in determiningthe power-handling capability of the disc resistor.

Due to this increase in cross-sectional area away from the center of the disc, it will be apparent that the current density is not constant in the direction of current flow, but diminishes toward the periphery of the disc.

It is a primary object of the invention to increase the power-handling capability of a radial-type coaxial attenuator by providing a novel disc or wheel configuration which gives a substantially constant current density through the radial resistor element for current flow from the central conductor toward the outer conductor.

' Another object of the invention is to provide a disc :resistor which has great mechanical strength to resist very sharp shocks and to withstand very high accelerations to which it may be subjected in certain known applications.

A further object is to provide a coaxial attenuator which 'is very eificient electrically and which conforms to the symmetry of the coaxial conductor so as to intro- ,duce the minimum of such discontinuities as tend to produce standing waves and other undesirable disturbances which reduce the electrical efficiency of the highfrequency coaxial system.

Still another object is to produce an attenuator of high precision, which maintains a constant value of attenuation over a wide range of current and frequency values. The attenuator of the invention is particularly suited for frequencies in the range up to 5000 megacycles and higher.

The above and other objects are attained according to the invention by providing, instead of the usual disctype resistor, a wheel-type resistor having, in effect, radial spokes of resistance material so arranged as to provide a substantially uniform resistor cross-section to current fiow in the radial direction between the inner and outer conductors of a coaxial system.

2,994,049 Patented July 25, 1961 The specific nature of the invention, as well as other objects and advantages thereof, will clearly appear from a description of a preferred embodiment as shown in the accompanying drawing, in which:

FIG. 1 is a central longitudinal section of a coaxial attenuator according to the invention;

FIG. 2 is an enlarged sectional view of the spokedwheel radial element of the attenuator taken on line 2-2 of FIG. 1;

FIG. 3 is a sectional view taken on line 3-3 of FIG. 2; and

FIG. 4 is an enlarged sectional view of the spring clip on the end of the central terminal pin.

Referring to FIG. 1, the attenuator is provided as a unit suitable for insertion into a standard coaxial line and is therefore provided with a male coaxial fitting 2 at one end and a female coaxial fitting 3 at the other end. The central male contact pin 2a is supported and centered by insulating member 4, and terminates in a spring clip 6, as best shown in FIG. 4, for the reception of central conductor 7, which in one form of the invention is a resistor element formed by coating an insulating central rod with a layer of resistive material applied in any known manner, but preferably as shown in my copending US. patent application Serial No. 671,098, filed July 10, 1957, for High Frequency Attenuator. The ends of the rod 7, which are inserted into spring clips or other contact devices, make good contact by virtue of a layer of conductive material such as silver paint, as is well-known in the art.

The other end of rod 7 is inserted into a similar spring clip 8 which is part of a clip assembly including also a similar clip 9 fastened to and in contact with the center of radial resistor 11. Clip 9 engages a central rod 12 similar to rod 7, the other end of which engages a clip 13, similar to the previously described clips, which is on the end portion of female fitting 3.

Radial resistor 11 is shown in detail in FIGS. 2 and 3. It comprises a wheel-shaped insulating member having a central hub 14 centrally apertur ed at 16, and a tirelike circumferential rim 17 connected to the hub 14 by a number of radial spokes 18, shown as four in number, which are symmetrically distributed circumferentially, i.e'., at equal central angles. This insulating wheel is accurately formed and smoothly surfaced. It is further prepared as a radial resistor in the following manner:

The entire wheel, including rim surface 17, its flat ends 19 and its entire inner cylindrical surface 20 as well as the entire hub surfaces are first coated with a thin film of, resistance material, e.g., a fihn of gold-platinum alloy so thin that it has the desired resistance, or a thin film of carbon deposited in any suitable known manner. The rim and hub are then given a coating of fired-on highly conducting material, e.g., silver, by any known technique used for this purpose. This leaves a film of resistive material on the surface of the spokes extending between the highly conductive hub surface and the highly conductive rim surface.

At the center of the radial resistor, contact is established with the hub by means of the two spring clips 8 and 9, which are screwed together as shown in FIG. 3 to provide a pressure contact with the fiat end of the hub.

Pressure contact is also provided at the rim by means of two metal sleeves 23, 24 made, for example, of brass, which are inserted with a loose friction fit into metal outer cylinder 26; cylinder 26 is threaded at its ends to receive the outer shells 27 and 28 respectively of the male and female conductor portions of the attenuator. Shells 27 and 28 are screwed in against the ends of sleeves 24 and 23 to press the end of the sleeves against the conductive outer surface of the rim 17 and thus establish good contact; the shells are then staked, e.g., by screws 31 and 32 to prevent the pressure from being relaxed. The male conductor is completed by a loose nut 33 internally threaded at 34 and held against axial displacement on shell 27 by any suitable means, such as spring clip 36, which permits the nut to rotate freely so that it may be thoroughly engaged with a mating female coaxial conductor shell similar to shell 28 on the other end of the attenuator.

Metal cylinder 26 is preferably provided with a series of circumferential fins 37 to facilitate heat dissipation from the attenuator and thus increase its power handling capacity.

It will be noted that the inner cylindrical surface 20 of the rim 17 is made substantially flush with the inner cylindrical surfaces of sleeves 23 and 24 to thus offer the minimum discontinuity of the inner surface and hence the least possible distortion of the high-frequency field. The same is true of the outer cylindrical surface of the hub 14, which is made substantially flush with the outer surfaces of clips 8' and 9 for the same purpose. The spokes 11 are relatively thin in the circumferential direction and relatively deep in the axial direction, therefore they permit the maximum undisturbed matching cylindrical surfaces to be provided in the direction of propagation of the current, i.e., the direction of propagation of the electromagnetic field. The major portion of the resistive material, it will be apparent, is provided in the form of thin sheets of resistive material on the axiallyextending sides of the spokes 11; these sheets are of uniform cross-section in the direction of current flow and therefore the current is uniformly distributed, provided the maximum power-handling capability, in contrast to the arrangements which provide less available cross-sectional area near the center than at the outer periphery, which limits the current handling capability to a value determined by said minimum cross-section.

The attenuator shown in FIG. 1 is in the form of a T- pad. An equivalent vr-pad attenuator can obviously be made by providing two radial resistors with an axial resistor between them, which can therefore be made somewhat shorter, but on the other hand is more expensive to make. Similarly, any other desired combination of axial and radial resistors of the type shown can be built up to obtain a desired characteristic.

It will be apparent that the embodiments shown are only exemplary and that various modifications can be made in construction and arrangement within the scope of my invention as defined in the appended claims.

I claim:

1. A high-frequency coaxial attenuator comprising a central inner conductive element and an outer conductive element coaxial therewith, and a shunt resistor between said elements, said shunt resistor comprising an insulating member having an outer hollow cylindrical portion of substantial axial length, an inner cylindrical portion coaxial with and spaced from said outer portion, and a plurality of spoke-like insulating radial members extending between and joining said inner and outer portions, said radial members having substantial length in the axial direction, a highly conductive coating on the end and inner surfaces of said outer portion and on the end and outer surfaces of said inner portion, a thin resistive coating on said radial members extending between and in contact with said highly conductive coatings, said inner and outer coaxial conductive elements being in good electrical contact with the highly conductive coatings on said inner and outer portions respectively.

2. The invention according to claim 1, said radial members being very thin in the circumferential dimension, which constitutes the thickness of said members.

3. The invention according to claim 2, said radial members being equally spaced circumferentially.

4. The invention according to claim 1, said thin resistive coating being of substantially uniform thickness on all sides of said spoke-like radial members.

5. The invention according to claim 4, and pressure means establishing pressure con-tact between at least a portion of said inner and outer conductive elements and the highly conductive coatings on said inner and outer portions respectively.

6. In combination, a coaxial conductor section and a shunt attenuator element therefor; said coaxial section comprising an inner conductor having a cylindrical outer conducting surface and a concentric outer conductor having a cylindrical inner conductive surface opposed to and spaced from said outer conducting surface; said attenuator element comprising an insulating member having an outer hollow cylindrical portion of substantial axial length and of the same internal diameter as said outer conductor and contiguous thereto, an inner cylindrical portion of the same diameter as said inner cylindrical conductor and contiguous thereto, and a plurality of spoke-like insulating members extending between and joining inner and outer portions, said radial members having substantial length in the axial direction, a highly conductive coating on the end and inner surfaces of said outer portion and on the end and outer surfaces of said inner portion, said conductive coatings constituting smooth continuations of the conducting surfaces of said coaxial conductor section, a thin resistive coating on said radial members extending between and in contact with said highly conductive coatings, said inner and outer coaxial conductive elements being in good electrical contact with the highly conductive coatings on said inner and outer portions respectively.

7. The invention according to claim 6, said radial members being very thin in the circumferential dimension, which constitutes the thickness of said members.

8. The invention according to claim 6, said thin resistive coating being of substantially uniform thickness on all sides of said spoke-like radial members.

9. The invention according to claim 8, and pressure means establishing pressure contact between at least a portion of said inner and outer conductive elements and the highly conductive coatings on said inner and outer portions respectively.

Johnson Dec. 2, 1952 Hancock Jan. 21, 1958

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