US2875418A - High power resistive attenuator devices - Google Patents

Description

Feb. 24, 1959 .L-CPROLFS I HIGH POWER RESISTIVE ATTENUA TOR DEVICES Filed Aug. 26, 1954 Fig-5- INVENTOR ATTORNEY e 2,875,418 1 HIGH rowan RESISTIVE ATTENUATOR DEVICES John c. Rolfs, Williston Park,'N.

Y., assignor to Sperry Rand Corporation,

a corporation of Delaware Application August 26, 1954, Serial No. 452,411 9 Claims. (Cl. 333-81) This invention relates to high-power resistive attenuator devices for dissipating microwave radio energy, and more particularly, to improvements in attenuators of the lossy wall wave guide type.

For dissipating relatively large amounts of average power of the order of 100 watts or more, such as are encountered in thefoperation of radar systems, it is necessary that theloss be distributed over a substantial area to prevent excessive localized heating from destroying the attenuator. structed to avoid producing regions of high electric field Furthermore, the attenuator must beconintensity to prevent voltage breakdown under the relatively high pulse powers, of the order of. 100 kilowatts or more, that occur inthe operation of pulse radar systems. These requirements have been met to some extent by attenuators of the type comprising a length of wave guide having walls made of a dissipative mixturesuch as material aggravates the difficulty by acting as a heat insulator.

The p1 incipa l object ,of-thisinventionis to provide improved attenuators of the lossy .wall type capable of dissipatingsafely large amounts of power.

Another object is to provide attenuators that can readily be designed to maintain a good impedance matchoto a wave guide throughout a wide frequency band. p

A more specific object is to provide a wave guide termination ordummy load having sufficient power capacity ice 'low grooves are formed on an interior wall surface of the wave guide, and these grooves are fiilledwith a dissipative material. The grooves are separated by regions of the conductive wall material, thus providing short low temperature gradient heat paths through the dissipative material to the conductive wall material.

In some cases this construction would require an impractically long wave guide to obtain an adequate attenuation if dissipative materials like those in. prior art lossy wall attenuators were used, particularly where all of the incident power is to be absorbed, as in dummy loads. Therefore it is preferable to use a dissipative material having substantially higher loss than the materials previously used in attenuators of this general type. A suitablematerial is described below. This material is also described in more detail and is claimed in copending U. S. patent application S. N. 452,469, filed August 26, 1954, by J. C. Rolfs and L. Brecht, entitled Lossy Materials for Microwave Attenuators.

Referring to the drawing, there is shown a preferred embodiment of the invention in a dummy load in the form of a rectangular hollow uniconductor waveguide.

The wave guide is made of conductivemateri'al such as aluminum.

to dissipate the entire output of, ahigh power radar trans mitter.

A fuitlier object is to provide attenuators of the lossy wall type wherein the dissipative material is arranged in shallow grooves or slots in the conductive walls of a wave guide, thus reducing the lengths of the heat paths through the lossy material and making them more numerous than in prior art devices;

Still another object is to provide attenuators of the described type wherein the properties of a special dissipative material, containing iron as a principal ingredient, are utilized to advantage.

The invention will be described with reference to the accompanying drawing, wherein:

Fig. 1 is a perspective view of a wave guide attenuator embodying the invention.

Fig. 2 is a transverse section of the structure of Fig. 1, and

Fig. 3 is a longitudinal section of in the plane 3-3 of Fig. 2.

According to the present invention, a number of shalthe device, taken The interior surfaces of the broad walls of the wave guide 1, 3 are each provided with a plurality of spaced parallel slots or grooves 5, 7, 9, 11 and 13 extending longitudinally of the guide as shown in Fig. 3. In the present example, there are five grooves in each broad Wall and the spacing between grooves is approximately equal to the groove width. The grooves are relatively shallow, having a depth approximately one half the width.

In order to provide a smooth transition from a nondissipative wave guide to the load, and to minimize the possibility of voltage breakdown across the central region of the guide at high power levels, the outer grooves 5 and 13 are made longer in the direction of the input end of the load, and are tapered toward points near that end. The intermediate grooves 7 and 11 are somewhat shorter, and the central groove 9 is still shorter.

The grooves are filled throughout their lengths with dissipative material 15, flush with the surface of the ridges of wave guide wall materialbetween the grooves, as shown in Fig. 2. The end of the wave guide remote fromthe input end may be provided with a cover, or simply left open, as shown in Fig. 1.

Since only a fraction of the internal surface of the guide consists of lossy material, it is desirable that this material provide a substantially higher loss per unit area than the materials used in previously known types of lossy wall attenuators. This is especially so whenthe dissipative or energy absorbing material is placed in the broad walls, as shown rather than in the narrow walls.

Iron powder, mesh 44 Calcined plaster of Paris 8.8 Carbon black 2.9 Powdered silicon carbide 220 mesh 42.6

Elemental sulphur 1.7

The dry powdered ingredients are thoroughly ball milled, then mixed with sutficient water to form a thick paste which can be spread into the grooves with a spatula or a suitable fixture. After drying in air for four hours or more, the surfaces of the molded material are smoothed with emery cloth, then the structure is baked for an extended period to remove excess moisture. Then the broad walls. may be painted with a heat resistant coating such as silicone base enamel, and the'parts' baked again and finally assembled.

In the operation of the described device, the power loss is distributed over a substantial portion of the length of the wave guide, thus avoiding hot spots. The grooved construction enables the use of thin sections of lossy material which are largely surrounded by the metallic wall material, facilitating the conduction of heat to the outside of the load. The above described lossy material retains its physical strength and electrical characteristics up to the melting point of aluminum, thus avoiding the operating temperature limitations of the materials used in prior art attenuators. A wave guide load constructed as described has'been found by tests to be capable of continuously absorbing power up to the full capacity of presently available high power generators.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the ac companying drawing shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A wave guide load for dissipating microwave power, comprising a length of tubing of metal forming a rectangular wave guide having anaxial passage of uniform transverse cross section along its full length and open on at least one end to the free passage of microwaves and provided with at least three grooves longitudinally disposed in at least one of the broad wall surfaces of said wave guide, the narrow wall surfaces of said wave guide being devoid of energy absorbing material, the spacing between adjacent grooves being approximately the same as the widths of said grooves, said grooves containing energy absorbing material.

2. Apparatus for dissipating microwave power, comprising a hollow wave guide of conductive material having an axial passage of uniform transverse cross section along substantially its full length and open on at least one end to the passage of microwaves and provided with longitudinally extending grooves in its interior wall surface, the depths of said grooves being substantially less than the widths, said grooves varying in length from a minimum near the regions of maximum electric field intensity to a maximum near the regions of minimum electric field intensity, all of said grooves being substantially longer than the transverse dimensions of said wave guide, and filled with energy absorbing material.

3. Apparatus for dissipating microwave power, comprising a hollow rectangular wave guide of conductive material having an axial passage of uniform transverse cross section along substantially its full length and open on at least one end to the free'passage of microwaves and provided with a plurality of longitudinally extending grooves in the interior of surfaces of its broad walls, the depths of said grooves being substantially less than the widths, said grooves varying in length from a minimum near the centers of said broad walls to a maximum near the sides, all of said grooves being substantially longer than the width of said wave guide, adjacent grooves being separated by ridges of conductive material of said guide and filled substantially flush with said ridges with an energy absorbing material.

4. The combination set forth in claim 3, wherein said energy absorbing material includes iron as a principal constituent.

5. Apparatus for dissipating microwave power, comprising a hollow rectangular wave guide of conductive material having an axial passage of uniform transverse cross section along substantially its full length and open on at least one end to the free passage of microwaves and provided with a plurality of longitudinal grooves in at least one of its walls, the grooves adjacent the edges of said walls extending further in the direction from which microwave power is to flow into the apparatus than the grooves near the center of said wall, said grooves being filled with energy absorbing material.

6. The invention as claimed in claim 5 wherein said grooves adjacent the edges of said walls are tapered at their ends extending in the direction from which microwave power is to flow into the apparatus.

7. The invention as set forth in claim 5 wherein the extensions of said grooves in the direction ,from which microwave power is to flow into the apparatus are graduated from a maximum near the edges'of said wall to a minimum near the center of vsaid wall. 8-. The combination as set forth in claim 1 wherein said grooves are disposed in both of the broad wall surfaces of said wave guide, each groove in one broad wall surface being opposite and substantially coextensive with a corresponding groove in the other broad wall surface of said wave guide.

9. The combinationas set forth in claim 1' wherein said grooves are filled substantially flush with the interior of said one broad wall surface of'said wave guide with said absorbing material.

ReferencesCited in the file of this patent UNITED STATES PATENTS 2,646,549 Ragan et al July 21-, 1953 2,669,696 Ward 'Feb. 16, 1954 2,676,307 Anderson Apr. 20, 1954 2,705,779 Weber Apr. 5, ,1955 2,764,743 Robertson.- Sept. v25, .1956

FOREIGN PATENTS 603,119 Great Britain June 9, 1948

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