US2908875A - Dummy load for microwaves - Google Patents

Description

Oct. 13, 1959 L. N. BLATT ET AL DUMMY LOAD FOR MICROWAVES Filed July 12, 1955 GLAZED TO RESIST MOISTURE NITRIDED SHJCON AND SHJCON CAFQBDE //Vl f/V7 O/4 5, 455 /V. BZATT W/LL/AM P. PfVSE/P SOL ONO/V LAP/D (/5 United States Patent f DUMlVIY LOAD FOR MICROWAVES Lee N. Blatt, Brooklyn, William P. Peyser, Valley Stream, and Solomon Lapidus, Flushing, N.Y., assignors to Bogart Manufacturing Corporation,.Brooklyn, N.Y., a corporation of New York Application July 12, 1955, Serial No. 521,516 4 Claims. (Cl. 333-22) This invention relates to microwave apparatus, and more particularly to a dummy load for dissipating microwave energy.

Dummy loads are used in testing of equipment in produetion, in field testing, and in laboratory work. They are also used with radar transmitters which are to be connected to an antenna for only brief intervals in order to'prevent counter measures.

The usual dummy load employs a lossy material, and heretofore has been rather large in dimension, and has been provided with radiating fins or other cooling arrangement to help dissipate energy without excessive heating. One type of dummy load which is favoredfor waveguide plumbing uses a piece of plumbing made of lossy material, and relies on energy absorption in the walls. The piecematches the waveguide at its entrant end, and is usually made with a two-stage taper in accordance with design formulas published .in Technique of Microwave Measurements by Montgomery, vol. 2, in the Rad. Lab. (Radiation Laboratory) series, pages 738-741. A lossy material there suggested is made up of graphite and cement, but this has not proved altogether satisfactory because'of a tendency toflakewhen heated, and because of the large size of the dummy load.

The primary objects of the present invention are to generally improve dummy loads, and to overcome the foregoing difliculties. More specific objectsare to provide dummy loads which are small and compact, yet capable of high power dissipation, and which are light in weight, thus adapting the same for air-borne equipment. Other objects are to provide :a dummy load which is characterized by minimum reflection or so-called voltage standing wave ratio, which is characterized by negli gible moisture absorption, which is shock-proof, which does not require fins or equivalent accessory for heat dissipation, and which will operate at high temperatures over long periods of time without deterioration.

Further objects of the invention are to adapt the 'dummy load for pressurized waveguide systems, to provide a metal housing for the same, and to prevent-difficulty arisingin consequence of a difierence intemperature coefficient of expansion between the lossy material on the one hand and the metal housing on'the other.

To accomplish the foregoing objects, and other more specific objects which will hereinafter appear, our invention resides in the dummy load elements including the lossy material thereof, and their relation one to another, as are hereinafter more particularly described in the following specification. The specification is accompanied by a drawing, in which:

Fig. 1 is a perspective view of a dummy load enibodying features of our invention;

Fig. 2 is a longitudinal section taken approximately in the plane of the line 22 of Fig. '1;

Fig. 3 is a longitudinal section taken approximately in the'plane of the line 3-3 of Fig. 2;

Fig. 4 is a partially sectioned view similar to Fig. 2,

2,908,875 Patented Oct. 13, 1959 but taken through the lossy material or core alone; and

Fig. 5 is a transverse section taken approximately in the plane of the line 5-5 of Fig. 2.

Referring to the drawing, the dummy load comprises a core 12 of lossy material, and a metal housing 14 about the core. This housing includes a flange 16 at the entrant end for attachment to waveguide plumbing. The flange is 'here shown of the simple cover type without a gasket, but it will be understood that the flange may be provided with an O ring or other gasket, and that the flange may be a choke flange instead of a simple cover flange.

The rectangular opening 18 in the flange conforms in dimension to the waveguide to which the dummy load is 'to be connected, and the entrant end 20 of the hollow in the body of lossy material 12 registers with the opening 18. The inside walls 22, 24 and 26 act to absorb microwave energy, and for this purpose the passage preferably tapers. The walls 22 have a slight taper toward the closed end, followed by Walls 24 which have an abrupt taper. The walls 26 also have a slight taper toward the closed end. This design is partially in accordance with the teachings in Technique of Microwave Measurements previously mentioned, except that because of the superiority of our lossy material, the length may be shortened to about one-half the length there specified. Most, though not all, of the shortening is accomplished in the gradually tapering section. The shortened dummy loads have slightly greater reflection than is sought by the textbook specification, but the reflection is kept well Within commercially acceptable specifications.

-We have discovered that a dramatic improvement in the operation of these dummy loads is obtained by using so-called nitrided silicon carbide as the lossy material. The specific material which we prefer to employ is a modification as described later of a material commercially available under the trademark Niafrax made by The Carborundum Company of Perth Amboy, N]. This Niafrax material is characterized by excellent heat conductivity, like that of a metal, and yet the material will withstand high temperatures, like a ceramic or other refractory. The material is also shock resistant even under high temperatures. This 'Niafrax material has been used as a refractory, and has been used in jet engines, but so far as We are aware it has not been used as the lossy material in a dummy load.

Silicon carbide itself (not nitrided) has been used to make electrical resistance rods, in large size for furnace heating, and in small size in radio and television receivers, and some attempt has been made to use silicon carbide for a dummy load. For this purpose it has been made in the form of a solid wedge; it has been large in dimension; and has not been satisfactory, for it has a high voltage standing Wave ratio, and a low resistivity, apart from being large in dimension.

With our nitrided silicon carbide, however, a dummy load in the X band, for example, is less than five inches long, and will provide 750 watts of power dissipation. The temperature may go up to, say, 800 F. for a power dissipation of 500 watts, whereas by Way of .contrast, other lossy materials, if kept down to so small a size, would be able to dissipate only one-tenth as much power. A graphite load would start to flake under a temperature rise of only 350 F.

In making Niafrax, silicon carbide is mixed with silicon to form aslurry which may be cast and evaporated to dry the same. Thus the hollow tapered core may be cast to desired configuration. At this time the mate- 'rial is still soft and it is easy to finish the same or to .it substantially non-absorptive or moisture-proof.

' the silicon to silicon nitride, and that the latter acts as a be held to a tolerance of plus or minus 0.005, and this tolerance can be met when working with the nitrided .silicon carbide or Niafrax of the present invention.

Heretofore the nitriding step was performed by em bedding the cast body in graphite, which would remove oxygen from the air by forming carbon dioxide, thus leaving a nitrogen atmosphere. In accordance with our invention, howeventhe core preferably is not embedded in graphite at all, and instead is heat treated in a furnace supplied with a nitrogen atmosphere. The firing is at a .temperature of, say 2400 F. We have found that the impregnation and acquisition of carbon is highly undesirable in microwave plumbing, and therefore we avoid using graphite.

One factor in practicing the present invention is that .the silicon should be fully nitrided. Heretofore themate- .rial was nitrided until there was an increase in weight of from 8% to 10%. We wish to minimize any residue in metal form. We accordingly provide for an increase in weight at the maximum end of the range, so that when using a mixture of silicon and silicon carbide in such proportions that the weight increase previously looked for was from 8% to 10%, we instead seek an increase of 10%. This increase in weight might, of course, be different when starting with a different ratio of silicon to silicon carbide.

The core forming the hollow is made of metal and was usually left in place during the heat treating steps. We provide for the removal of the metal core before the material is nitrided and heat treated. The inside walls are important, and uniformity of transformation of the material is wanted.

We have found that the nitrided silicon carbide is subject to moisture absorption, and in accordance with our invention the molded nitrided core is glazed to make The core material is self-glazing under high temperature treatment in an oxidizing atmosphere. There is probably a formation of silicon dioxide on the surface which makes the material highly resistant to moisture absorption. This kind of glaze is superior to applying a coating of a glazing material, which may tend subsequently to crack or craze under high temperature.

Referring to the drawing, the end wall 30 0f the hollow body must be smooth and flat so that it will come into perfect intimate contact with the-adjacent wall .of the flange 16. For this purpose the mold is so designed that there will be no parting line ridge left on the face 30. We have found that it will not do to simply mold the piece with a parting line ridge with the idea of removing the ridge after hardening, for the material is too hard to be worked, and indeed .one can dress grinding wheels with this material, instead of removing the material .by means of.a grinding wheel. However, the

material canbe worked before hardening.

The coeflicient of expansion 'of the lossy material is small compared to the coeflicient of expansion of the metal housing 14. The latter is preferably made of aluminum so that it will be light in weight. Assuming the use of alumium, its coeflicient of expansion is about 000001, which is aboutthree times the expansion of the core, it having a coefficient of about 0.000003. However,

there must be no crevice between the end 3 0 of the core 12 and the metal flange, for an electrical diSGO Ltinuit vmay lead to arcingand to reflections, etc;

the core is provided with undercuts or notches 32 near the entrant end. The aluminum housing 14 is preferably cast about the core. This has the advantage of providing a close or intimate contact between the metaland the core and of compensating for any surface irregularities which may exist on the outside of the core. In addition the cast metal has projections which fill the notches or undercuts 32, and the interlock of'the housing and core caused by these'undercuts at the entrant end serve to hold the end wall 30' of the core 12 against the flange 18. In other words, diflerences in expansion of the core and the housing at high temperature manifest themselves at the remote or closed end of the dummy load, and no crevice or opening develops at the entrant end.

The housing may be cast without the flange portion 16, in which case the latter is subsequently welded, as indicated at 34, or in other cases the flange may be cast integrally with the remainder of the housing. The flange is provided with threaded holes for mounting the dummy load, and it will be understood that the flange is preferably made thick enough for this purpose. The lossy material is almost impossible to work, and it is therefore not feasible to form threaded holes in the end of the core itself. The metal wall of the housing 14 may have a thickness much less than that of the flange, typically a thickness of A3".

The aluminum housing 14 is preferably coated with a black finishmade of enamel or other suitable coating in order to aid in heat dissipation.

If the load is to be worked at an extremely high temperature, a special metal may be used, for example Invar or stainless steel, which would permit going up to a temperature of, say, 2000 F. When using Invar or other metal having a very small coefficient of expansion, the anchoring notches 32 would not be needed.

It will be understood that while an advantage of this dummy load is that it does not require the use of fins,

' such fins may be used when desired and do aid in heat dissipation.

,Althoughwe have mentioned casting of the housing, it will be understood that the housing may be built up of pieces of metal bolted together, or pieces of metal welded together. The housing is preferably made gastight so that the dummy load may be used with a pressurized waveguide system. Indeed one advantage of using a complete housing is to make pressurization possible, for the core cannot itself be used as a gas-tight member. When making a dummy load which it is known will be used with a non-pressurized system, the metal housing is not necessary, but even in such case the left end of the housing, that is, the flange 16 together with a piece of the'body suficient to hold the core against the flange, would be employed solely for the convenience of attaching the core to the waveguide.

By way of example, we have made a dummy load for the X band (8.2 to 12.4 kilomegacycles) which is only 4% x 1% x 1 /2" in dimension and weighs only 1b., and yet will dissipate 750 watts average power or 0.29 megawatt peak power. This is used with a waveguide of A-N type RG-52/U.

A dummy load for the B band (7.05 to 10.0 kilomegacycles) is only 5% x 1% x 1%" in dimension and weighs only 1 1b., and yet will dissipate 850 watts average power or 0.46 megawatt peak power. This is used with a waveguide of A-N type RG-Sl/U.

A dummy load for the C band (5.85 to 8.20 kilomegacycles) is only 6 /8 x 1 /2 x 2% in dimension and weighs only 1% lbs., and yet will dissipate 1000 watts average power or 0.71 megawatt peak power. This is .used with a waveguide of A-N type RG-SO/ U.

age power or 2.0 megawatts peak power. This is used with a waveguide of A-N type RG-49/ U.

A dummy load for the S band (2.60 to 3.95 kilomegacycles) is only 13% x 2% x 3%" in dimension and weighs only 6 /2 lbs., and yet will dissipate 2500 watts average power or 3.2 megawatts peak power. This is used with a waveguide of A-N type RG-48/U.

A dummy load for the L band (1.12 to 1.70 kilomegacycles) is only 30 x 4% x 8" in dimension and weighs only 48 lbs., and yet will dissipate 4000 watts average power or 17.2 megawatts peak power. This is used with a waveguide of A-N type RG-69/U.

In all of these dummy loads the VSWR (voltage standing wave ratio) does not exceed 1.10 as a maximum.

In order to show the dimensions of the tapered termination inside the dummy load we have indicated the dimensions on the drawing using the letters A through G. We have successfully employed the dimensions given in the following table:

It will be understood that all of the foregoing dimensional and quantitative values have been given by way of illustration, and are not intended to be in limitation of the invention.

In the table of dimensions given above it will be seen that the length of the dummy load is only about onehalf that heretofore thought needed, as given, for example, in the Technique of Microwave Measurements mentioned above. We must explain that most lossy materials will not withstand high unit area temperatures. Therefore the use of such materials requires greater areas and longer lengths in order to limit the unit area temperature. Inasmuch as our lossy material can withstand very high temperatures we have experimentally determined that a commercially satisfactory, very high power dummy load can be produced using our material in about half the length shown in the above textbook, while keeping within a commercial VSWR limit of 1.10 to 1. Thus we gain the advantages of smaller size and lighter weight.

It is believed that the construction and method of manufacture of our improved dummy load, as well as the advantages thereof, will be apparent from the foregoing detailed description. It will also be apparent that while we have shown and described our invention in preferred forms, changes may be made without departing from the scope of the invention, as sought to be defined in the following claims.

We claim:

1. A dummy load for terminating a waveguide, said dummy load having an entrant end and comprising a core of a lossy material, said core having an entrant end and having undercuts near the entrant end, and a gasti-ght metal housing molded about the core, said housing having a flange at the entrant end, the interlock of the housing and core caused by the undercuts at the entrant end serving to hold the entrant ends in desired relation despite differences in expansion of the core and housing at high temperature.

2. A dummy load for terminating a waveguide, said dummy load having an entrant end and comprising a core of a lossy material, said core having an entrant end and having notches at its corners near the entrant end, and a gas-tight aluminum housing molded about the core, said housing having a flange at the entrant end, the interlock of the housing and core caused by the notches at the entrant end serving to hold the entrant ends in desired relation despite differences in expansion of the core and housing at high temperature.

3. A dummy load for terminating a waveguide, said dummy load having an entrant end and comprising a core of a lossy material, said core having an entrant end and having undercuts near the entrant end of said core, and a gas-tight metal housing molded about the core, said housing having a flange at the entrant end, the interlock of the housing and core caused by the undercuts at the entrant end serving to hold the entrant ends in desired relation despite differences in expansion of the core and housing at high temperature, said core being of the hollow or absorptive wall type, the lossy material of which is nitrided silicon and silicon carbide.

4. A dummy load for terminating a waveguide, said dummy load having an entrant end and comprising a core of a lossy material, said core having an entrant end and having notches at its corners near the entrant end of said core, and a gas-tight aluminum housing molded about the core, said housing having a flange at the entrant end, the interlock of the housing and core caused by the notches at the entrant end serving to hold the entrant ends in desired relation despite differences in expansion of the core and housing at high temperature, said core being of the hollow or absorptive wall type, the

lossy material of which is a silicon nitride-bonded silicon carbide material.

References Cited in the file of this patent UNITED STATES PATENTS 2,510,493 Bjorklund June 6, 1950 2,636,826 Nicholson Apr. 28, 1953 2,676,307 Anderson Apr. 20, 1954 2,695,425 Stott Nov. 30, 1954 2,701,861 Andrews Feb. 8, 1955 2,752,258 Swentzel June 26, 1956 2,804,598 Fano Aug. 27, 1957 FOREIGN PATENTS 494,671 Canada July 21, 1953

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