Method for fabricating a shaped dielectric antenna lens

Abstract

A light-weight, temperature-resistant spherical antenna lens is formed of homogeneous ceramic material to provide a uniformly-distributed dielectric constant. The ceramic material formulation can be varied to adjust the dielectric constant for various wavelengths. A protective sealant coating is applied to the exterior surface and, because of the very high porosity of the lens, the sealant is applied over an under coating having essentially the same dielectric and temperature characteristics as the lens matrix material. The shaped spherical body of the lens is provided by press-forming and fusing together a quantity of highly-porous ceramic granules each being the same in composition and each having essentially the same pore size. Pore size is controlled by first preparing a ceramic slip, mixing the slip with an organic `burn-out` material, such as stearic acid powder, and then subjecting the mix to a wet granulation process to provide granules of a particular size. A burn-out produces granules having the desired pore size which then are mixed with a mechanical binder, pressed to form the sphere and subsequently sintered to provide a rigid spherical structure capable of being machined and coated with the sealant glaze.

Parent Case Text

CROSS-REFERENCE TO RELATED APPLICATION

This application is a division of application Ser. No. 428,589 filed Dec. 26, 1973, now U.S. Pat. No. 3,866,234.

Description

BACKGROUND OF THE INVENTION

The present invention relates generally to spherical antenna lenses and, in particular, to the production of a porous, ceramic lens-like structure having a uniformly-distributed dielectric constant.

Spherical lenses of the type under consideration are particularly useful in transmitting and receiving microwave energy although, as will become apparent, the principles of the present invention are readily applicable to the transmission of other energy bands. Also, there is no present intention to limit the present principles to lenses of a particular shape, such as the spherical shape of the conventional microwave antenna lens. However, the microwave energy band is of particular concern since, as is known, lenses of the present type generally are not practical for use in a broadcast band. Other energy bands, such as the infrared, usually can employ ordinary optical principles.

Various types of spherical microwave lenses are in present use although for one reason or another, each of these known types presents some deficiency or disadvantage. Examples of these types include lenses formed of plastic foams, dielectric filled plastics, quartz glass and other similar materials. The quartz-glass type unfortunately are quite heavy so that their use for missile or aircraft antenna applications presents obvious weight problems. In this regard, a 10 inch diameter sphere of quartz may weight about 40 pounds whereas a ceramic lens, such as is presently contemplated, weighs less than 19 pounds. In addition, in the quartz lens the focal point of the energy beam transmitted by the lens is fixed inside the spherical structure of the lens and, as is known, such a fixed focal point reduces efficiency. It is preferred to have the focal point exterior to the lens surface or at least to be able to adjust the focal point so as to place it either inside or outside of the surface.

One especially well known plastic foam lens is known as the Luneberg lens which consists of a series of concentric, spherical shells each consecutive shell having a successively lower dielectric constant with the outer shell having the lowest dielectric. This shell structure, although advantageous, nevertheless does produce some distortion in that the consecutive shells cause the energy beam to bend during its transmission. Further, even though the Luneberg and other dielectric filled plastic lenses have relatively low density so as to be rather lightweight, their permissible operating temperatures usually have been found to be much too low for unprotected missile flight. Protection can be provided but it involves serious problems in transmission efficiency and distortion. In addition, the stepped or consecutive-shell construction imposes operational limitations which, for the most part, limit the use of these lenses to lower microwave frequencies.

OBJECTS OF THE INVENTION

A general object of the invention is to provide a temperature-resistant lightweight, ceramic antenna lens having a relatively high transmission frequency and low distortion characteristic. With regard to this general object, one of the features of the invention is the provision of a rigidly-shaped lens body formed of a quantity of porous ceramic granules which are fused one to the other to provide the rigidity, each of the granules being essentially uniform in composition and size and each further having a pore size smaller than a minimum wavelength of the energy band to be transmitted. Control of the pore size permits a uniformly distributed dielectric constant which reduces distortion and increases the efficiency.

A further important object is to provide a fabrication process for forming rigidly-shaped dielectric structures, such as the antenna lens, so as to provide a structure having a predetermined and adjustable dielectric constant as well as other advantageous physical properties such as light weight and temperature resistance.

With regard to the fabrication process of the last-mentioned object, a still further object is to provide a method of forming a quantity of porous granules from a particular ceramic composition which can be controlled to adjust the dielectric constant, the specific gravity, the hardness and the temperature resistance of the lens formed by these granules. More specifically, this object contemplates a method of providing the porous structure of each of the granules so as to assure a relatively uniform pore size matched to the wavelength of the energy to be transmitted by the lens.

Other objects are to provide particular formulations for use in particular fabrication process steps.

These and other significant objects are accomplished in manners which will become more apparent in the ensuing detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated in the accompanying drawings which:

FIG. 1 is a view of the spherical microwave antenna lens of the present invention and an associated antenna feedhorn for the lens;

FIG. 2 is a section taken along lines 2--2 of FIG. 1;

FIG. 3 is an enlarged sectional view of several adjacent ceramic granules showing primarily the porous structure of these granules;

FIG. 4 is a section taken along lines 4--4 of FIG. 2;

FIG. 5 is a somewhat schematic representation of a hydrostatic press employed in the lens fabrication process, and

FIG. 6 is a transmission plot for a typical lightweight, dielectric, spherical lens antenna of a type contemplated by the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings, FIG. 1 is intended to illustrate a microwave antenna lens having a spherical configuration that provides an omnidirectional capability. Comparable lenses can be exemplified particularly by the so-called "Luneberg" lens described by R. K. Luneberg in his "Mathematical Theory of Optics," Brown University Advanced Instruction and Research in Mechanics, Providence, Rhode Island, 1944. Luneberg lenses, as has been stated, are ceramic lenses formed of a series of consecutive ceramic shells designed to provide a particular dielectric constant. The particular advantage of the spherical symmetry is that a plane wave incident on one side of the sphere may be focused at a point on its other side and, conversely, a transmitting point source located on or near the surface of the sphere may be converted to a planar wave front by passing it through the lens. One advantage over the more common dish type antenna is that the focusing property of the lens does not depend upon the direction of the incident wave. For this reason such lenses may be used with so-called organ pipe scanners having switchable feedhorns to provide a very wide scanning angle. However, as shown in FIG. 1, lens 1 is fed by a single antenna feedhorn 20. Lenses of this type preferably should possess a number of characteristics beyond those that are more directly concerned with efficiency and lack of distortion. Thus, since these lenses are well-suited for use in missiles or high-speed aircraft, they should be as lightweight and their lightness should be coupled with acceptable structural strength capable of eliminating or minimizing the use of protective radomes and the like. Also, it is highly important that the structure be resistant to high temperatures which may be developed frictionally due to the high speed of the missile.

As shown in FIG. 1, lens 1 is formed of a spherical body portion 2 over which, preferably, is applied a protective sealant coating 3 constructed and formed in a particular manner which will be described in some detail. The ceramic body portion, as shown in FIG. 2, has an essentially homogeneous structure to the extent that it is formed of a quantity of substantially identical ceramic granules such as are shown in the drawing as dots 4. These granules 4, as stated, are essentially identical at least to the extent that each has a porous ceramic structure and each is formed of essentially the same composition and of the same dimensional size. In this regard, one of the features of the invention which also will be described subsequently, resides in the fabrication step used to assure the identity of these granules particularly with regard to the substantial uniformity of the pore size. As shown in FIG. 3, each of the granules is formed with a large number of pores 6 that are intended to be of approximately the same size.

FIG. 4 illustrates primarily the use of a so-called two part sealant coating 3, the two parts of this coating being a base layer 7 and a exterior sealant layer 8 preferably formed of a strong enamel glaze. Although this two part coating will be described subsequently, it presently can be noted that its base layer 7 has a porous structure adapted to match dielectric constant of body portion 2 of the lens while the sealant 8 is intended to be applied as a thin film having a thickness that is less than the wavelength of the electromagnetic energy transmitted by the lens itself. Consequently, the coating, as a whole, produces a minimum amount of distortion while providing operational strength and protection.

The foregoing description has attempted to describe generally the physical structure of the present spherical antenna lens. However, other specific structural features also are considered quite significant. These other features best can be understood by considering the manner in which the lens is fabricated and, of course, this fabrication process itself is an important feature of the present invention.

The first step in the fabrication of the lens is the preparation of a very porous, fired ceramic grain or quantity of granules which ultimately are pressed and fused together to provide the rigid spherical antenna lens. A quantity of granules is formed by a wet granulation process using a ceramic slip which preferably conforms to the following general formula:

Weight % ______________________________________ Titania 20-80 Silica 5-30 Boron phosphate 0-10 Ball clay 5-40 Alumina 0-50 Glass frit No. 1 0-6 Water 30-50 ______________________________________

The ingredients of this general formula are milled for about 4 hours in a porcelain ball mill according to general practices well known in the art of technical ceramics.

Specific formulation examples for the dielectric ceramic slip are as follows:

A B C ______________________________________ Titania 42.0 47.4 24.2 Silica 12.2 6.8 Ball clay 10.2 10.2 24.2 Alumina 24.2 Glass frit No. 1 3.4 3.4 Water 32.2 32.2 27.4 Grain sintering temperature 1080.degree.C 1100.degree.C 1400.degree.C Final sintering temperature 980.degree.C 980.degree.C 1300.degree.C ______________________________________

This general formula provides a wide latitude in adjusting the dielectric constant and the density of the antenna lens that is being formed. The dielectric constant may be adjusted by changing the ratio of titanium oxide to silica or alumina. Other properties, such as the specific gravity and fired hardness of the rigid lens structure, also are controllable by varying the percentages of boron phosphate, glass frit and ball clay. Specifically, specific gravity and fired hardness may be decreased by lowering the percentages of these materials. Higher operating temperatures can be achieved by eliminating glass frit and boron phosphate while increasing the amount of alumina used in the slip. Obviously variations or adjustments in the general formula are made with regard to the specific application or use for which the lens is intended. For example, a missile application may be concerned primarily with low weight and resistance to high operating temperatures. Also, in most applications the dielectric constant of the lens is a critical concern and this, as has been indicated, can be adjusted with regard to the wavelengths to be transmitted.

Following preparation of the slip, and following a settling step, excess water is decanted. The remaining ceramic slip is blended in a paddle-type blender with an organic burn-out material such as stearic acid powder. A general formula for this blend is as follows:

Weight % ______________________________________ Ceramic slip (from A) 38-60 Stearic acid powder 30-60 Cellulose powder 0-2 Water to create a thick, smooth paste ______________________________________

Specific examples of grain formulas are as follows:

A B C ______________________________________ Ceramic slip A 62 Ceramic slip B 62 Ceramic slip C 58 Stearic acid powder 38 38 38 Water 4 ______________________________________

After blending these materials, the wet granulation process is completed by removing excess water from the paste preferably by drying the paste on an absorbent surface to about 8-15% moisture content. Following the absorption of the moisture on the absorbent surface, the drying is continued concurrently with a step of breaking-up or comminuting the material by any suitable mechanical means until the resulting moist grain is capable of passing through a 28 mesh sieve. In this regard, it should be noted that sieve sizes used in the present description refer to the Tyler standard screen scale sieves. The various sieve openings of the Tyler standard scale are provided in a number of publications including Lange's "Handbook of Chemistry." More specifically, the screening of the moist grain is conducted in such a manner as to produce a range of granules sized to a plus 28 mesh, minus 48 mesh. Obviously, the range can be extended somewhat on either side to include mesh sizes within a range of 20-48 plus. The sieve openings produce granules of a size less than 1 millimeter which is to be compared with the wavelength the customary microwave band which is generally considered to have a wavelength of about 1 millimeter at its lower limit. After the sizing operation, the screened granules are further dried using warm circulating air preferably not to exceed 65.degree. centigrade and the drying is continued to produce a moisture content of less than about 0.1%.

The next fabricating step is one of burning out the organic burn-out material incorporated in the described blending of the slip. The preferred burn-out material is stearic acid powder. The principle objective in the burn-out step is to remove all of the organic material from the grain or granules without melting or appreciably changing the structure of each of the granules. A preferred way of achieving this result is to spread the dried grain about one-half inch deep on a flat tray having a bottom support surface formed of a 100 mesh stainless steel screen. The loaded tray is plunged into a hot gas flame under a vented hood. The flame should completely engulf the tray. Venting of the hood is important to assure an ample supply of oxygen to produce a rapid burning capable of igniting and removing the stearic acid without permitting the heat of the flame to get inside the granules where it would melt and destroy the granules. The loaded tray is left in the flame until all the stearic acid is charred and no further flaming from the stearic acid is evident. Usually the time required to achieve this result is about 3 to 5 minutes and the person conducting the operation will be capable of visually determining the completion point by noting the darkness of the granules. In a commercial process a burn-out procedure would utilize a conveyor belt capable of being loaded with the granules and passed into the flame for a fixed period of time. When the burn-out is completed, the charred grain is transferred to flat, open-refractory saggers, 1 to 3 inches deep, and fired to a designated grain sintering temperature for about one hour. The sintering temperature normally will be within the range of 1,000.degree. - 1,600.degree.C with the temperature choice depending upon the ceramic slip formula which has been given in the foregoing description. After sintering, another grain sizing step may be conducted by gently rubbing the sintered grain through a 20 mesh screen to break up agglomerates. Fines then are removed through a 48 mesh screen.

The use of stearic acid powder as an organic burn-out material is considered to be one of the features of the present fabrication process, since, as now should be clear, the burning out of this powder from each of the granules is the step that produces the porous structure of the granules. In this regard, reference again is made to FIG. 3 where it will be seen that each of the granules is formed with a number of interior pores 6 all of which are of a substantially uniform size which is only a fraction of a millimeter or, in other words, only a fraction of a minimum radar wavelength. Consequently, the particle size of the stearic acid powder is a matter of concern since its size determines the pore size of each of the granules. In actual practice, the stearic acid powder is a commercially-obtainable item and, as will be appreciated, the particle size dictates the use of a rather coarse powder.

As to the particular use of the stearic acid powder it will be recognized that any burn-out material might be substituted although a wide variety of materials have been tested and the stearic acid has been found to be significantly better than those that were tested. For example, in addition to stearic acid powder other tested materials included balsa dust, polystyrene spheres, acrylic spheres, paradichlorobenzene, filter paper pulp, pressed wood dust, cork granules and others. A principal difficulty with the other materials is the fact that they leave a substantial impurity in the form of ash when the burn-out is conducted and this ash impurity has an uncontrollable effect upon the dielectric constant and loss tangent of the antenna lens. Some of the materials present other difficulties such as the fact that the balsa dust is quite a difficult material to grind into the form needed for present use. Materials such as cork and acrylic spheres are found to expand excessively when heated. Other materials simply sublime when subjected to the burn-out procedure rather than igniting and burning. As has been indicated, test results certainly dictate the use of the stearic acid powder as the preferred burn-out material although other acceptable materials include the balsa dust and the polystyrene spheres. In selecting a material for this burn-out step, the factors to be taken into consideration are, in addition to those already mentioned, the ease with which the material can be ignited and burned and the strength of the material to provide support during the granulation and drying operations prior to burning.

The actual fabrication of the rigid antenna lens structure is conducted by a hydrostatic pressing operation. In this operation, the objective is first to mechanically press the granules into a cohesive spherical form which incorporates a flux material such as a glass frit so that the cohesive sphere ultimately can be fused into a rigid structure by use of a final sintering step. However, in pressing the granules to provide the cohesive sphere, care must be taken to preserve the porous structure of each of the granules. Of equal importance, the pressing should be conducted in such a manner that the density differential between the center of the sphere and its periphery is reduced to a minimum. For example, conventional pressing of the granules very possibly may result in a seriously degraded product either due to the fact that the pressure crushes the granules thereby destroying their desired porous structure or that it results in a sphere which is much denser at the periphery than at the center. Any destruction of the porous structure or any significant differential in density will materially affect the operation of the antenna lens by introducing distortion and reducing transmission efficiency.

With such considerations in mind, the sized and sintered granules are prepared for the hydrostatic pressing step first by blending the granules with a mechanical binder, such as methyl cellulose. After these two materials, i.e., the granules and binder, are thoroughly mixed, a flux material such as a glass frit is added to the mixture and carefully blended to assure complete dispersion. In particular, the mixture can be conducted using the following percentages by weight of the ingredients:

Sintered grain, -20+48 mesh 40-80 percent Methyl cellulose, 2 percent aqueous 20-30 percent solution Glass frit No. 1, 325 mesh 5-15 percent

It is quite important in mixing these ingredients to be sure that they are added slowly and are thoroughly mixed in the order given in the above example. The importance is dictated by the need to prevent the flux or glass frit from penetrating the individual granules and getting into the granular pores. When the methyl cellulose, or such other binders as may be used, is mixed with the granules prior to the introduction of the glass frit, the methyl cellulose costs the surfaces of each of the granules in such a manner that penetration by the frit is avoided.

FIG. 5 schematically illustrates the hydrostatic pressing apparatus, although, as will be recognized, the apparatus is fundamentally a conventional arrangement used for a variety of purposes. As shown in FIG. 5, the apparatus includes a latex, spherical forming mold 11 contained in an evacuation cylinder 12 provided with an appropriate cover shown as a transparent plastic plate 13, the cylinder having an exterior connection 14 for evacuating its interior chamber and another connection 16 which mounts a pressure gauge 17, this latter connection applying the hydrostatic pressure to the mold. The present operation is conducted by filling the latex forming mold with the granules coated with a moist mixture of methyl cellulose and glass frit. Air then can be evacuated from the mold which is sealed. The mold is removed and placed into a hydrostatic press. Isostatic pressure is then applied at about 100 to 200 psi to attain the desired density which, of course, is determined by the particular application to which the antenna will be put. Depending upon the particular structure being formed, the pressure may vary within a range of 100 - 1,000 psi. The term isostatic is used in the sense of a pressure application having hydrostatic equilibrium. Thus, the application of pressure equally on the outside of the mold produces a sphere which has a minimum pressure differential between the center and the periphery of the sphere. For example, a 7 inch sphere formed in this manner was cut into two parts and the density differential from center to surface was found to be considerably less than 1%.

Following the application of the pressure, the mold is stripped from the sphere and the sphere is slowly dried to about 100.degree.C. At this point, the sphere is capable of retaining its spherical form due to the cohesive binding achieved by the use of the methyl cellulose. However, this methyl cellulose binder also must be removed to prevent its presence from degrading the lens operation and its removal is achieved by conducting another burn-out step. In particular, the dried and cooled sphere is placed in a vented oven and heated to 400.degree.C at a rate not greater than 25.degree. per hour. This heating burns out the organic material. The controlled, relatively slow rate is used because excessive heat likely would cause a spalling or flaking off of the outer surface. Excessive or quick heating also would result in unequal expansion between the outer and the inner surfaces.

A final sintering step is then performed to fuse the granules one to another, this fusion being conducted at the final sintering temperatures which have been provided in the specific formulations given above. In this regard, it might be noted that the term `sintering` as used in the present description connotes the heating of an aggregate of fine particles at a temperature below the melting point of these particles so as to cause them to weld or fuse together and agglomerate. Also, the use of the term `flux` for the glass frit or other like materials is intended to encompass or include all compatible materials which are capable of promoting the sintering or solid state bonding of the grains. Further, with regard to the specific slip formulations which have been previously recited it should be noted that the formulations include such materials as silica and alumina both of which are high-temperature refractory materials. However, the combination of these two materials is known to promote sintering so that, to some extent, the grain formulations or compositions are restricted by the need to sinter these ceramic granules to produce the rigid end product.

At this point it will be appreciated that a rigid spherical structure has been formed. However, the sphere may be of a rough dimension due to the use of the hydrostatic pressing operation and further, as will be apparent, the exterior surface of the sphere is extremely porous so that, most suitably, it requires a protective coating to exclude moisture and mechanically protect its relatively fragile structure. Consequently, the next step is to dry machine the sphere to precise dimensions following which the sphere is coated with a special protective coating identified as coating 3 (FIGS. 1 and 3).

In particular, coating 3 is a two-part coating including a base layer 7 and a sealant layer 8. The need for such a two-part coating is dictated by several considerations. First, the sphere or shaped structure to be coated is a very highly porous structure so that the coating requires some thickness to provide a smooth glazed or glassy surface. If a relatively thick plastic or enamel sealant were applied directly to the sphere, the thickness would be such as possibly to interfere with the desired microwave transmission efficiency. Further, because of the fact that the lens must be resistant to high operating temperatures, some care must be taken to match the expansion of the coating to that of the sphere body. Other significant considerations applicable to the coating involve the fusion characteristics of the coating and the dielectric constant which should be such that it does not degrade energy transmission by the lens. These several conditions can be satisfied by first applying and sintering a base layer directly to the spherical body and then applying and sintering a relatively thin sealing glaze or outer layer 8. Base layer 7 may be formed by ball milling the following ingredients for about 1 hour:

Prefired grain 91 percent Glass frit No. 1 3 Ball clay 3 Alginate gum binder 3 Water -- sufficient to make thin slurry for spraying.

In the above formula it will be noted that prefired grain is employed and that this prefired grain most suitably is ceramic grain or granules formed in accordance with the foregoing description. In other words, the grain used in the base layer is essentially the same composition as the grains used in the sphere body. As a result, the dielectric constant of the sphere body is not degraded by the presence of the base layer. Further, the base layer matches the temperature expansion characteristics of the material of the sphere body. Obviously, other binders or fluxes can be substituted although the formulation which has been given has been found to produce excellent results. The mixture produced by this formulation is a relatively thin slurry capable of being sprayed onto the sphere following which sintering can be conducted at about 800.degree. to 1,300.degree.C for a period of about one hour.

Glass frit No. 1 17.7 percent Glass frit No. 2 29.6 Glass frit No. 3 11.8 Enamel clay 2.1 Alginate gum binder .2 Water 38.6

These materials are ball milled for about three hours and the slurry or slip so formed then sprayed onto the precoated sphere. The sphere then can be dried by placing it on ceramic points and subsequently sintered to 925.degree.C for about 30 minutes. Following the drying, the ceramic points are removed and the rough tips of the points smoothed by grinding. In the above formulation it will be noted that particular glass frits are used. These glass frits may be identified as follows:

Glass frit No. 1 Ferro frit No. 3291 Glass frit No. 2 Glostex frit No. 32 Glass frit No. 3 Glostex frit No. LB-88 Alginate gum binder W. S. Perkins "Wondergum" Enamel clay Ferro Green Label clay Ball clay Superblend ball clay

The use of the three glass frits in formulating the sealing glaze slurry is dictated by the desire to match the thermal expansion of the coating material with that of the undercoating and the sphere body. As will be apparent, other fluxes may be employed and, of course, other binders may be substituted in the coating formulation as well as the other formulations that have been recited. For example, the methyl cellulose used as a binder for cohesively forming the sphere can be replaced by gum arabic, gum tragacanth or other seaweed gums.

Following the final sintering of the sealing glaze, the spherical antenna lens is completed and ready for operation. Tests conducted with various spheres formed in the manner which has been described have demonstrated the following operational characteristics:

Sphere Diameter Density, Dielectric Beam width, Gain, Side lobes Formula inches gm/cc constant degrees db db down Pattern __________________________________________________________________________ A 7 0.71 2.83 3.3 31.6 20.0 Very good B 7 0.65 2.87 3.9 30.1 19.7 Good C 4 0.73 2.47 6.0 27.0 16.5 Very good (sealed) 4 0.75 2.33 6.3 21.2 16.7 Good __________________________________________________________________________

As probably will be apparent, the sphere formula referred to in the foregoing test results designates the specific slip formulas A, B, and C previously given in this description. A typical transmission plot for a lightweight ceramic dielectric material at 35 GHz is shown in FIG. 6 of the drawing. It readily will be noted from this plot that the efficiency of the lens is quite high and its distortion is very low.

With regard to the fabrication process, there are several general considerations that should be noted. In particular, the process employs a prefired grain which is most desirable in order to minimize the rupture of the sphere during the organic burn-out. Further, the ignition of the grain for carbonization is desirable to prevent melting and deformation. As to the coating of the sphere, this protection is needed to increase the lens strength, to prevent surface erosion, to seal out moisture and to provide a dielectric transition from the lens surface to the air. Such factors are obviously particularly applicable to the production of antenna lenses. Nevertheless, the coating is not necessarily limited to the lens structure and instead could be employed on any shaped structure which must be protected due to its highly porous nature.

The present antenna is intended specifically for use in the nose of a missile or other aircraft and, when so used, it possesses a number of quite advantageous properties. For example, it is capable of operating with or without a protective radome since it is capable of withstanding temperatures up to 1,200.degree.C. Further, it will handle considerably more RF power than plastic foam or plastic-filled lenses. As to its dielectric constant, it has been noted that this property can be adjusted to suit particular conditions. Most significantly it demonstrates excellent efficiency and minimum distortion, these facts being apparent from the well-defined, pencil beam, low side lobe radiation pattern shown in FIG. 6. Structurally, the lens has excellent strength-to-weight ratio and a specific gravity of less than one so that its lightness is appropriate for the intended uses. Another important factor is that the focus location for this lens can be placed either outside the sphere or on the sphere's surface. This fact is illustrated in FIG. 1 which includes a dot identified by numeral 20, this dot being intended to locate the focus of the lens. Further, the operating temperature of the present lens can approach 1,200.degree.C. This compares with a maximum operating temperature of about 150.degree. for a Luneberg lens and about 200.degree. for a dielectric filled plastic lens. Obviously, a quartz lens has a high operating temperature of about 1,200.degree.C but such a lens is relatively very heavy and not suited for missile operation because of its weight. Other distinct advantages which should be apparent include its simplicity and consequent low cost, its relative lightness coupled with strength and its applicability to millimeter wavelength radar which, as is known, is not readily available with a Luneberg lens.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

Claims

We claim:

1. A process for fabricating a rigidly-shaped dielectric structure for transmitting high-frequency electromagnetic energy, said structure having a predetermined and uniform dielectric constant and said process comprising the consecutive steps of:

providing a quantity of porous ceramic granules having a substantially uniform pore size no larger than the minimum wavelength of said electromagnetic energy, said pore size as well as the composition of the granules being a determinable function of said predetermined dielectric constant,

intimately mixing said quantity of porous granules initially with a cohesive organic binder material capable of cohesively binding one granule to another and then with a flux material, said initial intimate mixing applying said binder material to the exterior surfaces of each of said granules for inhibiting penetration of said granule pores by said flux material,

controllably pressing said granular mixture to bindably form the mixture into said shaped structure the application of the pressure being controlled to provide a desired structural density,

controllably heating said shaped structure sufficiently to remove said organic binder material and,

sintering said shaped structure for providing said rigidity.

2. The process of claim 1 wherein controlled pressing of the granular mixture is accomplished by isostatically pressing the mixture at between 100-1,000 psi to attain said desired density.

3. The process of claim 1 wherein said controlled heating is accomplished by increasing the temperature of the structure to about 400.degree.C at a rate sufficiently gradual to minimize spalling due to temperature differential between the inner and outer portions of the structure.

4. The process of claim 1 wherein the step of providing said quantity of porous granules includes the steps of:

intimately mixing a ceramic slurry formed of a liquid dispersion of ceramic material and a binder material,

combinably mixing said slurry with an organic burn-out material of a particular particle size determined by the desired pore size of the porous granules to be formed,

partially drying said combined materials to produce a moist granular mass,

screening said mass to produce a quantity of granules having a grain size range of about 20-48 mesh,

drying said sized grains, and

exposing said dried grains to a hot flame for a limited period of time sufficient to burn out said organic materials from the ceramic grains without destructively melting the ceramic material comprising the grains,

said burn out operation producing said quantity of porous granules each of which has a pore size approximating said particle size of the removed burn out material.

5. The process of claim 4 wherein said organic burn out material is stearic acid powder.

6. The process of claim 5 wherein said stearic acid powder is approximately 38 percent by weight, the balance of said combined mixture being essentially provided by said ceramic slurry.

7. The process of claim 4 wherein said ceramic slurry is formed in accordance with the following general formula:

said step of drying the sized grains being performed in a manner capable of producing a moisture content of less than 0.1 percent.

8. The process of claim 7 wherein said ceramic slurry is formed as follows:

9. The process of claim 4 wherein said ceramic slurry is formed as follows:

10. The process of claim 7 wherein said ceramic slurry is formed as follows:

11. The process of claim 7 wherein said organic burn out material is stearic acid powder, and

said step of exposing the grains to a hot flame includes:

supporting said grains thinly on a flat screen,

igniting said stearic acid powder by exposing said grains to a vented flame, and

stopping said flame exposure when said stearic acid powder is charred and ceases to flame.

12. The process of claim 7 wherein the intimate mixture of the porous and sized granules with the organic binder material and the flux material has the following composition by weight:

said granules being mixed with said methyl cellulose prior to being mixed with said glass frit.

13. The process of claim 4 further including the step of adhering a continuous protective thin-film sealant coating on the porous surface of said sintered structure, said coating having a thickness no greater than the wavelength of said electromagnetic energy to be transmitted.

14. The process of claim 13 wherein said ceramic slurry composition is selected to provide a temperature-resistant shaped structure,

said sealant coating being formed of a material having thermal characteristics approximating those of said shaped structure whereby said sealed structure has a temperature resistance comparable to said shaped structure.

15. The process of claim 14 wherein said sealant coating is applied over a base coating, said base coating being applied directly to said shaped ceramic structure, and being formed by:

preparing a thin slurry composed essentially of a quantity of material having the same composition as the porous ceramic granules of said shaped structure, a binder and a flux material,

coating said shaped ceramic structure with said thin slurry, and sintering said slurry-coated structure.

16. The process of claim 15 wherein said sealant coating is formed of an aqueous mixture of glass frit and a binder material, the coating being applied by:

spraying the mixture over said binder coating, and sintering said applied mixture.