Antenna And Interface Structure For Use With Radomes

Abstract

Where a planar microwave antenna is installed inside a curved radome, there is usually space between the surface of the antenna and the inside of the radome which forms a discontinuity adversely affecting performance. The antenna element can be formed over a domed substrate which takes up much of this space, but the curvature of the substrate and that of the radome seldom match so that a significant space still exists. A sheet of flexible, compressible material having a dielectric constant and dissipation factor substantially the same as that of the radome is interposed between the antenna and the radome to take up any space so that no significant discontinuity exists between the face of the antenna and the outside face of the radome.

Description

BACKGROUND OF THE INVENTION

Many applications for which small planar spiral element microwave antennas are useful involve installation on aircraft and require mounting the antenna structure on the inside surface of a radome. In such installations, the physical configuration of the radome is usually chosen for its aerodynamic properties rather than its electromagnetic propagation properties. A typical small planar spiral microwave antenna consists of a printed circuit pattern attached to a plastic or glass epoxy substrate and is mounted on a housing such that it is backed with a microwave cavity. When such an antenna is fastened to the inside surface of a curved radome, a space usually remains between the surface of the antenna itself and the interior surface of the radome. Since this is an air space, it becomes a discontinuity of substantial proportion at the microwave frequencies for which such antennas are usually employed. Such discontinuities affect performance of the antenna adversely, both as to loss of signal and as to pattern distortion, and it is highly desirable that they be eliminated or minimized. Recent requirements for extending operating frequencies with existing systems and radomes have resulted in air gap discontinuities which have been shown to be prohibitive.

SUMMARY

It has been found that the electrical characteristics of a typical microwave spiral antenna are not significantly affected if the substrate is formed in a somewhat domed configuration rather than perfectly flat. The curvature resulting from the dome-shaped substrate effectively removes a substantial portion of the space between the antenna surface and the inside surface of the radome, thus significantly reducing the electromagnetic propagation discontinuity referred to above and the resulting power dissipation. A curvature might be chosen such as to match a symmetrical mean dimension of the associated radome. Because of the many configurations of radomes and the numerous applications for the cavity-backed spiral antennas and because of manufacturing tolerances, it is not often practical to attempt to tailor the configuration of either the radome or the domed substrate to match the other. Consequently there remains, in most instances, a significant air space because of the differences in curvature between these adjoining parts. Even this limited space may constitute a significant percentage of one wave langth at the frequencies employed and thereby constitute a discontinuity degrading performance of the antenna. This discontinuity may be effectively removed by interposing between the domed surface of the antenna and the inside surface of the radome a layer of flexible, compressible material having essentially the same dielectric constant and dissipation factor as the material used in the radome such that it may be compressed between these curved surfaces to eliminate any space having the dielectric characteristics of air (unity).

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view, partially in section, of a typical radome structure to which is attached a cavity-backed microwave spiral antenna.

FIG. 2 shows an exploded view, partially in section, of the microwave antenna, the interposed dielectric layer, and a portion of the radome structure.

FIG. 3 is a plan view of the microwave antenna structure as shown in FIGS. 1 and 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, a curved radome 10 is shown partially broken away to show its interior surface 12. Such radomes are typically formed of glass epoxy resin having a dielectric constant of approximately 4.5 or 5.0. Formed as part of interior surface 12 are bosses 14 and 16. An antena structure 18 has a pair of mounting lugs 22 and 23 which are attached to the bosses 14 and 16 by means of screws 24 and 26. The usual antenna conductor may be attached to the antenna by means of the threaded member 28. The antenna 18 includes a dome-shaped substrate 30 upon which is formed a printed circuit spiral antenna pattern, and this is attached to the housing structure 32 which contains one or more microwave cavities which cooperate with the spiral pattern. A similar antenna is described in U.S. Pat. No. 3,441,937 (common assignee) except that it shows a flat substrate. Compressed between the inside surface 12 of the radome and the domed surface 30 of the antenna structure is a sheet of flexible, compressible material such as silicon rubber which has a dielectric characteristic very similar to that of the radome. Since this material is flexible, it fills all of the space between the radome and the antenna surface, thereby effectively eliminating air spaces which would otherwise adversely affect performance of the antenna.

FIG. 2 shows the working parts of FIG. 1 in an exploded view. In this view, the antenna structure 18 is shown disassembled from the radome 10 with the antenna surface 30 spaced away from the inside surface 12 of the radome and the flexible dielectric member 34. It will be appreciated that the area and thickness of member 34 may be tailored to the configuration of surfaces 30 and 12, although any member 34 of substantial thickness will be effective to deal with a number of variations not only in tolerances of parts but even of parts designed to have cured surfaces somewhat different from each other. FIG. 3 is a plan view showing the surface of the dome-shaped substrate 30 and a portion of the mounting lugs 22 and 23.

While only a single embodiment has been shown and described herein, those skilled in the art will recognize that the teachings herein are applicable to many different configurations of parts and, particularly, to many different contours of radome structure. Obviously there may be some situations in which the radome is sufficiently large that its curvature may be relatively slight even as compared with a flat antenna member, and the resulting space may be easily taken up by the flexible dielectric member. And while the above specification describes a spiral circular antenna, the teachings of the present disclosure are applicable to other antenna element shapes such as elliptical, dipoles in combination with circular or elliptical antennas, or even rectangular elements, so long as they are designed to operate at frequencies for which the air gap creates a substantial discontinuity adversely affecting propagation.

Claims

I claim:

1. An antenna structure comprising a radome having curved inside and outside surfaces, a generally circular, cavity-backed planar microwave antenna having a spiral antenna element pattern fastened to the inside surface of said radome, characterized in that said spiral antenna element pattern is formed on a dome-shaped substrate, and a layer of flexible compressible material having a dielectric constant and dissipation factor essentially like that of the radome material is interposed between said dome-shaped substrate and the inside surface of said radome whereby any space between said substrate and said inside surface is filled with said compressible material.

2. For use with a radome having inside and outside surfaces, a planar element antenna structure adapted to be attached to the inside surface of said radome, said antenna structure being formed on a substrate having a convex curved surface and being mounted on a housing containing at least one microwave cavity,

a layer of flexible compressible material having a dielectric constant essentially like that of the radome material interposed between said antenna structure and the inside surface of said radome, and

said housing including means for fastening said antenna structure to said radome and thereby compressing said flexible compressible material between said convex curved surface and the inside surface of said radome.

3. A planar element antenna structure as set forth in claim 2 wherein the dissipation factors of said flexible compressible material and said radome are substantially the same.

4. A method of eliminating losses due to discontinuities in a planar antenna structure mounted within a curved radome comprising the steps of:

a. forming a spacer member of a compressible material having a dielectric and dissipation factor approximately the same as that of said radome; and

b. interposing said spacer member between said antenna structure and said radome and fastening said antenna structure to the inside of said radome such that said spacer member takes up any space between said antenna structure and said radome.

5. A method of eliminating losses due to discontinuities in a planar antenna structure as set forth in claim 4 including the further step of attaching said antenna structure on a dome-shaped substrate.