US3576581A

US3576581A - Radomes

Info

Publication number: US3576581A
Application number: US752948A
Authority: US
Inventors: Gus P Tricoles; Eugene L Rope
Original assignee: General Dynamics Corp
Current assignee: General Dynamics Corp
Priority date: 1968-08-15
Filing date: 1968-08-15
Publication date: 1971-04-27
Anticipated expiration: 1988-04-27

Abstract

A radome especially suitable for circularly polarized waves and antennas is described. The radome has an anisotropic core made up of parallel dielectric strips which may be in the form of dielectric rings which extend circumferentially around the wall of dielectric material which makes up the radome. Alternatively, the strips may extend longitudinally along the wall.

Description

)(Rv 39576q58l [72] Inventors Gus P. Tricoles; 2,744,042 5/1956 Pace (343/872UX) EugeneL. Rope,SanDiego, Calif. 2,929,581 3/1960 Johnson,Jr.. ..(343/872RUX) [21] Appl. No. 752,948 3,175,220 3/1965 Schetne 343/872 [22] Filed Aug. 15,1968 OTHER REFERENCES 3 g m 8 C .R. M. Kubow et al., AVIATION AGE, Feb. 1958, pp 74 l 3] 78. Copy in Scientific Library, TL 501.A83. 343 872R Primary Examiner-Rodney D. Bennett, Jr. [54] RADOMES Assistant ExaminerRichard E. Berger 12 Claims, 5 Drawing Figs. Attorney-Martin Lu Kacher [52] U.S.Cl 343/872,

343/756 [51] hit. Cl ABSTRACT; A radome especially uitable for circularly Field Of Search olarized waves and antennas is described The radome has an 872 anisotropic core made up of parallel dielectric strips which may be in the form of dielectric rings which extend circum- [56] References cued ferentially around the wall of dielectric material which makes UNITED STATES PATENTS up the radome. Alternatively, the strips may extend longitu- 3,444,550 5/1969 Leitner 343/872 dinally along the wall.

l4 l l A I I I 1 11 u u 1/ H PATENTEDAPRZ'HQYI 3576.581

SHEET 1 0F 2 Fig.

'9). 605 F! TRICOLES EUGENE L. ROPE BY W. ATTY F lg. 2

RADOMES The present invention relates to radomes and particularly to radomes having improved transmittance for circularly polarized waves.

The invention is especially suitable for use in radomes which are designed to pass waves which are incident thereon at fairly high angles of incidence (by angle of incidence is meant the angle between a ray and a line perpendicular to the wall of the radome on which the ray is incident). The invention is also especially suitable for use in a radome for circularly polarized waves.

With the advent of needle-nose aircraft, it becomes necessary that the radome which forms the nose of the aircraft be capable of transmitting waves from an antenna which lies within the nose with maximum power transmission, even though the angle of incidence of such waves to the walls of the radome is fairly high. The problem becomes more acute in cases where circularly polarized waves are to be transmitted to the radome. Known radome constructions provide different transmission characteristics, particularly phase delays, between the perpendicular and the parallel polarization components which make up the circularly polarized wave. Thus, pattern distortion and internal reflections result which degrade the performance of the radar or other microwave system located within the radome.

It is therefore an object of the present invention to provide an improved radome which passes circularly polarized electromagnetic waves with minimum pattern distortion and internal reflection.

It is a further object of the present invention to provide an improved radome which has high transmittance even at high angles of incidence of electromagnetic waves with respect thereto.

It is a still further object of the present invention to provide an improved radome which is capable of transmitting electromagnetic waves over a broad band of frequencies (say percent of the center frequency of such waves).

It is a still further object of the present invention to provide an improved radome having improved structural strength as well as improved electrical properties.

Briefly described, a radome embodying the invention has a dielectric wall so structured as to have anisotropic electromagnetic wave propagation characteristics. In other words, the dielectric constant of the wall as a result of the anisotropy is higher for parallel than for perpendicular polarization of the electromagnetic radiation. An anisotropic radome wall in accordance with the invention includes a dielectric sheet having dielectric strips mounted thereon. The strips may be disposed transversely (e.g. edgewise) to the plane of the wall. The strips may form part of a core of an A-sandwich wherein the skins are relatively thin (much less than a quarter wavelength of the radiation to be transmitted). In the A-sandwich case, the strips reduce the difference between the phase delay for parallel polarization and that for perpendicular polarization. The resultant phase difference of the entire wall including the core is reduced below that of a radome having a hollow core or a skin alone.

In the event that the skin is relatively thick, say of the order of a half wavelength of the radiation to be transmitted, the strips may be in the form of circumferential rings or discs. The invention itself, both as to its organization and method of operation, as well as additional objects and advantages thereof will become more readily apparent from a reading of the following description in connection with the accompanying drawings in which:

FIG. 1 is an elevational view of a radome embodying the invention, the view showing a portion of the outer skin of the radome broken away to illustrate the construction of the core;

FIG. 2 is a fragmentary view of the core of a radome such as shown in FIG. 1;

FIG. 3 is a sectional view in elevation of a radome in accordance with another embodiment of the invention;

FIG. 4 is a curve of the power transmittance of the radome shown in FIG. 1; and

FIG. 5 is a curve similar to FIG. 4 showing the power transmittance of the radome shown in FIG. 3. Referring to FIG. 1, a conical radome is shown having an A-sandwich panel construction. The A-sandwich is made out of a thin outer skin 10 of dielectric material, such as fiber glass sheets laminated with epoxy plastic. Quartz cloth may also be used. The thickness of the skin 10 is, desirably, much less than a quarter wavelength at the frequency to be transmitted. Thus, for Ku-band radiation, a thickness of approximately 0.02 inches may be suitable. A thin inner skin 12 of material, similar to the outer skin and of a thickness similar to the thickness of the outer skin, is separated by a core made up of a plurality of thin strips 14 of dielectric material. These strips may be made of material similar to the skins (viz. a fiber glass laminate using an epoxy resin to laminate a pair of fiber glass sheets). The edges of the strips may be cemented to the skin so that the strips are perpendicular to the skins. The strips are desirably spaced equally from each other. Inasmuch as the spacing is desirably equal, some of the strips are shorter than others in order that the spacing may remain equal all the way to the apex of the conical surface of the radome. The spacing between the strips is a function of the frequency of the radiation to be transmitted. At a frequency of 9.225 GI-lz, a spacing of 0.65 inches was found suitable.

The dielectric constant of the radome is higher for parallel than for perpendicular polarization, particularly at high incidence angles of the radiation to the wall of the radome. By parallel polarization is meant that the E.-vector of the radiation is parallel to the plane of incidence, which contains the incident wave normal and the local normal to the surface. For perpendicular polarization, the E-vector is normal to the plane of incidence. In FIG. 1 the strips are approximately parallel to the plane of incidence. The strips reduce the difference between the phase delays for perpendicular and parallel polarizations. The effect of this reduction, on the transmission of circularly polarized waves, is explained as follows: 8 is defined as the difference between the insertion phase delay for perpendicular polarization and that for parallel polarization. Briefly 5 equalsI PD I PD jlhe IPD of the sandwich made up of the skins l0 and 12 and the core containing the steps 14 is the sum of the [PD of the core. 8 and the IPD of the skins, 6, and 8, That is, for the sandwich If 8 O and 8 0,6 can be reduced by making 8 0. This is what the core made up of the steps 14 does. The power transmittance for a circularly polarized plane wave for circular polarization is Where T and T are complex-valved amplitude transmittances for parallel and perpendicular polarizations respectively. Equation 2 shows minimizing 8 increases T In order to enhance the anisotropy of the radome, it is desirable to use a honeycomb of fiber glass material in the space between the longitudinal strips 14 (see FIG. 2). The cells 16 of the honeycomb are rectangular in shape. The anisotropy favoring the parallel polarized waves is enhanced by having the long dimension of the cells 16 substantially parallel to the longitudinal strips 14. The honeycomb core containing the strips 14 is strong electrically and mechanically. Accordingly, the radome constructed in accordance with the invention may be relatively large in size without the need for additional internal support.

The power transmission of the radome shown in FIGS. 1 and 2 is illustrated in the graph of FIG. 4. The abscissa of the graph is calibrated in terms of a gimbal angle. By gimbal angle is meant the angle between the-axis of the cone and the axis of an antenna, such as the horn antenna 20, shown in FIG. 3. When the cone axis and the antenna axis coincide, the gimbal angle is 0. Small gimbal angles therefore correspond to high angles of incidence of the radiation with respect to the wall of the radome. The dash line represents the radome, including the core having longitudinal strips, as shown in FIG. 1. The solid line represents the core without the longitudinal strips. In both cases, the curves were taken with circularly polarized radiation produced by a conical horn antenna, such as the antenna shown in FIG. 3. A significant improvement in transmittance is apparent from FIG. 4.

Referring to FIG. 3, there is shown a conical radome having a skin made of dielectric material. The skin is relatively thick as compared to the thin skins or sheets used in the A-sandwich radome construction shown in FIGS. 1 and 2. Specifically, the thickness of the skin or sheets 22 may be about one-half wavelength at the frequency of the radiation to be transmitted. The material of the conical sheet 22 is desirably of a ceramic nature, such as alumina. Supported within the conical sheet on the inner surface thereof are a series of equally spaced circular discs 24. These discs may be made of fiber glass sheets laminated with epoxy. The thickness of the strips may, for example, be about 0.04 inches and their spacing may be about 0.65 inches. The thickness of the sheets and the spacing is a function of the frequency of the radiation to be transmitted. The above given examples of thicknesses and spacings may be suitable for a frequency of approximately 16.0 GI-Iz. The antenna 20 which is shown disposed within the radome is designed to propagate circularly polarized waves. To this end, a plastic wedge 28 of material, such as polystyrene is disposed in the waveguide section of the horn 20 and delays the components of the radiation aligned with it so that the E-vector of the radiation rotates circularly. Accordingly, the radiation emitted by the horn 20 is circularly polarized.

The improved results obtained with the radome shown in FIG. 3 are illustrated in FIG. 5. The solid line curved represents the transmittance of the radome, including the discs 24. The same radome, but without the discs, provides the transmittance shown by the dash line curve. The power transmittance (lTl is calibrated in normalized form on the ordinance of the curve. Accordingly, it will be observed that the improvement in transmittance is very substantial.

From the foregoing description it will be apparent that there has been described different embodiments of a radome which is especially suitable for the transmission of circularly polarized radiation. Simplifications in the illustration have been made in order to more concisely and clearly explain the invention. Variations and modifications in the herein described embodiments will undoubtedly suggest themselves to those skilled in the art. Accordingly, the foregoing description should be taken merely as illustrative and not in any limiting sense.

We claim:

1. A radome comprising:

a. a sheet of dielectric material, and

b. an array of thin strips of dielectric material supported edgewise on said sheet with the edges of said strips in contact with said sheet, said strips being arranged in planes spaced from each other, said planes being parallel to the plane of incidence of at least one of two polarizations of radiation which are perpendicular to each other, said planes lying transverse to said sheet, said array providing anisotropy favoring said one polarization of radiation.

2. The invention as set forth in claim 1 wherein said strips are parallel to each other.

3. The invention as set forth in claim 2 wherein said strips each are at least a quarter wavelength in a direction along the plane of said one polarization of radiation and are a fraction of said quarter wavelength in thickness measured from side to side along their supported edges.

4. The invention as set forth in claim 3 wherein the spacing of said strips from each other is equal and is from one-half to 1 /2 of a wavelength of the frequency of the radiation to be transmitted through said radome.

5. The invention as set forth in claim 1 wherein said sheet has a honeycomb of cells of dielectric material thereon between said strips.

. The invention as set forth in claim 5 wherein said cells are rectangular in shape and have their longer dimension substantially parallel to said strips.

7. The invention as set forth in claim 1 wherein said sheet is conical in shape, said strips being disposed on the inner surface of the cone formed by said sheet.

8. The invention as set forth in claim 7 wherein said strips are circular discs having their axes along the axis of said cone.

9. The invention as set forth in claim 8 wherein said sheet is about one-half wavelength in thickness at the frequency of the radiation transmitted through said radome.

10. The invention as set forth in claim 1 wherein said sheet forms a conical surface and said strips are disposed along the axis of said conical surface equally spaced from each other and directed so that the intersection of lines extending therefrom is at the apex of said conical surface.

11. The invention as set forth in claim 10 wherein said radome is an A-sandwich having inner and outer skins of dielectric material, said outer skin being comprised of said sheet and wherein said strips comprise the core of said sandwich.

12. The invention as set forth in claim 11 wherein said core includes a honeycomb of rectangular cells of dielectric material disposed between said strips, the long dimension of said cells being substantially parallel to said strips.

Claims

1. A radome comprising: a. a sheet of dielectric material, and b. an array of thin strips of dielectric material supported edgewise on said sheet with the edges of said strips in contact with said sheet, said strips being arranged in planes spaced from each other, said planes being parallel to the plane of incidence of at least one of two polarizations of radiation which are perpendicular to each other, said planes lying transverse to said sheet, said array providing anisotropy favoring said one polarization of radiation.

4. The invention as set forth in claim 3 wherein the spacing of said strips from each other is equal and is from one-half to 1 1/2 of a wavelength of the frequency of the radiation to be transmitted through said radome.

6. The invention as set forth in claim 5 wherein said cells are rectangular in shape and have their longer dimension substantially parallel to said strips.