Current Sheet Antenna

Abstract

An antenna comprises a conductive ground surface element and at least one nductive radiator element in the form of an extended two dimensional sheet. The conductive radiator element is mounted parallel to the ground surface element and no more than one-fifteenth of an operating free-space wavelength from it. Signals are fed to at least two points on the radiator element wherein the signals at the points excite the radiator element symmetrically to cause current to flow across the outer surface of the conductive radiator element in the lowest mode of excitation.

Parent Case Text

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of Pat. application, Ser. No. 807,303 filed Mar. 14, 1969, now abandoned.

Description

The invention described herein may be manufactured and used by or for The Government of the United States of America without the payment of royalties thereon or therefore.

BACKGROUND OF THE INVENTION

This invention pertains to antennas and more particularly to antennas which can be mounted very close to conductive surfaces.

When small antennas such as half-wavelength dipoles operate in the vicinity of conducting surfaces the far field radiation differs considerably from the radiation by such an antenna in free space, i.e., remote from a conducting surface. The reason for the difference is because of the currents and charge distribution induced on the conducting surface. This can be seen by the relatively simple case of a half-wavelength dipole parallel to a perfectly conducting infinite plane. In such a case, the plane acts as a mirror and the image of the dipole is present behind the plane. Effectively, the antenna now appears as two half-wavelength dipoles separated by twice the distance of the real dipole from the plane. Both dipoles (the real dipole and the image dipole) radiate and the far field radiations from each dipole interact. More specifically, if the radiation resistance of this antenna system is calculated, it is found that this resistance falls off very rapidly as the spacing between the dipole and the conducting plane is reduced to less than one-quarter of a wavelength. The reason is that the currents in the real dipole and the image dipole are equal in magnitude but opposite in direction and hence the relative phase of their far fields approaches 180.degree. as their separation is reduced.

The reduction in radiation resistance increases the losses in the matching network required between the antenna and the input signal generator. Hence, it is usually undesirable to operate with the dipole at a distance of much less than one-quarter of a wavelength above a conductive ground plane. However, in many applications, particularly those involving moving vehicles such as aircraft, ships, etc., it is highly desirable to mount the antenna systems as close as possible to the conductive surface or "skin" of the vehicle.

SUMMARY OF THE INVENTION

It is accordingly a general object of the invention to provide an improved antenna which operates efficiently in close proximity to a conducting surface.

It is another object of the invention to provide an improved antenna which can be flush (or near-flush) mounted on the conducting surface of a body.

Briefly, the invention contemplates an antenna comprising a conductive ground surface element and at least one radiator element of a conductive material. The radiator element has an extended two-dimensional shape. Means connect the radiator element parallel to the ground surface element at a distance of no more than one-fifteenth of an operating free space wavelength therefrom. Signal feed means are connected to at least two points on the radiator element for exciting the radiator element symmetrically to cause current to flow across the outer surface thereof.

DESCRIPTION OF THE DRAWINGS

Other objects, the features and advantages of the invention will be apparent from the following detailed description when read with the accompanying drawing which shows by way of example and not limitation preferred embodiments of the invention.

In the drawings:

FIG. 1 is a plan view of an antenna having a polygon-shaped radiator element in accordance with one embodiment of the invention;

FIG. 2 is a cross-sectional view taken along the line 2--2 of FIG. 1;

FIG. 3 is a plane view of a portion of a phased array antenna system utilizing a circular shaped radiator element in accordance with another embodiment of the invention;

FIG. 4 is a cross-sectional view taken along the line 4--4 of FIG. 3;

FIG. 5 is a plan view of a portion of another phased array antenna system utilizing rectangular strip radiator elements in accordance with a further embodiment of the invention;

FIG. 6 is an enlarged cross-sectional view taken along the line 6--6 of FIG. 5;

FIG. 7 is a plan view of another embodiment of the invention wherein the ground surface element comprises a group of discrete conductive surfaces that are electrically interconnected;

FIG. 8 is a sectional view taken along the line 8--8 of FIG. 7;

FIG. 9 is a plan view of a further embodiment of the invention wherein the radiator element is partially surrounded by the ground surface element; and

FIG. 10 is a sectional view taken along the line 9--9 of FIG. 9.

PREFERRED EMBODIMENT OF THE INVENTION

FIGS. 1 and 2 show one embodiment of the antenna 10 comprising a ground surface element 12 of electrically conductive material and a radiator element 14 of electrically conductive material, conductively mounted via post 16 to the ground surface element. The elements are spaced by no more than one-fifteenth of an operating free-space wavelength or closer. A signal source 18 is connected by a coaxial cable balun 20 to feed points 22 and 24 on the underside of radiator element 14.

The feed points 22 and 24 are preferably symmetrically about the center of the radiator element 14 at post 16. A coaxial balun 20 is used to provide anti-phase signal feed to the points 22 and 24, i.e., the signal at point 22 is 180.degree. out-of-phase with the signal at point 24. The separation of the feed points can vary; in some cases the feed points can be at the periphery of the radiator element 14; in other cases they can be closer together. In order to improve the frequency response of the antenna, it may be desirable to connect tuning capacitors for tuning out the inductance 26 and 28 between the radiator element 14 and the ground surface element 12 adjacent (up to 0.03 wavelengths away) the edge of the element 14 as shown in FIG. 1 and 2.

With the feed configuration as shown, current flow takes place diametrically across the upper surface of radiator element 14 from one feed point to the other. Although current and charge distributions will also be built up on the underside of radiator element 14 and also on the part of ground surface element 12 below radiator element 14, these will not contribute to the far field. The far field is due principally to the current flow on the top side of radiator element 14 and the charges at the edges thereof. The far field pattern so obtained, approximately resembles that of a half-wavelength dipole above a ground plane, but since there are no cancelling image currents the radiation resistance is not detrimentally reduced as is the case with the dipole. The radiation will be linearly polarized. If circular polarization is desired, a second pair of feed points disposed on a line orthogonal to the line connecting the first pair is fed with a signal in phase quadrature to the signal fed to the first pair (See FIG. 7).

While radiator element 14 is shown as a regular hexagon, other polygonal shapes such as squares, rectangles and even circles can be used. It is only necessary that the radiator have an extended two-dimensional shape. In fact, the shape of the radiator element is preferably symmetrical about the direction of current flow. In addition, the following are the dimensions for the radiator element. The length, i.e., the dimension in the direction of current flow, should not exceed 0.7 operating wavelengths. The width, i.e., the dimension perpendicular to the direction of current flow, should be no less than 0.15 operating wavelengths.

It should be noted that the far field radiation patterns of the antenna have a maximum in a direction perpendicular to the radiator element away from the conducting surface element, and are polarized in the plane through the line joining the feed points. When the conducting ground surface element is of infinite extent the patterns, but not the radiation resistance, are similar to those obtained with a conventional dipole parallel to a similar infinite ground surface and at the same distance from the latter as the radiator element. Thus the field will be zero in all directions at the ground surface.

When a finite conducting ground surface element is used, the far field (well beyond the boundaries of the ground surface element) falls off much more slowly as the plane of the ground surface element is approached and may become significant even behind the ground surface. In fact, by a suitable choice of size and shape for the so-called ground surface element, a useful approximation to uniform hemispherical coverage may be obtained, representing a most useful type of antenna particularly when excited for circular polarization.

In FIGS. 3 and 4 there is shown a phased array antenna configuration of FIGS. 1 and 2. In particular, antenna system 40 includes ground surface element 42 and a plurality of radiator elements 44A to 44N. Each of the radiator elements 44 is connected to ground surface element 42 via an optional conductive stand-off 46 but so that the radiator elements are parallel to the ground surface element and spaced by no more than one-fifteenth of an operating wavelength therefrom. Just as with antenna 10 of FIGS. 1 and 2, each radiator element is fed by a co-axial balun 42. The baluns are connected to feed system and signal source 50. All the pairs of feed points such as points 52 and 54 of element 44A are aligned along diameter lines that are mutually parallel. In this case the radiator elements 44 are shown as circular disks having a little less than a half-wavelength diameter. They could have other polygonal shapes.

The radiator elements are shown having a triangular spacing geometry but a square spacing geometry could also be employed as long as the radiator elements are spaced by not much more than half an operating wavelength.

The phased array antenna system 60 of FIG. 5 exploits the fact that the field has nulls orthogonal to the lines connecting the feed points. In such a case the radiator elements can be joined in the direction of zero field. In particular, system 60 includes the usual ground surface element 62 and a plurality of strip radiator elements 64A to 64N. Each of the radiator elements has a plurality of pairs of feed points such as 66 and 68. Adjacent pairs of feed points are about one-half wavelength apart or closer and the lines connecting the feed points are parallel. Triangular spacing between feed pairs could be employed as was described in FIG. 3. Physically, the radiator elements 64 can be mounted on the ground surface element 62 through the agency of a layer of dielectric material 70. It should be noted that the previously described radiators 10 and 44 could have been similarly supported. See FIG. 6 which shows the details of a typical signal feed for broadband operation. The feed device is a branched length of coaxial cable whose first branch 72 feeds point 66 and whose second branch 74 feeds point 69. The branches are of equal length. The outer conductor of branch 72 is connected to the ground surface element 62 while the control conductor thereof is connected to point 66. The outer conductor of branch 74 passes through ground surface element 62 and the "center" of radiator element 64A and is connected thereto in the region of feed point 68 while the central conductor extends to the ground surface element at point 76.

The coaxial cable is connected to feed system and signal source 80 which also feeds the other feed points of the system. The signals fed to the cables can be programmed to carry out sweeping radiation patterns as is well known with phased array antenna systems.

In FIGS. 7 and 8 there is shown another embodiment of the invention wherein the ground surface element is discontinuous and also which is capable of radiating in an elliptically or circularly polarized mode.

Ground surface element 100 comprises a plurality of conductive strips 100A to 100N electrically connected by conductive jumpers 102A to 102N to form a conductive grid. While conductive strips are shown, other discrete shapes and even wires could be used. Affixed to ground surface element 100 via conductive stand-off 104 is radiator element 106. Affixed to radiator element 104 are a first pair of feed points 108A and 108B, and a second pair of feed points 110A and 110B. Each pair of feed points is fed in a similar manner. For example, the feed points 108A and 108B are fed by coaxial balun 112. Feed points 110A and 110B are also fed by another coaxial balun (not shown for the sake of clarity). Feed points 108A and 108B are symmetrical about the center of the radiator element 106, as are feed points 110A and 110B. The line connecting feed points 108A and 108B is orthogonal to the line connecting feed points 110A and 110B. If the pairs of feed points receive the same signals but which are in phase quadrature then the antenna will radiate in an elliptically polarized mode. If the amplitudes of the signals are the same then the radiation will be circularly polarized. In all other respects the antenna of FIG. 7 is the same as the antenna of FIG. 1 and can have the same variations and dimensions.

In FIGS. 9 and 10 the ground surface element 120 is shown as a partially closed surface in the form of an open top box of conductive material. The radiator element 122 is disposed within the box and facing the top thereof. Radiator element 132 is fixed to the bottom 124 of the box via conductive stand-off 126. Just as previously described, the feed points 128 and 130 of the radiator element 122 receive signals from coaxial balun 132. Except for disposing the radiator element within a partially closed ground surface element, all previously mentioned variations and dimensions apply.

There has thus been shown improved antennas which use extended surface radiator elements in close proximity to ground surface elements. While only representative embodiments have been disclosed, there are, of course, many other antenna configurations using the surface current concept of the invention. For example, the circular discs can be divided into quadrants that are excited by conventional turnstile feed systems.

As an additional example, although the radiator elements have been shown in a flat planar configuration, they may be curved surfaces following the contours of the ground surface element. Such configurations are useful when the antenna is fixed to the metallic skin of the fuselage or wings of an aircraft wherein the shape is determined by aerodynamic requirements. Furthermore, under such circumstances, the space between the radiator elements and the ground is preferably filled with solid dielectric material.

While only a limited number of embodiments have been shown and described in detail, there will now be obvious many modifications and variations satisfying many or all of the objects of the invention but which do not depart from the spirit thereof as defined in the following claims.

Claims

What is claimed is:

1. An antenna comprising an extended conducting ground surface element, at least one conducting radiator element of an extended two-dimensional shape substantially parallel to and no more than 0.15 operating wavelengths from said ground surface element, and electrical feed means for exciting said radiator element symmetrically to cause current to flow across the surface of said radiator element remote from said ground surface element in the lowest mode of excitation, and a pair of opposed tuning capacitors respectively positioned at a different edge of said radiator element between said radiator element and said conducting ground surface, the dimension of said radiator element in the direction of current flow being less than 0.7 operating wavelengths and the dimension perpendicular to the direction of current flow is at least 0.15 operating wavelengths.

2. The antenna of claim 1 wherein the shape of said radiator element is symmetrical about the direction of current flow.

3. The antenna of claim 1 wherein said electrical feed means comprises at least two feed points symmetrically placed about the center of said radiator element and the signals fed to said feed points being in anti-phase relationship.

4. The antenna of claim 1 further comprising a conductive support means for connecting said ground surface element to the center of said radiator element.

5. The antenna of claim 1 wherein the space between said ground surface element and said radiator element is at least partially filled with a solid dielectric material.

6. The antenna of claim 1 wherein said ground surface element and said radiator element have the same contour.

7. The antenna of claim 1 wherein said electrical feed means comprises two pair of feed points wherein the feed points of each pair are symmetrically placed with respect to the center of said radiator element and the lines connecting the feed points of each pair are mutually orthogonal and means for feeding the pairs of feed points in phase quadrature.

8. The antenna of claim 1 wherein said conducting surface element is at least partially closed and said radiator element being disposed adjacent to at least a part of a surface of said ground surface element.

9. The antenna of claim 1 wherein said ground surface element is continuous.

10. The antenna of claim 1 wherein said ground surface element comprises a plurality of discrete electrically conductive surfaces electrically interconnected.

11. The antenna of claim 1 wherein said ground surface element has an area at least as great as the area of said radiator element.

12. The antenna of claim 1 comprising a plurality of spaced radiator elements.

13. The antenna of claim 12 where the feed points of each radiator element are on a line passing through the center of the radiator element with each point being on an opposite side of the center and wherein said lines are substantially parallel.

14. The antenna of claim 12 comprising a plurality of similar rectangular spaced radiator elements wherein the long axis of each element extends in the same given direction and said elements being spaced from each other in a second direction perpendicular to said given direction, each of said rectangular radiator elements having a plurality of pairs of feed points, the lines joining said pairs of feed points being parallel to said second direction.