Zoom Interferometer Antenna

Abstract

In surface wave antennas, including single antenna structures and plural structures comprising two antennas disposed with a common boresight on opposite sides of a common reference plane radiating into free space as an interferometer or with a reflector for monopulse, having exponential aperture distributions A(y) = Ae.sup.+-.sup..alpha..sup.y the attenuation constant, .alpha., is altered by altering either the dielectric constant (permitivity), .epsilon., or the permeability, .mu., of a ferroelectric or ferromagnetic dielectric wave supporting surface by altering an electric or magnetic field therein; the gain and beamwidth are altered by altering the attenuation constant; in the interferometer, sensitivity or scale factor (difference is electrical phase per difference in angle of incidence) of incoming waves varies with variations in .alpha.; a monopulse embodiment utilizes a reflector, and the squint angle varies with variations in .alpha..

Description

BACKGROUND OF THE INVENTION

1. Field of Invention

This invention relates to antenna structures, and more particularly to a surface wave antenna having a variable attenuation constant, and concomitant variable gain, beamwidth and interferometer sensitivity or monopulse reflector spuint.

2. Description of the Prior Art

In a commonly owned copending application, Ser. No. 385,205, filed on even date herewith by B. R. Cheo et al, a phase interferometer antenna system which is capable of unambiguous detection of the angle of incidence of waves for angles of on the order of .+-.50.degree. is composed of two antennas disposed about a common reference plane, each antenna having an exponential aperture distribution over substantially its entire aperture. A true exponential aperture distribution provides unambiguous angular resolution of incoming waves between .+-.90.degree.. As disclosed in Cheo et al, ideal surface wave antennas inherently provide such an aperture distribution. However, it is difficult to approach ideal antenna characteristics with these classes of antennas. The result is that the scale factor or sensitivity does not diminish sufficiently off of boresight to provide unambiguous operation beyond .+-.50.degree..

The utilization of antennas having exponential aperture distributions as set forth in the aforementioned copending application is exemplary merely; there are additional applications, such as where a radiation pattern having small side lobes is desired.

SUMMARY OF INVENTION

One object of the present invention is to provide surface wave antennas having an exponential aperture distrubution with controllably variable attenuation constant, and therefore controllable gain and beamwidth.

Another object is to provide interferometer antenna systems with a controllably variable scale factor, or sensitivity.

According to the present invention, the attenuation constant of an isolated pair of surface wave antennas is controllably altered by altering the properties of the surface-supporting wave by means of an applied field. According to the invention in one form, the constant is controlled in a wave supporting surfaces comprising a ferromagnetic (such as a ferrite) material by controlling a magnetic field therein; according to the invention in another form, the dielectric constant of the wave supporting material is controlled by controlling an electric field impressed thereon.

In one embodiment of the invention, a pair of variable attenuation constant surface wave antennas are disposed back-to-back about a common isolating reference plane so as to form an interferometer antenna system, the variation in attenuation constant serving to alter the sensitivity or scale factor (electrical phase difference as a function of a difference in angle of incidence) of the interferometer system.

The present invention is readily implemented utilizing techniques and materials which are well known and readily available. The invention provides a simple method of adjusting the sensitivity or scale factor of a dual antenna, interferometer antenna system, thereby to achieve unambiguous operation over a broader range of angles of incidence.

Other objects, features and advantages of the present invention will become more apparent in the light of the following detailed description of preferred embodiments thereof, as illustrated in the accompanying drawings .

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a simplified, side elevation view of a surface wave interferometer antenna system;

FIG. 2 is a plot of aperture distribution of the antenna of FIG. 1;

FIG. 3 is a plot of interferometer antenna sensitivity or scale factor for various antenna systems;

FIG. 4 is a simplified, schematic, perspective view of an electric field embodiment of the present invention;

FIG. 5 is a partial schematic illustration of a modification of the current control circuit for the embodiment of FIG. 4;

FIG. 6 is a partial, simplified perspective view of a magnetic field embodiment of the present invention;

FIG. 7 is a partial section taken on the line 7--7 of FIG. 6;

FIG. 8 is a partial, simplified perspective view of a single antenna, magnetic field embodiment; and

FIG. 9 is a partial schematic illustration of a monopulse embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Surface wave antennas are devices which include a surface (or an effective surface) capable of supporting the propagation of surface waves together with a suitable feed for launching a wave onto the surface. Some commonly used structures include dielectric, ferromagnetic and ferroelectric sheets with or without a ground plane, dielectric rods, corrugated surfaces, ferrite or plasma sheets backed by a ground plane, and arrays of proximity-coupled dipoles, etc. Such antennas, and their characteristics and design requirements, are now well known, as illustrated by 1) Zucker, Francis J., "Surface -- And Leaky -- Wave Antennas," Chapter 16 of Antenna Engineering Handbook (Henry Jasic, Editor), McGraw-Hill, 1961 (Library of Congress Card No. 59-14455), and by 2) IRE Transactions On Antennas And Propagation, Volume Ap-7, Dec. 1959, Special Supplement, Pgs. S132-S296.

In the aforementioned Cheo et al application, it is shown, inter alia, that an interferometer antenna comprising a pair of surface wave antennas disposed back-to-back about a common reference plane ideally has a purely exponential aperture distribution in which the reception response amplitude of the aperture is given as

A(y) = Ae.sup.+-.sup..alpha.y (1)

where

A = maximum aperture amplitude,

e = Naperian operator,

y = distance along aperture from the reference plane,

and

.alpha. = attenuation constant of aperture distribution, a property of the dielectric material and thickness.

Such an interferometer antenna structure is illustrated briefly in FIG. 1 to comprise a pair of dielectric wave supporting surfaces 10, 12 disposed about a common conducting reference or ground plane 14, each surface having a wave launched thereon by a related wave launching mechanism 16, 18 which are connected with waveguides 20, 22 or other suitable feeds. In a theoretical sense, the antenna of FIG. 1 has an aperture distribution which falls off exponentially on either side of the reference plane as illustrated in FIG. 2. An ideal aperture distribution of the type shown in FIG. 2 is shown in equation (14) of the aforementioned Cheo et al application to provide a sensitivity or scale factor at boresight (BSF) as

Bsf = 2k.sub.o /.alpha. (2)

where

k.sub.o = 2.pi./.lambda..sub.o

Theoretically, the BSF can be increased to any arbitrarily high value simply by decreasing the value of the attenuation constant. In practice, however, as indicated by the dotted line in FIG. 3, this may cause ambiguity, for instance, whenever electrical phase differences (.DELTA..phi.) in excess of about 160.degree. are detected since it is impossible to determine whether they relate to an angle of between +40.degree. and +60.degree. or an angle below -60.degree.. On the other hand, if a lower boresight scale factor is chosen as shown by the solid line in FIG. 3, then the sensitivity or scale factor can be rendered unambiguous for angles of incidence (.theta.) between .+-.90.degree.. The dashed line of FIG. 3 indicates the ideal situation (for an ideal surface wave antenna) in which a high boresight scale factor can be achieved, and regardless of how high the boresight scale factor is, the scale factor (electrical degrees per degree of angle of incidence) will drop off with increasing angles of incidence such that the phase difference is unambiguous for any angle of incidence between -90.degree. and +90.degree..

Thus one utilization of the present invention is to provide a surface wave interferometer antenna in which the attenuation constant, .alpha., can be varied so as to supply a high boresight scale factor under less than ideal conditions as shown in the dotted line, or unambiguous operation, although with a low sensitivity, as shown by the solid line of FIG. 3, or variations in between those two.

Another useful parameter variation which can be achieved by varying the attenuation constant in accordance with the present invention is the alteration of the boresight gain, G, of a surface wave antenna according to the relationship

G .apprxeq. 1/.alpha. (3)

similarly, the beamwidth, B, of a surface wave antenna may similarly be altered due to the relationship

B .apprxeq. .alpha. (4)

for the case of the surface wave propagating on the ferromagnetic dielectric (ferrite) in the TM mode, it has been shown in IRE Transactions on Antennas and Propagation, Volume Ap-6, Jan. 1958 pp. 13-20, R. Pease, "On the Propagation of Surface Waves Over an Infinite, Grounded Ferrite Slab," that the attenuation constant, .alpha., is a function of the dielectric constant, .epsilon.; the dielectric thickness; the free space wavelength, .lambda.; the operating frequency, f; the gyromagnetic ratio, .gamma.; and the magnitude of the longitudinal magnetization, .beta., given by:

.alpha. .apprxeq. 4.pi. .sup.2 /.epsilon..lambda. t/.lambda. [.epsilon.(1 - (.gamma..sup.2 .beta..sup.2 /f.sup.2) - 1] (5)

Also for the case of the surface wave propagating on the ferroelectric dielectric in the TE mode, it has been shown by Cheo et al and in Antenna Engineering Handbook (supra) that the attenuation constant .alpha. is expressed in terms of the surface wave mode propagation constant, k, and the dielectric thickness, t, by

.alpha. = - k/tan (kt) (6)

where k is defined in terms of the dielectric constant .epsilon. and the free space wavelength .lambda. by

k = 1/.lambda. [4.pi..sup.2 (.epsilon.- 1) - (.alpha..lambda.).sup.2 ].sup.(1/2)

thus giving .alpha. as a function of .epsilon..

Thus, at any given frequency, the attenuation constant, .alpha., is a function of the magnetization or the dielectric constant of the material. As is known, the effective magnetization of a ferromagnetic material may be changed by providing a magnetic field thereto, and a dielectric constant of any ferroelectric dielectric material may be altered by the application of an electric field thereto. The second of these is achieved in an embodiment of the invention illustrated in FIG. 4, wherein the wave supporting surfaces comprising dielectric slabs 10, 12 are disposed on opposite sides of a metallic ground plane 14. However, in this case, the permitivity, .epsilon..sub.s, of the wave supporting surface structures 10, 12 can be altered by providing a variable electric field between a pair of edge-surface conducting plates 26, 28, because the voltage applied therebetween by a pair of lines 30, 32 is adjustable by means of a suitable adjustable voltage divider, such as a potentiometer 34 connected across a source, such as a battery 36. Alternatively, as seen in FIG. 5, the voltage between the lines 30, 32 may be controlled by means of a source of clock signals 40 periodically energizing an electronic switch, such as a field effect transistor 42, which shorts out one resistor 44 of a voltage divider which also includes a resistor 46. The modification of FIG. 5 will permit use in a digital system, such as a single antenna modification of a coarse-fine system described in Alcock, R. N., "A Digital Direction Finder," Phillips Technical Review, Volume 28, No. 5/6/7, July 1, 1967, pp. 226-230.

Another method for implementing a variable attenuation constant antenna in accordance with the present invention is illustrated in FIGS. 6 and 7 which comprises impression of a magnetic field into ferrite or other ferromagnetic wave supporting surface members 10b, 12b by means of wires 30b, 32b (fed in the same fashion as illustrated in FIGS. 4 or 5 hereinbefore) which are passed through a slot 46 formed between two thin conductive planes 14c, 14d (FIG. 7) which are separated by a suitable magnetic material 48, thereby providing a magnetic path having a lower reluctance at the end of the surfaces than across the slot 46, such that current flowing counterclockwise through the wires 32b, 30b, will cause a magnetic field to the left in the material 10b as shown by the arrow 50 and a magnetic field to the right in the material 12b and shown by the arrow 52. The conductive planes 14c, 14d may be formed by vapor deposition or similar processes and be only of molecular thickness, and the magnetic path 48 may be on the order of 2 mils high as seen in FIGS. 6 and 7. A similar magnetic piece is utilized at the opposite end of the wave launching structure so as to provide a low reluctance path thereat. The magnetic field alters the complex magnetic characteristics in a manner to affect the magnetization, so as to alter .alpha..

Because of the closed magnetic path the magnetization may be latched, i.e. the remanent magnetization is still considerable when the current is no longer passing through the wires.

It should be noted that the direction of the electric field or the magnetic field in either or both of the wave supporting surface materials is immaterial, but the preferred directions are as indicated.

A single antenna variation of the embodiments of FIGS. 6 and 7 is illustrated in FIG. 8. Therein, a low reluctance magnetic structure 48b is shaped so as to provide a return magnetic path in place of the wave supporting surface structure 12b of FIGS. 6 and 7, the wave supporting material 10b being provided with a thin conductive layer 14c across its lowest surface, in the same fashion as is described with respect to FIGS. 6 and 7 hereinbefore.

Alternatively, a magnetically varied, single, surface wave antenna may be provided with a solenoid wound coil in the manner illustrated in FIG. 1 of U.S. Pat. 2,921,308 to R. C. Hansen et al. However, the present invention differs from Hansen et al in that Hansen et al requires that the structure be so short as not to support a surface wave, in order that variations in permeability or permitivity will cause squinting of the angle of fire of the wave off the end of the structure; in contrast, the present invention presupposes a sufficiently large wave supporting surfaces such that substantially pure surface waves are developed thereon. Stated alternatively, the device of Hansen et al will not provide the characteristics of a true surface wave antenna, and therefore will not provide the relationships of equations (2), (3) and (4) hereinbefore with respect to variations in the attenuation constant, .alpha.. Similarly, an electrically varied single antenna structure is possible by an obvious modification of FIG. 4 in which the plates 26, 28 are applied to a single wave supporting structure.

An embodiment illustrated in FIG. 9 shows the embodiments of FIGS. 4, 6, used as a primary feed together with a parabolic reflector 60. It is well known that for purposes of determining the angle of squint in a parabolic reflector of focal length F, the feed aperture may be replaced by a point source at its phase center. In this embodiment the distance d of the phase center from the plane of reference is given by

d .about. 1/.alpha. (8)

and the squint angle .theta..sub.s is given by

.theta..sub.s .about. d/F (9)

or

.theta..sub.s .about. 1/F.sub..alpha. (10)

it is necessary to point out that it is only in this embodiment that a beam is squinted, the effect being produced by the geometry of the combination of the offset feed and the parabolic reflector.

In a single antenna embodiment (such as that shown in FIG. 8), it is immaterial whether a ground plane be provided or not, since such is required in the dual antenna interferometer antenna system embodiments for proper interferometer operation, rather than necessarily to support the generation and transmission of surface waves.

Although the description herein is oriented in terms of launching waves on the antenna surfaces, in the manner of a transmitter, it should be understood by those skilled in the art that due to the reciprocal nature of antennas, all the principles herein are equally applicable to antennas when receiving signals.

The embodiments of FIGS. 4-7 are exemplary merely, and serve to illustrate the nature of the present invention. Thus, although the invention has been shown and described with respect to exemplary embodiments thereof, it should be understood by those skilled in the art that the foregoing and various other changes and omissions in the form and detail thereof may be made therein without departing from the spirit and the scope of the invention.

Claims

Having thus described a typical embodiment of my invention, that which I claim as new and desire to secure by Letters Patent of the United States is:

1. A variable sensitivity, surface wave interferometer antenna system comprising:

a pair of surface wave antennas each comprising a surface wave supporting surface comprised of a dielectric material selected from the group consisting of ferroelectric materials and ferromagnetic materials and feed means for launching a surface wave on said wave supporting surface, each of said wave supporting surface structures being adjacent to a conducting reference plane which is between said two structures; and

field means for applying to said material of both said antennas a controllably variable field, selected from the group consisting of electric fields and magnetic fields, said selected field being an electric field in the case where said selected material is a ferroelectric material, said selected field being a magnetic field in a case where said selected material is a ferromagnetic material, thereby to alter the attenuation constant of said antenna, the variation in attenuation constant causing a variation in boresight sensitivity of said antenna system.

2. The antenna system according to claim 1 wherein said field means applies an electric field and said material comprises a ferroelectric dielectric material.

3. The antenna system according to claim 2 wherein said field means comprises a pair of plates disposed on opposite edges of said wave supporting surface material of both antennas, and means for impressing a variable voltage across said plates.

4. The antenna system according to claim 1 wherein said field means applies a magnetic field and said material comprises a ferromagnetic dielectric material.

5. The antenna system according to claim 4 wherein said field means comprises means including a low reluctance ferromagnetic path portion disposed adjacent to but separated from substantially all of said wave supporting surface material of both antennas, electric conductor means passing through the space between said magnetic path and said material, and means supplying a variable current to said electrical conductor.