US3922681A

US3922681A - Polarization rotation technique for use with two dimensional TEM mode lenses

Info

Publication number: US3922681A
Application number: US516244A
Authority: US
Inventors: Bernard L Lewis
Original assignee: US Department of Navy
Current assignee: US Department of Navy
Priority date: 1974-10-18
Filing date: 1974-10-18
Publication date: 1975-11-25
Anticipated expiration: 1992-11-25

Abstract

A non-dispersive, parallel-plate high-frequency lens-antenna has dipole elements which allow it to radiate or receive TEM mode electromagnetic energy polarized with the direction of polarization parallel to the plane of the lens.

Description

United States Patent Lewis 1 Nov. 25, 1975 POLARIZATION ROTATION TECHNIQUE FOR USE WITH TWO DIMENSIONAL TEM MODE LENSES [75] Inventor: Bernard L. Lewis, Oxon Hill, Md.

[73] Assignee: The United States of America as represented by the Secretary of the Navy, Washington, DC.

22 Filed: 0a. 18, 1974 211 Appl.No.:5i6,244

[52] US. Cl. 343/754; 343/756; 343/909 [51] Int. Cl. .,H01Q 15/04; H010 15/08; H010 i9/08 [58] Field of Search 343/727, 753, 754, 756, 343/799, 800, 909, 911 R [56] References Cited UNITED STATES PATENTS 2508.084 5/[950 Alford 343/727 Carson 4. 343/754 Brickey 343/756 Primary E.mminerPaul L. Gensler Attorney, Agent, or FirmR. S4 Sciascia; Arthur L, Branning; Norman V, Brown [57! ABSTRACT A non-dispersive, parallel-plate highirequency lensantenna has dipole elements which allow it to radiate or receive TEM mode electromagnetic energy polarized with the direction of polarization parallel to the plane of the lens.

4 Claims, 3 Drawing Figures U.S. Patent Nov. 25, 1975 POLARIZATION ROTATION TECHNIQUE FOR USE WITH TWO DIMENSIONAL TEM MODE LENSES BACKGROUND OF THE INVENTION The present invention relates to high-frequency an tennas. and more particularly to parallel-plate lens antennas. Parallelplate lens-antennas are generally utilized as two dimensional electromagnetic lenses to focus high-frequency electromagnetic energy into an antenna beam. Lenses of this type form the equivalent of a cylindrical optical lens and are especially useful in producing a radar beam having a beamwidth narrow in azimuth but large in elevation.

Prior non-dispersive parallel-plate TEM lens-antennas, although capable of radiation energy polarized with the direction of polarization normal to the plane of the lens, have heretofore been incapable of radiating energy polarized with the direction of polarization parallel to the plane of the lensa highly desirable polarization for many applications.

In order to transmit energy polarized parallel to the plane of the lens, prior parallel-plate antennas have had to be operated in the dispersive, non-TEM mode. Operation in any dispersive mode causes propagation in which the velocity of propagation through the lens is a function of frequency, termed dispersive propagation. Dispersive propagation is undesirable because it causes different frequency components of the signal to focus at different points, resulting in an objectionable electromagnetically fuzzy" focusing known as chromatic aberration.

The present invention utilizes a non-despersive parallelplate lens-antenna to transmit and receive TEM mode electromagnetic energy polarized in the same direction as the plane of the lens.

SUMMARY OF THE INVENTION DESCRIPTION OF THE DRAWINGS FIG. I shows in top view an embodiment of the present invention.

FIG. 2 shows in side view the embodiment of the present invention depicted in FIG. 1.

FIG. 3 depicts addition of quarter-wavelength chokes to a parallel-plate lens-antenna (shown incomplete, with only selected elements presented in the interest of clarity).

DETAILED DESCRIPTION OF THE INVENTION A parallel-plate lens-antenna 10, designed to radiate electromagnetic energy of wavelength A, is indicated generally in FIGS. 1 and 2. The lens-antenna has circular shaped upper and lower plates I2 and 14 arranged in a parallel stacked fashion. Each plate 12, I4 has a diameter, D, of one wavelength A or greater. The plates I2, I4 are constructed in conventional manner from any good conductor, such as aluminum, and are sufficiently thick and rigid so as to provide structural support for the lens assembly. The spacing distance, t, between the parallel plates 12, I4 is chosen to be less than Between and adjacent to the parallel-plates 12, I4 is a tapered low-loss dielectric material 16 having a dielectric constant, commonly termed k, of approximately 3.5. The dielectric material 16 is in the form of a circular disc with sloping sides in which the upper radius is smaller than the lower radius, thus forming a tapered region 18 at the disc periphery. The tapered region l8 typically has a linear taper. with the difference between upper and lower radii of approximately A plurality of conventional feed elements 26 adapted to radiate or receive high-frequency electromagnetic energy are attached to the upper plate 12. Each feed element 26 has a coaxial connector section 28 fastened to the upper plate 12, and an inner conductor element 30 which passes through an insulated aperture in the upper plate 12 and extends into the region between the upper and lower plates I2, 14. The end of each inner conductor element 30 is located adjacent and above the center of taper region 18.

The number of feed elements 26 utilized in the present invention is equal to where r is the radius from the center of the lens I0 to the feed elements 26.

Conventional quarter-wavelength chokes 32 may optionally be utilized in the embodiment of the present invention, as depicted in FIG. 3. For clarity, FIG. 3 depicts only selected portions of elements shown in FIG. 2, in addition to quarter-wavelength chokes 32. Each of the

parallel plates

12, 14 has a choke 32 disposed on its outer surface near its periphery. Each choke 32 is formed by a first flat conducting section 34 and a second conducting section 36. The first section 34 is shaped in the form of a washer and is disposed parallel to the outermost quarter-wavelength of the corresponding upper and lower plates I2, 14. Each first section 34 is terminated and connected at its inner end to corresponding second connecting section 36, shaped in the form of a ring. Connecting sections 36 are disposed perpendicularly to the respective top and bottom plates I2, 14, to which they are respectively attached.

Construction of parallel plate lens-antennas in general is fully discussed in various texts such as Antenna Engineering Handbook", edited by H. Jasik, McGraw 3 Hill. l96l, (at page 2726) and in Microwave Scanning Antennas, Volume I. beginning at p. 23], edited by R. C. Hansen, Academic Press, 1964.

Returning again to FIG. 1, a plurality of half-wave high-frequency dipoles 40 for radiating or receiving electromagnetic energy are attached to the periphery of upper and lower plates l2, 14. These dipoles 40 are at the heart of the present invention and are the means by which rotation of the direction of polarization of the electromagnetic energy is accomplished.

Each half-wave dipole 40 is formed from two tubular rods. An upper rod 42 is attached to the upper plate 12 and a lower rod 44 is attached to the lower plate 14. The upper rod 42 extends from the upper plate 12 in a downward and outward direction, and is bent at a point halfway between the plates so as to be approximately one-quarter wavelength long and parallel to the tangent at the point of attachment to the upper plate 12. The lower rod 44 is attached to a point on the lower plate 14 located directly below the point of attachment of the upper rod 42. Lower rod 44 is bent in a shape similar to that of the upper rod 42, but in opposite directions, so as to result in an overall dipole structure onehalf wavelength long aligned parallel to the plane of the parallel-

plates

12, 14.

In this embodiment of the present invention each dipole 40 is spaced away (in the radial direction by onequarter wavelength, from the peripheral edges of the parallel-

plates

12, 14, although in other embodiments of the present invention this spacing may be other than Dipoles 40 described above are of the narrowband type. Alternatively, broadband type dipoles may be used. Typical of such broadband dipoles are the triangular shaped assemblies, commonly referred to as bowtie" dipoles. the dipoles 40 utilized in the present invention are of conventional type, and their operation and construction is described in detail in many textbooks dealing with antenna theory, such as in the previously cited reference to Antenna Engineering Handbook" at page 24.25 through 24.26.

The maximum number of dipoles 40 that may be utilized is determined by that number of dipoles that can be fitted, without touching, around a periphery approximately one-quarter wavelength larger than that of the

plates

12, 14. A spacing between dipole centers of 0.7A has been found to work well and is utilized in the embodiment of the present invention.

In operation, the circular design of the parallel-plate lens creates a locus of focal points along a focal circle near the periphery of the lens. It is about this focal circle that feed-elements 26 are spaced in a symmetrical fashion. high-frequency electromagnetic energy. typically l 3 Hz, propagates from the energy source (not shown) through various switching means and coaxial cable (not shown) to the connectors 28 of selected monopole feed-elements 26. Each center conductor 30 acts as a monopole radiator which omnidirectionally radiates electromagnetic energy in the TEM mode.

It should be noted that although any feed-element 26 may be used to transmit or receive electromagnetic energy. this discussion will treat the case of transmitting 4 only, since the lens-antenna system concepts discussed herein are equally applicable to transmitting or receiving. Also, this discussion addresses operation of only one feed-element 26, although simultaneous operation of selected feed-elements 26 in either transmit or receive mode is possible.

Energy from feed-elements 26 propagates with the E field perpendicular to the plane of the lens-antenna (defined by the plane midway and parallel to the upper and lower plates l2, 14) as indicated by arrow 50.

Electromagnetic energy propagating from a feed-element 26 through the dielectric focusing section 16 (indicated by lines 52) is focused into an antenna beam 53 (indicated by lines 54), and coupled to space through the dipoles 40. Antenna beam 53 has a beamwidth B arc sin For a typical value of D 10A, the beamwidth is approximately 6. The manner in which focusing is accomplished by the dielectric section 16 in combination with the plates l2, 14 is wellknown, and is described, for example, in Microwave Scanning Antennas edited by R. C. Hansen, Academic Press, 1964 (at page 23] hereby incorporated by reference.

It should be noted that by choosing the spacing distance, t, between the

parallel plates

12, 14 to be less than possible undesirable propagation of modes of higher order than the TEM (i.e. dispersive modes) is precluded. Also, the choice of dielectric constant k 35 results in the focal points (formed by the lens) being at the periphery of the dielectric section 16.

Energy propagating between the parallel plates l2, 14 causes the plates l2, 14 to have an associated charge distribution of opposite polarity (in any given period of time). [t is well-known that the polarization of an electric field is in the direction from a positive charge distribution to a negative charge distribution Since the plates l2, 14 are oppositely charged, the energy between them has a direction of polarization normal to the plane of the lens 10 (as indicated by arrow 50).

In the present invention, the charge distribution experienced by each parallel plate l2, 14 causes the attached

dipole section

42, 44 to also experience the same charge distribution. Because of this, the sections 42, 44 (lying in the plane of lens 10) of each dipole pair 40 have charge distributions of opposite polarity (in any given period of time). Since all upper dipole sections 42 of each dipole 40 are disposed to the same side (e.g. to the right or counterclockwise) of all lower dipole section 44, the direction of polarization of the electric field when radiated from the dipoles 40 is parallel to the plane of the lens 10 (as indicated by arrows 51). in this manner, the arrangement of dipole elements 20 causes rotation of the direction of polarization from normal to the lens antenna 10 plane (while the energy is confined between the parallel plates 12, 14) to parallel to the lens plane upon radiation.

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

What is claimed and desired to be secured by Letters Patent of the United States is:

l. A lens-antenna for transmitting and receiving highfrequency electromagnetic energy polarized with the direction of polarization parallel to the plane of the lens comprising:

an upper conducting plate;

a lower conducting plate substantially identical to said upper plate and disposed parallel and below said upper plate, forming a focusing region between said plates;

a dielectric section disposed between said upper and lower plates. said section having flat upper and lower surfaces in contiguous contact with said plates and filling a substantial portion of said focusing region;

means disposed within said focusing region for propagating and accepting high-frequency electromagnetic energy from at least one selected point within said focusing region; and

dipole means connected to said parallel plates for radiating and receiving high-frequency electromagnetic energy, said dipole means disposed at the periphery of said parallel plates and arranged to have the radiating sections of said dipole means lie parallel to the plane of said parallel plate lens, one pole of said dipole means connected to said upper plate and the other pole of said dipole means connected to said lower plate;

whereby electromagnetic energy polarized with the direction of polarization normal to the plane of said lens is propagated by said propagating means, focused into a beam shape by said dielectric section, and rotated in polarization by and radiated by said dipole means, thereby radiating electromagnetic energy polarized with the direction of polarization parallel to the plane of said lens-antenna.

2. The lens-antenna of claim 1 wherein said plates are spaced apart by less than one wavelength divided by twice the square root of the dielectric constant of said dielectric section, whereby propagation of only TEM mode electromagnetic energy may be supported in the region between said plates.

3. The lens antenna of claim 2 wherein the dielectric constant of said dielectric section is 3.5.

4. The lens-antenna of claim 3 wherein said parallel plates have a circular form with a diameter greater than one wavelength, and wherein said dipole means are comprised of a plurality of dipole elements, each said element having a length of one-half wavelength and wherein the centers of said dipole elements are spaced apart by 0.7 wavelengths.

1' ll k l

Claims

1. A lens-antenna for transmitting and receiving high-frequency electromagnetic energy polarized with the direction of polarization parallel to the plane of the lens comprising: an upper conducting plate; a lower conducting plate substantially identical to said upper plate and disposed parallel and below said upper plate, forming a focusing region between said plates; a dielectric section disposed between said upper and lower plates, said section having flat upper and lower surfaces in contiguous contact with said plates and filling a substantial portion of said focusing region; means disposed within said focusing region for propagating and accepting high-frequency electromagnetic energy from at least one selected point within said focusing region; and dipole means connected to said parallel plates for radiating and receiving high-frequency electromagnetic energy, said dipole means disposed at the periphery of said parallel plates and arranged to have the radiating sections of said dipole means lie parallel to the plane of said parallel plate lens, one pole of said dipole means connected to said upper plate and the other pole of said dipole means connected to said lower plate; whereby electromagnetic energy polarized with the direction of polarization normal to the plane of said lens is propagated by said propagating means, focused into a beam shape by said dielectric section, and rotated in polarization by 90* and radiated by said dipole means, thereby radiating electromagnetic energy polarized with the direction of polarization parallel to the plane of said lens-antenna.