US3521288A

US3521288A - Antenna array employing beam waveguide feed

Info

Publication number: US3521288A
Application number: US743808A
Authority: US
Inventors: Allan C Schell
Original assignee: US Air Force
Current assignee: US Air Force
Priority date: 1968-07-10
Filing date: 1968-07-10
Publication date: 1970-07-21
Anticipated expiration: 1987-07-21

Description

UHUUQ [1 LI" LHLWUL:

swam

July 21 1970 ANTENNA ARRAY EMPLOYING BEAM WAVEGUIDE FEED Filed July 10. 1968 A. C. SCHELL.

2 Sheets-Sheet l gr F2 Rice/viz .SfSfiM 4124M a. 3mm

R 1" mm? July 21, 1970 A. c. SCHELL 3,521,288

ANTENNA ARRAY EMPLOYING BEAM WAVEGUIDE Filed July 10, 1968 2 Sheets-Sheet 2 BY w Arrm m United States Patent US. Cl. 343-781 7 Claims ABSTRACT OF THE DISCLOSURE This invention comprehends a high efliciency, broadband antenna system for operation in millimeter wavelength regions. It includes a mode converter adapted to generate and control millimeter electromagnetic wave modes, a plurality of reflector elements, and a beam waveguide system that terminates at each reflector element and at the mode converter. A system of power junctions and reflectors disposed within the beam waveguide system provides appropriate combination and division of electromagnetic wave power. The power junctions and reflectors are spaced to permit in-phase combination of received electromagnetic energy, and broadband operation is achieved by making the electromagnetic wave path lengths between reflectors and mode converter equal. A condensing lens is provided for coupling each reflector element with the beam waveguide. A particular feature of the invention is the use of quartz sheet power junctions to effect low-loss power splitting within the beam waveguide system.

BACKGROUND OF THE INVENTION The invention relates to antenna systems and in particular to antenna array feeds for systems designed to operate at frequencies between X band and infrared.

There currently exists the need for antennas that are capable of handling a large number of wavelengths at a given frequency. Such antennas would, for example, find great utility in airborne radar mapping ssytems where mutually incompatible parameters such as small size, high resolution and high efliciency call for many unwanted tradeofl's. Adequate resolution in an airborne radar mapping system might require an antenna that could handle 3600 wavelengths at 2 millimeters. This would suggest a totally impractical 28-foot dish type antenna. Doppler radar devices such as side looking radar have been tried in order to achieve simulated wide antenna apertures. Unfortunately the complexity of these systems together with the great amount of processing required makes them unattractive for most applications. The alternative approach to the problem has been to use an antenna array that can be distributed along the aircraft fuselage. Such an approach provides adequate resolution without the use of a. prohibitively large antenna element. However, state of the art feed systems, such as waveguides and magic tee power dividers, are highly ineflicient in the millimeter wave regions. Consequently, antenna arrays have not yet provided a completely satisfactory solution.

The present invention is directed toward overcoming the foregoing and other deficiencies prevalent in current state of the art antenna systems.

SUMMARY OF THE INVENTION The novel antenna array disclosed herein utilizes the concept of the so-called beam waveguide in combination with quartz power junctions and polished conducting mirrors to achieve a very low loss feed system. A unique mode converter is used to deliver optimum beam modes 3,521,288 Patented July 21 1970 to the system and condensing lenses are used to change the focal lengths of the mode patterns to the focal lengths of the antenna array reflector elements. One feature of the invention is the use of quartz power junctions. These are spaced in particular relationship to the other system components so as to achieve eflicient two-way operation and broadband response.

-'Beam waveguides of the type herein employed are disclosed in detail in US. Pat. No. 2,994,873, entitled Beam Waveguide Antenna, issued to George J. E. Goubau, Aug. 1, 1961. They are described analytically in the periodical article by George I. E. Goubau in IRE Transactions on Antenna and Propagation, volume AP-9 pp. 248 263, May 1961.

In essence, the beam waveguide is a transmission line adapted to propagate a beam of electromagnetic wave energy confined to a substantially cylindrical space without actual boundary. It comprises a plurality of appropriately designed phase-correcting plates, such as dielectric lenses of predetermined focal length, inserted into the path of a propagated electromagnetic wave beam at respective spaced intervals which are long in comparison to the radius of the beam and very long compared to its wavelength. Each phase plate or lens intercepts the beam and reshapes it by resetting the phase distribution in the beam cross-section to the original distribution, that is, it compensates for the diffractional expansion occurring in the beam after it passes the preceding phase plate. The phase correction required by the phase plates is given by the formula.

where 1/ is the phase advance at a distance r from the axis, D is the distance between successive phase plates and K is a constant. The phase plates may be designed as lenses each having a focal length of D/ 2. If ray-optical considerations were to apply, each phase plate or lens would image the preceding phase plate into the succeeding phase plate. However, since the operational frequencies are too low for ray-optical imaging, the distribution of the beam energy after passing a phase plate is determined by diffraction. A characteristic feature of the beam waveguide is that the diffraction pattern obtained has very small, or low level side lobes. These side lobes by-pass the phase plate and are eliminated from the propagated beam. The net effect is that the beam energy remains substantially confined within a cylindrical space the diameter of which is determined by the diameter of the phase plates.

The use of quartz or similar materials for the lenses in this type of waveguide and for the power junctions results in a low loss transmission system. Thus the feed system of the invention can provide efficiencies at millimeter wavelengths that are comparable to the efliciencies that are obtainable at UHF It is a principal object of the invention to provide an improved antenna system for operation in the millimeter wavelength regions.

It is another object of the invention to provide an antenna array and feed system for operation at millimeter wavelengths and above having much improved efficiency characteristics.

It is another object of the invention to provide a large antenna array and feed system that permits control of the amplitude and phase of the radiating elements.

It is another object of the invention to provide an antenna array and feed system for operation at X band and above having broadband response capabilities.

It is another object of the invention to provide an antenna array and feed system for operation at millimeter wavelengths employing high efliciency devices and techniques including beam waveguide means and quartz power junctions.

It is another object of the invention to provide an antenna array and feed system for operation at millimeter wavelengths incorporating the use of multiple modes on. beam waveguide means to produce high aperture efiiciency.

It is another object of the invention to provide an antenna array and feed system that can be readily incorporated into an airborne mapping system.

These, together with other objects, advantages and features of the invention, will become more readily apparent from the following detailed description when taken in conjunction with the illustrative embodiments in the accompanying drawings.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic illustration of one presently preferred embodiment of the invention;

FIG. 2 is a sectional view of one type of mode converter employed in the invention;

FIG. 3 is a sectional view of a second type of mode converter suitable for use in the invention;

FIGS. 4 and 5 illustrate, respectively, the power di vision and combination eifects of the transmit and receive modes of operation on a typical power junction; and

FIGS. 6 and 7 are diagrams illustrating respectively the transmit and receive modes of operation of the embodiment of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT One presently preferred embodiment of the invention is disclosed by FIG. 1, reference to which is now made. A transmitter-receiver system 1 either delivers millimeter electromagnetic wave energy to or receives it from the antenna array and feed system of the invention. The antenna array in the referenced illustrative example com prises reflector elements 5, 6, 7 and 8 and their

respective subreflectors

9, 10, 11 and 12. The feed system comprises

beam waveguides

13, 14, 15 and 16 and the mode converter comprising horn 2 and reflector 3. Beam waveguides 13, 14, 15 and 16 are of the type disclosed in US. Pat. 2,994,873. They include an appropriate number of lenses -8 of quartz or polyfoam or similar low loss material having effective curvatures designed so that beam modes or combinations of waves propagate with very low loss due to spreading along the device. The mode converter is directly connected to the transmitterreceiver system and is positioned such that its reflector 4 can receive electromagnetic wave energy from beam waveguide 13 or deliver such energy to it depending upon whether or not the system is in a receive or a transmit mode of operation. The beam waveguide system of the invention also includes a number of polished conductive mirrors 19 that are effective to direct the electromagnetic wave energy through the various beam waveguide branches. The electromagnetic wave power is combined or divided at the several beam waveguide branch intersections by means of power junctions 17 These power junctions effectively divide electromagnetic wave power dur ing the transmit mode of operation and combine it during the receive mode of operation in a manner hereinafter described with references to FIGS. 4 and 5. Finally, condensing lenses 20 are provided to adapt the mode pattern of the electromagnetic wave energy in the beam waveguides to the focal lengths of the reflector elements 5, 6, 7 and 8.

The mode converter for the present invention can be any appropriate means for providing a discrete mode pattern. Two such means are illustrated by FIGS. 2 and 3. The mode converter of FIG. 2 consists of a conventional horn 2 in combination with reflector element 3. rReflector element 3 has a stepped surface 4 that produces the desired mode by phase control. The mode converter of FIG. 3 comprises a long born 2?. having a dielectric lens 23 disposed in its mouth. Dielectric lens 23 has stepped protrusions 24 that provide phase control of the gen erated modes in a manner similar to the device of FIG. 2. In. operation the mode converter of FIG. 3 is oriented as shown in FIGS 6 and 7. Since the mode converters are designed to generate particular modes for transmission through the feed and antenna system, they also provide optimum receiving means for electromagnetic waves having such particular mode characteristics when the system is operated in a receive mode of operation.

Referring now to FIGS. 4 and 5, there are illustrated thereby diagrammatic representations of transmit and receive operations of the power junctions used in the invention. These power junctions 17 are fabricated of quartz sheet material and are oriented as shown. In the trans mit mode of operation, these quartz sheets act as power dividers to electromagnetic waves traveling through the beam waveguide. This is illustrated by FIG. 4 wherein electromagnetic wave E is divided into orthogonal components E/ 2 and E'/ 2. Quartz sheets have the property of being capable of splitting such power by an amount related to the thickness of the sheet and the wavelength. of the electromagnetic wave. Typically, a sheet of quartz approximately .011" thick produces almost equal power division of electromagnetic wave energy of two millimeter Wavelengths.

FIG. 5 illustrates the electromagnetic wave energy cornbining properties of the quartz sheet power junction 17. In the receive mode of operation, component waves E/2 and E/2 incident upon the power junction from the direction indicated by the arrows are combined to provide an outgoing wave E. This occurs only in the event that there is proper phase relationship between wave components. Appropriate phase relationship is achived by proper spacing of the power junctions relative to the reflector elements and the mode converter. Such spacing is fixed to effect cancellation of the wave components E/ 4 and E'/4.

The transmit mode of operation of the invention is here described having reference to the diagrammatic flow diagram of FIG. 6. Mode converter 22 excites modes in the beam waveguide system and directs the electromagnetic wave energy generated by it toward power junction 17. The electromagnetic wave energy is divided by power junction 17 such that one half travels through the beam waveguide system to power junction 17" and the other half is directed by conductive mirror 19 to power junction 17'. The electromagnetic waves delivered to power junction to power junction 17' and 17" are again divided by those devices substantially equally. The electromag netic wave portions thus divided are then directed through the various beam wave guide branches to antenna reflector sub-elements 9, 10, 11 and 12. Condensing lenses 20 change the focal lengths of the mode patterns to per mit optimum matching with the focal length of reflectors 5, 6, 7 and 8. The electromagnetic waves are, at the antenna element, reflected ofi? the sub-elements and pri mary reflectors and radiated into space as indicated,

An examination of the arrangement of the components of the antenna feed system disclosed in the drawings readily reveals that the electromagnetic wave path lengths between the mode converter and each radiator element are substantially equal. Such an arrangement provides for a very broadband device. It is also apparent from an examination of the drawings, that by properly combining and spacing of conductive mirrors and power junctions any arbitrary spacing of antenna radiator elements is pos sible without violating bandwidth and phasing con siderations.

FIG. 7 is a diagrammatic flow diagram of the receive mode of operation of the invention. In accordance with the principles discussed above, incoming electromagnetic wave energy received by antenna elements 5, 6, 7 and 8 are transmitted through the beam waveguide system and combined by power junctions 17. 17' and 17 to provide a combined electromagnetic wave that is delivered to the mode converter and to the receiver system.

While the invention has been described in its preferred embodiments, it is understood that the words which have been used are words of deescription rather than words of limitation and that changes within the purview of the appended claims may be made without departing from the scope and spirit of the invention in its broader aspects.

I claim:

1., An antenna system comprising a plurality of reflector elements, a beam waveguide system, said beam waveguide system having a termination at each said reflector element, a mode converter, said mode converter being adapted to deliver and receive electromagnetic wave energy to and from said beam waveguide system, power junction means and conductive mirror means, said power junction means and said conductive mirror means being disposed within said beam waveguide system and arranged to effect division and combination of electromagnetic wave energy therein.

2. An antenna system as defined in claim 1 wherein said power junction means and said conductive mirror means are physically spaced to provide in-phase com= bination of electromagnetic Wave energy.

3, An antenna system as defined in claim 2 wherein said beam waveguide system has dimensions adapted to provide substantially equal electromagnetic wave path lengths between said mode converter and each of said reflector element.

4., An antenna system as defined in claim 3 wherein said mode converter comprises a microwave horn in combination with a stepped reflector element.

5. An antenna system as defined in claim 3 wherein said mode converter comprises a long microwave horn having a stepped dielectric lens.

6., An antenna system as defined in claim 4 wherein each said power junction means comprises a quartz sheet,

7 An antenna system as defined in claim 6 including a condensing lens disposed at the junction of each said reflector element and said waveguide system.

References Cited UNITED STATES PATENTS 2,668,869 2/1954 Iams 343853 ELI LIEBERMAN Primary Examiner Ursa CL XGR;