WO1996010277A1

WO1996010277A1 - Planar high gain microwave antenna

Info

Publication number: WO1996010277A1
Application number: PCT/US1995/012496
Authority: WO
Inventors: Marion Hines
Original assignee: The Whitaker Corporation
Priority date: 1994-09-28
Filing date: 1995-09-28
Publication date: 1996-04-04

Abstract

A planar millimeter wave antenna is described formed of a sandwich structure having an upper planar phased array made from an etched metallized layer of dielectric material and a lower metallized dielectric layer having a guided wave feed. With the upper and lower dielectric layers forming parallel plate waveguides which are coupled to each other at a boundary zone having a parabolic reflector whereby millimeter waves are coupled between the waveguides with a desired distribution in the upper waveguide so as to enable the formation of a desired antenna beam from the phased array. Multiple feeds are described and spaced from the parabolic axis in order to create different azimuthal beams which have a desired angular spacing from each other. A circular polarized phased array formed of angled slits is described for the upper metallized layer.

Description

PLANAR HIGH GAIN MICROWAVE ANTENNA

Field Of The Invention

This invention relates to microwave antennas generally and more specifically to a planar slot array antenna for use at millimeter waves with highly direc¬ tive characteristics and multiple beam capability.

Background Of The Invention

Antennas operating at x-band frequencies have been proposed, such as by G. Broussaud in 1956 in an article entitled Un Nouveau Type d'Antenne de Structure Plane

(A New Type of Planar Structure Antenna) published in

Ann Radio Electricite, Volume 11, page 70 and published in 1956. This x-band (about 10 gigahertz) antenna is formed with a planar array of slots or holes formed in a planar parallel plate waveguide, somewhat like the antenna shown in U.S. Patents 5,177,496, 5,173,714, and 5,239,311, which list U.S. Patent 2,807,800 to Broussaud. In the Broussaud antenna a plane TEM wave in a parallel-plate waveguide is coupled to a plane wave in outer space by way of an array of slots. Broussaud also described a single layered parabolic reflection method for focusing the beam from a parallel plane wave in a gap to a simple small feed horn antenna connected to a waveguide.

Parabolic focusing in a parallel plate wave-guide medium was developed by the British during World War II and was called a cheese antenna. See "Cheese Aerials for Marine Navigational Radar," by D.G. Kelly, A.E. Collins, and G.S. Evans as published in the IEE Pro- ceedings Part II, Volume 98, No. 51 pages 37-43, January 1951. Such cheese-shaped antennas are illus¬ trated in U.S. Patents 4,819,003 and 5,049,895 which PC17US95/12496

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list earlier U.S. Patents to a Kelly, namely 2,929,064 and 3,063.049.

U.S. Patents 3,918,064 (Figure 1 to Gustincic and 3,235,869 to Van Atta et al . illustrate a folded structure wherein a lower waveguide feed expands towards a parabolic end wall at a gap that leads to an supper surface wave antenna structure in the Van Atta et al. patent and an end radiator in the Gustincic patent.

Although these various prior art structures provide useful antennas, they do not lead to a satis¬ factory flat construction useful at millimeter waves and which can be economically, conveniently and accu¬ rately made with consistently reproducible antenna performance characteristics, such as a narrow beam with low side lobes.

Summary Of The Invention

With a microwave antenna structure in accordance with the invention, a high degree of precise control in the design of a planar array type antenna is obtained. Such control is particularly desirable at millimeter wavelengths, such as occur at frequencies as high as 77 gigahertz. As a result, a single millimeter wave antenna in accordance with the invention can have one or several highly directive beams with a planar array of elements such as apertures or patches with low side lobes.

This is achieved with one microwave antenna for use at millimeter waves in accordance with the inven- tion by forming a planar sandwich structure having a lower microstrip feed and an upper microstrip slotted array antenna. The lower microstrip feed is formed of a thin, dielectric layer sandwiched between conductive materials. One conductive material is an upper metal cladding and the other, a metal plate.

The upper microstrip antenna is formed with an upper metallized dielectric layer which is placed over the lower microstrip feed. The upper dielectric layer is provided with a metal cladding which has a pattern of apertures selected to form an aperture array. The cladding for the lower dielectric layer is partially removed near an end wall of the plate so as to form a dielectric filled gap whose shape is selected to form a parabolic transfer zone through which millimeter wave signals can be efficiently coupled.

With an antenna in accordance with the invention performance can be achieved that approaches that of a dish antenna but with a compact planar configuration which is adaptable to low cost manufacture.

It is, therefore, an object of the invention to provide a microwave antenna which is of compact design, economic to manufacture and has excellent performance characteristics and is capable of generating and receiving multiple highly directive beams.

This and other objects and advantages of the invention can be understood from the following detailed description of an embodiment as shown in the drawings. Brief Description Of The Drawings

Figure 1 is a greatly enlarged perspective view of an antenna in accordance with the invention;

Figure 2 is section view of the antenna of Figure 1 taken along the line 2-2 in Figure 1;

Figure 3 is an exploded view of the antenna shown in Figure 1;

Figure 4 is a top plan partially broken away view of an antenna as shown in Figure 1; Figure 5 is a side view in elevation of the antenna as shown in Figure 1;

Figure 6 is an end view in elevation of the antenna shown in Figure 1;

Figure 7 is a partial section view of the upper and lower metallized dielectric layers used in the antenna of Figure 1 illustrating the mathematical relationship regarding beam direction;

Figure 8 is a plot pattern of an antenna in accor¬ dance with the invention illustrating the directive beam features;

Figure 9 is a perspective view of a support plate and two feeds used with an antenna in accordance with the invention;

Figure 10 is a perspective view of a support plate and a different feed used with an antenna in accordance with the invention; and

Figure 11 is partial plan view of a radiating aperture pattern for generating a circularly polarized beam.

Detailed Description Of Drawings

With reference to Figures 1-3 an antenna 20 in accordance with the invention is shown formed of a P

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lower microstrip feed 22 and an upper microstrip antenna 24 arranged in a sandwich structure to form a compact design. The upper microstrip antenna is made of a metallized dielectric material 26 having a cladding 28 of copper. The dielectric layer is usually a low loss lateral such as tetrafluoroethylene, also known as Teflon, a trademark of the Dupont Corporation. The conductive cladding 28 bears an array 30 of radiating apertures 32 selected to provide a desired antenna pattern. The array is sufficiently wide and long to provide a narrow beam with low sidelobes. Typically the array is of the order of about 40 to 70 wavelengths wide and about as long.

The lower microstrip feed 22 is formed of a conductive base plate 34 and a lower metallized dielec¬ tric layer 36 which is made of the same material as dielectric layer 26. Hence, layer 36 has a copper cladding 38, which is located adjacent the upper dielectric layer 26 in the sandwich construction as shown in Figure 2.

The base plate has an end wall 40 with a concave parabolically shaped surface 42 which faces like-shaped ends 44 and 46 of dielectric layers 26 and 36. The lower cladding 38 overlying the lower dielectric layer 36 is terminated short of end 46 so as to leave an uncovered segment 48. The width of segment 48 is selected so as to form a substantially dielectric filled gap 49 opposite surface 42 with which millimeter wave energy can be efficiently coupled between the dielectric layers 26 and 36. The basic requirement for the width "w" of the gap 49, i.e. the distance from surface 42 to the edge 51 of the lower cladding 38, is that the shunt capacitance discontinuity essentially tunes out the excess series inductive effect of the gap zone 49 where the waves reverse direction.

The base plate 34 has a recess 50 sized to receive the lower dielectric layer 26 and is bounded by a back edge 52 located at a particular distance of a multiple of 1/4 wavelengths from one or several feeds 54. In the embodiment illustrated in Figure 3 a multiple of apertures are provides for coupling to microwave transmission lines.

These apertures can, in a transmission mode, be considered as feeds though it is to be understood that in accordance with antenna practice a feed also can act as a receiving aperture. Hence, as used herein the term feed or beam and other terms employed to describe an antenna in accordance with the invention apply to both an antenna used for transmitting as well as receiving microwave energy.

In the embodiment shown in Figure 3 three rectan¬ gular waveguide feeds 54 are shown extending through the plate 34 from a backside 56 (see Figure 2) . Howev¬ er, it should be understood as shown in Figures 9 and 10 that different types and a greater or lesser number of feeds can be used.

The feeds 54 are as centrally located as possible with respect to the central axis 60 of the parabolic surface 42 of end wall 40. This means that the middle feed 54.2 is bisected by the central axis 60 and the other feeds 54.1 and 54.3 are closely spaced to the middle feed 54.2. The middle feed 54.2 is preferably located at or near the focal point of the parabolic surface 42. Hence, in a transmitting mode the energy radiated from the feeds 54 produce expanding waves toward the parabolic end zone where the gap 49 enables transfer to the upper dielectric layer by reflection from surface 42. The reflected wave 61 within the upper dielectric layer 26 will take on a plane wave characteristic with straight and parallel phase fronts as a result of the parabolic reflecting surface. The wave will then pass beneath the array 30 of apertures 32, and radiate a small fraction at each row. By appropriately selecting the shape of the apertures only a small fraction of the energy will be left at the end 62 of the array 30.

In order to reduce reflections from any remaining wave portions the base plate 34 is provided with a lossy strip material 64 extending along the width of the plate 34 and below the upper dielectric layer 26. Similarly, to reduce radiation and reflections from lateral sides of the lower dielectric layer 36, the plate 34 is provided with wave absorbing sections 66, 68 located beneath the layer 36 and placed in appropri¬ ately sized counter recesses 70, 72 formed in the bottom 74 of lateral sides of recess 50.

With reference to Figures 4, 5 and 7 the antenna 20 is shown oriented as it is most likely to be used. A beam emerges from the array 30 of apertures at a small angle, theta, from a perpendicular axis to the flat radiating front surface 74 of upper cladding 28. Hence, foi a forward looking narrow beam, the front surface 74 may slant either backward or forward by c few degrees. The internal wave 61 propagates below th- apertures vertically, either down or up depending o: the orientation of the antenna, with a horizontal phase front. The spacing of the apertures 32 in the array should be such that they all radiate in the proper phase relationship for the desired direction of the beam.

Various shapes and spacings can be adopted for the apertures 32. When a beam direction is desired to be perpendicular to the face of an antenna, the phases of excitation of each element of the array should be the same, or 360 degrees apart, or some exact multiple of 360 degrees. This can be achieved when the elements are exactly one or some other integer number of wave¬ lengths apart in the medium of propagation in the feed line or the dielectric-filled wave-guiding medium within the antenna. If the individual apertures cause waves to be scattered backward in that inner medium, and if they are spaced at exactly one wavelength apart, the reflected components may add together in phase, to interfere with wave propagation in the inner medium. This problem can be avoided either by reducing the degree of reflection of each aperture or by spacing the apertures at somewhat more or somewhat less than one wavelength in the medium. Figure 7 illustrates the mathematical relationship between the beam direction theta (θ) and the free-space wavelength (λ₀) when the spacing of apertures is S and the dielectric constant is epsilon (e) .

It is possible, by introducing complexities in the aperture pattern, to introduce an impedance matching effect for each aperture 32 and avoid the reflection stop band effect arising from a half or full wave length spacing between the rows of apertures. When the aperture pattern is uniform over the face of the array 30, the wave within the upper dielectric layer 26 exhibits an exponential decay as it propagates. The radiation pattern obtained with the array pattern 30 can be satisfactory. Better patterns can be obtained by carefully tailoring the apertures 32 dimensions and spacings as a function of the distance along the array.

The ability to make an antenna in accordance with the invention can be appreciated when these dimensional requirements are taken into consideration for the aper¬ tures 32. At the , expected wavelength of about 2.75 in he cλi ekct K-. millimeters the sensitivity of the array to dimensional

variations mandates a tight control over the formation of the apertures and their spacings. This is achieved by etching the apertures into the upper cladding 28 using photolithography processes as are well known in the art. Fine dimensional tolerances can then be maintained and a diverse range of aperture shapes can be accurately and repetitively accommodated.

•As illustrated in Figures 3 and 6 an advantage of an antenna in accordance with the invention resides in the ability to produce multiple beams in selected different directions. This is done by using a multiple of feeds or launchers 54. Three waveguides 80, 82, and 84 are shown in Figure 6 terminating at the bottom 74 of recess 50. Each waveguide is for one beam, see Figure 8 and terminates at the dielectric layer 26, whose thickness is about one half of the "b" dimensions of the waveguides. This reduces mismatches at the ends of the waveguides and any further corrections can be achieved with the judicious placement of discontinu¬ ities. The wall 52 is located about 1/4 or several multiples of a 1/4 wavelength from the waveguides ends so as to form a reflection that is in phase with launched waves from the waveguides. The output beam from the aperture array 30 is affected by the size and shape of the parabolic reflec¬ tor surface 42 in the transfer zone 49. When a rather deep parabola, of about 20 dielectric wavelengths, is used with a large lateral width of about 79 dielectric wavelengths and with the beam launcher located at the parabola focal point in the lower dielectric layer 36, a very narrow beam, about one degree in the azimuth direction, is obtained with low sidelobes about 36 db down. The deep parabola provides a strongly tapered illumination of the array 30 and results in a high beam efficiency.

A more shallow parabola provides more uniform illumination of the aperture array 30, but also causes more loss due to spill over from a spreading beam and exhibits higher sidelobes.

With the three waveguide inputs spaced from each other near the focal point of the parabola of the transfer wall 42, but away from the parabola axis 60, see Figure 3, three different azimuthal beams 86,88,90 can be obtained as illustrated in Figure 8. The beams are spaced three degrees apart with a two degree beam width at the three db point in the azimuth direction and three and a half degrees width in the elevation direction. Since severe beam distortion can occur for off-axis beam launching with a deep parabolic shape for the reflection surface 42, a somewhat shallower parabo¬ la is necessary.

For a multiple beam pattern similar to that shown in Figure 8 a parabolic focal length of about 27 dielectric wavelengths and a lateral width for the parabolic surface 42 of about 40 dielectric wavelengths is sufficient. Typically sidelobes can be suppressed to more than about 19 db for the central beam 88 and somewhat less, about 17 db, for the other beams 86 and 90. The elevation pattern is represented by curve 92 in Figure 8. A more desirable elevation pattern 92 is achievable with a proper tapering of the apertures 32.

Figure 11 shows a sketch of an array 96 of aper¬ tures 98 and 100, in the shape of differently oriented slits, capable of providing a circularly polarized beam. The apertures are arranged in rows 101, 102 with the aperture slits at +/-45 degree angles relative to the propagation direction 103 of the wave in the dielectric layer beneath the cladding 28' .

The rows are arranged in pairs, spaced apart by 1/4 dielectric wavelength. Reflections from the two rows of slits in each pair 101,102 induced in the underlying dielectric waveguide layer 26, tend to cancel each other because of the 1/4 wavelength spacing between the slits.

The different orientation of the slit apertures provides for 90 degree relative angles between the different slits. The pairs of rows 101, 102 are spaced at intervals of one dielectric wavelength. This leaves significant gaps between the second row of each pair and the first row of the next pair. Waves launched from the first row of each pair are all in phase, and those launched from the second rows of each pair are at 90 degrees time phase as well as 90 degrees in polariza¬ tion orientation. These conditions provide for circular polarization. Figure 9 shows a somewhat different antenna base plate 34' to illustrate the use of a pair of waveguides 104, 106 for launching waves in the lower dielectric layer 36. The waveguides 104, 106 are located at the focal distance from the parabolic reflecting surface 42 and are closely placed to the central axis 60. Such waveguide placement can be useful for a monopulse antenna.

Figure 10 shows still another antenna base plate 34" to illustrate the use of a coaxial feed 110 instead of a waveguide with the center conductor 112 of the coaxial conduit extending up into the dielectric layer

26.

Having thus described several embodiments of the invention its advantages can be understood. Variations of the embodiments can be made and may occur to one skilled in the art without departing from the scope of the invention as set forth by the following claims.

What is claimed is:

Claims

1. A microwave antenna having an array of radiating apertures and a reflector: characterized in that said reflector is a parabolic surface and said surface is coupled to a metallized dielectric layer having a parabolically shaped end.

2. A microwave antenna as recited in claim 1, wherein said metallized dielectric layer has a dielectric substrate between a conductive cladding and a conductive base plate.

3. A microwave antenna as recited in claim 2, wherein said wherein said parabolic surface is an end wall of said base plate and said metallized dielectric layer is planar and is disposed in a recess in said base plate.

4. A microwave antenna as recited in claim 3, wherein said conductive cladding is terminated short of said parabolically shaped end of said metallized dielectric layer.

5. A microwave antenna as recited in claim 2, wherein said array of radiating apertu^res is a microstrip antenna.

6. A microwave antenna as recited in claim 5, wherein said microstrip antenna is planar and is disposed on top of said conductive cladding of said metallized dielectric layer.

7. A microwave antenna as recited in claim 6, wherein said microstrip antenna has a dielectric substrate and a conductive cladding disposed thereon, said cladding having apertures therein.

8. A microwave antenna wherein said apertures are spaced so that microwaves emitted therefrom are in phase. 9. A microwave antenna as recited in claim 3, wherein said recess has a back edge opposite said end wall.

10. A microwave antenna as recited in claim 9, wherein said recess has atleast one feed located a distance of an integer multiple of one-quarter wavelength of said microwaves in said dielectric substrate from said back edge.

11. An antenna assembly comprised of a base plate and a radiating array, further characterized by a wave guiding medium formed beneath the array.

12. An antenna assembly in accordance with claim 11, characterized in that said base plate has an endwall facing said wave guiding medium, and said wave guiding medium has an essentially insulating filled gap.

13. An antenna assembly in accordance with either of claims 11 or 12 characterized in that said essentially insulating filled gap has a width selected so as to form a substantially dielectric filled gap opposite said endwall with which millimeter wave energy can be efficiently coupled between dielectric layers of said wave guiding medium and array.

14. An antenna assembly in accordance with any of claims 11 through 13, characterized in that said endwall and said essentially insulating filled gap are profiled as interfitting parabolic surfaces.

15. The antenna assembly of either of claims 12, 13, or 14 characterized in that the width of the insulating gap is defined such that the shunt capacitance discontinuity essentially tunes out the excess series inductive effect of the gap where the waves reverse direction. 16. A planar microwave antenna, comprising: a generally planar multiple layered microwave structure having a lower microstrip feed and an upper microstrip antenna; said lower microstrip feed being formed of a thin lower dielectric layer sandwiched between an intermedi^¬ ate conductive material and a lower conductive materi^¬ al; said upper microstrip antenna being formed of a thin upper dielectric layer overlying the intermediate conductive material and an upper conductive layer overlying the upper dielectric layer, with said upper conductive layer having an array of apertures distrib^¬ uted over a top surface and selected to form a desired antenna beam from the array of apertures; a microwave feed coupled to the lower microstrip feed to inject microwave signals and to couple received microwave signals; said lower conductive material and said upper conductive layer being effectively coupled to each other at one end of the sandwich shaped microwave structure and with said intermediate conductive materi^¬ al terminating short of said one end to form a gap sized to couple microwave energy between the upper dielectric layer and the lower dielectric layer; said one end of the sandwich shaped microwave structure being so shaped so as to form a parabolic transfer zone with which a desired energy distribution in said upper and lower dielectric layers is obtained for enhanced coupling of microwave energy onto said microwave feed and to said array of slots.

A planar microwave antenna, comprising:

X / _• a generally planar multiple layered microwave structure having a lower microstrip feed and an upper microstrip antenna; a conductive plate; said lower microstrip feed being formed of a thin lower metallized dielectric layer having a first conductive cladding with the lower dielectric layer placed on and facing the conductive plate; said upper microstrip antenna being formed of a thin upper metallized dielectric layer having an upper conductive cladding, with the upper dielectric layer facing and overlying the first conductive cladding, said upper conductive cladding having an array of apertures formed therein and distributed over its surface and selected to form a desired antenna beam from the array of apertures; a microwave feed extending through said conductive plate to inject millimeter wave signals into said lower metallized dielectric layer and to couple received millimeter wave signals; said conductive plate having an end wall facing and being adjacent to ends of said lower and upper metallized dielectric layers, with the first conductive cladding terminating short of the end of said lower metallized dielectric layer so as to form a dielectric layer filled gap sized to couple microwave energy between the upper and lower metallized dielectric layers; said end wall of the conductive plate and the ends of the upper and lower metallized dielectric layers being so shaped so as to form a parabolic transfer zone with which a desired energy distribution in said upper and lower metallized dielectric layers is obtained for enhanced coupling of microwave energy onto said microwave feed and to said array of apertures. . A planar microwave antenna, comprising: a generally planar multiple layered shaped micro¬ wave structure having a lower and an upper layer of dielectric material with a thin metallic patterned layer in intimate contact with the top of said upper layer, a thin metallic sheet between the lower and upper dielectric layers, with said lower dielectric layer having a lower surface in intimate contact with a metallic surface; said upper and lower dielectric layers acting as wave-propagating regions allowing energy flow in direc¬ tions parallel to and between metallic surfaces within the microwave structure; said metallic patterned layer having an array of radiating apertures which expose a portion of said upper dielectric layer, said radiating apertures acting as a phased array of antenna elements which in concert radiate into space, or receive radiation therefrom, along a directional microwave beam; said metallic surface below said lower dielectric layer having a coupling aperture through which a microwave transmission line is coupled to the lower dielectric layer; said lower and upper dielectric layers having a common boundary zone shaped along a parabolic curve, at which zone propagating waves in one of said dielectric layer is coupled to the other of said dielectric layer; said coupling aperture being located in the vicinity of the focal point of said parabolic curve such that energy from said coupling aperture will propagate in a diverging manner toward the paraboli¬ cally-shaped boundary and transfer to said upper dielectric layer to propagate therein in a substantial¬ ly non-diverging manner; said wave propagating in said upper dielectric layer being coupled to said array of apertures such that a small fraction of energy in the wave is trans¬ ferred to space above the dielectric layer at each aperture with a phase pattern selected to provide said directional microwave beam.

19. The planar antenna according to claim 18in which said array of apertures has a double-periodic pattern, spaced apart in the direction of the propagation of a wave in the upper dielectric layer by a distance less than one free-space wavelength of the wave.

20- The planar antenna according to claim18 in which said apertures are narrow slits arrayed in a pattern where the axis of one-half of the slits are oriented at a +45 degree angle with respect to the direction of wave propagation, and the other slits are oriented at a -45 degree angle with respect to that direction, in order to provide a circularly-polarized beam.

i • The planar antenna according to claim 18 and further including electromagnetic absorbent materials placed along selected side and end boundaries of said dielectric layers.

22 • A planar antenna according to claim IB and further including a second coupling aperture below said lower dielectric layer, said coupling apertures being placed side-by-side, near the focus of the said parabolic boundary to provide a multi-beam antenna with different spatial directions.

23. A method for the manufacture of a planar antenna according to claim 16 comprising the steps of: etching a metallized dielectric sheet to remove undesired metal and expose an underlying dielectric layer to form said array of apertures and etching a second metallized dielectric sheet to remove undesired metal and expose an underlying dielec¬ tric layer to form said boundary zone.