Microstrip antenna with parasitic elements

Abstract: A microstrip antenna (100) has a patch radiator (30) and parasitic elements (32, 34, 36) and is flush-mounted and conforming to the side of a guided missile. The antenna (100) produces an antenna beam which is tilted in a required direction, so that the antenna serves as a link or fuse antenna, by the presence of the elements (32, 34, 36) which direct the beam away from the antenna normal to the desired direction. The parasitic elements include a reflector (32) and one or more directors (34, 36).

Description: This invention relates to a patch antenna comprising: a plurality of antenna elements disposed in an array in a first direction on a reference surface, each of the antenna elements having a center, a width, and a length, with the lengths disposed in the said first direction and the widths disposed orthogonal to the said first direction, and the antenna elements including an active element, at least one reflector element, and at least one director element, and the reference surface being a first surface of a dielectric member having a second surface opposed to the first surface, the second surface having disposed thereon an electrically conductive coating. In guided missile (or simply missile) applications, fuse and link antennas often are required to be mounted conformally with the generally cylindrical shape of a missile. Antennas which adapt easily to conformal mounting usually produce beams with main lobes directed normally (or broadside to) the missile, whereas the required direction of main lobes of beams for fuse and link antennas is usually not normal (or broadside) to the missile. Thus, the main lobes of fuse antenna beams are typically pointed forward of the missile, while the main lobes of link antenna beams are usually pointed aft, say in a beam direction approximately twenty degrees from normal. To accomplish such an end, known link antennas are usually made of components that occupy critical area internally of the missile. The mass and volume of all components within the missile are critical to performance, and any decrease in the size and number of components is highly desirable. It is known in the art that microstrip patch antennas have a low profile and may be made conformal to a missile. Unfortunately, most patch antennas produce an antenna beam normally disposed to the aperture of the antenna. Different approaches have been used to change the antenna beam direction. Multiple patch antenna arrays have been used to steer the antenna beam direction. Such arrays have been built by using a stripline distribution network; however, such a network is complicated, with many connections required. A less complicated technique is desirable. It is also known in the art that parasitic elements may be used to control the direction of the beam of an antenna. For example, the well-known "Yagi" antenna uses parasitic elements in combination with at least one active element to control the direction of a beam. EP-A-0 018 878 (corresponding to US-A-4 384 290) describes a Yagi antenna consisting of a plurality of antenna elements disposed in an array on a reference surface in the form of one face of a dielectric plate, each of the antenna elements having a center, a width, and a length, with the lengths disposed in the said first direction and the widths disposed orthogonal to the said first direction. The antenna elements include two collinear active elements and two collinear reflector elements directly connected to a reference ground plane, and two director elements. These elements are provided in the form of a printed circuit on the dielectric plate, and are strip-like. The plate is to be mounted at the wall of an aircraft as part of an identification friend or foe interrogation system. The reference ground plane is in the form of an electrically conductive coating that extends over a side portion of the reference surface behind the reflector elements, around the edge of that side of the dielectric plate, and over a part of the other face of the dielectric plate below the reflector and active elements. The dipole constituted by the collinear active elements is supplied by a coaxial connector having its external conductor connected to the ground plane and its central conductor connected to one element of the dipole. A similar technique is known for use with parasitic slot array antennas, as described in an article by R.J. Coe and G. Held, I.E.E.E. Transactions on Antennas and Propagation, Vol.Ap-12, No.1, pp. 10-16, January 1964. In such an array, a reflector element and a director element are formed by cavity-tuned parasitic slots so that when a driven element (a slot) is excited, a beam is formed in the direction of the director in the plane of the elements. The parasitic slot array provides a flush mounting antenna suitable for an application where no projection above a plane surface is required. However, as noted previously, a fuse or link antenna usually requires an antenna beam direction approximately twenty degrees from normal or broadside of the face of the antenna so the parasitic slot array is hardly one to be used in a missile. US-A-3713162 describes a thin, flexible wrap-round antenna assembly for a missile. The assembly includes an inner cylindrical thin conductor that can be flush-mounted on the skin of the missile, and a shorter concentrically positioned outer cylindrical thin conductor having an axial length which is a quarter wavelength at the operating frequency. A cylindrical dielectric layer overlies the inner conductor and fills the space between the inner and outer conductors. One end of the outer conductor is disposed at one end of the inner conductor and these two ends are electrically shorted together by soldering. The quarter-wave coaxial cavity thus formed is excited by a signal supplied from within the missile by a coaxial line having its inner conductor connected to the input of a combination multiple feed and impedance-matching network supported on the exposed surface of the dielectric layer and coupling the inner conductor of coaxial line to the outer cylindrical conductor of the antenna assembly. Operating typically at 2.2 GHz, the antenna assembly produces a radiation pattern defining a figure of revolution about the missile axis and having deep nulls directly forwardly and rearwardly of the missile. DE-A-2138384 describes a printed Yagi antenna in which a rectangular plate of dielectric material supports a Yagi array of printed conductors, including a folded dipole active element and reflector element, on one face, and a reference plane coating on the other face. The reference plane covers a rectangular area of the bottom face of the plate, extending from one shorter edge of the plate to a position below the reflector element. The Yagi array is disposed on the top face in an area extending from the position of the reflector element to the other shorter edge of the plate, so that there is no conductive material below the dipole and the director elements. The printed conductors constituting the elements of the Yagi array are of negligible width. According to the present invention an antenna of the kind defined hereinbefore at the beginning is characterised in that the active element is a rectangular patch radiator element having a length and a width of between 0.3 and 0.5 of a wavelength of a beam of radio frequency energy to be radiated thereby, in that the said reflector element has a width of approximately 0.494 of the wavelength, a length of approximately 0.304 of the wavelength, and its center disposed approximately 0.570 of the wavelength from the center of the patch radiator element, in that the said director element has a width of approximately 0.266 of the wavelength, a length of approximately 0.114 of the wavelength, and its center disposed approximately 0.456 of the wavelength from the center of the patch radiator element, in that the electrically conductive coating extends below all the antenna elements, such that, in operation, the said beam of radio frequency energy is radiated in a direction oblique to the reference surface at approximately twenty degrees from a normal to the reference surface. A preferred embodiment of this invention provides an improved antenna which has a beam with a main lobe tilted approximately twenty degrees from a normal to a missile, while retaining the low profile, low volume attributes, and which is readily adaptable to flush-mounting on a missile. The preferred embodiment takes the form of a microstrip patch antenna with parasitic elements flush-mounted to the side of a missile to produce an antenna beam with a main lobe directed approximately twenty degrees from the normal to the missile, such antenna including a driven patch antenna, a reflector element and two director elements, with the reflector and director elements being parasitic elements, in combination with appropriate connector elements. The invention will now be described by way of example with reference to the accompanying drawings, wherein: FIG. 1 is a sketch showing generally the contemplated location of a microstrip antenna embodying the invention and the direction of the antenna beams; FIG. 2 is a sketch showing an isometric view, partially cut away for clarity of illustration, of a microstrip antenna embodying the invention; and FIG. 3 is a plan view of the microstrip antenna of Fig. 2. Description of the Preferred Embodiment Referring now to FIG. 1, a missile 10, here a semi-active missile, is shown to include a fuse antenna 12 and a link antenna 16. It is well known in the art that the main lobe of the beam from fuse antenna 12 must typically point forward from a normal to the missile 10 as illustrated by beam 14 because any target (not shown) would be ahead of the missile 10. In contrast, the main lobe of a link antenna 16 must typically point aft of a normal as illustrated by beam 18 because signals to (or from) the link antenna 16 come from (or are directed to) a station (not shown) located to the rear of missile 10. Referring now to FIG. 2, a microstrip antenna 100 as here contemplated is shown to include antenna elements 30,32,34,36 disposed on a slab 28 fabricated from a dielectric material. Such dielectric material may, for example, be the material known as "Duriod," or other teflon-fiberglass material. The antenna elements 30,32, 34,36 are formed by depositing an electrically conducting material (here copper) in any conventional manner as shown on the slab 28. The second side of slab 28 is covered with an electrically conductive coating to form a metallic ground plane 26. The antenna elements 30,32,34,36 are arranged in an array where a driven element (herein also referred to as "patch 30″) here is the second antenna element from the right. It will be observed that the patch 30, when actuated by itself, is operative to form a beam by reason of fringing fields around the periphery of such patch and that the main lobe of such beam is broadside to such patch. Further, it will be observed that the patch 30, when matched to a feed, is effectively equivalent to a resonant cavity. A shorting pin 38 in electrical contact centrally of the patch 30 is passed through the slab 28 to be attached to ground plane 26. The shorting pin 38 has no effect on radiation or impedance of the antenna being described, but simply allows a low frequency path between the patch 30 and the ground plane 36. The patch 30 here is fed by a coaxial line 20 affixed to the ground plane 26. Thus, an outer shield 24 of the coaxial line 20 is attached in any known fashion to ground plane 26. A center conductor 22 of the coaxial line 20 is attached to the patch 30 in any known fashion. Although the location of the point of connection between the patch 30 and the center conductor 22 does not affect the frequency of resonance, such location does affect input impedance of the antenna being described, so care should be taken to provide a proper impedance match with the impedance of the coaxial line 20. A reflector element 32, a first director element 34 and a second director element 36 make up the parasitic elements of the microstrip antenna 100. The parasitic elements are here effective to cause the direction of the main lobe in the beam radiated by the patch 30 to be changed as desired. Parasitic elements are inactive elements, meaning not fed or driven with a signal, placed on the face of the microstrip antenna 100 in close proximity to the patch 30. The advantage of this approach is that the direction of the main lobe in the beam may be changed without the penalty of beam narrowing associated with antenna feed networks. By varying the length, width, location and number of the parasitic elements, the direction of the main lobe in the beam is changed as required from the normal. There are two types of parasitic elements, a director type element and a reflector type element. The length of the parasitic element with respect to the narrow edge of the patch 30 determines the type of element, where shorter elements act as directors, while longer elements act as reflectors. Reflector element 32, here a parasitic element of the reflector type, tips the beam (not shown) away from the parasitic element. Director elements 34, 36, respectively, here parasitic elements of the director type, tip the beam (not shown) in the direction of the parasitic elements. The presence of the parasitic elements affects the impedance matching of the patch 30. A resulting mismatch of impedance can be compensated for by retuning the patch 30 with the parasitic elements present. This is easily accomplished either by changing the dimensions of the patch 30, or adjusting the location of the feed point. In connection with the latter method of adjustment, it will be noted that the feed point, i.e., the point at which the center conductor 22 is attached to the patch 30, is on a centerline of the patch 30. However the feed point is adjusted, the point of attachment should remain on that centerline. A high thermal protection window 50 (hereinafter also referred to as "window 50″) is attached to the missile 10 to overlie the slab 28 and antenna elements 30,32,34,36 when the microstrip antenna 100 is mounted on missile 10 (FIG. 1). The window 50 here is a ceramic, rigid, composite-fiber, insulation material, known as "HTP 12-22," developed by Lockheed Missiles and Space Company, Inc., Sunnyvale, California. HTP 12-22 provides good thermal shock resistance, low thermal conductivity, good strength and low dielectric constant for the window 50. The window 50 protects the microstrip antenna 100 from a harsh environment experienced while missile 10 (FIG. 1) is in flight. For further protection the window 50 may be treated with a silane polymer solution for moisture-proofing and an external reaction-cured glass-based coating for increased surface toughness and crack propagation resistance. Referring now to FIG. 3, a plan view of the microstrip antenna 100 is shown. It can be seen that patch 30 has a width C and a length D. Patch 30 is constructed such that the width C is equal in wavelength "L" to .380L and the length D is equal to .494L. Reflector element 32 has a width A and a length B. Reflector element 32 is constructed such that the width A is equal to .494L and the length B is equal to .304L. It should be noted that the center of reflector element 32 is separated from the center of patch 30 by the distance H which is equal to .570L. Microstrip antenna 100 also includes director element 34 and director element 36. Director element 34 has a width F and a length G. Director element 34 is constructed such that the width F is equal to .266L and the length G is equal to .114L. The center of director element 34 is separated from the center of patch 30 by the distance I which is equal to .456L. Director element 36 has a width F′ and a length G′. Director element 36 is constructed such that the width F′ is equal to .266L and the length G′ is equal to .114L. The center of director element 34 is separated from the center of director element 36 by the distance J which is equal to .228L. Having described this invention, it will now be apparent to one of skill in the art that the number and disposition of the parasitic elements may be changed without affecting this invention. For example, the number of director elements could be reduced to one or increased to three, thus further controlling the desired direction of the beam. It is felt, therefore, that this invention should not be restricted to its disclosed embodiment, but rather should be limited only by the spirit and scope of the appended claims.

Claims: A patch antenna comprising: a plurality of antenna elements (30,32,34,36) disposed in an array in a first direction on a reference surface, each of the antenna elements (30,32,34,36) having a center, a width, and a length, with the lengths disposed in the said first direction and the widths disposed orthogonal to the said first direction, and the antenna elements including an active element (30), at least one reflector element (32), and at least one director element (34), and the reference surface being a first surface of a dielectric member (28) having a second surface opposed to the first surface, the second surface having disposed thereon an electrically conductive coating (26), characterised in that the active element is a rectangular patch radiator element (30) having a length and a width of between 0.3 and 0.5 of a wavelength of a beam of radio frequency energy to be radiated thereby, in that the said reflector element (32) has a width of approximately 0.494 of the wavelength, a length of approximately 0.304 of the wavelength, and its center disposed approximately 0.570 of the wavelength from the center of the patch radiator element (30), in that the said director element (34) has a width of approximately 0.266 of the wavelength, a length of approximately 0.114 of the wavelength, and its center disposed approximately 0.456 of the wavelength from the center of the patch radiator element (30), in that the electrically conductive coating (26) extends below all the antenna elements (30,32,34,36), such that, in operation, the said beam of radio frequency energy is radiated in a direction oblique to the reference surface at approximately twenty degrees from a normal to the reference surface. An antenna according to claim 1, characterised in that there is one reflector element (32) and two director elements (34,36), the second director element (36) having a width of approximately 0.266 of the wavelength, a length of approximately 0.114 of the wavelength, and its center disposed approximately 0.228 of the wavelength from the center of the said one director element (34). An antenna according to claim 1 or 2, characterised in that a high thermal protection window (50) is attached to the first surface of the dielectric member (28). An antenna according to claim 3, characterised in that means are provided for so mounting the dielectric member (28) together with the antenna elements (30,32,34,36), and the high thermal protection window (50) in an opening formed therefor in a guided missile as to permit the beam of radio frequency energy to emerge through the said opening.

Inventors: ALEXANDER CLEO J, CAVALLARO NUNZIO M, TOTH JOHN F, ALEXANDER, CLEO J., CAVALLARO, NUNZIO M., TOTH, JOHN F.
Applicants: RAYTHEON CO, RAYTHEON COMPANY
Application year: 1990
Main classification: H01Q1
IPC codes: H01Q19, H01Q1