Patent Grant
10103444
The present invention relates to antenna arrays, and more particularly to conformal broadband directional antenna arrays useful for missile nose cones.
U.S. Pat. No. 5,023,623, entitled “Dual Mode Antenna Apparatus Having Slotted Waveguide and Broadband Arrays,” by Donald E. Kreinheder et al., the entire contents of which are incorporated herein by this reference, provides a description of conventional missile target detection and tracking systems. Briefly, one type of target tracking system is known as broadband anti-radiation homing (ARH). Such a system is passive, and tracks a target by receiving radiation emitted by the target.
Known conformal arrays for missiles employ conformal slot radiators and microstrip patch radiators. See Min Liu et al., “A 35 GHz Cone Conformal Microstrip 4×4 array” Proc. Of Asia-Pacific Microwave Conf. 2007 and D. Augustin et al., “Performance of an Original Microstrip Ring Array Antenna Lied on a Conical Structure” Antennas and Propagation 9th International Conf. 1995. These antennas are narrow band (e.g., a few % of the center frequency), and because of their physical and/or electrical characteristics they can not be inclined to enhance their forward radiation. The result is a limited field of view.
Conventional conformal mounting situates the antenna elements so they face normal to the missile surface resulting in poor radiation in the forward direction. This is because the antenna is situated so that the greatest amount of energy from each element is directed normally to the missile body. This makes radiation in the forward direction difficult. The problem is made worse for elements radiating with an E-field tangential to the metallic missile body. The metal surface will not support these fields and forces them to zero at the point of contact. This is a major problem for conformal arrays since their “view” to missile boresight is tangential from the cylindrical section and nearly tangential in the nose region.
U.S. Pat. No. 5,220,330, entitled “Broadband conformal inclined slotline antenna array” by Gary Salvail et al., which is hereby incorporated by reference, provides a description of a missile guidance antenna that is conformal to the missile surface, dual-polarized and broadband. An array uses broadband antenna elements with both the E and the H-plane elements inclined toward boresight to improve directivity in that direction. Any inclination angle between 0 and 90 degrees may be used in accordance with the invention, although 30° and 90° are preferred inclination angles. This offsets the nullifying effects of the metallic skin in the H-plane as well as enhances the performance of the E-plane. Tilting the elements also makes the antenna more compact, which helps in adapting it to conformal use. The antenna uses slotline (notch) elements, which have a flat profile. These elements are suitable for close packing in both the E and H-planes to prevent grating lobes in the antennas' field of view while the antenna is scanned to boresight. Slotline (notch) elements are broadband with greater than three-to-one bandwidths being achieved. Dual polarization is accomplished by combining the E and H-plane elements in a linear or circumferential manner. A single or dual polarized array can be mounted on the cylinder section, on the nose, or radially around the missile body. In the radial configuration, the elements still incline in the boresight direction. Any combination of array positions is possible. The slotline elements can be packed with spacing close enough to allow for electronic beam steering without creating grating lobes at the highest frequency of operation.
The following is a summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not intended to identify key or critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description and the defining claims that are presented later.
The present invention provides a conformal broadband directional antenna array for a missile that is compact and exhibits a desired radiation pattern inclined towards boresight.
In an embodiment, a missile nose cone has a payload (e.g., an optical or RF seeker, propellant, explosive or kinetic warhead) having a metal skin with a circular cross section normal to the boresight axis. An annular RF radome encircles the payload. An annular metal cover is positioned around and aft of the annular RF radome. An antenna array comprises a plurality of ½ flared notch radiators recessed within the nose cone and positioned in a circumferential arrangement around and extending radially from the payload's metal skin to contact the annular metal cover that provides a ground plane such that each ½ flared notch radiator is inclined towards the boresight axis. The payload's metal skin provides an image plane for each ½ flared notch radiator to launch RF energy with a radial polarization normal to the image plane forward through the annular RF radome.
In an embodiment, the compact antenna array occupies less than 50% of the volume of the nose cone.
In an embodiment, the image plane creates an image of each ½ flared notch element that together approximate a full flared notch element. The image plane and said ground plane each have a circular cross-section normal to the boresight axis.
In an embodiment, the payload is cylindrical about the boresight axis. Each ½ flared notch radiator launches RF energy that is nominally parallel to the boresight axis.
In an embodiment, the payload is conical about the boresight axis. Each ½ flared notch radiator launches RF energy that is inclined to the boresight axis at angle α where 0 degrees<α<90 degrees.
In an embodiment, each ½flared notch radiator includes a section that extends radially beyond the RF aperture to contact the ground plane.
In an embodiment, RF absorbing material is positioned aft of each ½ flared notch radiator both behind and radially beyond the RF aperture. RF absorbing material may also be positioned on the walls of the septums between radiators.
In an embodiment, each ½ flared notch radiator is confined in a 5½ sided waveguide that both confines the RF energy to be launched forward through the RF aperture and isolates adjacent ½ flared notch radiators. The payload's metal skin, an annular metal backplane, a pair of metal septums (sidewalls) to either side of the radiator and the annular metal cover define the 5½ sided waveguide. The septum-to-septum spacing is such that a cutoff frequency fc of the waveguide is below a useable bandwidth of the antenna array.
In an embodiment, a single ½ flared notch radiator in a 5½ sided waveguide is positioned to launch RF energy inclined to the missile's longitudinal axis backward through an RF window formed in the missile body. This configuration may, for example, be used as a data link antenna. The missile fuselage provides a ground plane, which has a circular cross section, opposite the image plane, which may be flat or have a circular cross section.
These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description of preferred embodiments, taken together with the accompanying drawings, in which:
The present invention describes a conformal broadband directional antenna array for a missile that is more compact and exhibits a more desirable forward radiation pattern inclined towards boresight than the inclined slotline antenna array described in U.S. Pat. No. 4,220,330. Additional embodiments further improve the directionality and isolation of adjacent antenna elements. In addition, one or more antenna elements may be configured to launch RF energy backwards inclined towards a longitudinal axis of the missile to form, for example, a data link.
The antenna elements are configured to send and receive “broadband” RF energy. Broadband is defined to mean a bandwidth of at least 100% of the center frequency, and more typically at least 3-to-1. The bandwidth occupies a portion of the spectrum from 50 MHz to 100 GHz, and typically within 100 MHz to 50 GHz. For example, an antenna array may be designed for 1-20 GHz or for 20-40 GHz.
Referring now to
A conformal broadband directional antenna array 40 comprises a plurality of ½ flared notch radiators 42 recessed within the nose cone 12 and positioned in a circumferential arrangement around and extending radially from the payload's metal skin 22 to contact the annular metal cover 28 that provides a ground plane such that each ½ flared radiator is parallel to boresight axis 16. An RF feed 43 is coupled to the slotline of the ½ flared notch radiator. In general, the ½ flared radiator is inclined towards boresight with an angle α measured off of boresight where 0≤α<90 degrees. The payload's metal skin 20 provides an image plane for each ½ flared notch radiator to launch RF energy 44 with a radial polarization 46 normal to the image plane forward through the annular RF radome 26 and RF aperture 30.
To improve directionality and isolation, each ½ flared notch radiator 42 is suitably confined in a 5½ sided waveguide 50. The payload's metal skin 20, an annular metal backplane 52 such as formed by a step in the payload, a pair of metal septums 54 to either side of the radiator 42 and the annular metal cover 28 define the 5½ sided waveguide. The ½ flared notch radiators 42 and septums 54 alternate about the circumferential arrangement. The annular metal cover 28 covers both the top of the waveguide and a portion (not necessarily ½) of the 6th side above (or below) the RF aperture 30. Instead of being flat on top, the ½ flared notch radiator 42 is extended so that it contacts the metal cover just outside and aft of RF aperture 30. The septum-to-septum spacing is such that a cutoff frequency fcut of the waveguide, which acts as a high pass filter, is below the bandwidth of the antenna array. Nominally, this spacing is approximately one-half the center wavelength λc.
In an embodiment, an RF absorbing material 60 is positioned aft of each ½ flared notch radiator 42 between the radiator and the annular metal backplane 52. RF absorbing material 60 is positioned both behind and radially beyond the RF aperture 30 to increase absorption. RF absorbing material may also be positioned on the septums between radiators. The RF absorbing material serves to absorb the energy (E-field) that is propagating backwards. By absorbing the energy, the material prevents the energy from bouncing off the back plane (which it would do if it were uncoated metal) and then propagating forward and adding constructively and destructively to the energy already going forward from the ½ flare. A common RF absorbing material is ECCOSORB® produced by Emerson & Cuming Microwave Products, Inc.
Referring now to
An example of a ½ flared notch radiator was described by Xavier Artiga et al., “Halved Vivaldi Antenna With Reconfigurable Band Rejection”, IEEE Antennas and Wireless Propagation Letters, Vol. 10, pp. 56-58, 2011, which is hereby incorporated by reference, in which only half of a Vivaldi antenna is used and placed over a flat ground plane. Artiga discloses the structure and principle of using the ground plane to create an image of the halved Vivaldi to approximate a full Vivaldi.
The current antenna element 100 differs from the Halved Vivaldi Antenna in a number of critical aspects. First, antenna element 100 includes ground planes 104 and 106 both below and above the ½ flared notch radiator whereas the Halved Vivaldi Antenna only includes the ground plane below. Second, ½ flared notch radiator 102 is extended to contact ground plane 106 outside and just aft of the RF aperture whereas the top surface of the Halved Vivaldi is flat. Third, ground planes 104 and 106 each have a circular cross-section owing to the shape of the missile payload and annular metal cover of the nose cone whereas the Halved Vivaldi Antenna's ground plane is flat.
Referring now to
In another embodiment the payload is conical about the boresight axis. Thus the image plane is inclined to boresight. Each ½ flared notch radiator launches RF energy nominally parallel to the image plane and inclined to the boresight axis at angle α where 0 degrees <α<90 degrees.
Referring now to
A conformal broadband directional antenna array 230 comprises a plurality of ½ flared notch radiators 232 recessed within the nose cone 202 and positioned in a circumferential arrangement around and extending radially from the payload's metal skin 210 to contact the annular metal cover 218 that provides a ground plane such that each ½ flared radiator inclined to boresight axis 206. An RF feed 233 is coupled to the slotline of the ½ flared notch radiator. RF absorbing material 234 is positioned between the ½ flared notch radiator 232 and a back plane 236. Each radiator may be positioned in a 5½ sided waveguide as previous described to improve directionality and isolation. In general, the ½ flared radiator is inclined towards boresight with an angle α measured off of boresight where 0 <α<90 degrees. The payload's metal skin 210 provides an image plane for each ½ flared notch radiator to launch RF energy 238 with a radial polarization normal to the image plane forward through the annular RF radome 216 and RF aperture 220.
Referring now to
The 5½ sided waveguide 312 comprises an image plane 313 (flat or circular) inclined towards longitudinal axis 308, a portion 314 of the missile body having a circular cross section opposite the image plane, an RF window 316 formed in the missile body aft of portion 314 and abutting one end of the image plane, a ground plane 318 having a circular cross section spaced apart from the image plane and abutting the missile body, a back plane 320 that abuts the image and ground planes and a pair of sidewalls (not shown) between the image and ground planes that abut the backplane and the portion of the missile body. Each side of the waveguide (e.g. the image plane, ground plane, back plane, portion of the missile body and sidewalls) are formed of metal except the RF window.
The ½ flared notch radiator 310 extends radially from the image plane 313 to contact the ground plane 318 and the portion 314 of the missile body such that the ½ flared radiator is inclined towards the longitudinal axis 308 and isolated within the 5½ sided waveguide 312. RF absorbing material 321 is positioned in the waveguide between the radiator and back plane 320. The image plane forms an imaged radiator of ½ flared notch radiator 310 to approximate a full flared notch radiator to launch RF energy 306 with a linear polarization normal to the image plane 313 rearward through the RF window 316.
While several illustrative embodiments of the invention have been shown and described, numerous variations and alternate embodiments will occur to those skilled in the art. Such variations and alternate embodiments are contemplated, and can be made without departing from the spirit and scope of the invention as defined in the appended claims.
| Number | Name | Date | Kind |
|---|---|---|---|
| 5023623 | Kreinheder et al. | Jun 1991 | A |
| 5220330 | Salvail et al. | Jun 1993 | A |
| 6043785 | Marino | Mar 2000 | A |
| 6407711 | Bonebright et al. | Jun 2002 | B1 |
| Entry |
|---|
| Sun et al., “A Broadband Conformal Phased Array Antenna on Spherical Surface,” International Journal of Antennas and Propagation, vol. 2014, ARticle ID 206736, 5 pages, 2014. |
| Augustin et al, “Performance of an Original Microstrip Ring Array Antenna Lied on a Conical Structure,” Antennas and Propagation, Apr. 4-7, 1995, Conference Publication No. 407,O IEE 1995. |
| Liu et al., “A 35GHz Cone Conformal Microstrip 4×4 Array,” Proceedings of Asia-Pacific Microwave Conference 2007. |
| Artiga et al., “Halved Vivaldi Antenna With Reconfigurable Band Rejection,” IEEE Antennas and Wireless Propagation Letters, vol. 10, 2011, pp. 56-58. |
| Number | Date | Country | |
|---|---|---|---|
| 20180013203 A1 | Jan 2018 | US |
