Abstract:
A wave-scanning antenna is disclosed that does not require a rotary joint. The antenna produces a collimated beam that can be scanned through 360 degrees. The beam is directed perpendicular to the antenna&#39;s axis of rotation to form a disc-like surveillance volume, or at an angle above or below the perpendicular to form a cone-shaped surveillance volume. The radar&#39;s structure contains a transmitter and receiver coupled to a horn protruding through open centers of the support bearing and driven gear into the antenna housing. Energy emitted by the horn proceeds upward until deflected through an angle of 90 degrees by an angled reflector located on the axis of rotation. The energy is collected by a dielectric lens and focused into a collimated beam. Reflected energy is collected by the lens and directed by the reflector to the horn, where it is fed to a waveguide coupled to the receiver.

Description:
BACKGROUND OF INVENTION 
     The present invention relates in general to continuous rotation scanning antennas for use in surveillance radars, and in particular to a scanning antenna configuration that does not require the use of a rotary joint. 
     The possibility of terrorist activity, compromise of military information, or material theft results in the need to protect various high value assets whether located in permanent or temporary sites. A desirable approach protecting such valuable assets is the establishment of a network of low power surveillance radars to provide automated perimeter security. For greatest versatility, the surveillance radars should be easily transportable and deployable in multiple emplacements in any desired positional configuration. Therefore, the surveillance radar should be small in size and have a weight low enough for single person installation. 
     A typical example of a multiple surveillance radar deployment is shown in FIG.  1 . An aircraft parking area  1  containing high value assets, such as aircraft  2 , is encompassed by a multiplicity of surveillance radars  3  spaced so that the detection volumes  4  provided by each radar form a continuous zone for the detection of intruders around the perimeter of the area  1 . The cost of such an installation should be affordable, and thus each surveillance radar should be designed and constructed in a manner to minimize cost while providing the required performance. The surveillance radars should provide an azimuthal scan of 360 degrees to allow for versatility of placement, and a scan rate sufficiently high that an intruder cannot traverse the radar&#39;s detection volume without being intercepted by a scan of the beam and thus detected. Operation in the millimeter wave region of the electromagnetic spectrum allows the use of a small, lightweight-scanning antenna that produces a narrow beam in azimuth for adequate resolution of target details. 
     The prior art employs various methods in the design of continuous rotation, 360-degree scan antenna systems, especially for microwave radars. To accomplish focusing of the transmitted beam, the antenna can employ either a parabolic reflecting element or a refracting element with a microwave feed located at the focal point of the element, a planar array made up of slotted waveguides, or equivalent electromagnetic structure, etc. One common technique for coupling the microwave signals from the antenna to the transmitter and receiver subelements is to place these subelements in the stationary portion of the radar. The transmitted and received signals are transferred to and from the antenna by a rotary joint placed upon the axis of rotation of the antenna. 
     Another technique is to locate much or all of the transmitter and receiver subelements with the antenna on the rotating structure, and transfer raw power and control signals from, and receiver output video to the stationary portion of the radar via slip rings coupled to the rotational axis. This technique has disadvantages of significant transmitter/receiver weight forming part of the rotating mass, a relatively uncontrolled environment for critical transmitter/receiver circuitry, and signal noise generated by the slip ring assembly. 
     The first described technique using a rotary joint is generally preferable. Rotary joints operating in the microwave region of the electromagnetic spectrum are widely used and provide adequate performance. However, those that operate in the millimeter wave region of the spectrum may not provide adequate performance and are prohibitively expensive for use in a low cost surveillance radar. 
     One example of prior art is the reference Waters et al., statutory invention reg. no. H966, published on Sep. 3, 1991. Waters provides a scanning antenna requiring no rotary joint for use in a shipboard environment. In the stationary portion of this design, the electromagnetic energy is collimated into a beam of significant diameter by means of a parabolic reflector. This beam is transmitted upward to a rotating assembly that by phase sensitive reflection produces two scanning, orthogonally polarized beans transmitted horizontally in opposite directions. The physical mechanism that supports the scanning assembly must provide unobstructed passage of the rather large diameter collimated beam from the stationary parabolic reflector to the rotating assembly. 
     In view of the above, there is a need for an improved method of transferring the millimeter wave electromagnetic energy between the rotating antenna and the transmitter and receiver subelements located in the stationary portion of the radar. Furthermore, there is a need to accomplish this without requiring the use of slip rings or a rotary joint, and by using a minimum of components in a lightweight configuration having reasonable cost. For these and other reasons, there is a need for the present invention. 
     SUMMARY OF INVENTION 
     The invention relates to a surveillance radar-scanning antenna requiring no rotary joint. The surveillance radar antenna of the invention includes a millimeter wave horn positioned on the vertical axis of rotation of the antenna and protruding through the open center of the antenna support bearing, driven gear, and a hole in the antenna housing. Divergent millimeter wave electromagnetic energy is emitted vertically by the non-rotating horn, then is deflected to the horizontal by an angled reflector before being focused by a dielectric lens into a collimated beam. The rotating antenna housing supports the angled reflector and dielectric lens. Provisions are made for vertical positioning of the dielectric lens to allow limited adjustment of the transmitted beam above or below the horizontal. Received energy reflected from distant targets is collected by the dielectric lens and directed by the angled reflector to the non-rotating horn where it is fed to a waveguide coupled to the receiver. 
     The present invention provides a method for the transfer of millimeter wave electromagnetic energy between a rotating antenna assembly and the transmitter and receiver subelements in the stationary structure of the radar. An advantage of the present invention is that a millimeter wave rotary joint, with its intrinsic requirement for extremely accurate tolerances and highly expensive manufacturing processes, is not required. Another advantage is that a surveillance radar incorporating the present invention does not have any moving mechanical parts in the waveguide portion of the electromagnetic energy path. Furthermore, the radar of the invention experiences no variation in energy loss due to variations in a mechanical rotary joint, and does not require the periodic replacement of an expensive rotary joint component. 
     In contrast to Waters, the present invention uses a support bearing and driven gear, which supports and drives the rotating antenna structure, with open inner diameters only sufficiently large to allow passage of a non-rotating millimeter wave waveguide assembly. The electromagnetic beam is emitted by a non-rotating horn and then collimated by an angled reflector and dielectric lens forming a part of the rotating portion of the antenna. Other aspects, embodiments, and advantages of the prior art will become apparent by reading the detailed description that follows, and by referring to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 illustrates an electronic fence made up of a multiplicity of radars to protect high value assets. 
     FIG. 2 is a cross-sectional view of a rotating antenna structure that does not require a rotary joint. 
     FIG. 3 shows a cross-sectional view of an alternate configuration of the rotating antenna structure with provisions added to allow vertical adjustment of the dielectric lens position for the purpose of aiming the transmitted beam above or below the horizontal. 
     FIG. 4 is a frontal view of the alternate configuration with the lens support plate in the foreground and the antenna housing in the background. 
    
    
     DETAILED DESCRIPTION 
     In the following detailed description of exemplary embodiments of the invention, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific exemplary embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized, and logical, mechanical, and other changes may be made without departing from the spirit or scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims. 
     The configuration of major components of a rotating antenna according to one embodiment of the invention is depicted in cross sectional view in FIG.  2 . Major components are positioned axially along two major axes  11 - 11 ′ and  12 - 12 ′ which are perpendicular to each other and intersect at point  13 . Stationary structure  14  contains the transmitter, receiver and signal processor subelements of the radar, and provides support for the rotating portion of the antenna. 
     The rotating portion of the antenna includes the upper portion of support bearing  15 , driven gear  16 , housing  17  and its included components, reflector support  18 , reflector  19  and lens  20 . Lens  20  is fabricated of a dielectric material having the capability of reducing the propagation velocity of millimeter wave electromagnetic energy while passing it with essentially no attenuation. The lower portion of support bearing  15  is rigidly coupled to the stationary structure  14  while its upper portion is rigidly coupled to the driven gear  16  which is in turn rigidly coupled to housing  17 . Support bearing  15 , and driven gear  16  and a hole in the lower portion of housing  17  are located coaxially along axis  11 - 11 ′. This rotating assembly revolves about axis  11 - 11 ′, being rotationally driven by drive gear  21  which is coupled to driven gear  16 . Drive gear  21  is coupled to a drive motor, which is not shown, by shaft  22 . 
     Housing  17  has the form of a cylinder, seen in cross sectional view in FIG. 2, with its axis being defined by axis  12 - 12 ′. The cylinder is terminated at one end by lens  20  and at the other by housing end plate  23 . Included within the housing is a reflector  19  coupled to a reflector support  18  that is in turn coupled to the housing end plate  23 . Reflector  19  is supported so that its front reflecting surface coincides with the intersection of axes  11 - 11 ′ and  12 - 12 ′ at point  13 , and the plane of reflector  19  is tilted to form angles of substantially 45 degrees with respect to both axes  11 - 11 ′ and  12 - 12 ′. 
     Millimeter wave feed and horn structure  24 , comprising a circular waveguide coupled to a conical horn, is coaxially positioned along axis  11 - 11 ′ so that it protrudes through the open inner diameters of support bearing  15 , driven gear  16  and the hole located in the bottom of housing  17 . Point  25  defines the apparent point of origin of millimeter wave electromagnetic rays emanating from the horn. Lens  20  has a finite thickness that must be considered for highly accurate determination of the paths of electromagnetic rays passing through the lens. However, for first order analysis, a point  27  can be defined which will approximate the location of an imaginary lens having equivalent focusing performance but zero thickness. The feed and horn structure  24  is positioned along axis  11 - 11 ′ so that the distance along axis  11 - 11 ′ from point  25  to point  13  plus the distance along axis  12 - 12 ′ from point  13  to point  27  is essentially equal to the focal length of lens  20  at the frequency of operation. Millimeter wave feed and horn structure  24  is physically coupled to the stationary structure  14  and maintains a constant position as the rotating portion of the antenna rotates about axis  11 - 11 ′. 
     The preferred embodiment of the present invention operates at a frequency of substantially 35.5 Gigahertz. Housing  17  has outer and inner dimensions of substantially 16.5 and 15.2 centimeters respectively; its overall length is defined by the appropriate spacing of lens  20  with respect to reflector  19  and a selected length of reflector support  18  to provide equal mass distribution of the housing and its coupled components fore and aft of axis  11 - 11 ′. The horn portion of the horn and feed structure  24  is an axisymmetric conical structure having a cone half angle of substantially 24 degrees with respect to axis  11 - 11 ′, and having a length of substantially five centimeters from apex to aperture. The horn is fed by a circular waveguide having an internal diameter optimized to the frequency of operation by the use of principles well known to those skilled in the art. 
     Reflector  19  is fabricated of Aluminum or similar material being highly reflective of millimeter wave energy and has reflecting surface dimensions that exceed the area impinged by the electromagnetic energy emanating from the horn. The reflector surface finish and flatness are several orders of magnitude less than the wavelength the reflected millimeter waves. Lens  20  has a piano-convex form being fabricated of a polypropylene dielectric material, and has an aperture and focal length of substantially 15.2 and 17.8 centimeters respectively. The combined distances from points  25  to  13  and from  13  to  27  are adjusted to be effectively equal to the focal length of lens  20 . The angles of reflector  19  with respect to axes  11 - 11 ′ and  12 - 12 ′ causes the apparent point of origin of rays emanating from the horn, point  25 , to appear to be located at point  26  on axis  12 - 12 ′ when viewed from the position of lens  20 . Adjusting the vertical position of horn and feed structure  24  can be accomplished to optimize the focus of the beam emanating from lens  20 . After adjustment, its position is fixed with respect to the stationary structure  14 . 
     In transmit operation, millimeter wave electromagnetic energy proceeds up the circular waveguide portion of the horn and feed structure  24  to point  25  and then is dispersed into a conical volume by the horn. Each elemental segment of this energy forms a ray that proceeds from the horn appearing to have come from point  25 , until it impinges upon reflector  19  to be reflected in accordance with well known laws of reflection from a flat reflective surface. Upon leaving the surface of reflector  19 , the solid cone of electromagnetic rays proceeds to the rear surface of lens  20 . While passing through the dielectric lens the rays are focused into a substantially collimated beam having an initial diameter essentially equal to the aperture of lens  20 , or 15.2 centimeters. A central ray  28  proceeds from point  25  along axis  11 - 11 ′ until reaching reflector  19  at point  13 , next proceeds to point  27  located in lens  20 , and then passes through the center of the lens undeviated continuing along a path that is an extension of axis  12 - 12 ′. The path of this central ray  28  defines the direction of propagation of the beam formed by the antenna. 
     Ray  29  and ray  30  are peripheral rays defined by the maximum aperture limit of lens  20 . After being reflected by reflector  19 , these rays appear to have originated at point  26  and proceed to the lower and upper regions of lens  20 . They are then diffracted by their angles of incidence with respect to the first surface and the curvature of the lens at the points of ray exit in accordance with the dielectric constant of the lens and the well-known Snell&#39;s law. The paths of rays  29  and  30  proceeding from the lens are substantially parallel to that of the central ray  28 . Although FIG. 2 is a two-dimensional depiction of that vertical plane which contains both axes  11 - 11 ′ and  12 - 12 ′, those skilled in the art will recognize that reflecting surface  19  is a two-dimensional surface, lens  20  has a circular aperture, and that rays emanating from point  25  will, after reflection from reflecting surface  19 , substantially fill the planer aperture of lens  20 . The electromagnetic energy exiting lens  20  has the form of a collimated beam, with diameter essentially the same as the aperture of the lens. Factors such as spherical aberration and manufacturing tolerances of low cost dielectric lenses result in some spreading of the emitted beam. One example of the preferred embodiment provided a transmitted beam width of some 3.6 degrees. 
     During receive operation, a portion of the transmitted beam is reflected from the target back to the antenna where the received energy impinging upon the lens  20  follows essentially a reverse path through the antenna until it arrives at point  25  and proceeds down the circular waveguide to the receiving subsystem within the stationary structure  14 . The described configuration produces a linearly polarized beam with the polarization rotating as the antenna structure sweeps through a 360-degree search pattern. Both analysis and experiment have shown that the area illuminated on targets of interest by the radar beam typically has a surface roughness significantly exceeding a half wavelength of the operational frequency, which is some 4.2 millimeters. Therefore, the rotating polarized beam has no effect on overall radar performance. 
     When deploying a multiplicity of radars incorporating the present invention in configurations similar to that shown in FIG. 1, it may be found that the terrain is not flat and thus it may be necessary to place a radar in a depression or at the top of a knoll with the requirement that the radar maintain surveillance of the area surrounding its position. The antenna configuration shown in FIG. 2 produces a beam pattern having the form of a horizontal disc with the radar rotating antenna at its center. If deployed at the bottom of a depression, the search range would be limited due to the radar beam impinging upon the sides of the depression a short distance away from the radar. If placed on a knoll, the disc-like beam pattern would be located progressively further above the surface as the distance from the radar increased, possibly allowing an intruder to crawl under the beam. Such situations make it highly desirable to adjust the antenna so that the path of the beam will be either above or below the plane formed by the rotation of axis  12 - 12 ′ about axis  11 - 11 ′. 
     FIG. 3 presents an alternate configuration for housing  17  and the coupling of the lens  20  thereto. A portion of the cylindrical housing nearest the lens is replaced with a conical section  40  that is coupled to a mounting plate  41 . FIG. 4 shows a front view of the alternate configuration with the mounting plate  41  in the background and a lens support plate  42  in the foreground. Lens  20  is coupled to the lens support plate  42  that is held in position against the mounting plate  41  by four fasteners  43 . Four slots  44  located in the mounting plate  41  and four holes for the fasteners similarly located in the lens support plate  42  allow fasteners  43  to be used to couple the lens support plate to the mounting plate in a range of vertical positions with respect to the cylindrical axis of the housing  17 . A vertical axis  45 - 45 ′ passes through the center of the lens  20  and is parallel to the axis  11 - 11 ′, about which the antenna rotates. A horizontal axis  46 - 46 ′ is coincident with and orthogonal to the axis  12 - 12 ′ and defines the vertical center of the housing  17 . A horizontal axis  47 - 47 ′passes through the center of the lens  20  and can occupy any of a number of positions above, on, or below the axis  46 - 46 ′, with its limits defined by the extent of the positions of the fasteners  43  in the slots  44 . The lens  20  is positioned above the axis  46 - 46 ′ in both FIGS. 3 and 4. An axis  48 - 48 ′, seen in FIG. 3, is orthogonal to both axes  45 - 45 ′ and  47 - 47 ′, and is parallel to the axis  12 - 12 ′. 
     In FIG. 3, the position of the lens  20  has been raised with respect to that which it occupied in FIG.  2 . No changes have been made in the positions of the feed and horn structure  24  or reflector  19 ; therefore, the apparent point of origin of rays emanating from the horn, point  25 , continues to appear to be located at point  26  on axis  12 - 12 ′ when viewed from the position of the lens  20 . A ray  50  can be traced from point  25  to point  49  on the reflector  19  where its path is reflected toward point  27  at the center of the lens  20  in accordance with the laws of reflection well known to those skilled in the art. A ray  50  passes through the center of the lens  20  undeviated and proceeds from the lens making a small positive angle with respect to the axis  48 . Note that the ray  50  can be considered to have come from point  26 , proceeding in a straight line through points  49  and  27  toward distant targets. 
     Peripheral rays  51  and  52  are defined by the maximum aperture of the lens  20 . After being reflected by the reflector  19 , these rays appear to have originated at point  26  and proceed to the lower and upper regions of the lens  20  where they are diffracted by the dielectric constant of the lens and the ray angles of incidence with respect to the first surface and the curvature of the lens at the points of ray exit. The paths of rays  51  and  52  proceeding from the lens are substantially parallel to that of the central ray  50 . Although FIG. 2 is a two-dimensional depiction of that vertical plane which contains the axes  11 - 11 ′,  12 - 12 ′, and  48 - 48 ′, those skilled in the art will recognize that the reflecting surface  19  is a two-dimensional surface, the lens  20  has a circular aperture, and that rays emanating from point  25  will, after reflection from the reflecting surface  19  substantially fill the planer aperture of the lens  20 . The electromagnetic energy exiting the lens  20  has the form a collimated beam, with diameter essentially the same as the aperture of the lens. 
     The axis  48 - 48 ′ is parallel to the axis  12 - 12 ′ with the separation between them being defined by a distance  53 . When the distance  53  is not zero, the angle that the radar beam makes with respect to a horizontal plane is approximately given by Beam angle=arctan (distance  53 /lens  20  focal length). In the preferred embodiment of the present invention, slots  44  have a length sufficient to provide an adjustment range of the distance  53  of plus and minus 1.5 centimeters that allows elevating or depressing the beam angle by a maximum of approximately five degrees. 
     It is noted that, although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement is calculated to achieve the same purpose may be substituted for the specific embodiments shown. For instance, the values presented above in conjunction with FIGS. 2,  3 , and  4  describe one embodiment of the present invention. Those skilled in the art will recognize that equivalent performance will be provided by operation at other wavelengths, and in particular at some 76 Gigahertz, and with other dimensions for the various components. A rectangular waveguide and horn structure can also be used in lieu of the circular structures. This application is intended to cover any adaptations or variations of the present invention. Therefore, it is manifestly intended that this invention be limited only by the claims and equivalents thereof.