Patent Publication Number: US-6219005-B1

Title: Method and apparatus for reducing sidelobes of antennas within radomes

Description:
FIELD AND BACKGROUND OF THE INVENTION 
     The present invention relates to antennas and radomes, and particularly to a method and apparatus for reducing sidelobes of an antenna covered by a radome. 
     Radomes are structures designed to cover antennas and thereby to protect them from direct exposure to aerodynamic and environmental conditions, while being as transparent as possible to the antenna&#39;s electromagnetic (EM) radiation. However, many types of radomes include various forms of discontinuities or blockages. For example, radomes on high-speed, airborne platforms are usually equipped with a metallic tip to protect the radome against rain erosion, and/or with a pitot probe to measure the pressure of the fluid stream. Dual-sensor infrared/radio frequency (IR/rf) homing systems may include an IR seeker mounted at the radome&#39;s nose area. Small Streamline radomes commonly include dimensional and geometrical constraints which lead to an EM discontinuity in the radome&#39;s nose area. In all the above radome constructions, the radome may produce a blockage at the antenna&#39;s aperture. 
     When illuminated by the main beam of the antenna, the blockage acts as a secondary source that radiates in antiphase to the primary beam of the antenna. The exact pattern of this new source depends on the blockage dimensions. This radiation pattern interferes with the antenna&#39;s main beam and sidelobes, leading to degradation in the gain and sidelobes levels of the antenna&#39;s radiation pattern. 
     An object of the present invention is to provide a method, and also apparatus, for reducing the sidelobes of an antenna induced by such blockages in a radome. 
     SUMMARY OF THE INVENTION 
     According to the present invention, there is provided a method of reducing sidelobes of an antenna induced by a blockage in a radome covering the antenna, comprising providing at least one passive tuning element between the antenna and the blockage, and adjusting the position of the tuning element to reduce the blockage-induced sidelobes. 
     According to further features in the preferred embodiment of the invention described below, the passive tuning element is provided along the line of sight between the antenna and the blockage. Preferably, the passive tuning element is a circular metal member coaxial with the line of sight between the antenna and the blockage and is supported on a thin metal rod coaxial with the line of sight. 
     As will be described more particularly below, the provision of such a passive tuning element substantially reduces the sidelobes of an antenna produced by the above-described blockages in a radome. A single tuning element reduces sidelobes in a narrow angular range, but may cause energy increase of adjacent sidelobes. According to a further feature of the present invention, a broader angular effect may be achieved by using multiple tuning elements. Thus, the interelement distance shifts the angular position of the sidelobe null; the larger the separation between the discs, the closer the null to the mainlobe. 
     The radii of the passive tuning elements are typically about the same order as the blockage radius, preferably between 0.5 and 1.5 times of the blockage radius. In addition, the tuning elements are preferably spaced from the radome tip, and from each other, a distance of about 0.5 to 1.5 the operating (middle) wavelength of the antenna. 
     The invention also provides apparatus including an antenna and a radome, in which the sidelobes induced by the blockage in the radiation pattern of the antenna are reduced according to the above method. 
     Further features and advantages of the invention will be apparent from the description below. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: 
     FIG. 1 schematically illustrates an antenna system including a radome, in which sidelobes of the antenna induced by a metal rain tip of the radome are reduced in accordance with the present invention; 
     FIG. 2 more particularly illustrates the parameters of the passive tuning elements included in the antenna system of FIG. 1; and 
     FIG. 3 includes a set of curves illustrating the reduction in the sidelobes produced by the passive tuning elements in the antenna system of FIGS.  1  and  2 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 schematically illustrates an antenna  2 , as might be carried by an airborne platform such as an aircraft or missile, enclosed by a radome  4  to protect the antenna from direct exposure to aerodynamic and environmental conditions. Radome  4  may be provided with a metal tip  6  to protect the radome against erosion caused by rain. 
     As briefly described above, such a construction of antenna and radome is conventional. However, as also described above, the metal tip  6  of the radome  4 , provided to protect the radome against rain erosion, produces a discontinuity or blockage in the radome&#39;s tip or nose area, which induced undesirable sidelobes in the antenna radiation pattern. Such sidelobes can be reduced by the provision of a passive tuning device, generally designated  10  in FIGS. 1 and 2, within the radome  4  between the blockage  6  and the antenna  2 . 
     In the construction illustrated in FIGS. 1 and 2, the passive tuning device  10  is a two-element device. It includes two circular metal members  11  and  12  coaxial with the line of sight LOS between the antenna  2  and the blockage  6 . Such circular metal members are supported on a thin metal rod  13  coaxial with the line of sight LOS. 
     The circular metal members  11 ,  12 , may be circular discs or circular rings. Preferably, the radii of each metal member  11 ,  12 , is between 0.5 and 1.5 times that of the blockage. In addition, the circular metal members  11 , 12 , are spaced from the blockage  6 , and also from each other, a distance which is preferably about 0.5 to 1.5 times the middle, or operating wavelength of the antenna. 
     In the example illustrated in FIG. 1, the radome  4  is a half-wave streamline radome, 12-13λ in length, with a fineness ratio of about 2. The metal tip  6  at the nose of the radome is about one half λ (the middle frequency of the antenna) in diameter, and its area blocks 10-15% of an x-band antenna aperture. The two circular metal members  11 ,  12  are identical metallic discs of a diameter of about 0.8 that of the rain tip  6 , and are supported on a thin steel rod  13 . For an efficient sidelobes blanking in the angular range of 30-60° C. aside of the main beam, both the distance between the first disc  11  and the rain tip, as well as the distance between it and the second disc  12 , is 5λ/4. 
     FIG. 3 illustrates the effect such a tuning device  10  has on the H-plane radiation pattern of an x-band antenna covered by a half-wave ogive radome. Thus, curve A (long broken lines) illustrates the radiation pattern of the antenna alone without the radome or the tuning device; curve B (shorter broken lines) illustrates the radiation pattern of the antenna with a radome; and curve C (full lines) illustrates the radiation pattern when the tuning device  10 , as described above with respect to FIGS. 1 and 2, is inserted into the radome. 
     While the invention has been described with respect to one preferred embodiment, it will be appreciated that many other variations, modifications and other applications of the invention may be made.