Patent Publication Number: US-6219000-B1

Title: Flared-notch radiator with improved cross-polarization absorption characteristics

Description:
TECHNICAL FIELD OF THE INVENTION 
     This invention relates to antenna elements for radar arrays, and more particularly to a flared-notch radiator having a reduced aperture return loss to cross-polarized incident plane waves. 
     BACKGROUND OF THE INVENTION 
     Single-polarization, flared-notch radiators are typically designed by optimizing the radiation performance of the element in one plane (co-polarized), without regard to its performance characteristics in the plane orthogonal to the radiator (cross-polarized). For a wave impinging upon an antenna comprised of these flared-notch radiators, this design approach results in a radiator that provides maximum absorption of the co-polarized component of the incident wave, but minimal absorption of the cross-polarized component from the radiator tips. 
     It would therefore be an advantage to provide a technique to improve this cross-polarization absorption component. 
     SUMMARY OF THE INVENTION 
     A flared-notch radiating element in accordance with the invention has a body portion tapering to an element tip region, the radiating element having a first thickness through a body element portion. The element tip region has reduced thickness in relation to the first thickness, the reduced thickness improving the absorption of the cross-polarized component of an incident wave. 
     The reduced thickness of the tip region can be provided in several ways. For example, there can be a single step reduction in the element thickness, or the tip region can have multiple stepped reductions in thickness. Another alternative is to smoothly taper the thickness from the thickness of the element body portion to an end tip thickness. 
     A typical application for a flared-notch radiator in accordance with the invention is in an array of flared-notch radiator elements. The array includes typically a plurality of metal sticks disposed in aligned rows, each stick defining a plurality of flared notches, with adjacent ones of the metal sticks being separated by a separation distance so as to define a respective channel between each adjacent pair of sticks. The co-polarized component of the incident wave is parallel to the channels, and the cross-polarized component is transverse to the channels. Thus, the thickness dimension being reduced in accordance with the invention is the dimension transverse to the channels between the array sticks. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     These and other features and advantages of the present invention will become more apparent from the following detailed description of an exemplary embodiment thereof, as illustrated in the accompanying drawings, in which: 
     FIG. 1A is an isometric view of a portion of a conventional single-polarization, flared-notch radiator stick. 
     FIG. 1B is an end view of the radiator stick of FIG.  1 A. 
     FIG. 2 is an isometric view of a first embodiment of a flared-notch radiator in accordance with the invention. 
     FIGS. 3A,  3 B and  3 C are respectively side, end and top views of the flared-notch radiator of FIG.  2 . 
     FIG. 4 is an isometric view of a second embodiment of a flared-notch radiator in accordance with the invention. 
     FIGS. 5A,  5 B and  5 C are respectively side, end and top views of the flared-notch radiator of FIG.  4 . 
     FIG. 6 is an isometric view of a third embodiment of a flared-notch radiator in accordance with the invention. 
     FIGS. 7A,  7 B and  7 C are respectively side, end and top views of the flared-notch radiator of FIG.  6 . 
     FIG. 8 is a graph plotting simulation results of the return loss versus frequency performance of a conventional flared-notch radiator and of a flared-notch radiator in accordance with the invention. 
     FIG. 9 is an end view of an array of sticks of the flared-notch radiator of FIG.  2 . 
     FIG. 10 is an isometric view of a portion of the array of FIG.  9 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1A shows a portion of a conventional flared-notch radiator stick,  10 , comprising a plurality of flared-notch radiator elements. An antenna array will typically include a number of the sticks arranged in parallel. An exemplary array is illustrated in U.S. Pat. No. 5,659,326, the entire contents of which are incorporated herein by this reference. The radiating elements such as element  12  include conductive body structures  14 A,  14 B that taper to a tip  16 . As shown in the end view of FIG. 1B, however, the conductive body structures  14 A,  14 B are of uniform thickness. While two body structures  14 A,  14 B are illustrated, and typically sandwich a balun circuit (not shown), the radiating element could be fabricated of one body structure or more than two body structures. This is true as well for radiating elements embodying this invention. 
     FIG. 2 is an isometric view of a portion of a flared-notch radiator stick  20  embodying a first embodiment of a flared-notch radiator element in accordance with this invention. FIGS. 3A-3C further illustrate the stick  20  in respective side, end and top views. As illustrated therein, the radiating elements  22  of the stick  20  have a tip region of reduced thickness, to act as an impedance transformer for the cross-polarization component of an impinging wave as it transitions from free space to the parallel-plate region between the flared-notch radiator sticks of an antenna. Thus, as shown in FIG. 3B, the radiating elements of the stick  20  have a thickness T 1  at the balun region, and a reduced thickness T 2  at the tip. The region of reduced thickness has a length L. In this embodiment, there is a sharp thickness transition between the tip region of reduced thickness and the body region of the radiating element. In an exemplary embodiment, T 1  is 0.400 inch, T 2  is 0.300 inch, and L is 0.800 inch, and the radiating elements operate over a frequency range of 2 Ghz to 18 Ghz. 
     FIGS. 9 and 10 illustrate an exemplary array  100  of the sticks  20  of the radiating elements  22 . The sticks are arranged in parallel in spaced relation, defining regions  102  between adjacent sticks that can be analyzed as parallel-plate channels. In some embodiments, an optional energy absorbing material can be placed at the bottom of the regions  102 , providing loading which can absorb any incident energy that is not absorbed by the radiating elements. 
     FIGS. 4-5 illustrate a stick  50  of radiating elements employing a second embodiment of a flared-notch radiating element in accordance with the invention. Here, the tips of the radiating elements are formed with a plurality of regions of progressively reduced thicknesses. Thus, radiating element  52  has a body region  52 A of thickness T 1 , a first reduced thickness region  52 B, a second reduced thickness region  52 C, and a fourth reduced thickness region  52 D. In an exemplary embodiment, T 1  is 0.400 inch, T 2  is 0.375 inch, T 3  is 0.300 inch, T 4  is 0.300 inch, and the respective regions  52 B,  52 C,  52 D have respective lengths 0.275 inch, 0.275 inch and 0.275 inch along the longitudinal axis  54  of the radiating element  52 . When the overall tapering length is short compared with the wavelength of the incident wave, a single-step or multi-step tip will provide better performance (lower return loss) over a specified frequency range than a smoothly tapered tip. 
     FIGS. 6-7 illustrate a stick  70  of radiating elements employing a second embodiment of a flared-notch radiating element in accordance with the invention. Here the tips of the radiating elements  72  are smoothly tapered from the thickness T 1  of the body of the element to a reduced thickness T 2  at the tip. The tapered region  72 B has an effective length L 1 =1 inch, with T 1 =0.400 inch, and T 2 =0.300 inch, in an exemplary embodiment. 
     FIG. 8 illustrates results of a simulation of the return loss performance of the flared-notch radiator of FIG.  1  and that of the flared-notch radiator of FIGS. 6-7. An exemplary frequency range of operation is from 2 Ghz to 18 Ghz. 
     It is understood that the above-described embodiments are merely illustrative of the possible specific embodiments that may represent principles of the present invention. Other arrangements may readily be devised in accordance with these principles by those skilled in the art without departing from the scope and spirit of the invention.