Abstract:
A radar altering structure comprises: a structure; and at least one layer of conductive material disposed at at least one surface of the structure, the layer comprising a plurality of conductive paths arranged in a specular pattern to reduce the radar cross section of the structure.

Description:
This application claims the benefit of the U.S. Provisional Application No. 60/737,959, filed Nov. 18, 2005. 

   BACKGROUND OF THE INVENTION 
   Electro-thermal heating has become an effective choice for airfoil and structure deicer heaters, especially when composite materials are used for the airfoils and/or structures being deiced. An electro-thermal heater may be used wherever icing conditions exist, including applications such as: airfoil leading edges of wings, tails, propellers, and helicopter rotor blades; engine inlets; struts; guide vanes; fairings; elevators; ships; towers; wind turbine blades; and the like, for example. In electro-thermal deicing systems, heat energy is typically applied to the surface of the airfoil or structure through a metallic heating element via electrical power supplied by the aircraft or appropriate application generators. 
   An exemplary electro-thermal deicing apparatus is shown in the cross-sectional illustration of  FIG. 1 . The apparatus comprises a heater element layer of electrically conductive circuits  10  which may be configured as metal foils, wires, conductive fabrics and the like, for example, disposed in a pattern over a surface  12  of an airfoil or other structure  14 . A deicing system  20  controls the voltage and current to the electrical circuits of layer  10  via a plurality of leads  16  to protect the surface  12  from accumulating ice. Generally, the heater element conductive pattern is implemented over or under the skin of the airfoil or structure, or embedded in the composite material itself. 
   An exemplary heater element pattern  10  is shown in the illustration of  FIG. 2 . Electro-thermal deicer patterns of this type have a tendency to give off a larger than desired cross-sectional radar image in response to radar illumination. This has become a particular problem when such deicer heater patterns are applied to military aircraft or other structures that may be illuminated by enemy radar systems. To protect an aircraft or structure from becoming a target, it is desired to keep the radar cross-section of the structure as small as possible. Accordingly, the metallic/conductive patterns of the circuits of heater element layer  10  render present electrothermal deicing apparatus impractical for use on structures where radar attenuation is of concern. 
   SUMMARY OF THE INVENTION 
   In accordance with one aspect of the present invention, a radar altering structure comprises: a structure; and at least one layer of conductive material disposed at at least one surface of the structure, the layer comprising a plurality of conductive paths arranged in a specular pattern to reduce the radar cross section of the structure. 
   In accordance with another aspect of the present invention, electrothermal deicing apparatus with radar altering properties comprises: a heating element comprising at least one layer of conductive material disposable at at least one surface of a structure for deicing the surface, the layer comprising a plurality of conductive paths arranged in a specular pattern to reduce the radar cross section of the structure; and a control unit coupled to the heating element for controlling the heating energy thereto to deice the surface. 
   In accordance with yet another aspect of the present invention, apparatus for creating different radar signatures of a structure to an illuminating electromagnetic radiation source comprises: at least one layer of conductive material disposable at at least one surface of a structure, the layer comprising a plurality of conductive paths arranged in a specular pattern to reduce the radar cross section of the structure; and a switching unit coupled to the layer of conductive material to selectively apply electrical energy thereto for creating different radar signatures of the structure to the illuminating electromagnetic radiation source. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross-sectional schematic illustration of exemplary electro-thermal deicing apparatus. 
       FIG. 2  is an illustration of an exemplary heater element pattern currently comtemplated for use in electro-thermal deicing apparatus. 
       FIGS. 3-8  are examples of specular conductive patterns  1 - 6 , respectively, suitable for embodying the broad principles of the present invention. 
       FIG. 9  is a cross-sectional schematic illustration of a radar altering structure switching apparatus suitable for embodying another aspect of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   For military applications, it is well known that structures, such as aircraft surfaces, for example, are designed to operate stealthily against radar illumination. However, when an electro-thermal heater element with circuit patterns such as those exemplified in  FIG. 2  are applied to the surface of such structures, the heater element circuit patterns alter the radar cross-section of the structure rendering the structure more vulnerable to radar illumination. Note that the circuit pattern design of  FIG. 2  comprises conductive circuit paths that are substantially transverse to electromagnetic illumination by a point source monostatic radar from the front, the rear or either side. Accordingly, the circuit paths of such patterns create intense reflected electromagnetic waves directly back to the point source radar to magnify the radar cross-section of the structure. 
   The radar cross-section altering embodiments of the present invention which will be described in greater detail herein below involve the modification and enhancement of the specular characteristics for the electromagnetic properties of the electro-thermal heater elements to provide additional magnetic and electrical energy loss due to reflective and interference mechanisms. In the present embodiments, this energy loss is designed to occur when an electromagnetic wave of energy is applied by a radar source at a desired frequency of utilization (MHz or GHz) and over a broadband range to maximize absorption of electromagnetic energy by normal or modified conductors of the heater element and dampen the radar signals returned thereby to the radar source. Note that the heater elements via conductive paths  16  are electrified by the deicing system  20  as illustrated in  FIG. 1 . 
   Specular pattern designs  1 - 6  of the various embodiments of the conductive paths of the heater element  10  are shown by way of example in  FIGS. 3-8 , respectively. Preferably, round wire may be used for the conductive paths because of its inherent reflective properties to reduce returns from illumination by a point source monostatic radar. However, it is understood that the conductive paths of the various heater element patterns may be etched foil, metallic coated fabric or the like without deviating from the broad principles of the present invention. Likewise, the preferred application of the heater element patterns is integration into composite non-metallic structures. However, applying the heater element patterns over or under metallic or non-metallic surfaces of a structure will work as a radar altering structure just as well. 
   Each of the specular patterns  1 - 6  comprises six (6) conductive paths with a supply lead and return lead for each path, rendering twelve (12) connecting leads for each pattern. The connecting leads for each specular pattern  1 - 6  are found in  FIGS. 3-8  at  16   a - 16   f , respectively. The conductive paths of each of the specular patterns  1 - 4  and  6  start and end in the same vicinity. For example, the two outer leads of  16   a  in  FIG. 3  are the supply and return connector leads of one conductive path, and the next two outer leads going inward are the supply and return connecting leads of another conductive path, and so on. In each specular pattern  1 - 4  and  6 , the conductive paths are juxtaposed and electrically isolated from one another with one conductive path being circumscribed by another extending outwardly until a final outer conductive path completes the overall pattern. 
   The specular pattern  5  of  FIG. 7  is slightly different from the others having conductive paths that are juxtaposed and electrically isolated from one another, except that the conductive paths are not circumscribed by each other. Rather, each conductive path starts at one end of the specular pattern and runs back and forth forming a plurality of three sided subpatterns one within the other extending across the overall pattern. Thus, the conductive paths end at the other end of the specular pattern  5 . 
   The conductive paths of the specular patterns  1 - 4  and  6  comprise short zig-zag and angular straight line runs of repeating subpatterns which are designed to provide opposing perpendicular lines of electromagnetic reflectance at a forty-five degree (45°) angle with respect to the line of sight a point source monostatic radar creating destructive zones of interference from any unabsorbed electromagnetic waves. The specular pattern  5  is different from the others as noted above and comprises larger subpatterns made from conductive paths of longer runs which are wavy line paths and not straight line paths as in specular patterns  1 - 4  and  6 . Notwithstanding the difference of specular pattern  5 , each of the specular patterns  1 - 6  function to reflect the electromagnetic waves away from returning to their source or to create a destructive interference between the electromagnetic waves. In either case, the electromagnetic waves returned to the radar source from the structure are altered in such a way that reduces the radar cross-section of the structure. 
   While the specular patterns of conductive paths have been described herein above as an electro-thermal heater element as illustrated in  FIG. 1 , it is understood that this is merely one possible application. In general, each of the different patterns of conductive paths as exemplified in  FIGS. 3-8  is intended to alter the radar cross-sectional area of the structure to which it is applied. In other words, the specular patterns of conductive paths may be applied to a structure and used as a stealth agent to cloak the structure from enemy radar, i.e. render it substantially transparent to radar. For example, a chosen pattern of conductive paths may be integrated into composite material forming a skin of the structure, like an airfoil of an aircraft, for example. With the addition of the pattern of conductive paths, the structure becomes a radar altering structure (RAS) so that the radar cross-sectional area of the structure is substantially reduced. 
   It is further understood that the same pattern of conductive paths need not be applied to the overall structure. For example, it may be desired that one pattern be applied to the top of an airfoil and a different pattern be applied to the bottom thereof. Or, one pattern may be applied to the front surface of the airfoil while a different pattern may be applied to the rear surface thereof. Different specular patterns may be even applied in a plurality of layers to the structure. Accordingly, to render the structure a radar altering structure may involve applying one or more patterns of conductive paths to respective portions of the structure and electrifying the conductive paths thereof. 
   In addition, once applied to the structure, the pattern of conductive paths may be controlled to create special radar signatures of the structure to illuminating radars. For example, the conductive paths  16  of the pattern  10  may be coupled to a RAS switch system  30  as shown in the schematic illustration of  FIG. 9  and operated as a special antenna to illuminating radars. Referring to  FIG. 9 , the system  30  may be operative to connect and disconnect the conductive paths to a voltage source or ground, for example. Thus, when connected, the conductive paths  16  become closed circuits and render the structure transparent to the illuminating radar, and when disconnected, the paths  16  are open-circuits and floating, i.e. ungrounded, and render the structure apparent to the radar. Therefore, the pattern of conductive paths may be controlled by closing and opening the circuits thereof to respond differently to illuminating radar signals, and possibly, send out false radar return signals to mislead the enemy. 
   While the present invention has been described herein above in connection with one or more embodiments, it is understood that such presentation is merely by way of example with no intent of limiting the present invention in any way by any single embodiment. Rather, the present invention should be construed in breadth and broad scope in accordance with the recitation of the claims appended hereto.