Patent Publication Number: US-7589683-B2

Title: Broadband blade antenna assembly

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a 371 of PCT US05/25621 filed Jul. 20, 2005. 
   This application claims rights under 35 USC 119(e) from U.S. application Ser. No. 60/608,264 filed Sep. 9, 2004, the contents of which are incorporated herein by reference. 

   STATEMENT OF GOVERNMENT INTEREST 
   This invention was made with U.S. Government support under Contract No. MDA972-00-9-0009 with Defense Advanced Research Project Agency (DARPA), and the U.S. Government has certain rights in the invention. 

   BACKGROUND OF THE INVENTION 
   1. Technical Field 
   The present invention relates to antennas and, more particularly, to broadband blade antennas. Even more particularly, the invention relates to an antenna which is formed by applying metallized surfaces to the surface of an airfoil in a specific pattern and in which three specific metallized areas are provided which are connected in series and provided with tuning components to provide for a tuned response. 
   2. Background Information 
   Airborne applications severely constrain the antenna design in terms of size/shape and weight. This problem is magnified for broadband applications. By using a nominal airfoil design as the basis for the antenna structure, the design becomes inherently suitable for the intended environment. 
   Typical blade antenna structures encase a radiating element in layers of glass or other support structure which form an airfoil to meet the airborne requirements. Typically, these blades are resistively loaded to control elevation lobing to avoid radiation nulls at the horizon. 
   The resistively tapered blade has two major limitations, (1) it does not improve the low-frequency match of the antenna and (2) the resistive taper is present electrically at all frequencies typically limiting the efficiency of the antenna to less than 50%. Additionally, due to construction techniques, the surface area of the radiating element can be relatively small compared to the surface area of the airfoil encasing it. 
   Also, it is a well known technique for electrically small wire monopoles, to provide inductance half way up the monopole, to draw current up the antenna and increase the radiation resistance. However, these wire antennas, which are typically used for CB radios, are narrow band. 
   There is, therefore, a need for an antenna which overcomes such limitations of the prior art. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention is an antenna which is integrated into the surface of an airfoil. By means of appropriate geometry features and reactive loading, a lightweight broadband omni-directional antenna assembly is realized. 
   Another aspect of the present invention is the formation of three metallized areas on the surface of the airfoil such as by the use of a metallized foil, metallized paint, flexible circuit board bonded to the airfoil, or the like. 
   A further aspect of the present invention is to provide three separate metallized areas, the first of which extends about the leading edge of the airfoil with the second metallized area extending about the trailing edge of the airfoil and spaced from the ends of the first metallized areas forming gaps therebetween which provides a capacitive coupling across the gaps for achieving a tuned response. The third area extends over the cap of the airfoil and is spaced from and extends along the first metallized area and is electrically connected to the first metallized area at the leading edge of the airfoil and is electrically connected to the second metallized area at the trailing edge of the airfoil to place the three metallized areas in a series electrical relationship. 
   Still another feature of the invention is to form the third metallized area with one or more gaps or grooves across which can be mounted parallel RLC networks which are electrically connected across the third metallized area segments to provide broadband impedance matching of the antenna. 
   Another aspect of the invention is to provide a transmission line connection at the lower end of the leading edge of the airfoil for connection to radio frequency (RF) electronics, and in which the airfoil is mounted on a support surface which functions as the ground plane for the antenna. 
   Another feature of the invention is to provide the three metallized areas on the airfoil whereby the first and second metallized areas covering the leading and trailing edges of the airfoil provide a fixed design for the antenna with the third metallized area covering the cap portion of the airfoil, providing flexibility by enabling various tuning components to be incorporated therein to tune the antenna to achieve the desired antenna characteristics. 
   A further feature of the invention is to incorporate the metallized areas of the invention into either a monopole or dipole antenna. 
   Still another aspect of the invention is to provide an antenna in which the radiation pattern is omnidirectional for a first portion of the band and then transitions to a unidirectional behavior beyond said first portion because the metallized pattern creates a traveling-wave notch element at higher frequencies. 
   The foregoing advantages, construction and operation of the present invention will become more readily apparent from the following description and accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     The present invention is further described with reference to the accompanying drawings wherein: 
       FIG. 1  is a side perspective view of a preferred embodiment of the antenna structure of the present invention; 
       FIG. 1A  is a top plan view of the airfoil of  FIG. 1  showing a modified type of RLC circuit. 
       FIG. 2  are graphs showing VSWR for the antenna shown in  FIG. 1 ; 
       FIG. 3  are graphs showing gain, directivity and reflected power vs. frequency for the antenna shown in  FIG. 1 ; 
       FIG. 4  is a side elevational view of a first embodiment of the antenna of the present application; 
       FIG. 5  are graphs showing measured impedance and VSWR for the antenna shown in  FIGS. 1 and 5 ; 
       FIG. 6  is a graph showing measured and modeled gain for the antenna shown in  FIGS. 1 and 5 ; 
       FIG. 7  is a diagrammatic perspective view of a second embodiment of the antenna structure of the present invention; 
       FIG. 7A  is an enlarged view of one of the encircled portions of  FIG. 7 ; 
       FIG. 7B  is an enlarged fragmentary view of another encircled portion of  FIG. 7 ; 
       FIG. 8  is a plot showing the measured impedance of the antenna shown in  FIG. 7 ; 
       FIG. 9  is a graph showing the VSWR of a 46 inch dipole antenna and a 40 inch dipole antenna similar to that shown in  FIG. 7 ; 
       FIG. 10  is a graph showing the measured and modeled gain for the antenna shown in  FIG. 7 ; 
       FIG. 11  are diagrammatic views of a sample blade antenna containing the metallized pattern for creating a traveling-wave notch element at high frequencies; 
       FIG. 12  is a chart and a graph showing the broadband impedance match and VSWR of the traveling-wave antenna of  FIG. 11 ; 
       FIG. 13  is a chart showing the swept gain of the traveling-wave antenna of  FIG. 11 ; and 
       FIG. 14  is a chart showing the pattern transitions from an omni monopole mode to a directional notch mode for the antenna of  FIG. 11 . 
   

   Similar numbers refer to similar parts throughout the specification. 
   DETAILED DESCRIPTION OF THE INVENTION 
   A preferred embodiment of the invention is shown in  FIG. 1  and is indicated generally at  1 , and is an example of one type of airfoil in which the improved antenna can be incorporated. Airfoil  1  is of a usual construction having a leading edge  3  and side edges  5  which are tapered rearwardly toward a trailing edge  9 , and which includes a generally teardrop-shaped cap end surface  11 . Airfoil  1  is shown in  FIG. 5  mounted on a conductive support surface  13  which will function as a ground plane for certain applications of the antenna when incorporated into airfoil  1 . Airfoil  1  can be a self-standing blade antenna or can be a particular airfoil structure of an aircraft such as the wing, tail, rudder etc. Likewise, it could be an airfoil-shaped blade antenna mounted at various positions on the aircraft whereby support surface  13  would be that portion of the aircraft structure on which the antenna is mounted and extends outwardly therefrom. 
   In accordance with the invention, a first metallized area indicated generally at  15 , is formed or mounted on airfoil  1  and extends about front leading edge  3  and rearwardly toward trailing edge  9 , a distance generally more than one half of the longitudinal length of side surfaces  5 . A second metallized area indicated generally at  17 , extends about trailing edge  9  and forwardly along side surfaces  5  toward leading edge  9 , a distance less than half of the longitudinal length of airfoil  1 . Second metallized area  17  terminates before contacting first metallized area  15  and forms a gap indicated generally at  18 . Gap  18  includes a pair of opposed, generally vertically extending gap sections  19 , one on each side surface  5 . Metallized area  17  preferably includes a triangular-shaped area  21  which extends forwardly toward leading edge  3  and is spaced from first metallized area  15  by an angularly extending gap section  23  which is a portion of gap  18  and merges into gap section  19  thereof. 
   A third metallized area indicated generally at  25  is formed on airfoil cap surface  11  and preferably extends throughout the longitudinal and cross-sectional length of cap surface  11  as shown particularly in  FIGS. 1 and 1A , and is spaced from first metallized area  15  by a gap  20 . 
   In accordance with a first embodiment, metallized area  25  is formed with a plurality of tuning gaps or spaces  27 , four of which are shown in  FIG. 1  which extends completely across the cross-sectional width of metallized area  25 . RLC circuitry, each of which is indicated generally at  29 , is provided across each gap  27  in order to provide a tuned response at a significantly lower frequency than the natural resonance of the antenna and to provide a damping effect s thereto as discussed further below. Metallized area  25  is electrically connected to the first metallized area  15  at leading edge  3  by a connection  31  and is connected to the second metallized area  17  by one or more electrical connections  33  adjacent trailing edge  9 . This places the three metallized area in an electrical series relationship. A connection  35  preferably formed at the lower end of front edge  3  of metallized area  15  to which is connected a transmission line  37  which extends to the appropriate transmit/receive equipment and associated components of an antenna system. 
   A slightly modified damping arrangement is shown in  FIG. 1A  wherein a single tuning gap or space  41  is provided, in a modified metallized area indicated at  45 . Gap  41  is provided with electrical connections  43  extending across the gaps of the individual spaced areas of metallized area  45  to provide for a RLC circuit therebetween. The remaining features of the particular metallized area  45  shown in  FIG. 1A  is the same as that of metallized area  25  discussed above including its various connections to the first and second metallized areas  15  and  17 . 
   In the present invention, the surface area of the airfoil is metallized with a specific pattern to achieve a reduction in the resonant frequency relative to the response of a uniformly metallized structure. In this way, the antenna surface area is maximized for a given airfoil. The geometry of the metallization yields a tuned response at a low frequency but suffers from an anti-resonant condition limiting its ability to achieve broadband gain and input match. By inserting a number of RLC sections on the top surface of the airfoil, this effect can be sufficiently damped. Alternatively, a lossy transmission-line can be used in place of the RLC circuit to provide damping. The relatively large surface area of the airfoil provides a good thermal sink for the high-power resistors in the case of high-power communications or electronic attack applications. 
   A 20″ high×24″ chord blade design was numerically modeled and reduced to practice. The structure is shown in  FIGS. 1 and 4 . 
     FIG. 2  shows the response of blade antenna  1  with and without loading. Note that the low-frequency is extended by 50%. The unloaded structure exhibits a narrowband series resonant behavior at about 55 MHz followed by an ant-resonance at 60 MHz. Thus the gain has a spike at the tuned frequency but does not exhibit a broadband nature. Additionally, the VSWR is high below 80 MHz which would require an external matching circuit or an isolator. The gain, directivity and reflected power vs. frequency is shown in  FIG. 3 . The loaded structure presents much less reflected power to the power amplifier while maintaining good broadband gain. 
   For the prototype antenna as is shown in  FIGS. 1 and 4 , the measured impedance and VSWR is shown in  FIG. 5 . Measured gain along with predicted gain from simulation tools is shown in  FIG. 6 . 
   Those skilled in the art will appreciate that the antenna of the present invention utilizes the surface area of the airfoil for the antenna in order to maximize radiation efficiency and provides broadband omni-directional radiation in excess of a 5:1 bandwidth with a good VSWR. Good VSWR performance can eliminate the need for a circulator or isolator stage between the power amplifier and the antenna thereby reducing system complexity, cost and weight. 
   The unique features of the present invention can be incorporated into a monopole antenna as shown in  FIGS. 1-6  and described above, as well as in a dipole antenna as shown in  FIGS. 7-10  and described briefly below.  FIG. 7  shows a dipole antenna indicated generally at  50 , which includes two generally similar half-sections indicated at  51  and  52 , which are connected at its central area  53  to a feed board assembly indicated generally at  54 . Assembly  54  includes a pair of micro strip feed lines  55 A and  55 B, each of which is connected to a respective member  51  and  52  of the dipole antenna  50 , and which includes a 4:1 matching balun  56 . 
     FIG. 7A  shows the third metallized area  57  provided at the cap of the airfoil being formed with multiple gaps  58  for receiving RLC circuits indicated generally at  60 , which are similar to that shown at  29  in  FIGS. 1 and 1A . Each dipole section  51  and  52  will have the first metallized area  62  extending about the leading edge of the air foil and second metallized area  63  extending about the trailing edge of the airfoil and spaced from the first metallized area by gaps  64 . Each gap  64  will include an angularly extending section  64 A and a generally linear section  64 B. Gap sections  64 B extend generally parallel to the leading edge of the airfoil and angular gap sections  64 A extend in opposite outward directions from adjacent feed assembly  54 . Each antenna section  51  and  52  is separated by a gap  65  as shown in  FIG. 7B . Each cap portion of the airfoil is covered with the metallized area  57  and separated from metallized area  62  and  63  by a gap  67  as shown in  FIG. 7A . 
   The operation and features of the dipole antenna  50  are similar to that described above for the monopole antenna of airfoil  1 . The measured impedance of a 46 inch longitudinal length airfoil dipole is shown in  FIG. 8  with the measured VSWR being shown in  FIG. 9  for both a 46 inch and a 40 inch dipole having a configuration similar to that shown in  FIG. 7 .  FIG. 10  shows the test results (measured) gain of the 46 inch dipole compared to the computer model results for such a dipole antenna according to the present invention, showing that the measured results are very comparable to that of the computer-modeled results. Each half section  51  and  52 , in the preferred embodiment for a 46 inch tapered dipole as shown in  FIG. 7 , will have a base chord of 6 inches and a top chord of 3 inches. Such an antenna will have an operation frequency range from 30 to 150 MHz. 
   It will also be appreciated that the various embodiments of the antenna of the present invention have the following additional advantages and capabilities. 
   The antenna provides efficient broadband radiation from an electrically short antenna structure that meets airborne requirements such as air drag, side-load pressure and weight 
   The design is applicable to high-power applications (5 KW or more) due the construction technique and the fact that it is amenable to RAM air cooling. 
   Relative to a standard resistively tapered blade, the antenna provides improves efficiency at the high end of the band and improves the input match at the low-end of the band. The improved match can eliminate the need for isolator/circulators or external matching circuits saving weight and system complexity. 
   The antenna can be configured to operate in an agile tuned mode if desired to improve narrowband gain at low frequencies. 
   The antenna can be designed on an arbitrary airfoil and is therefore suitable for integration directly into an airframe. 
   Given the embodiment disclosed in  FIG. 4 , the antenna impedance behavior remains well matched over a greater than 10:1 bandwidth. The radiation pattern is omnidirectional over the first 5:1 band and transitions to a unidirectional behavior above that. This unidirectional pattern results in increased gain in a given direction and may be useful for applications which require sectorized coverage. This behavior is a result of the shape of the leading edge metallization creating a traveling-wave “notch” element at high frequencies which results in the broadband unidirectional patterns. 
   This feature and results are shown for a prototype antenna having the shape and size as shown in  FIG. 11  which contains the metallized pattern as shown for the antenna of  FIG. 4  and described above. The broadband impedance match which is indicative for traveling-wave antenna, and the VSWR plot of  FIG. 12  shows the transition between omnidirectional to unidirectional, with the swept gain being shown in the plot of  FIG. 13 . The pattern transitions from the omni monopole mode to the directional notch mode of this antenna is shown by the graph of  FIG. 14 . 
   The angle of the notch as well as the length, curvature, etc. can vary without affecting the concept of the invention but will change the characteristics of the antenna while still providing the smooth transition from omnidirectional to unidirectional. In the antenna example of  FIG. 11 , it has been found that a height “H” of the metallized area above the ground plane which is approximately equal to or greater than 0.10 wavelength will start to change the radiation pattern from omnidirectional to unidirectional. 
   The various frequency responses of the antenna can be classified into two regions as follows: 
   At the lower frequencies, the antenna is resonant, and loaded with extra L and C due to the shape and due to L and Cs placed on the top of the antenna. This resonance is damped using a loss mechanism, in order to achieve broad band matching. At the higher frequencies, the base of the antenna forms a broadband traveling wave monocone, or a traveling wave “notch” element, independent of the top structure. 
   At the lowest frequencies the dominant response of the antenna is reactive, due to the low radiation resistance. Low radiation resistance, and the consequent degraded radiation efficiency, is a standard issue for electrically small antennas. The antenna forms an inductive/capacitive (LC) reactance: an initial inductive hook, with enhanced LC reactance on the top surface, and a series capacitance to ground at the bottom of the inductive hook. 
   Compared to a standard resistively tapered monopole or wedge, this hook causes more inductance and path length, and also causes larger capacitance to ground at the end of the hook. The enhanced L and C provide a lower tuned frequency response. 
   At the LC loaded resonance of the antenna, the antenna is electrically a short wire hook connected to a capacitor to ground. The antenna is less than a tenth of a wavelength high. Tuning is achieved due to the reactive LC cancellation. The current is larger flowing up the initial feed/base (metal  1 ) of the antenna, compared to the current flowing down the capacitive far end of the antenna (final metal  3 ). Hence radiation occurs. 
   There is a trade-off between improved match versus radiation resistance at the low resonance frequency. Improved match occurs at this low resonance frequency, due to the LC reactive cancellation, resulting in improved radiated power, regardless of the radiation resistance. The trade-off is that, this smaller opposite current down the capacitive final end partially reduces the radiation resistance at these very low frequencies. 
   Damping of the resonance is necessary for the following reason. At a frequency just above the LC loaded resonance, a large anti-resonance or mismatch can occur. More power radiates at the frequency of the anti-resonance when the top reactance is damped, and a perfect mismatch is avoided. One interpretation is that this anti-resonance is due to in-phase reflections from various parts of the antenna. One reflection is due to the top reactive loading. A second reflection is due to the capacitive end of the antenna. A third influence might be the shunt capacitance between the 1 st  and 3 rd  metal pieces, which may provide a parallel current path to the top reactance. These reflections add in phase and create an anti-resonance. If instantaneous bandwidth is desired, reflections should be dampened with a loss mechanism in the top reactive loading. 
   This loss mechanism is de-emphasized or by-passed at the higher frequencies, when the capacitance between the 1 st  and 3 rd  metal acts as a shunt capacitance to short out the top damped reactance. Hence close to 100% radiation efficiency is possible at the higher frequencies. 
   At the higher frequencies, the radiation is strong at the base of the antenna, due to the wedge shape. Much of the current radiates in the bottom quarter wave of the antenna. Any current that does reach the top is partially dampened in the loss of the LCR tuning components. The very base of the antenna can be shaped as a notch or a wedge. Using a notch, the patterns can be designed to be directional. Using a symmetrical wedge, the patterns are designed to be more symmetrical, i.e., the pattern would be more omni-directional or bi-directional. 
   While the present invention has been described in connection with the preferred embodiments of the various figures, it is to be understood that other similar embodiments may be used or modifications and additions may be made to the described embodiment for performing the same function of the present invention without deviating therefrom. Therefore, the present invention should not be limited to any single embodiment, but rather construed in breadth and scope in accordance with the recitation of the appended claims.