Abstract:
An antenna design, having two symmetrical phased array blade antenna elements which provide improved lateral target coverage with an increased effective radiated power and exhibits smooth null-free unidirectional antenna patterns. The direction of the unidirectional antenna pattern is dictated by a switching command under control of a user. Each blade antenna element is coupled to a 90-degree hybrid coupler and an RF switching device by a semi-rigid RF cable. Each blade antenna element is also connected to a sub-resonant choke balun for improved impedance matching and resultant distortion-less antenna patterns.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0001]    The invention described herein may be manufactured and used by or for the government of the United States of America for governmental purposes without the payment of any royalties thereon or therefore. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field Of The Invention 
         [0003]    The present invention relates generally to an electronically steered radio frequency beam transmitted by a phased array blade antenna assembly for use on an airborne platform whose mission is electronic warfare. The electronically steered radio frequency beam is dynamically directed to either the left or right side of the airborne platform in order to directionally maximize the power of the radiated radio frequency signal. 
         [0004]    2. Description of the Prior Art 
         [0005]    The industry has a number of airborne antennas used with different airborne amplifiers and covering a broad range of frequencies. However, most of the antennas, especially those covering lower frequency bands, provide less than optimal pattern coverage and thus reduced Effective Radiated Power (ERP) performance. An attempt to address these problems can be found in a U.S. Pat. No. 7,280,083 issued to this inventor. Although the &#39;083 patent reference provides added benefits of good impedance stability versus frequency and stable monotonic antenna pattern characteristics in proximity to an irregular ground plane, the overall design approach imposes the following limitations: the design has an inherent bidirectional antenna pattern vice a unidirectional antenna pattern, a lower amount of radiated power is directed towards the intended target as a result of the bidirectional antenna pattern, the antenna pattern is fixed to radiate symmetrically from the host platform vice a left only or right only radiation pattern, the phased array blade antenna described in the &#39;083 patent reference is not capable of switching the direction of radiation in response to control commands and most importantly, the reference results in a lower gain and thus a shorter standoff distance between a jammer and a target. 
         [0006]    There are no known airborne antenna designs in the prior art that will operate in the desired frequency range, at the desired power level, respond to main lobe directional control commands and increases the standoff distance between the jammer and the target. The increased standoff distance provided by the directionally controlled antenna pattern is important in raising the probability of mission success by moving the jamming aircraft further from potential threats. In addition, the unidirectional antenna pattern provides enhanced control of undesirable ownership EMI effects and a potential of significant fratricide reduction. 
       SUMMARY OF THE INVENTION 
       [0007]    An Electronically Steered Phased Array Blade Antenna (EASB) Assembly is an apparatus comprised of a pair of dipole antenna elements spaced apart from one another by a preset distance where the preset distance is a function of the radiated wavelength. The pair of dipole antenna elements are comprised of two symmetrical antenna blades with each antenna blade having a fan out angle of approximately 45 degrees. An electronically controlled radio frequency (RF) switching device and a 90-degree hybrid coupler, in combination, route the RF input signal to one of the two symmetrical antenna blades. 
         [0008]    A pair of coiled RF feed lines connect the pair of dipole antenna elements to the RF switching device and the 90 degree hybrid coupler combination, each of the pair of coiled RF feed lines operates as a balun for impedance matching purposes. All of the above described components are rigidly mounted to a dielectric substrate. Clamps are used to mount the dielectric substrate within a radome. 
         [0009]    The antenna design comprises a two-element phased array blade antenna (PAB) assembly which provides improved lateral target coverage with an increased effective radiated power and exhibits a smooth null-free unidirectional lateral antenna pattern. Each blade is coupled to the RF switching device and 90 degree hybrid coupler combination by a semi-rigid Radio Frequency (RF) cable through a sub-resonant choke balun for improved impedance matching. 
         [0010]    This antenna design provides a superior antenna input Voltage Standing Wave Ratio (VSWR), smooth pattern coverage and large antenna gain. Moreover, broadband antenna performance is achieved with a unique antenna blade design that not only improves the usable frequency range of the antenna, but also provides for a light weight construction that is required for most airborne antenna systems. 
         [0011]    Another unique feature of this antenna array design is the fact that it does not require any impedance matching networks since the antenna blade construction features a nominal input impedance of fifty ohms. This feature is a major antenna design simplification directly reducing construction cost, increasing reliability and also reducing RF insertion loss. The transformation of an unbalanced RF input coaxial cable to a balanced configuration is accomplished with two sub-resonant choke baluns, each made out of a coiled semi-rigid RF feed cable. This approach gives the lowest cost antenna balun implementation with more than adequate performance and most of all maximum design simplicity. The innovative antenna blade design provides a well-behaved antenna input impedance characteristic that covers approximately 22% of the bandwidth around the center or target design frequency. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The features described above, other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where: 
           [0013]      FIG. 1  is a view of the Electronically Steered Phased Array Blade (ESPAB) assembly and mounting structure. 
           [0014]      FIG. 2  is a view of the preferred embodiment, a 90 degree hybrid coupler followed by and connected to an RF switching device, in a combination. The combination is also referred to as the coupler switching device, where the RF switching device is a Transfer Switch. 
           [0015]      FIG. 3  is a view of the RF switching device followed by and connected to a 90-degree hybrid coupler, in a combination. The combination is also referred to as the coupler switching device, where the RF switching device is an Absorptive SPDT RF Switch, another embodiment. 
           [0016]      FIG. 4  is a view of the antenna blade construction for the ESPAB assembly of  FIG. 1 . 
           [0017]      FIG. 5  is a polar plot for the ESPAB simulated radiating elevation pattern as predicted before being installed on an aircraft. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0018]    The preferred embodiment of the electronically steered phased array blade (ESPAB) antenna assembly  20  is shown in  FIG. 1 . The ESPAB antenna assembly  20  fits within the constraints of a radome intended for use on an aircraft. Clamps  21 ,  22 ,  23  and  24  are mounted on a structure made of a dielectric substrate  25  and placed as shown to anchor the ESPAB antenna assembly  20  to the radome. The quantity of clamps mounted on the dielectric substrate  25  can be increased in order to achieve the desired stability. 
         [0019]    The ESPAB antenna assembly  20  of  FIG. 1  contains two modified dipole antenna elements  26  and  27  spatially separated by a distance approximated by λ/4. This design requires that the dipole element separation be set to accommodate the center frequency of interest. Due to the radome&#39;s internal spatial constraints, the dipole element spacing deviation from λ/4, within +/−20%, will also provide acceptable performance of the antenna assembly  20 . 
         [0020]    The RF signal that feeds the ESPAB antenna assembly  20  is split equally in amplitude and shifted 90 degrees out of phase. This signal split and phase shift is accomplished with a device known in the art as a broadband high power 90-degree hybrid coupler  28 , which is commercially available for either low or high RF power applications from a number of vendors. The input RF is connected to the 90-degree hybrid coupler  28  via an RF input cable  29  which is connected directly to the input port  30  of the broadband high power 90 degree hybrid coupler  28 . The benefit of using the 90-degree hybrid coupler  28  is that it dissipates common mode currents as heat into a dummy load  31  which is connected to an unused isolation input port  82  of the 90 degree hybrid coupler  28 . A pair of signal output ports ( 38  and  39 ) of the 90-degree hybrid coupler  28  is connected to a pair of input ports ( 58  and  56 ) of an RF transfer switch  55  by a pair of RF lines ( 49   b  and  49   a ). 
         [0021]    Referring to  FIG. 2 , one suitable RF transfer switch  55  has a part number of (KC)310C00100 and is available from Dow Key Microwave products. The preferred RF transfer switch  55  accepts an input switching command  87  for use in switching the RF at input port  56  to either RF output port  85  or to RF output port  86 . The ability of the RF transfer switch to select either RF output port  85  or RF output port  86  effectively controlling the relative 90-degree phase lag-lead between the two ports, per the input switching command  87 , is the mechanization by which the transmitted RF is directed out of either the left side or the right side of the radome (not shown). 
         [0022]    Referring to  FIG. 1 , the RF output port  85  and the RF output port  86  are symmetrically connected to the antenna dipoles  26  and  27  via two separate semi-rigid RF feed cables  32  and  33 , each having equal electrical length. The RF feed cables  32  and  33  are specially formed to serve as both a high power RF feed line and as a sub-resonant choke balun, which is necessary for correct antenna operation. The balun consists of the semi-rigid RF feed cable wound in the form of a coil. The coils  34  and  35  which form the balun, provide a high impedance inductive load as seen by the outside surface of the RF cable  32  and the RF cable  33 . The purpose of the balun is to suppress the unbalanced currents attempting to flow on the surface of the outer conductor of RF cables  32  and  33 . 
         [0023]    The components that comprise the ESPAB antenna assembly  20  described above are mounted onto the dielectric substrate  25 . The basis of the ESPAB antenna assembly&#39;s strength and rigidity is attributed to the mechanical properties of the dielectric material used as the dielectric substrate  25 . The dielectric material chosen for the dielectric substrate  25  has the characteristics of having a low relative permittivity constant, preferably in the range of 2 to 3, and possesses a low Loss Tangent property. The use of the dielectric substrate  25  is necessary for mechanical strength and rigidity to support the assemblage. The selected dielectric material should be as electrically transparent as possible, including low loss tangent, so as not to interfere with the environmental and electrical operation of the antenna assembly  20 . 
         [0024]    Referring to  FIG. 4 , each antenna dipole element  42  consists of two symmetrical blades  26  and  36 . The design of each blade  26  and  36  affords lightweight construction and also minimizes wind loading should the antenna be used in other than airborne applications. Antenna blades  26  and  36  have a fan out angle (item  41 ) of 45 degrees as shown. The fan out angle (item  41 ) of 45 degrees contributes to the dipole element impedance reduction from 73 ohms down to 50 ohms to match the impedance of the semi-rigid RF feed cable  32  and  33  and thus obviates the need for an impedance matching network. This desirable broadband antenna impedance behavior and thus ultra low VSWR are all attributed to the peculiar three-prong antenna shape and the taper angle (item  41 ) of 45 degrees, where the symmetrical blade elements (items  26  and  36 ) make up the blade array antenna  42 . The geometry of each blade (items  26  and  36 ) has an effect of extending the antenna frequency bandwidth to about 22%. 
         [0025]    The calculated antennae pattern of  FIG. 5  shows the unidirectional characteristic inherent in the ESPAB antenna assembly  20 . The polar plot  90  depicts the idealized antenna pattern  92  alongside the simulated antenna pattern points  94 . From the polar plot  90  it is evident that the calculated main lobe  96  and back lobe  98 , constructed by connecting the measured antenna pattern points  94 , exhibits a unidirectional shape. That is, the main lobe  96  contains a larger portion of the radiated energy which is transmitted in the commanded direction. By commanding the RF transfer switch to radiate in the opposite direction the main lobe and back lobe can be switched 180 degrees allowing the main lobe to transmit in the opposite direction. 
         [0026]    Referring to  FIG. 1 , the RF feed line connection  84  to the antenna blade elements  26  and  36  can be accomplished in a number of ways depending upon whether it is desired to have the blades interchangeable or not. The simplest method would be to drill holes corresponding to the outer and inner diameter of the feed line  34  in the two antenna blades  26  and  36 , respectively. Another method to achieve blade design commonality would be to have standard feed-thru adapter connectors installed in the threaded RF input orifices, one for the outer conductor and the other for the inner conductor, ensuring that precise electrical isolation exists between the two conductors. If the antenna blades  26  and  36  are constructed of a material that is dissimilar to that of the RF feed line&#39;s  34  metallic outer and inner conductor, a special design approach must be considered to prevent the dissimilar metal galvanic corrosion phenomenon. Towards that end, preventing moisture penetration is a critical strategy to inhibit galvanic corrosion if direct contact between two dissimilar metals cannot be avoided. 
         [0027]    The preferred embodiment is most useful in a scenario in which a jamming aircraft is configured to carry an external pod with the ESPAB assembly installed. The aircrew may then fly the jamming aircraft along a flight profile, and using the orientation of the target location relative to the external pod, the aircrew may command the RF to radiate in a direction that focuses the main lobe of the antenna pattern on the target. As the jamming aircraft changes the orientation of the target location relative to the external pod the aircrew may refocus the main lobe of the antenna pattern on the target by sending the RF switching command. 
         [0028]    Referring to  FIG. 3 , another embodiment of the ESPAB uses an absorptive RF switching device  53  in place of the RF transfer switch ( FIG. 2  item  55 ). Additionally, the position of the 90 degree hybrid coupler  28  and the absorptive RF switching device  53  are swapped. The swapped position results in the absorptive RF switching device  53  accepting the RF input  30  and switching the RF input  30  to either output port  45  or output port  47 , depending upon the state of the input switching command  87 . The RF available at either output port  45  or  47  is then fed into the 90 degree hybrid coupler  28 , controlling the necessary relative 90-degree phase lag-lead shift, for further RF coupling to the respective antenna blade via RF feed line connection  84 , as shown in  FIG. 1 . 
         [0029]    One advantage of using the absorptive RF switching device  53  is that no dummy load is required. One disadvantage of using the absorptive RF switching device  53  is that it must have much higher RF power rating as compared to the preferred embodiment using the RF transfer switch implementation ( FIG. 2  item  55 ). An absorptive switch produced by Comtech PST Corporation has a peak power limit of two kilowatts and an average power limit of eighty watts. Due to manufacturing difficulties, the average RF power handling constraints of the absorptive RF switching device  53  is usually orders of magnitude lower than that of the reflective electro-mechanical RF transfer switch  55 . For some applications, a lower average power rating may be adequate. 
         [0030]    From the foregoing, it may readily be seen that the present invention comprises a new, unique and exceedingly useful and effective electronically steerable blade array antenna system which includes a pair of lightweight dipole blades designed to fit within a radome which constitutes a considerable improvement over the known prior art. Many modifications and variations of the present inventions are possible in light of the above teachings. It is therefore to be understood that within the scope of the claims the invention may be practiced otherwise than as specifically described