Abstract:
An apparatus comprising an array of antenna elements, a beamformer for adjusting signals to and from the elements to form a first beam pattern and a second beam pattern, and wherein the first beam pattern is a sum pattern and the second beam pattern is a null pattern. A method of beamforming for sidelobe cancellation is also provided.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to antenna systems, and more particularly to such systems that include spatial sidelobe cancellation. 
       BACKGROUND OF THE INVENTION 
       [0002]    In monopulse radar systems, Identification; Friend or Foe (IFF) systems, as well as in many other systems, the antenna is an array of individual elements whose elemental signals are combined to form two main signal channels. One of these channels (commonly called the Sum channel) includes a narrow main beam directed along a pointing angle, or boresight, and having high directivity and a plurality of inherent undesired residual sidelobes which are off boresite. The Sum channel is generated by summing all of the antenna elements. Many times prior to any summing and/or signal combining, the individual antenna array elemental signals are pre-scaled (or Weighted) to achieve application specific optimization of such parameters as beamwidth, and main beam gain as well as limited control of sidelobes. Irregardless of whether Weighting has been incorporated, the main beam of the Sum channel is the desired portion, however because of the inherent undesired residual sidelobes, undesired energy will be transmitted and received at azimuth angles other than the pointing angle (assuming a horizontally aligned array, and correspondingly at elevation angles for a vertically aligned array). As a result undesired incoming signals and returns can result from reflected energy, jammers or other sources not in the direction of interest. This unwanted energy can corrupt systems such as radar and Identification; Friend or Foe (IFF), etc. 
         [0003]    To reduce the effects of the undesired Sum channel sidelobes, a common method employs a second auxiliary channel, called a Difference channel. The classic Difference channel is used to provide a second signal which can be compared to the Sum signal channel to determine if received signal energy is at boresite and valid or not. The Difference channel has a characteristic response such that its gain in the direction of the Sum channel pointing angle is lower than the Sum channel, but the Difference channel gain in other directions is intended to be higher than the gain of the Sum channel sidelobes. When signals are received, an amplitude comparison is made between the Sum and Difference channel outputs to distinguish (and eliminate) undesired signals that arrive at the undesired angles. This is sometimes referred to as sidelobe cancellation. 
         [0004]    A classic type Difference channel is not only used to provide sidelobe cancellation of the unwanted signals but sometimes to allow for Amplitude Monopulse Ratio (AMR) direction finding. When received energy is in a predetermined boresite angular sector, a comparison of the two channels can be used to find the angular direction of the incoming signal. This is due to the Difference channel characteristics that include a sharp null, which occurs in the same angular sector as the Sum channel main beam points, and the amplitude ratio value of these two channels thus varies with angle. 
         [0005]    The Difference channel also contains spatial sidelobes. In order to properly discern good signals from undesired ones so that sidelobe cancellation of undesired signals results, the sidelobes of the Difference channel should be higher than the Sum channel sidelobes. This is generally quite difficult to achieve since Difference channel sidelobes frequently tend to dip down below the Sum channels sidelobe levels resulting in what is known as punch through. 
         [0006]    It would be desirable to provide methods and apparatus that include spatial sidelobe cancellation while avoiding the deficiencies of the classic Difference channel approach. 
       SUMMARY OF THE INVENTION 
       [0007]    This invention provides an apparatus comprising an array of antenna elements, a beamformer for adjusting signals to and from the elements to form a first beam pattern and a second beam pattern, and wherein the first beam pattern is a sum pattern and the second beam pattern is a null pattern. 
         [0008]    In another aspect the invention provides a method of beamforming for sidelobe cancellation, the method comprising the steps of producing a sum channel having a main beam oriented along a boresight, and a plurality of sidelobes, and producing a null channel having a null oriented along the boresight. The null channel includes an omni-like pattern overlapping the plurality of sidelobes and having a greater gain than the sidelobes, to provide a greater margin and eliminate a punch through condition. 
         [0009]    The invention further encompasses a method of direction finding comprising the steps of: producing a sum channel having a main beam oriented along.a boresight, and a plurality of sidelobes; producing a null channel having a null oriented along the boresight, and an omni-like pattern overlapping the plurality of sidelobes; and comparing the sum channel to the null channel to determine an Amplitude Monopulse Ratio. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a block diagram of an antenna system constructed in accordance with the invention. 
           [0011]      FIGS. 2 ,  3 ,  4 , and  5  are plots of beamformed antenna patterns. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0012]    Referring to the drawings,  FIG. 1  is a block diagram of an antenna system  10  in accordance with the invention. The system includes an antenna array  12  having a plurality of individual antenna elements  14 ,  16 ,  18 ,  20 ,  22 ,  24 ,  26 ,  28  and  30 . In this example, the individual antenna elements are arranged in a linear array and are evenly spaced with respect to each other. Those skilled in the art will realize that the antenna array doesn&#39;t necessarily need to be a linear one, but in general and for most cases it should be symmetric about the center. A Beamforming block  32  is used to control the signals that are transmitted from or received by the antenna elements. 
         [0013]    During the receive mode, Beamforming block  32  may be a simple summation of the antenna element signals (or may take the form of a weighted summation for a more application specific need). Subsequent to this beamforming, signals are then combined in Sum block, also referred to as a sum element,  34  (via an addition) to produce the final Sum channel signal output on line  36  resulting in a main beam positioned in the boresight direction indicated by line  38 . The system includes a Delta block, also referred to as a difference element  40 , which combines the antenna element signals (via subtraction) into the final Null channel signal on line  42  having a null positioned in a boresight direction. A level compensator  44  is connected between the center element  22  and Delta block  40 . The purpose of the level compensator is to optimize nulling. For embodiments using a level compensator  44  having unity gain as an RF hardware type implementation choice, a single hardware component called a Sum/Difference Hybrid may be used to take the place of all three blocks  34 ,  40  and  44  thus reducing the amount of hardware required. Lastly, Transceiver  46  receives and processes signals from both Sum block  34  and Delta block  40 . 
         [0014]    During transmit, the transceiver supplies a signal up to Sum block  34  via line  36 . Sum block  34  will then split this signal equally into two output signals that exit out the top of Sum block  34 . One of these output signals feeds antenna element  22  directly. Beamformer  32  takes in the other Sum block  34  output signal at its bottom and internally splits it equally amongst the antenna elements it connects to at the top. For application specific optimization, Beamformer  32  may also weight (i.e., scale) each of the signals prior to its final application to the individual antenna elements. The Delta block  40  and Level Compensator  44  are not needed during transmit. 
         [0015]    For the purposes of this description, signals are mainly described as if the system is in a receive mode. However, those skilled in the art will recognize that the transmit mode forms similar antenna patterns. 
         [0016]    Spatial sidelobe cancellation techniques are used to reduce or eliminate the effects of unwanted received energy from directions other than boresite for a variety of system types. Such energy is a result of external emitters as well as an undesired signal that is transmitted and reflected back from directions other than the boresite. Spatial sidelobe cancellation is normally achieved by using two beam patterns. A main (Sum) channel is directional and has lower gain at the undesired azimuths. Another auxiliary channel (normally a Difference type) has a center main beam null and is designed with attempts for its sidelobe structure to always be higher than that of the Sum channel. For the best case, the auxiliary channel would be desired to be omni-like off boresite which a classic Difference channel cannot achieve. 
         [0017]      FIG. 2  is a plot of an antenna Sum pattern  50  of a prior art antenna. The Sum pattern includes a main lobe  52  and a plurality of sidelobes  54 .  FIG. 3  is a plot of an antenna Difference pattern  60  of a prior art antenna. The Difference pattern includes a null  62  at the boresight and a plurality of sidelobes  64 .  FIG. 3  depicts a typical classic Difference pattern whose sidelobes periodically dips down at or near sidelobe null points and will cause punch through. 
         [0018]    Systems constructed and operated in accordance with this invention do not contain the classic Difference auxiliary type channel but rather include an omni-like Null (or notched) auxiliary type channel. This Null channel can be configured to have a very good omni-like pattern that extends over a wide angle such as + 90  degrees azimuth (in lieu of having the sidelobe content and associated multiple sidelobe nulls that a classic Difference type channel exhibits). The Null channel also resembles a spatial notch filter with the notch at an angle which corresponds to the Sum channel main beam center. The Null channel provides the needed additional margin against punch through while it&#39;s notch, which is not quite exactly the same as that of the Difference channel null, allows for some Amplitude Monopulse Ratio (AMR) Direction Finding capability. 
         [0019]      FIG. 4  is a plot of an antenna Sum pattern  70  constructed in accordance with this invention. The Sum pattern includes a main lobe  72  and a plurality of sidelobes  74 . FIG.  5  is a plot of an antenna Null pattern  80  constructed in accordance with this invention. The Null pattern includes a null  82  at the boresight and an omni-like pattern  84  off boresight.  FIGS. 2 and 4  are example Sum channel patterns each depicting similar sidelobe levels.  FIG. 5  depicts the proposed Null channel pattern of this invention, which inherently does not have the same periodic dipping sidelobe structure, thus overcoming the punch through problem. 
         [0020]    Two basic methods to accomplish more omni-like auxiliary beam patterns will be described. The first method produces a pattern that is better in shape than a classic Difference channel but not quite as good in terms of omni-like performance as the second method described which is a true Null type channel. 
         [0021]    In the first method a Modified Difference channel is created by using fewer antenna array elements (than that used by the Sum channel). This is accomplished by symmetrically not using elements from each of the outer ends of the array and using only the centrally located elements to form the Modified Difference channel. As the number of outer end elements is reduced, the Difference channel pattern will spread out and form a pattern shape less crude than the classic Difference channel in terms of it&#39;s sidelobe structure. This spreading will result in fewer undesired sidelobes and fewer associated sidelobe nulls and less chances for punch through, however it will exhibit lower gain than the typical Difference channel and attain a much wider, far less sharp null characteristic, which is less desirable. When this Modified Difference channel uses as few as only the two center elements, no sidelobe nulls may exist, but at the same time the null will broaden very significantly which is not so desired. 
         [0022]    The second method requires forming a true omni type notched Null channel which is characteristically opposite that of the Sum channel main beam pattern and is described as follows. First the Sum channel main beam pattern can be expressed or approximated as a spatial weighting function of: 
         [0000]        w ( x )=ƒ( x )*Σ{( x−mX )} 
         [0000]    where w(x) is the weighted sum of all of the elements (representing the Sum channel output  36  transfer function), ƒ(x) is a user defined weighting function, δ is the impulse function representing a single antenna element of the array, X is the element spacing and m is the summation index (allowing as many summations as required in order to combine all the antenna elements that exist in the array). The index m is also symmetrical and centered about  0  and attains the value of  0  when the center array element is added in. 
         [0023]    The Sum antenna pattern characteristic versus sin(O) is the Fourier Transform of w(x), where  0  is the azimuth angle for a horizontally aligned array (and the elevation angle for a vertically aligned array). 
         [0024]    By using an odd number of elements in the array and knowing that δ(x) has a Fourier Transform which is a constant, a more complete omni-like Null channel n(x) is formed as follows: 
         [0000]        n ( x )= k *, 5 (x)−w(x) 
         [0000]    where k is a constant, chosen based on fix), such that null depth is optimized (via the level compensator  44 ). Since  6 (x) only relates to the center element, n(x) is formed exactly the same as w(x) with the exception of the way the center element is combined into it. With this, all elements except the center element can be combined or summed by Beamformer  32  as in  FIG. 1 . The final Sum channel would then simply add in the middle element via Sum block  34  to result in the signal on line  36 , and the Null channel would be formed via a subtraction with the middle element path by Delta block  40  to achieve the omni-like Null channel signal on line  42 . 
         [0025]    For an antenna array consisting of an even number of elements, a very similar approach can be taken. For the even element array, no middle element exists and an equivalent center element must first be synthesized. This is done by first pre-adding two (or more) symmetrically centrally located antenna elements into a single output. This subconfiguration output would then take the place of the center antenna element  22  of  FIG. 1 , and the rest of  FIG. 1  is the same for the remaining elements. It should noted that pre-adding symmetrically centrally located antenna elements, as described above, is not limited to a symmetrical even element array and can also be done with an odd element antenna array if so desired. The benefit of combining central elements in either case is increased channel gain. 
         [0026]    This invention uses a Null auxiliary channel which is omni-like instead of a classic Difference auxiliary channel. The Null auxiliary channel is not only easy to create but it doesn&#39;t have the problematic varying sidelobes and associated sidelobe nulls the classic Difference channel exhibits, as its characteristic is omni-like for directions other than boresite. 
         [0027]    The signal levels of the two channels can be compared. If the Sum channel is greater than the auxiliary channel, then the signal is an at boresite, valid signal. If the auxiliary channel is greater than the sum channel, then the signal is an off boresite, invalid signal, that can be ignored. 
         [0028]    While the invention has been described in terms of several embodiments, it will be apparent to those skilled in the art that various changes can be made to the described embodiments without departing from the scope of the invention as set forth in the following claims.