Abstract:
A feedhorn driving method and apparatus allows the establishment of multiple phase centers using only a single multimode feedhorn. At least two higher-order modes are extracted from the feedhorn and weighted in amplitude and phase. The phase center separation is established in accordance with an assigned weights. The feedhorn has application in i.a. moving target indication systems.

Description:
[0001]     This application claims benefit of the filing date of U.S. Provisional Application No. 60/480,742 filed on Jun. 24, 2003. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The invention relates generally to radio wave antennae, and more particularly to multiple phase center radio wave antennae.  
         [0003]     Multiple phase center antennae are used in some specialized communications and radar applications. Specific radar applications may include ground or airborne moving target indication (MTI), along track interferometry and maritime surveillance. In MTI systems it may become difficult to discern a target from stationary background clutter when the target is moving slowly with respect to the terrain. Clutter is the term used in radar applications, to describe confusing or unwanted reflections that interfere with the observation of desired signals on a radar indicator. Clutter may be suppressed by receiving reflected radiation beams via multiple radar channels and employing adaptive filtering to identify stationary clutter from the moving target.  
         [0004]     A multiple channel radar receiver may be implemented using multiple antennae, each antenna typically comprising a separate reflector excited by a feedhorn. This approach has several disadvantages, one being that the antenna directivity is limited to that of each individual antenna and not that implied by the physical span of the collective multiple antennae. Another disadvantage is that the phase center separation is mechanically fixed which also fixes the constant phase beamwidths. Finally, the system noise temperature increases linearly with the number of mismatched antenna apertures.  
         [0005]      FIG. 1 . shows an alternative approach where a single reflector antenna  100  is coupled to two feedhorns  102  and  104 . feedhorns  102  and  104  are inclined at an angle to the centerline  106  of the reflector  100  thus establishing a pair of separated phase centers  110  and  112  at the antenna aperture  114 . The separation increases with inclination angle of the feedhorns  102  and  104  to centerline  106 .  
         [0006]     The antenna configuration shown in  FIG. 1  also suffers from several disadvantages. For maximum gain, the phase center of each of the feedhorns  102  and  104  should be at the focus  108  of reflector  100 , but this is obviously impossible and hence a loss of antenna gain in the resulting radiation beam patterns must be suffered. Where more than two phase centers are required the problem is further exaggerated. Another disadvantage is that close placement of the feedhorns  102  and  104  commonly results in mutual coupling which may affect receiver discrimination. Furthermore, since MTI relies to a great extent on channel homogeneity the, the driving network for the feedhorns becomes increasingly complex requiring the provision of facilities for the calibration of the multiple beams. Yet another disadvantage is that, again, the phase center separation can only be changed by mechanical means. Furthermore for radar antenna that require rotation at high angular velocity, the added mass and pointing stability may also become an issue.  
         [0007]     Accordingly there is a need for an antenna system that mitigates some of the above disadvantages.  
       SUMMARY OF THE INVENTION  
       [0008]     The invention provides a method and apparatus for establishing multiple phase centers for a reflector antenna by using only a single multimode feedhorn.  
         [0009]     One aspect of the present invention provides a method for extracting a received radiation beam from a feedhorn by separating the received radiation beam into least two higher order modes and combining the higher order modes in accordance with a weighting such that at least two separated phase centers are established.  
         [0010]     Another aspect of the present invention provides a feedhorn for a multiple phase center reflector antenna. The feedhorn has a horn section for receiving a beam and at least two ports coupled to the horn section, each port for extracting a higher order mode such that the beam is received via at least two separated phase centers.  
         [0011]     The invention is advantageous in that there is a minimal loss of gain in the beam pattern over that for a comparative single phase center antenna. Another advantage is that the phase center separation and constant phase beamwidths may be adjusted by adjusting the drive parameters. A further advantages arises from the fact that the multiple phase centers are extracted from a single physical aperture which is intrinsically matched, thus reducing the overall system noise temperature. Yet another advantage is that the invention may be easily adapted to provide an antenna responsive to different polarizations.  
         [0012]     Advantageously the invention allows an antenna to be operated with a single phase center for a transmission and multiple phase centers for a reception without any substantial increase in complexity.  
         [0013]     Additional advantages and features of the invention will become apparent from the description which follows and may be realized by means of the instrumentalities and combinations particularly pointed out in the appended claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     Embodiments of the present invention will now be described by way of example only with reference to the following drawings in which:  
         [0015]      FIG. 1  is a schematic view of a prior art dual phase center antenna;  
         [0016]      FIG. 2  is a perspective view of a multimode feedhorn for vertical polarization;  
         [0017]     FIGS.  3 -A to  3 -C are a series of graphical depictions of the combination of the E-field in the feedhorn shown in  FIG. 2 ;  
         [0018]      FIG. 4  is a schematic view of a dual phase center antenna in accordance with an embodiment of the invention;  
         [0019]      FIG. 5  is a graphical depiction of phase center separation and antenna gain for a series of differing amplitude ratios;  
         [0020]      FIG. 6 -A is a graphical depiction of the antenna gain pattern for TE 11  excitation of the feedhorn of  FIG. 2 ;  
         [0021]      FIG. 6 -B is a graphical depiction of the antenna phase for TE 11  excitation of the feedhorn of  FIG. 2 ;  
         [0022]      FIG. 6 -C is a graphical depiction of the antenna gain pattern for excitation of the feedhorn of  FIG. 2  in both the TE 11  and the TE 21  modes according to the weight 0.6.TE 11 +0.4.j.TE 21 ;  
         [0023]      FIG. 6 -D is a graphical depiction of the antenna phase for excitation of the feedhorn of  FIG. 2  in both the TE 11  and the TE 21  modes according to the weight 0.6.TE 11 +0.4j.TE 21 ;  
         [0024]      FIG. 7 -A is a perspective view of a feedhorn for horizontal polarization; and  
         [0025]      FIG. 7 -B is a perspective view of probe used to receive the TM 01  mode in the feedhorn shown in  FIG. 7 -A. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0026]     For an understanding of the invention, reference will now be made by way of example to a following detailed description in conjunction with the accompanying drawings in which like numerals refer to like structures.  
         [0027]     In accordance with a first embodiment of the invention,  FIG. 2  shows a multimode feedhorn  200  comprising a lower circular waveguide  202  and a circular waveguide horn section  204  joined by a tapered waveguide section  206 . A pair of rectangular waveguides  208  and  210  are transversely connected to opposite sides of the feedhorn  204 . The diameter of waveguide  202  is selected such that, at the design frequency, only the dominant TE 11  mode is able to propagate. The diameter of the horn section  204  is chosen such that a TE 21  secondary mode is able to co-exist with the TE 11  mode.  
         [0028]     The TE 11  mode is extracted via port  212  and the TE 21  mode is symmetrically extracted via transversely located waveguides  208  and  210 . The desired phase center separation is achieved by assigning amplitude and phase weightings to the TE 11  and the TE 21  modes in accordance with a pair of complex weights. The complex weights define a power ratio and relative phase between the modes and may be written as:  
                       a   ·     TE   11       +     b   ·     TE   21                     a   ·     TE   11       -     b   ·     TE   21               }           Equation   ⁢           ⁢   1             
 
 where a and b are complex numbers. 
 
         [0030]      FIG. 3 -A is a graph of gain vs. angle for the TE 11  E-field of feedhorn  200 . Similarly  FIG. 3 -B is a graph of gain vs. angle for the TE 21  E-field. The resultant E-field gain patterns for two different combinations of the TE 11  and the TE 21  modes are graphed in  FIG. 3 -C. The curve  300  represents the combination of the modes according to the weight: 
 0.5.TE 11 +0.5TE 21 .  
 Curve  300  is symmetrical around 0° indicating that for a simple in-phase combination of the TE 11  and the TE 21 , there is no phase center separation. Curve  302  depicts the combination of modes according to a complex weight: 
 0.5.TE 11 +j0.5TE 21 ,  
 i.e. pattern  302  depicts a combination of modes where the TE 21  mode is of equal in power, but out of phase by 90°, with respect to the TE 11  mode. Curve  302  indicates that the peak angular gain of the feedhorn moves away from 0° when the modes are out of phase. In the case shown, the phase center is angularly shifted to point  304 . In general while it is optimal that the TE 11  and TE 21  modes be 90° out of phase, phase center separation may also be achieved for phase differences other than 90°. 
 
         [0033]     Note that for a second complex weight: 
 
0.5.TE 11 −j0.5TE 21 , 
 
 pattern  302  will be symmetrically displaced to the opposite side of the 0° point creating a second angularly shifted phase center (not shown). 
 
         [0035]     In one embodiment received modes TE 11  and the TE 21  are extracted via feedhorn  200 . Each of the complex weights in Equation 1, when applied to the amplitude of the received modes, yields a separate phase center. Conveniently, in an embodiment of the present invention the complex weights may be algorithmically assigned by a software or hardware controller thus removing the need for any mechanical or electrical adjustments to establish a particular phase center separation. Furthermore, the complex weights may be selected for a particular set of application dependent criteria. For example in MTI radar applications it is desirable to maximize both the phase center separation and the constant phase beam width, while simultaneously minimizing losses in the antenna gain relative to the conventional reflector antenna. Other applications may require different criteria and hence different complex weights.  
         [0036]      FIG. 4  shows an antenna system comprising a multimode feedhorn  200  and a reflector antenna  100 . Feedhorn  200  is coupled via waveguides  410 ,  412  and  414  to a duplexer  416 . Waveguide  414  couples the TE 11  mode port to circular waveguide section  202  for both transmit and receive operations. Waveguides  410  and  412  are only operative during a receive operation when they extract the TE 21  component from received radiation beams  400  and  402 . Duplexer  416  is also operative to connect the transmitter  418  and the receiver  420  to the feedhorn according to synchronization signals supplied by a timer  422 . The focus of the reflector  100  is at or near point  108 .  
         [0037]     In a receive operation feedhorn  200  establishes two laterally displaced phase centers according to complex weights assigned by duplexer  416 . Essentially this implies that two separated beams  400  and  402  are received. Phase centers  404  and  406  are laterally displaced from the conventional TE 11  radiator phase center  110  by a distance d as indicated in the figure. The separation between phase centers  404  and  406  is thus  2   d  and this separation increases as the power in the TE 21  mode is increased relative to the power in the TE 11  mode as graphically depicted in  FIG. 5  (for a 90° phase difference between the modes). As can be seen from the graph, the phase centers are initially co-incident (the separation is zero) when no power provided to the TE 21  mode. The phase centers separate with increasing TE 21  power until at equal power (when the ratio is 0.5/0.5) the separation is approximately 15 cm. Note that with increasing phase center separation there is a slight reduction in the antenna gain (&lt;5 dB at equal power) indicating that a compromise may need to be established between gain and phase center separation.  
         [0038]      FIG. 6 -A is a gain plot for conventional single TE 11  mode excitation and  FIG. 6 -B is a corresponding phase plot.  FIG. 6 -C is a gain plot for a multimode extraction of TE 11  and TE 21  modes according to the weight 0.6.TE 11 +0.4.j.TE 21 . Again,  FIG. 6 -D is the corresponding phase plot. The multimode gain pattern ( FIG. 6 -C) is only slightly altered from the single mode pattern in  FIG. 6 -A, with some of the gain shifting into the side lobes  600 . For MTI where constant phase beam width is an important parameter, the actual location of the phase center is taken as the point where the constant phase beam width is maximum. This point is indicated at  602  on the phase plot of  FIG. 6 -D and as can be seen from  FIG. 6 -B and  FIG. 6 -D, the constant phase beam width is not significantly compromised for the multimode case.  
         [0039]     Antenna reciprocity dictates that the antenna system characteristics are essentially the same regardless of whether an antenna is transmitting or receiving electromagnetic energy. Accordingly, reciprocity allows most radar and communications systems to operate with only one antenna. For an MTI radar it is advantageous to transmit only the TE 11  mode i.e. the TE 21  mode is not excited during transmission. A single phase center TE  11  radiation beam is thus transmitted from the phase center at  110  in  FIG. 4 . However in the receive mode, the reflected beams are received by feedhorn  200  which separates out TE 11  and TE 21  modes into waveguides  202  and  208 / 210  respectively. By combining the TE 11  and TE 21  modes in accordance with a predetermined complex weight the antenna, in receive mode, has two apparent phase centers at  404  and  406 .  
         [0040]     The feedhorn  200  shown in  FIG. 4  results in a vertically polarized radiation pattern with the E-field oriented orthogonal to the plane of the page. In another embodiment shown in  FIG. 7 -A, the resultant radiation pattern is horizontally polarized. Horizontal polarization may have some advantages in specific applications, such as maritime surveillance, where its use reduces the false alarm rate due to sea clutter.  
         [0041]     In  FIG. 7 -A, a horizontally polarized feedhorn  700  comprises a circular a waveguide  702  and a circular waveguide horn section  704  joined by a tapered section  706 . A rectangular waveguide  708  is connected the side of circular waveguide  702 . The rectangular waveguide propagates the TE 11  mode. Waveguide  702  is dimensioned to also propagate the TM 01  mode, which has an axial electric field distribution. In this embodiment the TM 01  mode is excited by a coaxial probe  710 .  
         [0042]     The coaxial probe  710  is shown in more detail in  FIG. 7 -B. Probe  710  comprises a metal cone  712  which is coupled to a coaxial conductor  714 . The coaxial conductor comprises a central conductor  716  and an outer conductor  718 . The metal cone  712  is connected to the central conductor  716 .  
         [0043]     In an alternative embodiment the interior volume of feedhorns  200  and  700  may be filled with a dielectric material, enabling the reduction of the physical size of these elements.  
         [0044]     The feedhorn embodiments described in relation to  FIG. 2  and  FIG. 7 -A both establish a pair of separated phase centers when appropriately driven. To establish more than two phase centers, the feedhorns need to be excited by additional TE or TM modes. For example, by selecting feedhorn dimensions to permit a TE 11  , a TE 21  and a TM 01  mode to propagate, a triple phase center antenna gain pattern may be established.  
         [0045]     The reflector antenna  100  in  FIG. 4  may be any type of reflector including a dual reflector like a Cassegrain or Gregorian type. A Cassegrain antenna utilizes a hyperbolic sub-reflector to intercept reflected waves before their normal focal point and re-reflect them back to a rear mounted feedhorn. The Gregorian antenna differs from the Cassegrain in that the hyperbolic sub-reflector is replaced by an elliptical sub-reflector allowing use at longer wavelengths. Practically, the separated phase centers are realized by receiving beams via a reflector antenna and focusing these beams into a multimode feedhorn. However the reflector part of the antenna is not necessarily altered, the change being made to the feedhorn in order to allow multiple modes to propagate therein. Accordingly, many different types of reflector may be used to couple the beams to the multimode feedhorn, and the selection of an appropriate complex weight will establish a particular phase center separation for the combination of feedhorn and receiving reflector.  
         [0046]     As will be apparent to those skilled in the art in light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the spirit or scope thereof.