Abstract:
A lensed antenna system is provided. The lensed antenna system includes a first column of radiating elements having a first longitudinal axis and a first azimuth angle, and, optionally, a second column of radiating elements having a second longitudinal axis and a second azimuth angle, and a radio frequency lens. The radio frequency lens has a third longitudinal axis. The radio frequency lens is disposed such that the longitudinal axes of the first and second columns of radiating elements are aligned with the longitudinal axis of the radio frequency lens, and such that the azimuth angles of the beams produced by the columns of radiating elements are directed at the radio frequency lens. The multiple beam antenna system further includes a radome housing the columns of radiating elements and the radio frequency lens. There may be more or fewer than two columns of radiating elements.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. application Ser. No. 14/244,369 filed Apr. 3, 2014 and U.S. Provisional Application Ser. No. 61/875,491 filed Sep. 9, 2013, which are hereby incorporated by reference in their entirety. 
    
    
     BACKGROUND 
     The present inventions generally relate to radio communications and, more particularly, to multi-beam antennas utilized in cellular communication systems. 
     Cellular communication systems derive their name from the fact that areas of communication coverage are mapped into cells. Each such cell is provided with one or more antennas configured to provide two-way radio/RF communication with mobile subscribers geographically positioned within that given cell. One or more antennas may serve the cell, where multiple antennas commonly utilized are each configured to serve a sector of the cell. Typically, these plurality of sector antennas are configured on a tower, with the radiation beam(s) being generated by each antenna directed outwardly to serve the respective cell. 
     A common wireless communication network plan involves a base station serving three hexagonal shaped cells or sectors. This is often known as a three sector configuration. In a three sector configuration, a given base station antenna serves a 120° sector. Typically, a 65° Half Power Beamwidth (HPBW) antenna provides coverage for a 120° sector. Three of these 120° sectors provide 360° coverage. Other sectorization schemes may also be employed. For example, six, nine, and twelve sector sites have been proposed. Six sector sites may involve six directional base station antennas, each having a 33° HPBW antenna serving a 60° sector. In other proposed solutions, a single, multi-column array may be driven by a feed network to produce two or more beams from a single aperture. See, for example, U.S. Patent Pub. No. 20110205119, which is incorporated by reference. 
     Increasing the number of sectors increases system capacity because each antenna can service a smaller area. However, dividing a coverage area into smaller sectors has drawbacks because antennas covering narrow sectors generally have more radiating elements that are spaced wider than antennas covering wider sectors. For example, a typical 33° HPBW antenna is generally two times wider than a common 65° HPBW antenna. Thus, costs and space requirements increase as a cell is divided into a greater number of sectors. 
     To solve these problems, antennas have been developed using multi-beam forming networks (BFN) driving planar arrays of radiating elements, such as the Butler matrix. BFNs, however, have several potential disadvantages, including non-symmetrical beams and problems associated with port-to-port isolation, gain loss, and a narrow band. Classes of multi-beam antennas based on a classic Luneberg cylindrical lens (Henry Jasik: “Antenna Engineering Handbook”, McGraw-Hill, New York, 1961, p. 15-4) have tried to address these issues. And while these lenses can have better performance, the costs of the classic Luneberg lens (a multi-layer, cylindrical lens having different dielectric in each layer) is high and the process of production is extremely complicated. Additionally, these antenna systems still suffer from several problems, including beam width stability over the wide frequency band and high cross-polarization levels. Accordingly, there is a need for an antenna system that solves these problems to provide a high performance multi-beam base station antenna at an affordable cost. 
     SUMMARY OF THE INVENTION 
     In one example of the present invention, a multiple beam antenna system is provided. The multiple beam antenna system includes a first column of radiating elements having a first longitudinal axis and a first azimuth angle, a second column of radiating elements having a second longitudinal axis and a second azimuth angle, and a radio frequency lens. The radio frequency lens has a third longitudinal axis. The radio frequency lens is disposed such that the longitudinal axes of the first and second columns of radiating elements are aligned with the longitudinal axis of the radio frequency lens, and such that the azimuth angles of the beams produced by the columns of radiating elements are directed at the radio frequency lens. One or more columns of radiating elements may be slightly tilted in elevation plane against the axis of radio frequency lens. The multiple beam antenna system further includes a radome housing the columns of radiating elements and the radio frequency lens. 
     There may be more or fewer than two columns of radiating elements. In one example, the multiple beam antenna system includes three columns of radiating elements. Each of the columns of radiating elements produces a beam having a −10 dB beam width of approximately 40° after passing through the radio frequency lens. The columns of radiating elements are arranged such that the beams have azimuth angles of −40°, 0°, 40°, respectively, relative to boresight of the antenna system. 
     In one example, the radio frequency lens is a cylinder having a diameter in the range of approximately 1.5-5 wavelengths of the nominal operating frequency of the columns of radiating elements. The radio frequency lens may be longer than the columns of radiating elements. 
     In another aspect of the present invention, the radio frequency lens comprises dielectric material having a substantially homogenous dielectric constant, which may be in the range of 1.5 to 2.3. The radio frequency lens may comprise a plurality of dielectric particles. In another aspect of the invention, the radiating elements are dual polarized radiating element, having dual linear +/−45° polarization. 
     In another aspect of the invention, the radiating elements are configure to have azimuth beam width monotonically decreasing with increasing of frequency. For example, the radiating elements may comprise a box-type dipole array. The radiating elements may further include one or more directors for stabilizing a beam formed by lensed antenna. 
     In another aspect of the invention, each of the columns of elements may comprise two or more arrays of radiating elements adapted to operate in different frequency bands. For example, a column of radiating elements may include high band elements and low band elements. In one example, the number of high band radiating elements is approximately twice the number of low band elements. The high band radiating elements may produce a beam having azimuth beamwidth that is narrower than a beamwidth of a beam produced by the plurality of lower band elements before passing through the radio frequency lens. This allows the beams after passing through the radio frequency lens to be of approximately equal beamwidths. 
     In one example, the high band radiating elements include directors to narrow the beamwidth. In another example, the high band elements are located in two lines in parallel to line of low band elements to narrow the beamwidth produced by the high band elements. 
     In another aspect of the invention, the multiple beam antenna system may further include a sheet of dielectric material disposed between the radio frequency lens and one or more of the columns of radiating elements. The sheet of dielectric material may further include wires disposed on the sheet of dielectric material. The sheet of dielectric material may further include slots disposed on the sheet of dielectric material. A second sheet of dielectric material may be included for improving port-to port isolation of multi-beam antenna. 
     In another aspect of the present invention, the multiple beam antenna system may further include a secondary radio frequency lens disposed between the columns of radiating elements and the radio frequency lens. The secondary lens may comprise a dielectric rod. Alternatively, the secondary lens may comprise dielectric blocks located at each radiating element. 
     The present invention is not necessarily limited to multi-beam antennas. In another example of the present invention, an antenna system may include at least one column of radiating elements having a first longitudinal axis and an azimuth angle; a radio frequency lens comprising a plurality of dielectric particles and having a second longitudinal axis, the radio frequency lens disposed such that the second longitudinal axis is substantially aligned with the first longitudinal axis and the azimuth angle is directed at the second longitudinal axis; and a radome housing the column of radiating elements and the radio frequency lens. 
     The plurality of dielectric particles may incorporate wires. In another example, the dielectric particles may comprise at least two types of particles uniformly distributed in the volume of the radio frequency lens. In another example, some of the dielectric particles contain left handed material. 
     In another aspect of the invention, the radio frequency lens (either for single beam or multi-beam antennas) may include two different kinds of dielectric material with different anisotropy. For example, one of the dielectric materials has anisotropy. In another example, the two different kinds of dielectric material comprise two different anisotropic materials. In another example, the two anisotropic materials are mixed in unequal proportions. In another example, the two anisotropic materials have different values of dielectric constant in a direction of the second longitudinal axis and an axis perpendicular to the second longitudinal axis. 
     In another aspect of the invention, the radio frequency lens (either for single beam or multi-beam antennas) may include a reflector covering a back area of the antenna system. The antenna may further include an absorber located between the column of radiating elements and the reflector. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1 a    is a diagram showing an exploded view of an exemplary lensed multi-beam base station antenna system; 
         FIG. 1 b    is a diagram showing a cross-sectional view of an exemplary assembled lensed multi-beam base station antenna system; 
         FIG. 2  is a diagram showing an exemplary linear array for use in a lensed multi-beam base station antenna system; 
         FIG. 3 a    is a diagram showing a top view of an exemplary box-style dual polarized antenna radiating element; 
         FIG. 3 b    is a diagram showing a side view of an exemplary box-style dual polarized antenna radiating element; 
         FIG. 3 c    is a diagram of equivalent dipoles of an exemplary box-style dual polarized antenna radiating element; 
         FIG. 4  is a diagram showing measured plots of antenna azimuth beamwidth against frequency for an exemplary assembled lensed multi-beam base station antenna system; 
         FIG. 5 a    is a diagram showing a first example of a secondary lens for use in a lensed multiple beam base station antenna system for azimuth beam stabilization; 
         FIG. 5 b    is a diagram showing a second example of a secondary lens for use in a lensed multiple beam base station antenna system for azimuth beam stabilization; 
         FIG. 5 c    is a diagram showing a third example of a secondary lens for use in a lensed multiple beam base station antenna system for azimuth beam stabilization; 
         FIG. 6  is a diagram showing an exemplary system of crossed directors for use in a lensed multi-beam base station antenna system; 
         FIG. 7 a    is a diagram showing a first example of an antenna compensator for use in a lensed multi-beam base station antenna system; 
         FIG. 7 b    is a diagram showing a second example of an antenna compensator for use in a lensed multi-beam base station antenna system; 
         FIG. 7 c    is a diagram showing a third example of an antenna compensator for use in a lensed multi-beam base station antenna system; 
         FIG. 7 d    is a diagram showing a fourth example of an antenna compensator for use in a lensed multi-beam base station antenna system; 
         FIG. 7 e    is a diagram showing a fifth example of an antenna compensator for use in a lensed multi-beam base station antenna system; 
         FIG. 7 f    is a diagram showing a sixth example of an antenna compensator for use in a lensed multi-beam base station antenna system; 
         FIG. 8  is a diagram showing a measured elevation pattern for an exemplary multi-beam base station antenna system with and without a lens; 
         FIG. 9  is a diagram showing a measured azimuth co-polar and cross-polar radiation patterns for a central antenna beam of an exemplary three-beam lensed based station antenna system. 
         FIG. 10  is a diagram showing a measured radiation patterns in azimuth plane for all three beams of an exemplary three-beam lensed base station antenna system; 
         FIG. 11  is a diagram showing nine sector cell coverage by three exemplary three-beam lensed base station antenna systems. 
         FIG. 12  is a diagram showing a side view of another exemplary lensed base station antenna with cylindrical lens having hemispherical ends; 
         FIG. 13  is a diagram showing a column of radiating elements of two different frequency bands for use in a dual band lensed multi-beam base station antenna system; 
         FIG. 14  is a diagram showing an another exemplary column of radiating elements of two different frequency bands for use in a dual-band lensed multi-beam base station antenna system; and 
         FIG. 15  is a diagram showing another exemplary column of radiating elements of two different frequency bands for use in a dual-band lensed multi-beam base station antenna system. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings, and initially to  FIG. 1 a , 1 b   , an exploded view of one embodiment of a multi-beam base station antenna system  10  is shown in  FIG. 1 a   , and its cross-section is shown in  FIG. 1 b   . In its simplest form, the multi-beam base station antenna system  10  includes one or more linear arrays of radiating elements  20   a ,  20   b , and  20   c  (also referred to as “antenna arrays” or “arrays” herein) and a radio frequency lens  30 . Arrays  20  may have approximately the same length with lens  30 . The multi-beam base station antenna system  10  may also include a first compensator  40 , a second compensator  42 , a secondary lens  43  (shown in  FIG. 1 b   ), a reflector  52 , radome  60 , end caps  64   a  and  64   b , absorber  66  and ports (RF connectors)  70 . In description below, azimuth plane is orthogonal to axis of radio frequency lens  30 , and elevation plane is in parallel to axis of lens  30 . 
     In the embodiment shown in  FIG. 1 a , 1 b   , the radio frequency lens  30  focuses azimuth beams of arrays  20   a ,  20   b , and  20   c , changing, for example, their 3 dB beam widths from 65° to 23°. In the embodiment shown in  FIG. 1 a , 1 b   , three linear antenna arrays  20   a ,  20   b , and  20   c  are shown, but any number and/or shape of arrays  20  may be used. The number of beams of a multi-beam base station antenna system  10  is the same as number of ports  70  of arrays  20   a ,  20   b , and  20   c . In  FIG. 1 a , 1 b   , each of arrays  20  has 2 ports, one for +45° and another for −45° polarization. 
     In operation, the lens  30  narrows the HPBW of the antennas arrays  20   a ,  20   b , and  20   c  while increasing their gain (by 4-5 dB for 3-beam antenna shown in  FIG. 1 ). For example, the longitudinal axes of columns of radiating elements of the antenna arrays  20   a ,  20   b , and  20   c  can be parallel with the longitudinal axis of lens  30 . In other embodiments, axis of antenna arrays  20  can be slightly tilted (2-10°) to axis of lens  30  (for example, for better return loss or port-to-port isolation tuning), but axis of an array and axis of lens are still located in the same plane. All antenna arrays  20  share the single lens  30  so each antenna array  20   a ,  20   b , and  20   c  has their HPBW altered in the same manner. 
     The multi-beam base station antenna system  10  as described above may be used to increase system capacity. For example, a conventional 65° HPBW antenna could be replaced with a multi-beam base station antenna system  10  as described above. This would increase the traffic handling capacity for the base station. In another example, the multi-beam base station antenna system  10  may be employed to reduce antenna count at a tower or other mounting location. 
     A cross-sectional view of an assembled multi-beam base station antenna system  10  is illustrated in  FIG. 1 b   .  FIG. 1 b    is also illustrating how 3 beams are formed (BEAM 1, BEAM 2, BEAM 3). The azimuth position angle of the beams provided by the antenna arrays  20   a ,  20   b , and  20   c  are shown by dotted lines in  FIG. 1 b   . Preferably, the azimuth angle for each beam will be approximately perpendicular to the reflector of the array  20 . For example, in the embodiment shown in  FIG. 1   b, − 10 dB beamwidth of each beam is close to 40° and the directions of beams are −40°, 0°, 40°, respectively. 
     One difference of lens  30  compared to known Luneberg lenses is its internal structure. As shown in  FIG. 1 b   , the dielectric constant (“Dk”) of lens  30  is homogenous, in the contrast with known Luneberg lenses which have multiple layers with different Dk. A lens  30  having a homogenous Dk is generally easier and less expensive to manufacture. Also, it can be more compact, having 20-30% less diameter. In one embodiment, a lens having a Dk of approximately 1.8 and diameter of about 2 wavelengths λ focuses beams and provides azimuth patterns with low sidelobes (less than −17 dB), as shown in  FIGS. 10 and 11 . In the case of an antenna system  10  having three beams, a lens  30  having a diameter of approximately 2 wavelengths and Dk=1.9 provides a beam width about 30% less than an equivalent prior art antenna system including a planar array based on the Butler matrix type BFN, as one can see from measured HPBW: 
     
       
         
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Lensed antenna 
                 Prior art 
                 Narrowing coeff. 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 1.71 GHz  
                 25.9 
                 33.3 
                 29% 
               
               
                   
                 1.8 GHz 
                 24.9 
                 31.7 
                 27% 
               
               
                   
                 1.9 GHz 
                 23.3 
                 30.0 
                 29% 
               
               
                   
                   
               
             
          
         
       
     
     It was also confirmed that homogeneous cylindrical lens (when diameter of lens is 1.5-5 wavelength in free space) has about 1 dB more directivity compare to multi-layer Luneberg lens with the same diameter and compare to predicted by geometric optics. Performance of dielectric cylinder in this case can be explained as combination of dielectric travelling wave antenna (end fire mode) combined with lens mode (focusing mode) of operation. The 1.5-5 wavelength diameter embodiment is applicable for forming 2 to 10 beams, which includes most of current multi-beam applications for base station antennas. Compactness is one of the key advantages of a proposed multi-beam base station antenna system; the antenna is narrower compared to known multi-beam solutions (based on Luneberg lens or Butler matrix). 
     A conventional Luneberg lens is a spherically symmetric lens that has a varying index of refraction inside it. Here, the lens  30  is preferably shaped as a circular cylinder (if, for example, each beam need the same shape) and is homogeneous (not multilayer) as shown in  FIGS. 1 a  and 1 b   . Alternatively, or additionally, the lens  30  may comprise an elliptical cylinder, which may provide additional performance improvements (for example, the sidelobes reduction of a central beam). Other shapes may also be used. 
     In some embodiments, the lens  30  may comprise a structure such as the ones described in U.S. patent application Ser. No. 14/244,369, filed Apr. 3, 2014, which is hereby incorporated by reference in its entirety. As described in that application, the lens  30  may comprise various segmented compartments to provide additional mechanical strength. 
     The lens  30  may be made of particles or blocks of dielectric material. The dielectric material particles focus the radio-frequency energy that radiates from, and is received by, the linear antenna arrays  20   a ,  20   b , and  20   c . The dielectric material may be artificial dielectric of the type described in U.S. Pat. No. 8,518,537 which is incorporated by reference. In one example, the dielectric material particles comprise a plurality of randomly distributed particles. The plurality of randomly distributed particles is made of a lightweight dielectric material. The range of densities of the lightweight dielectric material can be, for example, 0.005 to 0.1 g/cm 3 . At least one needle-like conductive fiber is embedded within each particle. By varying number/orientation of conductive fibers inside particle, Dk can be vary from 1 to 3. Where there are at least two conductive fibers embedded within each particle, the at least two conductive fibers are in an array like arrangement, i.e. having one or more row that include the conductive fibers. Preferably, the conductive fibers embedded within each particle are not in contact with one another. 
     Base station antennas are subject to vibration and other environmental factors. The use of compartments assists in the reduction of settling of the dielectric material particles, increasing the long term physical stability and performance of the lens  30 . In addition, the dielectric material particles may be stabilized with slight compression and/or a backfill material. Different techniques may be applied to different compartments, or all compartments may be stabilized using the same technique. 
     Antennas with traditional Luneburg cylindrical lenses can suffer from high cross-polarization levels. The use of a isotropic (homogeneous) dielectric cylinder can also provide depolarization of the incident EM wave based on its geometry (nonsymmetrical for vertical (V) and horizontal (H) components of the electric field). When the EM wave crosses a cylinder, polarization along the axis of cylinder (“VV”) will have a bigger phase delay than polarization perpendicular to cylinder axis (“HH”), causing depolarization. 
     This depolarization can be reduced by constructing a radio frequency lens  30  with dielectric materials having different DK for the VV and HH directions. To compensate for depolarization, the DK for VV polarization must be less than the DK for HH polarization. The difference in DK, may depend on a variety of factors including the size of cylinder and the relationship between beam wavelength and the diameter of the cylinder. In other words, reduction of the naturally occurring depolarization caused by a cylindrically shaped lens  30  can be achieved using anisotropic dielectric materials. Similarly, circular polarization can be created, if needed, on the other hand by using anisotropic material to create a difference in phase of 90°. 
     Anisotropic material can be, for example, the dielectric particles having conductive fibers inside described in U.S. Pat. No. 8,518,537, which is incorporated by reference. By mixing, or arranging, different particles with different compositions and/or shapes, different values of DK in direction of parallel and perpendicular to axis of cylinder can be achieved. For example, an incident wave linearly polarized with polarization +/−45° will have a cross-polarization level of about −8 dB after passing through a dielectric cylinder with a DK of 2 and a diameter of approximately two wavelengths, This level may be unacceptable for certain commercial applications where a cross-polarization level of approximately −15 dB is desired. This increased cross-polarization is occurring because the VV component of the electric field has a phase difference of about −30° compare to the HH component and the elliptical polarization is created with an axial ratio of about 8 dB. Artificial dielectric particles based on conductive fibers such as those described in U.S. Pat. No. 8,518,537, which is hereby incorporated by reference in its entirety, have a +20° phase difference between H and V field components (i.e. a phase difference in the opposite direction). By mixing regular dielectric with artificial dielectric, phase differences between VV and HH components can be obtained close to 0° and antenna cross-polarization can be minimized (see  FIG. 10 ) and Spec&lt;−15 dB can be met in wide frequency band, say 1.7-2.7 GHz. In one embodiment, a mix of approximately 40% regular dielectric and 60% artificial dielectrics (called also in literature left handed material for its unusual characteristic) are used. Other ratios also may be used. 
     Referring to  FIG. 2 , an exemplary linear antenna array  200  for use in a multi-beam base station antenna system  10  is shown in more detail. The array  200  includes a plurality of radiating elements  210 , reflector  220 , phase shifter/divider  230 , and two input connectors  70 . The phase shifter/divider  230  may be used for beam scanning (beam tilting) in the elevation plane. Each radiating element  210  includes two linear orthogonal polarization (slant +1-45°  311 ,  312 ), as shown in more detail in  FIG. 3 c   , where 4 equivalent dipoles  313 - 316  are shown forming two orthogonal polarization vectors  311 ,  312 . Four dipoles  310  are arranged in a square, or in the “box”, as shown in  FIG. 3 a    and supported by feed stalks, as illustrated in  FIG. 3 b   . The configuration of radiating element  210  and reflector  220  provide a special shape of antenna pattern in the azimuth plane with a close to linear dependence of Azimuth beamwidth with frequency. For example, for a three beam antenna shown in  FIG. 1 , measured −3 dB beamwidth of radiating element  210  is plotted against frequency in  FIG. 4  (plot  410 ) and vary from 62° (1.7 GHz) to 46° (2.7 GHz). As a result of lens  30 , the azimuth beamwidth of the total antenna is stabilized in the frequency band (see plots  430  for 3 dB beamwidth and  420  for −10 dB beamwidth). As one can see from plot  420 , −10 dB beamwidth is very close to desirable 40°: 40+/−3° was measured over 45% bandwidth). Beam width and beam position stabilization is important for multi-beam antennas to provide appropriate cell coverage. If a radiating element without this specific frequency dependence is used, beam variations of total antenna will be too much, i.e., −10 dB beamwidth may vary from 30° to 50° as a function of frequency, and illumination of assigned sector will be very poor. For example, these may be big gaps (up to 30 dB at the highest frequency) between sectors (drop signal) or big overlapping between sectors at lower frequency, which is also not acceptable because of interference. 
     The effect of azimuth beam stabilization over frequency can be explained by  FIG. 1 b   , where azimuth beamwidth of is written φ for antenna arrays  20  and Θ for lens  30 . The radio frequency lens is providing a focusing effect, so φ&gt;Θ. Θ is in inverse proportion to frequency f and also in inverse proportion to illuminated lens aperture S: Θ=k 1 /fS, where k 1  coefficient depends on amplitude and phase distribution (see J. D. Kraus, Antennas, McGraw-Hill, 1988, p. 846), and S=R 2 sin(φ/2) 
     For beam stabilization, the condition Θ(f 1 )=Θ(f 2 ) should be satisfied, or:
 
sin [(φ( f   1 )/2]/sin [(φ( f   2 )/2 ]=f   2   /f   1   (1)
 
As one can see from equation (1), for lensed antenna  10  beam stabilization, linear antennas  20   a ,  20   b ,  20   c  should have azimuth beam width monotonically decreasing with frequency. For small φ, φ(f 1 )/(φ(f 2 )≈f 2 /f 1 , i.e., azimuth beamwidth of antenna element  210  is in inverse proportion to frequency. This simplified analysis illustrates the importance of the frequency dependence of azimuth beam width of linear antennas  20 . For example, to get maximum gain for lowest frequency, the entire focus area of should be used, or S=D, where D is diameter of lens. It means that for optimal wideband/ultra-wideband performance, a full lens should be illuminated for lowest frequency of bandwidth, and central area for highest frequency.
 
     Another example using a “box” or square radiating element is shown in U.S. Pat. No. 6,333,720, which is hereby incorporated by reference in its entirety. An array of Box-type four dipole radiating elements has monotonically decreasing beamwidth with frequency because array factor is linearly reverse to frequency. When a box style radiating element is used without a lens, the array factor primarily contributes to its achieving significant frequency dependence (see plot  410  in  FIG. 4 ). As shown in  FIG. 4 , with proper selection of antenna element ( 4  dipoles arranged in square or box element), the Azimuth beamwidth of the lensed antennas can be stabilized (plots  420 ,  430 ). 
     Furthermore, linear antenna array can have “box” elements of different frequency bands, interleaved with each other as shown in U.S. Pat. No. 7,405,710 (which is incorporated by reference), where first box-type dipole assembly is coaxially disposed within a second box-type dipole assembly and located in one line. This allows a lensed antenna to operate in two frequency bands (for example, 0.79-0.96 and 1.7-2.7 GHz). For similar beam widths of lensed antenna in both bands, central box-type element (high band element) should have directors ( FIG. 6 ). In this case, a low band element may have, for example, a HPBW of 65-50°, and a high band element may have a HPBW of 45-35°, and in the result, the lensed antenna will have stable HPBW of about 23° (and beam width about 40° by −10 dB level) across both bands. 
     The multi-beam base station antenna system may include one or more secondary lenses. These secondary lenses  43  can be placed between array  20   a ,  20   b , and  20   c  and lens  30  for further azimuth beamwidth stabilization, as shown in  FIG. 1B . The secondary lenses may comprise dielectric objects, such as rods  510  and  520  or cubes  530  as shown in  FIGS. 5 a -5 c   , respectively. Other shapes may also be used. 
     As shown in  FIG. 6 , directors  610  can be also placed on the top of radiators for further beamwidth stabilization in the wide frequency band. The directors  610  can vary in length, which can be selected, for example, so as to narrow the radiation pattern for the higher frequency band while leaving the radiation pattern in the lower portion of frequency band unchanged. This configuration can result in more a sharp dependence of azimuth pattern of the arrays  20   a ,  20   b , and  20   c  against frequency. 
     By utilizing a combination of specially selected element  210  shapes, dielectric pieces/secondary lenses  510 ,  520 ,  530 , and/or directors  610  above array elements  210 , a stable pattern in the very wide frequency band can be provided (e.g. greater than 50%). For example, as shown in  FIG. 4 , a −10 dB beamwidth for a three-beam antenna  420  is 40+/−4° in 1.7-2.7 GHz band (40° is optimal for sector coverage). In prior art, this beamwidth can vary from 28-45°, which is not acceptable for cell sectors because too narrow beams can lead to drop signals in beam-crossing directions, and wide beams)(&gt;45° can lead to undesirable interference between sectors due to overlapping. 
     As shown in  FIG. 8 , the use of a cylindrical lens significantly reduces grating lobes (and other far sidelobes) in the elevation plane (compare plot  810  is for antenna without lens, and plot  820  for the same antenna with lens). Typically, 5 dB grating lobe reduction was observed for 3-beam antenna shown in  FIG. 1 . The 5 dB grating lobe reduction is correlated with 5 dB gain advantage of lensed antenna  FIG. 1  against original linear arrays  20 . The grating lobe&#39;s improvement is due to the lens focusing the main beam only and defocusing the far sidelobes. This allows increasing spacing between antenna elements. For prior art, the spacing between array elements depends on grating lobe and is selected by criterion: d max /λ&lt;1/(sin θ 0 +1), where d max  is maximum allowed spacing, λ—wavelength and θ 0  is scan angle (see Eli Brookner, Practical Phased Array Antenna Systems, Artech House, 1991, p. 4-5). In lensed antenna, spacing d max  can be increased: d max /=1.2˜1.3[1/(sin θ 0 +1)]. So, the lens  30  allows the spacing between radiating elements  210  to be increased for the multi-beam base station antenna system  10  while reducing the number of radiating elements by 20-30% for comparable prior art systems. This results in additional cost advantages for the multi-beam base station antenna system  10 . 
     As shown in  FIG. 7 a   , compensators  40  and  42  are, in the simplest case, dielectric sheets  710  with certain dielectric constant and thickness. The Dk and thickness of the compensator  40  and  42  can be selected for wideband return loss tuning (&gt;15 dB at ports  70 ) and providing desirable port-to-port isolation between all ports  70  (usually need &gt;30 dB). Also, second compensator  42  may also compensate reflection from the outer boundary of lens  30 , for further improvement of port-to-port isolation. Compensators  40  and  42  can have a variety of shapes, such as shapes  710 ,  720 ,  730 ,  740 ,  750 , and  760  shown in  FIGS. 7 a   - 7   f.    
     Alternatively, or additionally, short conductive dipoles (with length&lt;&lt;λ) may also be used on the surface of compensators  40  and  42  to compensate depolarization of isotropic dielectric cylinder. When an EM wave crosses the dipole, maximum phase delay will occur when vector E is parallel to the dipoles and minimum when perpendicular. So, the process of depolarization can be controlled by placing different orientations of wires on compensators  40  and  42 . For example, depolarization of linear polarization can be decreased (axial ratio&gt;20 dB), or, if needed, can be converted to circular (axial ratio close to 0 dB). For example, compensators  720  and  730  includes short wires printed on a dielectric sheet, as shown in  FIGS. 7 b  and 7 c   , respectively; compensator  720  has lateral wires,  730  has longitudinal wires. Referring to  FIGS. 7 d  and 7 e   , similar functions for polarization tuning can be achieved with compensators  740 .  750  having slots in the dielectric. In another example, compensator  760  comprises thin dielectric rods, as shown in  FIG. 7 f   . So, compensators  42 ,  40  are used for return loss and port-to-port isolation improvements and (or) antenna polarization control. Alternatively, or additionally, wires may be disposed on the surface or lens  30  for providing similar benefits. 
     End caps  64   a  and  64   b , radome  60 , and tray  66  provide antenna protection. Radome  60  and tray  66  may be made as one extruded plastic piece. Other materials and manufacturing processes may also be used. In some embodiments, tray  66  is made from metal and acts as an additional reflector to improve antenna back lobes and front-to-back ratio. In some embodiments, an RF absorber (not shown) can be placed between tray  66  and arrays  20   a ,  20   b , and  20   c  for additional back lobes&#39; improvement. The lens  30  is spaced such that the apertures of the antennas arrays  20   a ,  20   b , and  20   c  point at a center axis of the lens  30 . Mounting brackets  53  are used for placing antenna on the tower. 
     In  FIG. 8 , radiation patterns of the multi-beam base station antenna system  10  of  FIG. 1  is shown, measured in elevation plane (plot  820 ) for beam tilt 10° and d/2=0.92. For comparison, a radiation pattern without a radio frequency lens  30  is shown (plot  810 ) which has 5 dB higher grating lobe. In  FIGS. 9, 10 and 11 , radiation patterns of the multi-beam base station antenna system  10  of  FIG. 1  are shown, measured in azimuth plane. In  FIG. 9 , co-polar ( 910 ) and cross-polar ( 920 ) azimuth patterns are shown for central beam. As one can see from  FIG. 9 , good antenna performance is achieved, including low cross-polarization level (&lt;−20 dB), low sidelobes (&lt;−18 dB) and low back lobes. In contrast, prior art analogous antenna based on classical Luneberg has cross-polarization level 10-12 dB higher. In wireless communications, low cross-polarization of antenna benefits to diversity gain and MIMO performance, and reduction of side and back lobes reduce the interference. In  FIG. 10 , all three beams are shown together ( 1010 ,  1020 , 1030 ). Please note that all three beam have the same shape, which is an advantage compared to prior art Butler matrix multi-beam solutions, where outer beams are not symmetrical and have different shape and gain compare to central beam.  FIG. 11  illustrates a configuration of three multi-beam base station antenna systems of  FIG. 1  providing uniform 360° cell coverage with low overlap between beams, which is desirable for LTE. 
     In  FIG. 1 , radio frequency lens  30  has flat top and bottom areas, as it is convenient from mechanical/assembling point of view (simple flat end cups  64   a ,  64   b  can be used). But in some cases, as shown in  FIG. 12 , a radio frequency lens  1200  with rounded (hemispherical) ends  1210 ,  1220  may be used. For simplicity, only one linear array  20  is shown in  FIG. 12 , which can be analogous to linear array  20  presented in  FIG. 2 . Hemispherical lens ends  1210 ,  1220  provide additional focusing in elevation plane for edge radiating elements  1230 ,  1240  resulting in advantage of obtaining of additional gain ΔG≈10 log(1+D/L), [dB], where D is lens diameter. For a three beam antenna as shown in  FIG. 1 , ΔG≈1 db. Configuration of  FIG. 12  can be an economically effective way for improving antenna gain, because the additional gain ΔG is obtained without increasing lengths of arrays  20  and number of their radiating elements. 
     In addition to single band antennas, the dual and/or multiband antennas are in demand. Such antennas may include, for example antennas providing ports for transmission and reception in the, 698-960 MHz+1.7-2.7 GHz bands, or, for example, 1.7-2.7 GHz+3.4-3.8 GHz. Use of cylindrical lenses gives good opportunity for creating dual-band multi-beam BSA. A homogeneous cylindrical radio frequency lens works well when its diameter D=1.5−6λ (wavelength in free space). This is applicable for both BSA dual-band cases mentioned above. A challenge is providing the same the azimuth beamwidth for all bands and all beams. To get this, azimuth beam width of a low band antenna array (before passing through a radio frequency lens) should be wider compare to a high band antenna array, approximately in proportion of central frequency ratio between the two bands. 
     In  FIG. 13-15 , solutions for dual-band antenna arrays (which are part of multi-beam lensed antenna) are schematically shown. These dual band arrays contain radiators of 2 different bands and these arrays can be placed around lens in similar way as it is shown in  FIG. 1  for single band arrays. 
     In  FIG. 13 , lower band (LB) radiating elements  1300  and higher band (HB) radiating elements  210  are placed in the same line in the center of reflector  1310 . Both LB and HB radiating elements are box-type dipole array to provide azimuth beam width monotonically decreasing azimuth beam with increasing of frequency. Also, each HB element  210  has directors  610  which help HB azimuth beamwidth to be narrower, than LB azimuth beamwidth. In the result, after passing the radio frequency lens  30 , LB and HB radiation patterns have similar beamwidth (as it was detailed discussed above). If, for example, for array  1310  LB azimuth HPBW is 65°-75°, HB can be about 40°, and the resulting HPBW of multi-beam lensed antenna is about 23° in both bands. 
     In  FIG. 14 , another dual band array is shown, with another approach for narrowing HB azimuth beam. Inside LB element  1300 , single HB element  210  is placed, but between LB elements, a pair of HB elements  1400  are placed. These HB elements  1400  can be, for example, crossed dipoles, as shown in  FIG. 14 . By variation of spacing between elements  1400  in azimuth plane, azimuth HB beam can be adjusted to required width, so that beamwidth after passing through the radio frequency lens  30  is of a desired HPBW. 
     In  FIG. 15 , one more dual band array is shown. Pairs of HB elements  1400  are connected by 1:2 power divider  1500  and feedlines  1510  to phase shifter/divider  230 . By variation of spacing between elements  1400  in azimuth plane, azimuth HB beam can be adjusted to required width, for optimal covering of cell sector. 
     While the foregoing examples are described with respect to three beam antennas, additional embodiments including, for example, 1-, 2-, 4-, 5,-6, N-beam antennas sharing a single lens are also contemplated. Additional configurations are also contemplated. 
     So, proposed multi-beam antenna solution, compared to known Luneberg lens and Butler matrix feed network solutions has reduced cost, has less weight, is more compact and has better RF performance, including inherently symmetrical beams and improved cross-polarization, port-to-port isolation, and beam stability. 
     Though the invention has been described with respect to specific preferred embodiments, many variations and modifications will become apparent to those skilled in the art upon reading the present application. For example, the invention can be applicable for radar multi-beam antennas. The invention is therefore that the apprehended claims be interpreted as broadly as possible in view of the prior art to include all such variations and modifications.