Abstract:
Several electromagnetic scattering structures are designed to improve antenna radiation patterns. The electromagnetic scattering structure has a conductive layer with certain patterns, and is applied on the radome of the base-station sector antenna. The electromagnetic waves radiating from the antenna therein induce scattering effects, which, together with the electromagnetic diffractions from the rear metal panel of the antenna, can substantially reduce the back lobe and the fields in regions not covered by the antenna. Thus, the antenna radiation patterns are improved so that a lower possibility of co-channel interferences between adjacent base stations can be achieved and therefore better efficiency of the base-station coverage also can be obtained.

Description:
RELATED APPLICATION 
   The present application is based on, and claims priority from, Taiwanese Application Ser. No. 92126026, filed Sep. 19, 2003, the disclosure of which is herein incorporated by reference herein in its entirety. 
   BACKGROUND 
   1. Field of Invention 
   The present invention relates to an antenna apparatus. More particularly, the present invention relates to method and apparatus for improving antenna radiation patterns. 
   2. Description of Related Art 
   Mobile telephones are portable and wireless telephone devices installed on conveyances, such as vehicles and ships, or carried by a user. Mobile telephones are different from wireless extensions of the wired telephones or long distance radio transceivers. Mobile telephones provide users with the benefits of the same functions of and greater convenience than wired telephones. Connecting with international direct dialing, mobile telephone users can communicate with any other person in the world within coverage of a mobile telephone system. 
   A mobile telephone network system comprises mobile telephone switching offices (MTSO), base stations (BS) and mobile stations (MS). Generally, the mobile telephone network system may have one or more than one MTSO&#39;s, which comprise switches and communication devices, and govern a certain number of base stations. The communication devices of the MTSO are connected to the base stations. 
   A cellular wireless network is composed of several cells, and every cell has its own transmitting/receiving module (TRM), control channels (CC) and communication channels. The base station services one or more cells according to coverage requirements and with different antenna designs. The quantity, coverage regions, and frequency bands of the channels can be selected according to the service requirement of the cellular wireless network. 
   The mobile station is the mobile telephone mentioned above, which is a radio transceiver and a control unit for sending signals to the base station. When the mobile station intents to communicate with any other person, signals are transmitted to the MTSO via a wireless channel of the base station assigned to the mobile station, and then are forwarded to a public switched telephone network (PSTN). Every mobile station can receive and make a call from a subscribed MTSO, and also can make a call from other MTSO&#39;s by roaming. Besides, when a mobile station in use moves from one cell to another, the channel of the mobile station is automatically switched to that of the new cell. 
   From the above descriptions, when the base station services a larger quantity of mobile stations in a region such as a downtown area with a large population, the base station must have a larger capacity for dealing simultaneously with the higher communication load from many mobile stations. The mobile telephone network system applies a frequency reuse technique to increase the system capacity, and thus a large quantity of the base stations must be deployed. 
   Generally, the antenna used in the base station is a directional antenna, called a sector antenna, and the advantage of the antenna is that the energy of the antenna can be concentrated on a sector region. For the mobile telephone network system, the directional antenna is very helpful to the deployment of the base stations. However, in practice, the antenna radiation pattern of the sector antenna has an unwanted back lobe that radiates energy backward to other cells. 
   Since a cellular system adopts the above-mentioned frequency reuse technique, two signals of the same channel arriving at a mobile station from different base stations will interfere with each other. Thus, a larger back lobe of the base-station antenna radiation pattern will cause more interference to the service regions of other base stations. Also, the base-station antennas are usually mounted on top of buildings. Therefore, the back lobe radiations will more likely cause co-channel interferences to adjacent base stations assigned with the same frequencies. 
   In the prior art, in order to prevent mobile stations from co-channel interferences between different base stations and the communication quality thereof from being affected, the usual solution is to increase the distances between base stations using the same channels. However, this solution reduces the quantity of base stations in a certain area, and decreases the signal strengths over some regions in the certain area. Also, if the base-station antenna is mounted on top of a building, a conventional approach for reducing the co-channel interferences is to place a metal-grid panel behind the base-station antenna to shield the back-lobe radiation. However, this conventional method would affect the outer appearance of the base station, increase the wind resistance of the base-station antenna, require higher cost, need more construction efforts, and yet only provide smaller improvement. 
   SUMMARY 
   Accordingly, the invention uses the electromagnetic scattering principle to design electromagnetic scattering structures configured on a radome of an antenna to improve the antenna radiation patterns. The material of the electromagnetic structure is conductive. According to the electromagnetic principles, when electromagnetic waves illuminate conductive materials, induced currents will be excited on the conductive materials. Therefore, currents will be induced on the electromagnetic scattering structures due to the electromagnetic waves from the base-station antenna, and the induced currents then generate secondary radiation electromagnetic waves. The secondary radiation electromagnetic waves will interfere with the electromagnetic waves from the base-station antenna, and thus improve the antenna radiation patterns of the base station. 
   It is therefore an objective of the present invention to provide a method for improving antenna radiation patterns, which effectively reduces the back lobe of the antenna radiation patterns, to decrease the energy radiating to areas not covered by the base station, and mitigate interferences between adjacent base stations. 
   It is another objective of the present invention to provide a method for improving antenna radiation patterns in the horizontal plane, of which the energy outside the service region is reduced to mitigate the co-channel interferences or the adjacent-channel Interferences between the base stations. Or, the service region is increased to enlarge the coverage area of the base station. 
   It is still another objective of the present invention to provide a method for improving antenna radiation patterns in the vertical plane, of which the down-tilted angle of the main lobe is varied, so that the service of a base station is improved for the mobile stations positioned below. 
   It is still another objective of the present invention to provide an electromagnetic scattering structure, which is configured on a radome of the base station to adjust easily the antenna radiation patterns without any change of the size of the base-station antenna. Thus, one may replace the metal-grid panel, which is more expensive, hard to construct, and only a smaller improvement. 
   It is still another objective of the present invention to provide a radome with different functions for mobile communication system operators to choose according to requirements in different areas, so as to reduce the energy in regions not covered by the base station, and enlarge the coverage area of the base-station antenna or enhance the energy radiating downward from a base station on top of a building to the mobile stations positioned below. 
   In accordance with the foregoing and other objectives of the present invention, method and apparatus for improving antenna radiation patterns are provided. The electromagnetic scattering structure has a conductive layer with certain patterns, and is applied on the radome of the base-station antenna. The electromagnetic waves radiating from the antenna therein induce scattering effects, which, together with the electromagnetic diffractions from the rear metal panel of the antenna, can substantially reduce the back lobe and the fields in regions not covered by the antenna. Thus, the antenna radiation patterns are improved. 
   In the electromagnetic scattering structure of the present invention, the pattern of the conductive layer is designed according to a central working frequency of the antenna, and has variations of patterns and arrangements according to different demands. The units of the pattern, whether the type thereof is single or mixed, have a variety of modifications in their sizes, arrangements or quantities, and the purpose thereof is to improve the antenna radiation patterns. For example, the electromagnetic scattering structures, which comprises units of the same type but with different lengths, can adjust the antenna radiation pattern in the horizontal plane to change the level of the back lobe, the half-power beam width of the main lobe, or the energy radiating to certain directions from the base station. 
   According to preferred embodiments of the invention, the material of the conductive layer is metal, such as copper, and an adhesive layer is configured between the patterned conductive layer and a shell of the radome to stick the patterned conductive layer on the shell. Moreover, the present invention further comprises a protective layer, which is configured on one side of the patterned conductive layer opposite to the shell, to protect the patterned conductive layer. In addition, the patterned conductive layer is configured on an inner wall or an outer wall of the shell. 
   The pattern of the conductive layer comprises a plurality of units, such as strip units, cross units, U-shaped units, meandered square units or their combinations. The length of one of these units is a half, an integer multiple, or a certain multiple of a corresponding wavelength at the central working frequency of the antenna. 
   According to another preferred embodiment of the present invention, the patterned conductive layer is directly embedded in the shell, and the shell of the radome is therefore used to protect the conductive layer. 
   According to another preferred embodiment of the present invention, the pattern of the conductive layer comprises a plurality of slot units, such as spiral slot units. The slot units are a plurality of openings of the conductive layer, i.e. the slots of the conductive layer. The electromagnetic waves are scattered when they are transmitted through the discontinuous place, such as the interface between the conductive layer and the openings of this preferred embodiment. Therefore, the slot unit of the opening type can also be used in the present invention to form the pattern. 
   The embodiments of the invention provide several patterns for the conductive layer. Most can suppress the electromagnetic diffractions from the rear metal panel of the antenna, such that the energy radiating backward is reduced. The application of the same type but with different lengths can adjust the antenna radiation pattern in the horizontal plane to change the level of the back lobe, the half-power beam width of the main lobe and the energy radiating to certain directions from the base station. In addition, the electromagnetic scattering structures comprising different units can decrease the energy radiating to areas not covered by the base station, as well as decreasing the back lobe of the antenna radiation pattern. 
   In conclusions, the invention can decrease the energy radiating to areas not covered by the base station, and increase the energy radiating to the service regions of the base station or change the direction of the radiation of the antenna. Therefore, besides decreasing the cost of installing more base stations, the inventions further allow the energy of the antenna radiate more effectively to the coverage area. Moreover, the application of the invention is easy and convenient, and performs well, thus providing an economic and practical method and apparatus. 
   It is to be understood that both the foregoing general description and the following detailed description are examples, and are intended to provide further explanation of the invention as claimed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and 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: 
       FIG. 1  illustrates a front view of the base-station antenna; 
       FIG. 2A  illustrates a schematic view of the first embodiment of the present invention; 
       FIG. 2B  illustrates a far-field antenna radiation pattern in the horizontal plane of the first embodiment; 
       FIG. 2C  illustrates a far-field antenna radiation pattern in the vertical plane of the first embodiment; 
       FIG. 3A  illustrates a schematic view of the second embodiment of the present invention; 
       FIG. 3B  illustrates a far-field antenna radiation pattern in the horizontal plane of the second embodiment; 
       FIG. 3C  illustrates a far-field antenna radiation pattern in the vertical plane of the second embodiment; 
       FIG. 4A  illustrates a schematic view of the third embodiment of the present invention; 
       FIG. 4B  illustrates a far-field antenna radiation pattern in the horizontal plane of the third embodiment; 
       FIG. 4C  illustrates a far-field antenna radiation pattern in the vertical plane of the third embodiment; 
       FIG. 5A  illustrates a schematic view of the fourth embodiment of the present invention; 
       FIG. 5B  illustrates a far-field antenna radiation pattern in the horizontal plane of the fourth embodiment; 
       FIG. 5C  illustrates a far-field antenna radiation pattern in the vertical plane of the fourth embodiment; 
       FIG. 6A  illustrates a schematic view of the fifth embodiment of the present invention; 
       FIG. 6B  illustrates a far-field antenna radiation pattern in the horizontal plane of the fifth embodiment; 
       FIG. 6C  illustrates a far-field antenna radiation pattern in the vertical plane of the fifth embodiment; 
       FIG. 7A  illustrates a schematic view of the sixth embodiment of the present invention; 
       FIG. 7B  illustrates a far-field antenna radiation pattern in the horizontal plane of the sixth embodiment; 
       FIG. 7C  illustrates a far-field antenna radiation pattern in the vertical plane of the sixth embodiment; 
       FIG. 8A  illustrates a schematic view of the seventh embodiment of the present invention; 
       FIG. 8B  illustrates a far-field antenna radiation pattern in the horizontal plane of the seventh embodiment; 
       FIG. 8C  illustrates a far-field antenna radiation pattern in the vertical plane of the seventh embodiment; 
       FIG. 9A  illustrates a schematic view of the eighth embodiment of the present invention; 
       FIG. 9B  illustrates a far-field antenna radiation pattern in the horizontal plane of the eighth embodiment; 
       FIG. 9C  illustrates a far-field antenna radiation pattern in the vertical plane of the eighth embodiment; 
       FIG. 10A  illustrates a schematic view of the ninth embodiment of the present invention; 
       FIG. 10B  illustrates a far-field antenna radiation pattern in the horizontal plane of the ninth embodiment; and 
       FIG. 10C  illustrates a far-field antenna radiation pattern in the vertical plane of the ninth embodiment. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts. 
   The following descriptions use a base-station antenna for third generation mobile communications to be an example for illustrating several embodiments of the present invention. As illustrated in  FIG. 1 , a base-station antenna  100  comprises an array antenna  102  and a radome  104 . The array antenna  102  comprises a plurality of antenna units  112 , and the antenna units are enclosed in the radome  104 . The base-station antenna  100  is a sector antenna, of which the central working frequency is about 2 GHz. The electromagnetic wavelength corresponding to the central working frequency is about 150 mm. The length of the radome  104  is 1302 mm, the width thereof is 155 mm, the depth thereof is 69 mm and the thickness thereof is 2 mm. The relative dielectric constant of the material of the radome  104  is 2.73. Under these conditions, the characteristics of the antenna radiation patterns for the base-station antenna  100  are: the half-power beam width of the main lobe in the horizontal plane (θ=90°): 64°; the half-power beam width of the main lobe in the vertical plane (φ=0°): 6.5°; the front-to-back ratio: 26 dB; and the first side lobe level: −13.7 dB. 
   The electromagnetic scattering structure of the present invention comprises a patterned conductive layer, which is configured on the radome  104  in  FIG. 1 , and, more particularly, on the shell of the radome  104 . For example, the conductive layer can be adhered on an inner wall or an outer wall of the radome  104  by an adhesive layer. Moreover, the present invention further comprises a protective layer, which is configured on one side of the patterned. conductive layer opposite to the shell, to protect the patterned conductive layer. According to another preferred embodiment of the present invention, the patterned conductive layer also can be directly embedded in the shell, and the shell of the radome is therefore used to protect the conductive layer. 
   In the following embodiments, the material of the conductive layer is metal, such as copper or other conductive metals. 
   The First Embodiment 
     FIG. 2A  illustrates a schematic view of the first embodiment of the present invention. In this embodiment, the electromagnetic scattering structure comprises a plurality of strip units  202 . The length of the strip unit  202  is half of the corresponding wavelength at the central working frequency, which is about 76 mm, and the width of the strip unit  202 , which is not critical, is 2 mm in this embodiment. The strip units  202  are arranged periodically in two rows and in front of the antenna inside the radome  104  (i.e. the array antenna  102  as illustrated in  FIG. 1 ). The two rows are configured on the surface of the radome  104 , and each is spaced a quarter wavelengths from each closer edge of the radome  104 . 
     FIG. 2B  illustrates far-field antenna radiation patterns in the horizontal plane of the first embodiment, and the radial axis thereof represents the relative field value in dB. The curve  222  is the antenna radiation pattern without the electromagnetic scattering structure of the embodiment, and the curve  224  is the antenna radiation pattern with the electromagnetic scattering structure.  FIG. 2C  illustrates far-field antenna radiation patterns in the vertical plane of the first embodiment, and the radial axis thereof represents the relative field value in dB. The curve  232  is the antenna radiation pattern without the electromagnetic scattering structure of the embodiment, and the curve  234  is the antenna radiation pattern with the electromagnetic scattering structure. 
   From  FIGS. 2B and 2C , the level of the back lobe of the embodiment is about 14 dB lower than that of the antenna radiation pattern without the electromagnetic scattering structure, and the front-to-back ratio is increased to about 40 dB. In addition, the half-power beam width of the main lobe is almost unchanged, while the fields in other angles of the horizontal plane are reduced significantly by applying the electromagnetic scattering structure; for example, the level at the azimuth 120° is 8 dB lower. 
   The Second Embodiment 
     FIG. 3A  illustrates a schematic view of the second embodiment of the present invention. In this embodiment, the electromagnetic scattering structure comprises two strip units  302 . The length of the strip unit  302  is the same as the length of the radome  104 , and the width of the strip unit  302 , which is not critical, is 2 mm in this embodiment. The two strip units  302  are in front of the antenna inside the radome  104  (i.e. the array antenna  102  as illustrated in  FIG. 1 ), and are configured on the surface of the radome  104   
     FIG. 3B  illustrates far-field antenna radiation patterns in the horizontal plane of the second embodiment, and the radial axis thereof represents the relative field value in dB. The curve  322  is the antenna radiation pattern without the electromagnetic scattering structure of the embodiment, and the curve  324  is the antenna radiation pattern of the antenna with the electromagnetic scattering structure.  FIG. 3C  illustrates far-field antenna radiation patterns in the vertical plane of the second embodiment, and the radial axis thereof represents the relative field value in dB. The curve  332  is the antenna radiation pattern without the electromagnetic scattering structure of the embodiment, and the curve  334  is the antenna radiation pattern of the antenna with the electromagnetic scattering structure. 
   From  FIGS. 3B and 3C , the level of the back lobe of the embodiment is about 34 dB lower than that of the antenna radiation pattern without the electromagnetic scattering structure, and the front-to-back ratio is increased to about 60 dB. In addition, the half-power beam width of the main lobe is almost unchanged, while the fields in other angles of the horizontal plane are reduced significantly by applying the electromagnetic scattering structure; for example, the level at the azimuth 120° is 13 dB lower. Therefore, the embodiment decreases the energy radiating to areas not covered by the base-station antenna so that interferences between adjacent base stations can be mitigated. 
   The Third Embodiment 
     FIG. 4A  illustrates a schematic view of the third embodiment of the present invention. In this embodiment, the electromagnetic scattering structure comprises a plurality of cross units  402 . Each of the cross units  402  has two strip portions  412   a  and  412   b  with identical lengths. The length of the strip portions  412   a  and  412   b  is a half the corresponding wavelength at the central working frequency, and the widths of the strip portions  412   a  and  412   b , which are not critical, are both 2 mm in this embodiment. The cross units  402  are in front of the antenna inside the radome  104  (i.e. the array antenna  102  as illustrated in  FIG. 1 ), and are configured in two rows interleaving on the surface of the radome  104 . 
     FIG. 4B  illustrates far-field antenna radiation patterns in the horizontal plane of the third embodiment, and the radial axis thereof represents the relative field value in dB. The curve  422  is the antenna radiation pattern without the electromagnetic scattering structure of the embodiment, and the curve  424  is the antenna radiation pattern of the antenna with the electromagnetic scattering structure.  FIG. 4C  illustrates far-field antenna radiation patterns in the vertical plane of the third embodiment, and the radial axis thereof represents the relative field value in dB. The curve  432  is the antenna radiation pattern without the electromagnetic scattering structure of the embodiment, and the curve  434  is the antenna radiation pattern of the antenna with the electromagnetic scattering structure. 
   From  FIGS. 4B and 4C , the level of the back lobe of the embodiment is about 18 dB lower than that of the antenna radiation pattern without the electromagnetic scattering structure, and the front-to-back ratio is increased to about 44.5 dB. Therefore, the embodiment effectively reduces the energy radiating backward; the half-power beam width of the main lobe in the horizontal plane becomes 75°, and thus the coverage sector is increased by 11° more than the antenna radiation pattern without the electromagnetic scattering structure. 
   The Fourth Embodiment 
     FIG. 5A  illustrates a schematic view of the fourth embodiment of the present invention. In this embodiment, the electromagnetic scattering structure comprises a plurality of U-shaped combinations  502 . Each of the U-shaped combinations  502  has a first U-shaped unit  512   a  and a second U-shaped unit  512   b , which are placed opposite to each other. The length of the first U-shaped unit  512   a  is equal to the corresponding wavelength at the central working frequency, and the length of the second U-shaped unit  512   b  is two times the corresponding wavelength at the central working frequency. The widths of the two U-shaped units  512   a  and  512   b , which are not critical, are both 2 mm in this embodiment. The U-shaped combinations  502  are arranged in a row and in front of the antenna inside the radome  104  (i.e. the array antenna  102  as illustrated in  FIG. 1 ). 
     FIG. 5B  illustrates far-field antenna radiation patterns in the horizontal plane of the fourth embodiment, and the radial axis thereof represents the relative field value in dB. The curve  522  is the antenna radiation pattern without the electromagnetic scattering structure of the embodiment, and the curve  524  is the antenna radiation pattern of the antenna with the electromagnetic scattering structure.  FIG. 5C  illustrates far-field antenna radiation patterns in the vertical plane of the fourth embodiment, and the radial axis thereof represents the relative field value in dB. The curve  532  is the antenna radiation pattern without the electromagnetic scattering structure of the embodiment, and the curve  534  is the antenna radiation pattern of the antenna with the electromagnetic scattering structure. 
   From  FIGS. 5B and 5C , the level of the back lobe of the embodiment is about 8 dB lower than that of the antenna radiation pattern without the electromagnetic scattering structure, and the front-to-back ratio is increased to about 34 dB. 
   The Fifth Embodiment 
     FIG. 6A  illustrates a schematic view of the fifth embodiment of the present invention. In this embodiment, the electromagnetic scattering structure comprises a plurality of meandered square units  602 . The circumference of each of the meandered square units  602  is an integer multiple of the corresponding wavelength at the central working frequency. The width of the meandered square unit  602 , which is not critical, is 6 mm in this embodiment. The meandered square units are arranged in a row and in front of the antenna inside the radome  104  (i.e. the array antenna  102  as illustrated in  FIG. 1 ). 
     FIG. 6B  illustrates far-field antenna radiation patterns in the horizontal plane of the fifth embodiment, and the radial axis thereof represents the relative field value in dB. The curve  622  is the antenna radiation pattern without the electromagnetic scattering structure of the embodiment, and the curve  624  is the antenna radiation pattern of the antenna with the electromagnetic scattering structure.  FIG. 6C  illustrates far-field antenna radiation patterns in the vertical plane of the fifth embodiment, and the radial axis thereof represents the relative field value in dB. The curve  632  is the antenna radiation pattern without the electromagnetic scattering structure of the embodiment, and the curve  634  is the antenna radiation pattern of the antenna with the electromagnetic scattering structure. 
   From  FIGS. 6B and 6C , the level of the back lobe of the embodiment is about 6.2 dB higher than that of the antenna radiation pattern without the electromagnetic scattering structure, and the front-to-back ratio is reduced to about 19.8 dB. Although the level of the back lobe of the antenna radiation pattern with the electromagnetic scattering structure is higher than that of the antenna radiation pattern without the electromagnetic scattering structure, the energies radiating in the azimuths 60° and 300° are about 12.2 dB less than those of the antenna radiation pattern without the electromagnetic scattering structure, and the half-power beam width of the main lobe is reduced to about 46.5°. Therefore, the embodiment reduces the coverage region of the antenna radiation pattern, and thus mitigates the interferences from or to the adjacent base stations in the directions around the azimuths 60° and 300°. 
   The Sixth Embodiment 
     FIG. 7A  illustrates a schematic view of the sixth embodiment of the present invention. In this embodiment, the electromagnetic scattering structure comprises a plurality of cross units  704  and a plurality of strip units  702   a  and  702   b . Each of the cross units  704  has two strip portions  714   a  and  714   b  with identical lengths. The lengths of the strip portions  714   a  and  714   b  are both 0.45 times the corresponding wavelength at the central working frequency. The length of each of the strip units  702   a  and  702   b  is a half the corresponding wavelength at the central working frequency. The widths of the strip portions  714   a  and  714   b  and the strip units  702   a  and  702   b  are not critical. In this embodiment, the widths of the strip portions  714   a  and  714   b  are 8 mm, and the widths of the strip units  702   a  and  702   b  are 2 mm. 
   The cross units  704  and the strip units  702   a ,  702   b  are arranged in rows and in front of the antenna inside the radome  104  (i.e. the array antenna  102  as illustrated in  FIG. 1 ). Moreover, the rows of the strip units  702   a  and  702   b  are configured separately on the two sides of the radome  104 , and each of the cross units  704  is placed between two corresponding strip units  702   a  and  702   b.    
     FIG. 7B  illustrates far-field antenna radiation patterns in the horizontal plane of the sixth embodiment, and the radial axis thereof represents the relative field value in dB. The curve  722  is the antenna radiation pattern without the electromagnetic scattering structure of the embodiment, and the curve  724  is the antenna radiation pattern of the antenna with the electromagnetic scattering structure.  FIG. 7C  illustrates far-field antenna radiation patterns in the vertical plane of the sixth embodiment, and the radial axis thereof represents the relative field value in dB. The curve  732  is the antenna radiation pattern without the electromagnetic scattering structure of the embodiment, and the curve  734  is the antenna radiation pattern of the antenna with the electromagnetic scattering structure. 
   From  FIGS. 7B and 7C , the level of the back lobe of the embodiment is about 6 dB lower than that of the antenna radiation pattern without the electromagnetic scattering structure, and the front-to-back ratio is increased to about 32 dB. Besides, the energies radiating in the azimuths 60° and 300° are about 18.4 dB less than those of the antenna radiation pattern without the electromagnetic scattering structure, and the half-power beam width of the main lobe is reduced to about 46.5°. 
   The Seventh Embodiment 
     FIG. 8A  illustrates a schematic view of the seventh embodiment of the present invention. In this embodiment, the electromagnetic scattering structure comprises a plurality of meandered square units  804  and a plurality of strip units  802   a  and  802   b . The length of each of the strip units  802   a  and  802   b  is half of the corresponding wavelength at the central working frequency. The circumference of each of the meandered square units  804  is an integer multiple of the corresponding wavelength at the central working frequency. The widths of the strip units  802   a  and  802   b  and the meandered square units  804  are not critical. In this embodiment, the widths of the meandered square units  804  are 6 mm, and the widths of the strip units  802   a  and  802   b  are 2 mm. 
   The meandered square units  804 , the strip units  802   a  and  802   b  are arranged in rows and in front of the antenna inside the radome  104  (i.e. the array antenna  102  as illustrated in  FIG. 1 ). Moreover, the rows of the strip units  802   a  and  802   b  are configured separately on the two sides of the radome  104 , and each of the meandered square units  804  is placed between two corresponding strip units  802   a  and  802   b.    
     FIG. 8B  illustrates far-field antenna radiation patterns in the horizontal plane of the seventh embodiment, and the radial axis thereof represents the relative field value in dB. The curve  822  is the antenna radiation pattern without the electromagnetic scattering structure of the embodiment, and the curve  824  is the antenna radiation pattern of the antenna with the electromagnetic scattering structure.  FIG. 8C  illustrates far-field antenna radiation patterns in the vertical plane of the seventh embodiment, and the radial axis thereof represents the relative field value in dB. The curve  832  is the antenna radiation pattern without the electromagnetic scattering structure of the embodiment, and the curve  834  is the antenna radiation pattern of the antenna with the electromagnetic scattering structure. 
   From  FIGS. 8B and 8C , the level of the back lobe of the embodiment is only increased by about 1.8 dB compared with that of the antenna radiation pattern without the electromagnetic scattering structure, and the front-to-back ratio is reduced to about 24.2 dB. The energies radiating in the azimuths 60° and 300° are about 38 dB less than those of the antenna radiation pattern without the electromagnetic scattering structure, and the half-power beam width of the main lobe is reduced to about 46.5°. Therefore, the embodiment reduces the coverage region of the antenna radiation pattern, and thus mitigates the interferences from or to the adjacent base stations in the directions around the azimuths 60° and 300°. 
   The Eighth Embodiment 
     FIG. 9A  illustrates a schematic view of the eighth embodiment of the present invention. In this embodiment, the electromagnetic scattering structure comprises a plurality of meandered square units  904 . The circumference of each of the meandered square units  904  is an integer multiple of the corresponding wavelength at the central working frequency. The widths of the meandered square units  904  are not critical. In this embodiment, the widths of the meandered square units  904  are 6 mm. 
   The meandered square units  904  are arranged in rows and in front of the antenna inside the radome  104  (i.e. the array antenna  102  as illustrated in  FIG. 1 ). Moreover, the meandered square units  904  are configured only on the lower half of the radome  104 . 
     FIG. 9B  illustrates far-field antenna radiation patterns in the horizontal plane of the eighth embodiment, and the radial axis thereof represents the relative field value in dB. The curve  922  is the antenna radiation pattern without the electromagnetic scattering structure of the embodiment, and the curve  924  is the antenna radiation pattern of the antenna with the electromagnetic scattering structure.  FIG. 9C  illustrates far-field antenna radiation patterns in the vertical plane of the eighth embodiment, and the radial axis thereof represents the relative field value in dB. The curve  932  is the antenna radiation pattern without the electromagnetic scattering structure of the embodiment, and the curve  934  is the antenna radiation pattern of the antenna with the electromagnetic scattering structure. 
   From  FIGS. 9B and 9C , the level of the back lobe of the embodiment is about 5.5 dB higher than that of the antenna radiation pattern without the electromagnetic scattering structure, and the front-to-back ratio is reduced to about 20 dB. The energies radiating in the azimuths 60° and 300° are reduced, and the half-power beam width of the main lobe is reduced to about 50°. Moreover, the width of the main lobe in the vertical plane becomes greater, of which the half-power beam width is about 8.5°. In addition, the direction of main lobe is down-tilted by 2.5°. 
   Because the base stations are usually installed at high locations, in order to enhance the service to the mobile stations below, the prior art usually down-tilts the base-station antenna mechanically, and thus changes the direction of the main lobe in the vertical plane. Another conventional method to tilt the main lobe down is by properly adjusting the relative phase angles and amplitudes of the input currents of the antenna units  112 . The embodiment provides another new way to increase the down-tilted angle of the main lobe for increasing the energy radiating downward in the vertical plane of the antenna radiation pattern. Hence, the embodiment improves the coverage quality of an elevated base station for the mobile stations positioned below. 
   The Ninth Embodiment 
     FIG. 10A  illustrates a schematic view of the ninth embodiment of the present invention. In this embodiment, the electromagnetic scattering structure comprises a plurality of spiral slot units  1002 . The spiral slot units  1002  are a plurality of openings of the conductive layer, and the widths of the spiral slot units are 4 mm. The material of other portions  1004 , which are not the spiral slot units, is a conductive material such as, for example, metal, such as, for example, copper. The spiral slot units  1002  are arranged in rows and in front of the antenna inside the radome  104  (i.e. the array antenna  102  as illustrated in  FIG. 1 ). 
     FIG. 10B  illustrates far-field antenna radiation patterns in the horizontal plane of the ninth embodiment, and the radial axis thereof represents the relative field value in dB. The curve  1022  is the antenna radiation pattern without the electromagnetic scattering structure of the embodiment, and the curve  1024  is the antenna radiation pattern of the antenna with the electromagnetic scattering structure.  FIG. 10C  illustrates far-field antenna radiation patterns in the vertical plane of the ninth embodiment, and the radial axis thereof represents the relative field value in dB. The curve  1032  is the antenna radiation pattern without the electromagnetic scattering structure of the embodiment, and the curve  1034  is the antenna radiation pattern of the antenna with the electromagnetic scattering structure. 
   From  FIGS. 10B and 10C , in the vertical plane, the antenna radiation pattern of the embodiment varies significantly from that without the electromagnetic scattering structure. In the horizontal plane, the levels of the antenna radiation pattern in side directions are lower than those without the electromagnetic scattering structure, and the half-power beam width of the main lobe is reduced to 52°. 
   In conclusion, the invention effectively and conveniently changes the antenna radiation pattern without any change of the original size and appearance of the base-station antenna. The invention has various practical implementations, such as a film sticker having the electromagnetic scattering structure of the invention manufactured for being adhered directly on the radome of the base station. Moreover, optional radomes with different functions can be provided for base stations of a certain model. According to requirements of different areas, the mobile communication operators properly choose and replace radomes to improve the performance of the base-station antennas for the service regions thereof. Alternatively, when the base-station antennas are manufactured, the radomes with different functions can also be prepared to be selected by the mobile communication operators. 
   It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.