Patent Publication Number: US-11380983-B2

Title: Radome for base station antenna and base station antenna

Description:
RELATED APPLICATION 
     The present application claims priority to and the benefit of Chinese Patent Application No. 201911246929.1, filed Dec. 9, 2019, the content of which is hereby incorporated herein in its entirety. 
     FIELD OF THE INVENTION 
     The present invention relates to communication systems and, more particularly, to radomes for base station antennas and base station antennas. 
     DESCRIPTION OF RELATED ART 
     Most base station antennas include a radome, which is used to protect internal electronic components of the antenna from the external environment and to reduce wind loads on the antenna.  FIG. 1  is a schematic horizontal cross-sectional view of a conventional base station antenna. The conventional base station antenna includes a dielectric radome  1  mounted on a mounting plate  20  and an array  30  of radiating elements  31  that are mounted on the mounting plate  20  adjacent the inner side of the radome  1 . In the example shown in  FIG. 1 , each row of the array  30  of radiating elements (the rows extend along a width direction of the base station antenna, which is also referred to as a horizontal direction) includes four radiating elements  31 . Each T-shaped portion in the figure represents a column (along a length direction, which is also referred to as a vertical direction, of the base station antenna) of radiating elements  31 , where each column may include one or more radiating elements  31 . The mounting plate  20  may function as a reflector and a ground plane for the radiating elements  31 . 
     SUMMARY 
     Embodiments the present invention are directed to a radome for a base station antenna and the base station antenna suitable for use in a communication system. 
     A first aspect of the present invention is a radome for a base station antenna. The radome for a base station antenna includes: a first dielectric layer having a first dielectric constant and a first thickness; a second dielectric layer having a second dielectric constant and a second thickness, the second dielectric layer being positioned on an outer side of the first dielectric layer; and a third dielectric layer having a third dielectric constant and a third thickness, the third dielectric layer being positioned on an outer side of the second dielectric layer. Each of the first and third dielectric constants is greater than the second dielectric constant. 
     Another aspect of the present invention is a base station antenna. The base station antenna can include: an array of radiating elements; and a radome described above. The first dielectric layer can be closer to the array of radiating elements than the third dielectric layer. 
     A third aspect of this disclosure is to a base station antenna that includes: an array of radiating elements configured to emit an electromagnetic wave; a radome including a first dielectric layer, the first dielectric layer having a first dielectric constant and a first thickness; and a dielectric plate that is extending between the array of radiating elements and the radome, the dielectric plate having a second dielectric constant and a second thickness. There is a first gas between the dielectric plate and the radome, and each of the first and second dielectric constants is greater than a dielectric constant of the first gas, and a shape of the dielectric plate matches a shape of a corresponding portion of the radome. 
     Other features of the present invention and advantages thereof will become explicit by means of the following detailed descriptions of exemplary embodiments of the present invention with reference to the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which constitute a part of the specification, illustrate embodiments of the present invention and, together with the description, serve to explain the principles of the present invention. 
         FIG. 1  is a schematic horizontal cross-sectional view of a conventional base station antenna. 
         FIG. 2  is a schematic view showing incident angles at which electromagnetic waves are received by the radome in  FIG. 1 . 
         FIG. 3  is a schematic horizontal cross-sectional view of a base station antenna according to an embodiment of the present invention. 
         FIG. 4  is a partially enlarged schematic horizontal cross-sectional view of a small portion of the radome in  FIG. 3 . 
         FIG. 5  is a schematic horizontal cross-sectional view of a base station antenna according to another embodiment of the present invention. 
         FIG. 6  is a partially enlarged schematic horizontal cross-sectional view of a small portion of the radome in  FIG. 5 . 
         FIGS. 7A and 7B  are schematic horizontal cross-sectional views of base station antennas according to further embodiments of the present invention. 
         FIGS. 8A and 8B  are schematic horizontal cross-sectional views of base station antennas according to further embodiments of the present invention. 
         FIG. 9  is a schematic horizontal cross-sectional view of a base station antenna according to an additional embodiment of the present invention. 
         FIGS. 10A and 10B  are schematic horizontal cross-sectional views of base station antennas according to further embodiments of the present invention. 
         FIGS. 11A and 11B  are simulations of radiation patterns on the azimuth plane of a single column of radiating elements, including a radiation pattern without a radome, a radiation pattern with the radome in  FIG. 1 , and a radiation pattern with the radome and the dielectric plate in  FIG. 7A . 
         FIGS. 12A and 12B  are simulations of radiation patterns on the elevation plane of a single column of radiating elements, including a radiation pattern without a radome, a radiation pattern with the radome in  FIG. 1 , and a radiation pattern with the radome and the dielectric plate in  FIG. 7A . 
     
    
    
     Note that, in some cases the same elements or elements having similar functions are denoted by the same reference numerals in different drawings, and description of such elements is not repeated. In some cases, similar reference numerals and letters are used to refer to similar elements, and thus once an element is defined in one figure, it need not be further discussed for following figures. 
     In order to facilitate understanding, the position, size, range, or the like of each structure illustrated in the drawings may not be drawn to scale. Thus, the disclosure is not necessarily limited to the position, size, range, or the like as disclosed in the drawings. 
     DETAILED DESCRIPTION 
     The present invention will be described with reference to the accompanying drawings, which show a number of example embodiments thereof. It should be understood, however, that the present invention can be embodied in many different ways, and is not limited to the embodiments described below. Rather, the embodiments described below are intended to make the disclosure of the present invention more complete and fully convey the scope of the present invention to those skilled in the art. It should also be understood that the embodiments disclosed herein can be combined in any way to provide many additional embodiments. 
     The terminology used herein is for the purpose of describing particular embodiments, but is not intended to limit the scope of the present invention. All terms (including technical terms and scientific terms) used herein have meanings commonly understood by those skilled in the art unless otherwise defined. For the sake of brevity and/or clarity, well-known functions or structures may be not described in detail. 
     Herein, when an element is described as located “on” “attached” to, “connected” to, “coupled” to or “in contact with” another element, etc., the element can be directly located on, attached to, connected to, coupled to or in contact with the other element, or there may be one or more intervening elements present. In contrast, when an element is described as “directly” located “on”, “directly attached” to, “directly connected” to, “directly coupled” to or “in direct contact with” another element, there are no intervening elements present. In the description, references that a first element is arranged “adjacent” a second element can mean that the first element has a part that overlaps the second element or a part that is located above or below the second element. 
     Herein, the foregoing description may refer to elements or nodes or features being “connected” or “coupled” together. As used herein, unless expressly stated otherwise, “connected” means that one element/node/feature is electrically, mechanically, logically or otherwise directly joined to (or directly communicates with) another element/node/feature. Likewise, unless expressly stated otherwise, “coupled” means that one element/node/feature may be mechanically, electrically, logically or otherwise joined to another element/node/feature in either a direct or indirect manner to permit interaction even though the two features may not be directly connected. That is, “coupled” is intended to encompass both direct and indirect joining of elements or other features, including connection with one or more intervening elements. 
     Herein, terms such as “upper”, “lower”, “left”, “right”, “front”, “rear”, “high”, “low” may be used to describe the spatial relationship between different elements as they are shown in the drawings. It should be understood that in addition to orientations shown in the drawings, the above terms may also encompass different orientations of the device during use or operation. For example, when the device in the drawings is inverted, a first feature that was described as being “below” a second feature can be then described as being “above” the second feature. The device may be oriented otherwise (rotated 90 degrees or at other orientation), and the relative spatial relationship between the features will be correspondingly interpreted. 
     Herein, the term “A or B” used through the specification refers to “A and B” and “A or B” rather than meaning that A and B are exclusive, unless otherwise specified. 
     The term “exemplary”, as used herein, means “serving as an example, instance, or illustration”, rather than as a “model” that would be exactly duplicated. Any implementation described herein as exemplary is not necessarily to be construed as preferred or advantageous over other implementations. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the detailed description. 
     Herein, the term “substantially”, is intended to encompass any slight variations due to design or manufacturing imperfections, device or component tolerances, environmental effects and/or other factors. The term “substantially” also allows for variation from a perfect or ideal case due to parasitic effects, noise, and other practical considerations that may be present in an actual implementation. 
     Herein, certain terminology, such as the terms “first”, “second” and the like, may also be used in the following description for the purpose of reference only, and thus are not intended to be limiting. For example, the terms “first”, “second” and other such numerical terms referring to structures or elements do not imply a sequence or order unless clearly indicated by the context. 
     Further, it should be noted that, the terms “comprise”, “include”, “have” and any other variants, as used herein, specify the presence of stated features, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, steps, operations, elements, components, and/or groups thereof. 
     A radome for a base station antenna should have sufficient mechanical strength and good electrical performance such as high transmissivity (which means low reflectivity) over the entire operating frequency band of the base station antenna with respect to all scanning angles of the array of radiating elements. In the fifth generation of mobile communications, the frequency range of communications includes a dominant frequency band (which is in specified portions of the 450 MHz˜6 GHz range) and an extended frequency band (24 GHz˜52 GHz, namely a millimeter wave frequency band, primarily 28 GHz, 39 GHz, 60 GHz and 73 GHz). The frequency ranges that will be used in fifth generation mobile communications include frequency bands that are higher than those used in previous generations of mobile communications. Therefore, it is desirable that radomes for fifth generation base station antennas have high electrical performance in these higher frequency ranges. The dielectric material of the radome for the base station antenna is typically frequency-selective to electromagnetic waves. The higher the frequency of the electromagnetic wave is, the greater effect the dielectric material may have on the electromagnetic wave, for example, the worse transmissivity and the higher reflectivity. The deterioration of the transmissivity may decrease the intensity of electromagnetic wave signals, and hence the gain of the base station antenna. The higher the reflectivity is, the more electromagnetic waves will be reflected back from the radome. These reflected waves may be superimposed with the electromagnetic waves emitted from the radiating elements to cause jitter and ripple in the pattern. These are all undesirable effects. 
     The incident angles at which electromagnetic waves impinge upon a radome also may impact the performance of a base station antenna. In particular, when an electromagnetic wave has a large incident angle, the electrical performance of the radome may decline significantly. In a communication system, the free space or “path” loss increases with increasing frequency. Thus, for high frequency communications, multiple input multiple output (MIMO) technology is typically employed to compensate for the path loss. In the fifth generation of mobile communications, especially within the millimeter wave frequency band, the base station antenna may generate radiation patterns or “antenna beams” that have high gain and small beamwidth by using massive MIMO technology, and may perform electronic beam scanning by changing the pointing directions of the antenna beams in the azimuth and/or elevation planes (where the pointing direction of an antenna beam may refer to the direction where the antenna beam exhibits peak gain) so as to cover a predetermined spatial range within a predetermined time of period to improve signal coverage and reduce interference.  FIG. 2  illustrates several antenna beams that are electronically scanned to point in a variety of different horizontal directions. Each antenna beam shown in  FIG. 2  has a different incident angle with respect to the radome, where the straight line P 1  schematically represents the projection of a plane of the radome for receiving electromagnetic waves in the cross-sectional view, the antenna beam B 1  points in a direction D 1  in which the scanning angle is 0 degree, the antenna beam B 2  points in a direction D 2  in which the scanning angle is 30 degrees, and the antenna beam B 3  points in a direction D 3  in which the scanning angle is 60 degrees. Directions D 4  and D 5  are directions that are symmetric with directions D 2  and D 3 , respectively, although the corresponding antenna beams are not shown. Incident angles of the antenna beams corresponding to the directions D 2  to D 5 , with respect to the radome, are shown as θ 2  to θ 5  respectively. The antenna beam B 1  corresponding to the direction D 1  in the figure has an incident angle of 0 degrees with respect to the radome (not shown). When the antenna beam points in a direction in which the scanning angle is larger, the incident angle of the antenna beam with respect to the radome is larger as well. When the incident angle is large, the electrical performance of the radome will be degraded, for example, the transmissivity may be degraded and the reflectivity may be increased. The massive MIMO technology may execute beam scanning in three-dimensional space within a sweeping angle range of ±60 degrees, that is, a range of sweeping angles of −60 degrees to +60 degrees may be used in both horizontal and vertical directions. For example,  FIG. 2  may be used to show some scanning an antenna beam of the base station antenna of  FIG. 1  in the vertical direction as well as the incident angles of these beams with respect to the radome. Therefore, there is a need for the radome to have a desirable transmissivity and reflectivity for electromagnetic waves having a large incident angle (for example, an incident angle of ±60 degrees). 
     Conventional radomes (for example, the radome  1  shown in  FIG. 1 ) are divided into two types, radomes with thin-walled structures and radomes with thick-walled structures, both of which have low reflectivity. Radomes having a thin-walled structure may be less than λ/20 thick, where λ, is the wavelength of the electromagnetic wave in the dielectric of the radome. Such a thin-walled structure may allow for good transmissivity and reflectivity even when the electromagnetic wave has a large incident angle (for example, an incident angle of 70 degrees). However, for high frequencies such as 3.5 GHz, a thin-walled radome is less than 2 mm thick (calculated in terms of the dielectric constant of the radome being 4), which may be insufficient to meet the mechanical strength requirements for the radome. With respect to higher frequencies, the thickness of the radome will be reduced even further and such a radome will not be usable in a base station antenna. The radome with a thick-walled structure has a thickness of about λ/2 which, for high frequency electromagnetic waves, may meet the mechanical strength requirements for the radome. However, thick-walled radomes may have a small bandwidth, such as a bandwidth that is 5% of the center frequency of the operating frequency band. For example, for electromagnetic waves with a center frequency of 3.5 GHz, the radome with a thick-walled structure may only support a bandwidth of 175 MHz. This makes the radome obviously not suitable for using in the fifth generation of mobile communication system, where the operating frequency bands tend to exceed 5% (e.g., the operating frequency band for the 3.5 GHz frequency band is about 400 MHz). 
       FIGS. 3 and 4  schematically illustrate a base station antenna in accordance with an embodiment of the present invention. The depicted base station antenna includes a radome  10  in accordance with an embodiment of the present invention. The base station antenna further includes a mounting plate  20  and an array  30  of radiating elements  31  mounted on the outer side of the mounting plate  20 . The radome  10  includes, from its inner side to outer side, dielectric layers  11  to  13 . The inner side of the radome  10  refers to the side that is closest to the array  30  and the outer side refers to the side that is farther away from the array  30 . The dielectric layer  11  forming the inner side of the radome  10  has a dielectric constant ε1 and a thickness h1, the dielectric layer  12  on the outer side of the dielectric layer  11  has a dielectric constant ε2 and a thickness h2, and the dielectric layer  13  on the outer side of the dielectric layer  12  that forms the outer side of the radome  10  has a dielectric constant ε3 and a thickness h3. The dielectric constants of the dielectric layers  11  to  13  meet the following relationship: dielectric constant is greater than dielectric constant ε2, and dielectric constant ε3 is greater than dielectric constant ε2. Dielectric materials that have high density and strength, such as reinforced glass fiber materials, may be selected to form the dielectric layers  11  and  13  having higher dielectric constants, in order to ensure the mechanical strength of the radome  10 . Dielectric materials having low loss tangent and low density, for example, gaseous materials such as vacuum or gases, solid materials that are for example solid, honeycombed, foamed, porous and/or meshed, and even a suitable liquid material, may be selected to form the dielectric layer  12  having a lower dielectric constant, so as to allow the radome  10  to be less heavy. In addition, the solid materials that are for example honeycombed, foamed, porous and/or meshed may have a light weight even in case of being thick, and at the same time may have a high mechanical strength. Therefore, the radome  10  with such a structure may reach a high mechanical strength with a light weight, i.e., achieving a great strength-to-weight ratio. 
     Further, the radome  10  may exhibit good electrical performance as well. As shown in  FIG. 4 , electromagnetic waves that are transmitted through the radome  10  may pass through four interfaces S 1  to S 4 . Transmission and/or reflection may occur at each of these interfaces. The dotted lines in the figure schematically illustrate incident waves having an incident angle of 0 degree and reflected waves having a reflection angle of 0 degree potentially occurring at each of the four interfaces S 1  to S 4 . Designers of the radome  10  may adjust reflections of electromagnetic waves emitted by the array  30  on each of the four interfaces S 1  to S 4  by designing the thicknesses h1 to h3 and the dielectric constants ε1 to ε3 of each dielectric layers  11  to  13 , such that these reflected waves are superimposed out of phase or even reverse phase so as to reduce the reflectivity of the entire radome  10 , thereby enabling the entire radome  10  to meet transmissivity and reflectivity design goals. 
     A design process for the radome  10  may comprise designing a range of total thickness of the radome  10  according to the requirements for the mechanical strength and spatial size of the radome  10  of the base station antenna, and then designing the thicknesses and dielectric constants of the individual dielectric layers  11  to  13  within the range of total thickness so as to meet the requirements for electrical performance of the radome  10 . Upon design of the dielectric constants of the individual dielectric layers  11  to  13 , the materials of the dielectric layers  11  and  13 , for example, a material commonly used for manufacturing the radome (such as ASA engineering plastics or the like) may be determined first, and then the dielectric constant of the dielectric layer  12  is adjusted and determined as required. In the event that the dielectric layer  12  is made of a solid material that is for example honeycombed, foamed, porous, and/or meshed, the dielectric constant of this material may be controlled precisely by controlling the density of voids in the material. Upon design of the thicknesses of the individual dielectric layers  11  to  13 , the thicknesses of the dielectric layers  11  and  13  may be determined before the thickness of the dielectric layer  12  is adjusted and determined as required; alternatively, the thickness of the dielectric layer  12  may be determined before the thicknesses of the dielectric layers  11  and  13  are adjusted and determined as required. 
     In the embodiment shown in  FIG. 4 , the thickness h2 of the dielectric layer  12  is greater than both the thickness h1 of the dielectric layer  11  and the thickness h3 of the dielectric layer  13 . With such a configuration, in the process of passing through the radome  10 , the path over which the electromagnetic wave travels through the high-dielectric constant dielectric is shortened, thereby reducing the losses of the electromagnetic waves due to the radome  10 . This may not only increase the transmittance of electromagnetic waves through the radome  10 , but also increase the bandwidth of the radome  10 . In some embodiments, the thickness h2 of the dielectric layer  12  is 2 to 15 times at least one of the thickness h1 of the dielectric layer  11  and the thickness h3 of the dielectric layer  13 . For example, the thicknesses h1 and h3 of the dielectric layers  11  and  13  may both be 0.2 to 0.8 mm, the thickness h2 of the dielectric layer  12  may be 0.4 to 12 mm, and the total thickness of the radome  10  may be 0.8 to 13.6 mm. In some embodiments, the thickness h2 of the dielectric layer  12  is equal to or less than one quarter of the wavelength of the electromagnetic waves emitted by the array  30 , in the dielectric layer  12 . In the design flow of determining the thickness of the dielectric layer  12  before determining the thickness of the dielectric layers  11  and  13 , the thickness of the dielectric layer  12  may be determined first according to the wavelength of the electromagnetic wave emitted by the array  30 , in the dielectric layer  12 , and then the thicknesses of the dielectric layers  11  and  13  are adjusted and determined based on the relationship between the thicknesses of the individual dielectric layers as described above. 
     In some embodiments, for ease of design, the dielectric layers  11  to  13  may be symmetrically configured, that is, the dielectric constants ε1 and ε3 of the dielectric layers  11  and  13  are equal, and/or the thicknesses h1 and h3 of the dielectric layers  11  and  13  are also equal. In some embodiments, the dielectric layer  13  on the outer side of the dielectric layer  12  may be a protective layer applied on the outer side of the dielectric layer  12 . For example, a coating layer applied on the outer surface of the dielectric layer  12 . When the conventional radome is formed of, for example, a woven fabric, a protective layer may be applied to the outer side thereof to resist water, dust or the like. In the radome according to embodiments of the present invention, the dielectric layer  13  of the radome  10  may be implemented as a protective layer. In some embodiments, the dielectric layers  11  and  13  may be made of glass fiber, and the dielectric layer  12  may be made of foam plastic, corrugated paper, or the like. In some embodiments, the dielectric layers  11  and  13  may be made of ASA engineering plastic, polyvinyl chloride (PVC), polycarbonate (PC), ABS plastic, or the like, and the dielectric layer  12  may be formed of air. 
     In some embodiments, the dielectric layers  11  through  13  are monolithic. The radome  10  may be integrally formed by an injection molding process. For example, after a molten plastic (may be any one or more of the plastic materials mentioned above) is injected into a mold, a gas is introduced into a portion corresponding to the dielectric layer  12  so that this portion includes air holes so as to form a foam plastic. In this example, the dielectric layers  11  and  13  are made of a higher density plastic, and the dielectric layer  12  is made of a lower density plastic, such that the dielectric constants of the dielectric layers  11  and  13  are larger than the dielectric constant of the dielectric layer  12 . For another example, after the molten plastic is injected into the mold, impurities having a higher dielectric constant (such as ceramic particles) are doped into portions corresponding to the dielectric layers  11  and  13 , so that the dielectric constants of the dielectric layers  11  and  13  are greater than the dielectric constant of the dielectric layer  12 . For a further example, a mold whose intermediate layer is used for manufacturing a hollow layer may be used. The molten plastic is injected into this kind of mold and solidified, and then an integrally formed radome  10  in which the dielectric layers  11  and  13  are plastic and the dielectric layer  12  is air is obtained. 
     In the case where the dielectric constants of dielectric layers  11  to  13  and the thicknesses of the dielectric layers  11  and  13  are fixed, the larger the incident angle of the electromagnetic wave with respect to the radome  10 , the greater the thickness that dielectric layer  12  will need to have in order to provide optimal transmittance through the radome  10 . Thus, in some embodiments shown in  FIGS. 10A and 10B , the radome  10  may be designed such that the dielectric layer  12  has a thickness that increases from a center portion R 1  to an edge portion R 2  (including one or more of left edge portion, right edge portion, upper edge portion, and lower edge portion) of the radome  10 . The thickness h2 of the dielectric layer  12  may be increased smoothly (as shown in  FIG. 10A ) or may be increased stepwise. It is even possible to design the thickness h2 to include two thickness values, as shown in  FIG. 10B , a smaller first thickness value in the center portion R 1  of the radome  10 , and a larger second thickness value in the edge portion R 2  of the radome  10 . 
       FIGS. 5 and 6  schematically illustrate a base station antenna that includes a radome  90  in accordance with another embodiment of the present invention. The base station antenna also includes the mounting plate  20  and the array  30  of radiating elements  31  which are the same as or similar to those in the embodiment as described above. The radome  90  includes, from the inner side to the outer side, dielectric layers  91  to  95  having respective dielectric constants of ε1 to ε5 and respective thicknesses of h1 to h5. The dielectric constants meet the following relationship: ε1&gt;ε2, ε3&gt;ε2, ε3&gt;ε4, and ε5&gt;ε4, and the thicknesses meet the following relationship: h2&gt;h1, h2&gt;h3, h4&gt;h3, and h4&gt;h5. The dielectric layers  91  to  93  and the dielectric layers  93  to  95  may have configurations similar to those of the dielectric layers  11  to  13  as described in the above embodiment respectively, and thus will not be described herein again. Compared to the above embodiment, the radome  90  may be designed in more dimensions and thus is more likely to achieve better performance. 
       FIGS. 7A and 7B  schematically show base station antennas according to further embodiments of the present invention. The base station antenna includes a radome  40  in addition to the mounting plate  20  and the array  30  of radiating elements  31  which are the same as or similar to those in the above described embodiments. The radome  40  may be any known radome including a dielectric material having a first dielectric constant and a first thickness. The base station antenna further includes a dielectric plate  50  that is extending between the array  30  and the radome  40 . A shape of dielectric plate  50  matches a shape of a corresponding portion of the radome  40 . For example, in the embodiment shown in  FIG. 7A , the radome  40  includes a substantially flat portion  41  and a curved portion  42 . At the substantially flat portion  41  of the radome  40 , the dielectric plate  50  may correspondingly have a substantially flat portion  51  opposite thereto; and at the curved portion  42  of the radome  40 , the dielectric plate  50  may correspondingly have a curved portion  52  opposite thereto. In an embodiment, the dielectric plate  50  may also include only the substantially flat portion  51  corresponding to and disposed opposite to the portion  41 , without including the portion  52  (may be similar to the dielectric plate  80  in  FIG. 9 ). In the embodiment shown in  FIG. 7B , the entire radome  40  has a curved shape. The entire dielectric plate  50  has a curved shape accordingly and extends substantially parallel to the radome  40 . 
     The dielectric plate  50  has a second dielectric constant and a second thickness. There is a gas A (such as vacuum, air or other gases) between the dielectric plate  50  and the radome  40 , and the gas A has a third dielectric constant and a third thickness (i.e., a distance between the dielectric plate  50  and the radome  40 ). Each of the first and second dielectric constants is greater than the third dielectric constant so that the dielectric plate  50 , the gas A, and the radome  40  combine to form a structure similar to the dielectric layers  11  to  13  of the radome  10  as described in the above embodiments, which can produce a similar effect. The structure of the base station antenna according to this embodiment makes it possible to readily improve the conventional base station antenna without modifying the manufacturing process of the conventional radome, and thus has low costs. The thicknesses and dielectric constants of each dielectric layers, i.e., the dielectric plate  50 , the gas A, and the radome  40 , may be determined with reference to the relevant description in the above embodiments. In some embodiments, the distance between the dielectric plate  50  and the radome  40  (i.e., the third thickness) at various portions is substantially identical. In some further embodiments, the third thickness is increased from a location of the dielectric plate  50  close to the center of the array  30  (e.g., a location near the portion  51 ) to a location of the dielectric plate  50  close to the edge portion of the array  30  (e.g., a location near the portion  52 ). 
     For a single column of radiating elements  31  in the conventional base station antenna shown in  FIG. 1  and a single column of radiating elements  31  in the base station antenna according to the embodiment of the present invention shown in  FIG. 7A , radiation patterns are simulated at 5 GHz and a 60-degree incident angle with respect to the radome. The simulation results are shown in  FIGS. 11A through 12B . In the simulation settings, the material of the radome  1  in  FIG. 1  is ASA engineering plastic (dielectric constant is 3.3 and loss tangent is 0.025), and the thickness thereof is uniform everywhere at 2.5 mm. The material of the radome  40  and the dielectric plate  50  in  FIG. 7A  is ASA engineering plastic (dielectric constant is 3.3 and the loss tangent is 0.025), and the thickness thereof are uniform everywhere at 0.5 mm both. The gas A is vacuum (dielectric constant is 1), and the thickness thereof is uniform everywhere (that is, the distance between the radome  40  and the dielectric plate  50  does not change) at 5 mm.  FIGS. 11A and 11B  are simulations of radiation patterns on the azimuth plane, where curve L 1  corresponds to the case without radome, curve L 2  corresponds to the case with radome  1  in  FIG. 1 , and curve L 3  corresponds to the case with the radome  40  and the dielectric plate  50  in  FIG. 7A . For the main lobe of the antenna beam where is interest (for example, within the range of azimuth Phi from about −60 degrees to about 60 degrees), the gain of the curve L 2  is significantly smaller than that of the curve L 1 , and the curve L 2  distorts compared to the curve L 1 , while the gain and shape of the curve L 3  are both basically agree with the curve L 1 .  FIGS. 12A and 12B  are simulations of radiation patterns on the elevation plane, where curve L 4  corresponds to the case without radome, curve L 5  corresponds to the case with radome  1  in  FIG. 1 , and curve L 6  corresponds to the case with the radome  40  and the dielectric plate  50  in  FIG. 7A . For the two main lobes of the antenna beam where is interest (for example, the elevation angle Theta is near 30 or 150 degrees), the gain of the curve L 5  is significantly smaller than that of the curve L 4 , and the curve L 5  distorts compared to the curve L 4 , while the gain and shape of the curve L 6  are both basically agree with the curve L 4 . It can be seen that at 5 GHz and the incident angle with respect to the radome is 60 degrees, the combination of the radome  40  and the dielectric plate  50  in the base station antenna according to the embodiment of the present invention may have better transmittance of electromagnetic waves and may improve radiation patterns compared with the radome  1  in the conventional base station antenna. 
       FIGS. 8A and 8B  schematically show base station antennas according to further embodiments of the present invention. The base station antenna includes, in addition to the mounting plate  20 , the array  30  of radiating elements  31  and the radome  40 , which are the same as or similar to those in the above described embodiments, dielectric plates  61  and  62  that are disposed between the array  30  and the radome  40 . Dielectric plates  61  and  62  may extend substantially parallel to the radome  40 . There is a gas B (such as vacuum, air or other gases) between the dielectric plates  61 ,  62  and the radome  40 . The dielectric plates  61  and  62  are disposed apart from each other in regions corresponding to two opposite edge portions of the radome  40  (for example, the left edge portion and the right edge portion, the upper edge portion and the lower edge portion, the lower left edge portion and the upper right edge portion, etc.). Thus, in the center of the base station antenna, electromagnetic waves are radiated outside through the radome  40  rather than through the dielectric plate  61  or  62 , whereas at the edge portions of the antenna, electromagnetic waves are radiated outside through the radome  40  and the dielectric plate  61  or  62 . Therefore, at the edge portions of the base station antenna, the dielectric plate  61  or  62 , the gas B and the radome  40  combine to constitute a configuration similar to the dielectric layers  11  to  13  as described in the above embodiments, which can improve the performance of the base station antenna in the case of the incident angle of the electromagnetic wave being large. The shape of the dielectric plate  61  or  62  may match the shape of respective corresponding portions of the radome  40 . In the embodiment shown in  FIG. 8A , for each of the dielectric plates  61 ,  62 , the distance between the dielectric plate  61  or  62  and the radome  40  (i.e., a thickness of the gas B) increases from a portion of the dielectric plate that is close to a center portion of the array  30  to a portion of the dielectric plate that is close to an edge portion of the array  30 . In the embodiment shown in  FIG. 8B , the distance between the various portions of each of the dielectric plates  61 ,  62  and the radome  40  is substantially identical. 
       FIG. 9  schematically shows a base station antenna according to an additional embodiment of the present invention. In addition to the mounting plate  20  and the array  30  of radiating elements  31  that are the same as or similar to those in the above described embodiments and the dielectric plate  80  that is similar to the dielectric plate  50  in the above embodiment, the base station antenna further includes a radome  70  similar to the radome  10  as described in the above embodiment, where the dielectric layers  71  to  73  are similar to the dielectric layers  11  to  13  in the radome  10 , respectively. There is a gas C between the dielectric plate  80  and the radome  70 , such that the dielectric plate  80 , the gas C, and the dielectric layers  71  to  73  of the radome  70  combine to form a configuration similar to the dielectric layers  91  to  95  of the radome  90  as described in the above embodiments, which may produce similar effects. 
     Although some specific embodiments of the present invention have been described in detail with examples, it should be understood by a person skilled in the art that the above examples are only intended to be illustrative but not to limit the scope of the present invention. The embodiments disclosed herein can be combined arbitrarily with each other, without departing from the scope and spirit of the present invention. It should be understood by a person skilled in the art that the above embodiments can be modified without departing from the scope and spirit of the present invention. The scope of the present invention is defined by the attached claims.