Patent Publication Number: US-11050161-B2

Title: Antenna feeding network comprising coaxial lines with inner conductors connected by snap-on fingers and a multi-radiator antenna formed therefrom

Description:
TECHNICAL FIELD 
     The invention relates to the field of antenna feeding networks for multi-radiator antennas, in which the feeding network comprises at least two interconnected coaxial lines. 
     BACKGROUND OF THE INVENTION 
     Multi-radiator antennas are frequently used in, for example, cellular networks. Such multi-radiator antennas comprise a number of radiating antenna elements, for example, in the form of dipoles for sending or receiving signals, an antenna feeding network and an electrically conductive reflector. The antenna feeding network distributes the signal from a common coaxial connector to the radiators when the antenna is transmitting and combines the signals from the radiators and feeds the signals to the coaxial connector when receiving. A possible implementation of such a feeding network is shown in  FIG. 1 . 
     In such a network, if the splitters/combiners consist of just one junction between 3 different 50 ohm lines, impedance match would not be maintained, and the impedance seen from each port would be 25 ohm instead of 50 ohm. Therefore, the splitter/combiner usually also includes an impedance transformation circuit that maintains 50 ohm impedance at all ports. 
     A person skilled in the art would recognize that signal feeding is fully reciprocal in the sense that transmission and reception can be treated in the same way, and to simplify the description of this invention only the transmission case is described below. 
     The antenna feeding network may comprise a plurality of parallel coaxial lines being substantially air filled, each coaxial line comprising a central inner conductor at least partly surrounded by an outer conductor with insulating air in between the inner and outer conductors. The coaxial lines and the reflector may be formed integrally with each other. Splitting of the inner conductors may be done via crossover connections between inner conductors of adjacent coaxial lines. In order to preserve the characteristic impedance, the lines connecting to the crossover element include impedance matching structures. 
     Published application number US 2013/01355166 A1 discloses an antenna arrangement comprising an antenna feeding network including at least one antenna feeding line comprising a coaxial line having a central inner conductor and a surrounding outer conductor. The inner conductor is suspended inside the outer conductor with the help of dielectric support means. Published application number US 2013/0135166 A1 suggests use of a crossover element to connect two inner conductors of two adjacent coaxial lines. The crossover element is galvanically connected to the inner conductors by means of, for example, screws, soldering, gluing or a combination thereof, and thus a direct physical contact between the electrically conductive inner conductor and the crossover element is established. Where two conductors need to be connected, the wall between the two coaxial lines is partially or completely removed, and the crossover element is placed in the opening. The antenna arrangement according to published application number US 2013/0135166 has the disadvantage that such an antenna arrangement may be difficult and time consuming to assemble or manufacture. A further disadvantage with this arrangement is that the mechanical connection formed by the screwed, glued or soldered connection between the lines may introduce passive intermodulation (PIM). 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to overcome at least some of the disadvantages of the prior art described above. 
     These and other objects are achieved by the present invention by means of an antenna feeding network comprising at least two coaxial lines and a multi radiator antenna comprising such an antenna feeding network according to the technology described herein. 
     According to a first aspect of the invention, an antenna feeding network for a multi-radiator antenna is provided, the antenna feeding network comprising at least two coaxial lines. Each coaxial line comprises a central inner conductor and an elongated outer conductor surrounding the central inner conductor. At least a first inner conductor and a second inner conductor of the at least two coaxial lines are indirectly interconnected. 
     In other words, the antenna feeding network comprises at least a first coaxial line and a second coaxial line, wherein the first coaxial line comprises a first inner conductor and an elongated outer conductor surrounding the first inner conductor, and wherein the second coaxial line comprises a second inner conductor and an elongated outer conductor surrounding the second inner conductor. The first inner conductor, the second inner conductor, and optionally further inner conductors, are indirectly interconnected or interconnectable. The coaxial lines may be parallel. 
     The invention is based on the insight that an antenna feeding network, which is easy to assemble, yet provides high performance and low passive intermodulation, may be achieved by indirectly interconnecting inner conductors of the coaxial lines instead of connecting the inner conductors galvanically. Such an indirect interconnection, i.e. capacitive or inductive interconnection or a combination of the two, between the lines may provide an interconnection, which does not suffer from the disadvantages associated with mechanical/galvanical connections discussed above. 
     It is understood that the term coaxial line refers to an arrangement comprising an inner conductor and an outer conductor with insulating or dielectric material or gas there between, where the outer conductor is coaxial with the inner conductor in the sense that the outer conductor completely or substantially surrounds the inner conductor. Thus, the outer conductor does not necessarily have to surround the inner conductor completely, but may be provided with openings or slots, which may even extend along the full length of the outer conductor. 
     The at least two coaxial lines may each be provided with air between the inner and outer conductors. The air between the inner and outer conductors thus replaces the dielectric often found in coaxial cables. 
     In embodiments, at least one, or each, coaxial line of the at least two coaxial lines is provided with at least one support element configured to support the central inner conductor, the support element being located between the outer and inner conductors. 
     In embodiments, at least one, or each, coaxial line of the at least two coaxial lines is furthermore provided with at least one dielectric element to at least partially fill the cavity between the inner and outer conductors. Such dielectric element(s) is/are preferably slidably movable inside the outer conductor(s) to co-operate with the coaxial line(s) to provide a phase shifting arrangement. Phase shifting is achieved by moving the dielectric element that is located between the inner conductor and the outer conductor of the coaxial line. It is a known physical property that introducing a material with higher permittivity than air in a transmission line will reduce the phase velocity of a wave propagating along that transmission line. This can also be perceived as delaying the signal or introducing a phase lag compared to a coaxial line that has no dielectric material between the inner and outer conductors. If the dielectric element is moved in such a way that the outer conductor is filled with more dielectric material, the phase shift increases. The at least one dielectric element may have a U-shaped profile such as to partly surround the inner conductor in order to at least partly fill the cavity between the inner and outer conductors. 
     In embodiments, two of the at least two coaxial lines form a splitter/combiner. When operating as a splitter, the inner conductor of a first coaxial line is part of the incoming line, and the two ends of the inner conductor of the second coaxial line are the two outputs of the splitter. Thus, the second coaxial line forms two outgoing coaxial lines. In such an embodiment, the dielectric element may be arranged in the second coaxial line in such a way that by moving the dielectric part, different amounts of dielectric material is present in the respective outgoing coaxial lines. Such an arrangement allows the differential phase of the outputs of a splitter to be varied by adjusting the position of the dielectric part within the splitter. A reciprocal functionality is obtained when the coaxial line functions as a combiner. Such splitters/combiners having variable differential phase shifting capability are advantageously used in antennas, having radiators positioned in a vertical column, to adjust the electrical antenna tilt angle by adjusting the relative phases of the signals feeding the radiators. 
     In embodiments where the coaxial line(s) is/are provided with support element(s), dielectric element(s) or other components inside the outer conductor(s), the coaxial line(s) may be described as substantially air filled because these components occupy part of the space inside the outer conductor, which would otherwise be filled with air. 
     In embodiments, the antenna feeding network comprises a connector device configured to indirectly interconnect the at least first and second inner conductors. 
     Herein the word indirectly means that conductive material of the connector device is not in direct physical contact with the conductive material of the first inner is conductor and the second inner conductor, respectively. Indirectly thus means an inductive, a capacitive coupling or a combination of the two. 
     In embodiments, there may be at least one insulating layer arranged in between the conductive material of the connector device and the conductive material of the inner conductor. This at least one insulating layer may be arranged on the connector device and thus belong to the connector device and/or it may be arranged on the first inner conductor or on the second inner conductor or on both inner conductors. The at least one insulating layer may alternatively comprise a thin film which is arranged between the conductive material of the connector device and the conductive material of the inner conductor. The at least one insulating layer may also be described as an insulating coating. The insulating layer or insulating coating may be made of an electrically insulating material such as a polymer material or a non-conductive oxide material with a thickness of less than 50 μm, such as from 1 μm to 20 μm, such as from 5 μm to 15 μm, such as from 8 μm to 12 μm. Such a polymer or oxide layer may be applied with known processes and high accuracy on the connector device and/or on the inner conductor(s). 
     In embodiments, the connector device may be configured to be removably connected to the first inner conductor and the second inner conductor. This allows a quick reconfiguration of the antenna feeding network, if necessary or can be used for troubleshooting in antenna production. 
     In preferred embodiments, the connector device may be realized as a snap-on element comprising at least one pair of snap-on fingers and a bridge portion, whereby the snap-on fingers may be connected to the bridge portion and wherein the snap-on fingers are configured to be snapped onto the first or the second inner conductor. The bridge portion may be configured to connect with the other of the first or the second inner conductor, which is not engaged by the pair of snap-on fingers, when the snap-on element is snapped onto the first or second inner conductor. The snap-on element may comprise two pairs of snap-on fingers which are connected by the bridge portion, wherein the two pairs of snap-on fingers may be configured to be snapped onto the first inner conductor and the second inner conductor, respectively. These preferred embodiments are advantageous since they allow convenient assembly of the antenna feeding network, where the connector device is simply snapped onto the first and/or second inner conductors. The connector device may also be arranged with two or more bridge portions, connecting three or more pairs of snap-on fingers. 
     In an alternative embodiment, one of the inner conductors comprises a cavity and another of the inner conductors comprises a rod-shaped protrusion configured to extend into and engage with the cavity. An insulating layer is provided in the cavity and/or on the rod-shaped protrusion, or alternatively, an insulating layer is provided as an insulating film between the cavity and the rod-shaped protrusion. Thus, an indirect connection may be provided between two inner conductors. These embodiments are advantageous since they allow convenient assembly of the antenna feeding network, where the inner conductors are interconnected simply by pushing the rod-shaped protrusion into the cavity. Also, this arrangement will reduce the risk for PIM. The cavity may have a depth corresponding to a quarter wavelength. 
     In yet an alternative embodiment, the connector device comprises at least two engaging portions. Each of the at least first and second inner conductors comprises corresponding engaging portions, each adapted to engage with a corresponding engaging portion of the connector device. The engaging portion is in the form of a cavity or rod-shaped protrusion. An insulating layer is provided in the cavity and/or on the rod-shaped protrusion, or alternatively, an insulating layer is provided as an insulating film between the cavity and the rod-shaped protrusion. Thus, an indirect connection may be provided between two inner conductors. The connector device may in embodiments be provided with three legs, each being provided with an engaging portion at its end to interconnect three inner conductors. For example, the connector device may be provided with cavities at each end of the legs, and three inner conductors may be provided with rod-shaped protrusions adapted to fit and engage in a respective cavity. The cavity or cavities may have a depth corresponding to a quarter wavelength. The connector device may also be arranged such as to connect four or more inner conductors. 
     The embodiments described above may be combined in any practically realizable way. 
     According to a second aspect of the invention, a multi radiator base station antenna is provided, in which the antenna comprises an electrically conductive reflector, at least one radiating element arranged on the reflector and an antenna feeding network as described above. 
     In an embodiment of the multi-radiator antenna according to the second aspect of the invention, the electrically conductive reflector may comprise at least one opening; the opening may be located on either the front side or the back side of the reflector, so that the connector device can be installed on the first and second inner conductor via the opening. The opening may advantageously be adapted to the size of the connector device. An opening may be assigned to each inner conductor pair of the antenna feeding network so that all inner conductors in the electrically conductive reflector may be connected by connector devices. 
     According to a third aspect of the invention, a method for assembling an antenna feeding network for a multi-radiator antenna is provided. The method comprises providing at least two coaxial lines, wherein each coaxial line is provided with a central inner conductor and an elongated outer conductor surrounding the central inner conductor and interconnecting at least two inner conductors of the coaxial lines indirectly. 
     In an embodiment of the method according to the third aspect of the invention, the method further comprises providing a connector device, and providing an to insulating layer on the connector device and/or on the at least first and second conductors. Alternatively, an insulating layer is provided between the connector device and the at least first and second conductors. The embodiment further comprises connecting the connector device between the at least first and second inner conductors, wherein the connector device preferably is realized as a snap-on is element comprising snap-on fingers adapted to be snapped onto the at least first and second inner conductors. 
     In embodiments of a method according the third aspect of the invention, the method provides for assembling an antenna feeding network according to the first aspect of the invention or embodiments thereof. Embodiments of the method comprises performing steps to achieve features corresponding to any of the above described embodiments of the antenna feeding network. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be described, for exemplary purposes, in more detail by way of embodiments and with reference to the enclosed drawings, in which: 
         FIG. 1  schematically illustrates a multi-radiator antenna; 
         FIG. 2  schematically illustrates a perspective view of an embodiment of a multi-radiator antenna according to a second aspect of the invention; 
         FIG. 3  schematically illustrates a perspective view of an embodiment of an antenna feeding network according to a first aspect of the invention; 
         FIG. 4  schematically illustrates another perspective view of parts of an embodiment of an antenna feeding network according to the first aspect of the invention; 
         FIG. 5  schematically illustrates a front view into two neighboring coaxial lines of an embodiment of an antenna feeding network according to the first aspect of the invention; 
         FIG. 6  schematically illustrates parts of another embodiment of an antenna feeding network according to the first aspect of the invention; and 
         FIG. 7  schematically illustrates parts of yet another embodiment of an antenna feeding network according to the first aspect of the invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  schematically illustrates an antenna arrangement  1  comprising an antenna feeding network  2 , an electrically conductive reflector  4 , which is shown schematically in  FIG. 1 , and a plurality of radiating elements  6 . The radiating elements  6  may be dipoles. 
     The antenna feeding network  2  connects a coaxial connector  10  to the plurality of radiating elements  6  via a plurality of lines  14 ,  15 , which may be coaxial lines, which are schematically illustrated in  FIG. 1 . The signal to/from the connector  10  is split/combined using, in this example, three stages of splitters/combiners  12 . Turning now to  FIG. 2 , which illustrates a multi-radiator antenna  1  in a perspective view, the antenna  1  comprises the electrically conductive reflector  4  and radiating elements  6   a ,  6   b , and  6   c.    
     The electrically conductive reflector  4  comprises a front side  17 , where the radiating elements  6   a - 6   c  are mounted and a back side  19 . 
       FIG. 2  shows a first coaxial line  20   a  which comprises a first central inner conductor  14   a , an elongated outer conductor  15   a  forming a cavity or compartment around the central inner conductor, and a corresponding second coaxial line  20   b  having a second inner conductor  14   b  and an elongated outer conductor  15   b . The outer conductors  15   a ,  15   b  have square cross sections and are formed integrally and in parallel to form a self-supporting structure. The wall which separates the coaxial lines  20   a ,  20   b  constitute vertical parts of the outer conductors  15   a ,  15   b  of both lines. The first and second outer conductors  15   a ,  15   b  are formed integrally with the reflector  4  in the sense that the upper and lower walls of the outer conductors are formed by the front side  17  and the back side  19  of the reflector, respectively. 
     Although the first and second inner conductors  14   a ,  14   b  are illustrated as neighboring inner conductors they may actually be further apart thus having one or more coaxial lines, or empty cavities or compartments, in between. 
     In  FIG. 2 , not all longitudinal channels or outer conductors are illustrated with inner conductors, it is however clear that they may comprise such inner conductors. 
     The front side  17  of the reflector comprises at least one opening  40  for installation of a connector device  8 . The opening  40  extends over the two neighboring coaxial lines  20   a ,  20   b  so that the connector device  8  can engage the first and second inner conductors  14   a ,  14   b.    
     Although the invention is illustrated with two neighboring inner conductors  14   a ,  14   b , alternative embodiments have an opening (not shown) that extends across more than two coaxial lines  20   a ,  20   b  and provide a connector device  8  than can bridge two or even more inner conductors. Such a connector device (not shown) may thus be designed so that the connector device extends over a plurality of coaxial lines between two inner conductors or over empty cavities or compartments. Such a connector device (not shown) may also be used to connect three or more inner conductors. 
     In  FIG. 3 , an enlarged view of the opening  40  and the connector device  8  arranged therein is illustrated. The connector device  8  is clipped or snapped onto the first inner conductor  14   a  and the second inner conductor  14   b  thereby providing a removable connection between the first inner conductor and the second inner conductor. The connection between the first inner conductor  14   a  and the second inner conductor  14   b  is electrically indirect, which means that it is either capacitive, inductive or a combination thereof. This is achieved by providing a thin insulating layer of a polymer material or some other insulating material (e.g., a non-conducting oxide) on the connector device  8 . The insulating layer may have a thickness of 1 μm to 20 μm, such as from 5 μm to 15 μm, such as from 8 μm to 12 μm, or may have a thickness of 1 μm to 5 μm. The insulating layer may cover the entire outer surface of the connector device  8  or at least the portions of the connector device  8  that engage the first and second inner conductors  14   a ,  14   b.    
     The connector device  8  comprises a bridge portion  32  and two pairs of snap-on fingers  30 ,  30 ′. One of the two pairs of snap-on fingers  30 ′ is arranged close to one end of the bridge portion  32  and the other of the two pairs of snap-on fingers  30  is arranged close to the other end of the bridge portion  32 . The two pairs of snap-on fingers  30 ,  30 ′ may be connected to the bridge portion  32  via connecting portions configured such that the bridge portion  32  is distanced from the first and second inner conductors  14   a ,  14   b . In other embodiments, the snap-on fingers  30 ,  30 ′ are connected directly to the bridge portion  32 . The connecting portions, as well as the other portions of the connector device, are shaped to optimize the impedance matching of the splitter/combiner formed by the connector device and the coaxial lines. The shape, or preferably the diameter of the connecting inner conductors may also contribute to the matching of the splitter/combiner. 
     As can be seen from  FIG. 3 , the vertical separating wall portion  22  is reduced to about two-thirds to three-quarters of its original height in the area of the opening  40  so that the connector device  8  does not protrude over the front side  17  of the electrically conductive reflector  4 . In other embodiments, the wall portion  22  is reduced all the way to the floor of the outer conductors. The remaining height of the wall portion is adapted together with the other components, such as the connector device to optimize the impedance match. 
     It may be possible (not shown in the figures) to provide only one pair of snap-on fingers, for example the pair of snap-on fingers  30 ′ engaging the first inner conductor  14   a  providing an indirect connection, and to contact directly the other end of the bridge portion  32  to the second inner conductor  14   b  without insulating layer or coating. Such a direct connection can be provided by connecting the bridge portion  32  to inner conductor  14   b  by means of a screw connection, by means of soldering, by making the bridge portion an integral part of inner conductor  14   b , or by some other means providing a direct connection. 
       FIG. 4  shows another view of parts of an embodiment of the antenna feeding network. The connector device  8  engages the first and second inner conductors  14   a ,  14   b . The connector device  8  and the inner conductors  14   a ,  14   b  together form a splitter/combiner. When operating as a splitter, the inner conductor  14   a  is part of the incoming line, and the two ends of the inner conductor  14   b  are the two outputs of the splitter. The U-shaped dielectric element  9  can be moved along the inner conductor  14   b , which, together with an outer conductor (not shown), forms first and second coaxial output lines on opposite sides of the connector device  8 . The dielectric element, thus, has various positions along those coaxial output lines. 
     First, consider the case when the dielectric element  9  is placed in a central position, equally filling the first and second output coaxial lines. When a signal is received at the input coaxial line  14   a , the signal is divided between the first output coaxial line and the second output coaxial line, and the signals outputted from the two output coaxial lines is equal in phase. If the dielectric element  9  is moved in such a way that the first output coaxial line is filled with more dielectric material than the second output coaxial line, the phase shift from the input to the first output increases. At the same time the second output coaxial line would be filled with less dielectric material, and the phase shift from the input to the second output decreases. Hence, the phase at the first output lags the phase at the second output. If the dielectric element is moved in the opposite direction, the phase of the first output leads the phase of the second output. The splitter/combiner may thus be described as a differential phase shifter. 
       FIG. 4  illustrates how the connector device  8  engages the first and second inner conductors  14   a ,  14   b  in circumferential recessed areas or grooves  42  of the first and second inner conductors  14   a ,  14   b . These grooves may be used to position the connector device  8  correctly along the longitudinal direction of the inner conductors  14   a ,  14   b.    
       FIG. 5  illustrates a view into the first and second coaxial lines  20   a ,  20   b  where the connector device  8 , bridging the first inner conductor  14   a  and the second inner conductor  14   b  is visible. The snap-on fingers  30 ,  30 ′ are not so well visible since the snap-on fingers  30 ,  30 ′ engage the first and second inner conductors  14   a ,  14   b  in areas with a smaller diameter than the rest of the first and second inner conductors  14   a ,  14   b .  FIG. 5  further illustrates that the bridge portion  32  is not extending beyond the front side  17  of the electrically conductive reflector  4 . 
     The embodiment of the connector device  8  has been described having a thin insulating layer on the connector device  8 . It may however be possible to provide the first and second inner conductors  14   a ,  14   b  respectively with a very thin insulating layer of a polymer material and provide the connector device without any insulating layer. The insulating layer may cover the entire outer surface of the first and second inner conductors  14   a ,  14   b , or at least the portions where snap-on fingers  30 ,  30 ′ of the connector device  8  engage the first and second inner conductors  14   a ,  14   b . In other embodiments, an isolating material in the form of a thin foil is placed between the snap-on fingers  30 ,  30 ′ and the inner conductor  14 . 
     Further, the connector device  8  has been described illustrating a first and a second inner conductor  14   a ,  14   b  in the antenna arrangement  1  ( FIG. 1 ). The antenna arrangement  1  may however comprise more than one connector device  8  and a plurality of inner conductors  14   a ,  14   b.    
       FIG. 6  schematically illustrates parts of another embodiment of an antenna feeding network according to the first aspect of the invention. In  FIG. 6 , a cross section view is shown of a first inner conductor  14   a ′ and a second inner conductor  14   b ′. The first inner conductor  14   a ′ comprises a cavity  50  extending axially into one end of the first inner conductor  14   a ′. The second inner conductor  14   b ′ comprises a rod-shaped protrusion  51  extending axially from one end of the second inner conductor  14   b ′. The protrusion  51  is adapted to extend into the cavity  50  of the first inner conductor. An insulating layer  52  is provided in and around the cavity to provide an indirect electrical connection between the conductors. In other embodiments, the insulating layer may be provided on the protrusion  51  or as a separate insulating film between the conductors. The insulating layer may be provided as a polymer material or some other insulating material (e.g., a non-conducting oxide) on either or both inner conductors  14   a ′ or  14   b ′, completely or partially covering inner conductors  14   a ′ or  14   b ′, or the insulating material may be provided as a thin insulating foil inserted between inner conductors  14   a ′ and  14   b′.    
       FIG. 7  schematically illustrates parts of yet another embodiment of an antenna feeding network according to the first aspect of the invention. In  FIG. 7 , a cross section view is shown of three inner conductors  14   a ″,  14   b ″ and  14   c ″ and a three-legged h-shaped connector device  8 ′. Each leg of the connector device  8 ′ is provided with a cavity  50   a ,  50   b , and  50   c  extending axially into respective leg ends. The inner conductors  14   a ″,  14   b ″, and  14   c ′″ each comprises a rod-shaped protrusion  51   a ,  51   b  and  51   c  extending axially from one end of the inner conductors  14   a ″,  14   b ″, and  14   c ″. The protrusions  51   a ,  51   b , and  51   c  extend into corresponding cavities  50   a ,  50   b , and  50   c  of the connector device. Insulating layers  52   a ,  52   b , and  52   c  are provided in and around the cavities to provide an indirect electrical connection between the conductors. In other embodiments, the insulating layers may be provided on the protrusions, or as separate insulating films between the conductors and the connector device. The h-shaped connector device  8 ′ may be mounted in a similar manner as the connector device  8 , i.e. by reducing a separating wall between two adjacent outer conductors. In other embodiments, the connector device  8 ′ is provided with protrusions, and the inner conductors  14   a ″,  14   b ″, and  14   c ″ are provided with cavities. 
     The description above and the appended drawings are to be considered as non-limiting examples of the invention. The person skilled in the art realizes that several changes and modifications may be made within the scope of the invention. For example, the number of coaxial lines may be varied and the number of radiators/dipoles may be varied. Furthermore, the shape of the connector element (if any) and inner conductors and the placement of the insulating layer or coating may be varied. Furthermore, the reflector does not necessarily need to be formed integrally with the coaxial lines or may, on the contrary, be a separate element. The scope of protection is determined by the appended patent claims.