Patent Publication Number: US-6906666-B2

Title: Beam adjusting device

Description:
FIELD OF THE INVENTION 
   The present invention relates to a device for adjusting the beam direction at an antenna. 
   RELATED ART AND BACKGROUND OF THE INVENTION 
   Such a device is previously known from the document WO 96/37922 (Allgon AB). The known device comprises a feed line structure integrated with a stationary array of antenna elements so as to enable adjustment of the direction of the beam radiated from the array. The feed line structure includes a feed conductor line pattern disposed on a fixed carrier plate at a distance from and in parallel to a fixed ground plate, and a movable dielectric plate located therebetween. The feed line pattern is elongated in the same direction as the movement direction of the dielectric plate. The propagation velocity of the signal components is reduced by the presence of the dielectric plate between the respective feed line and the ground plate. Accordingly, by displacing the dielectric plate in the longitudinal direction, the phase difference between the various signal components may be controlled. 
   In the previously known device, the feed line pattern is configured basically in meander-like loops with several loop portions extending back and forth in the longitudinal direction. Accordingly, the signal paths are relatively long, and the losses of microwave power being transferred in the device is relatively high. Moreover, because of the various meander-like loops extending in parallel to each other, the device is necessarily relatively wide in a transverse direction. Therefore, the overall dimensions of the device are relatively large. 
   SUMMARY OF THE INVENTION 
   Against this background, a main object of the present invention is to provide such a device having a feed line structure which inherently involves low losses and which is smaller and less expensive to manufacture than the previously known device. 
   Accordingly, the feed line structure is generally configured as a star with at least four line segments extending from a source connection terminal, at the centre of the star, to the respective feed connection terminals. At least two line segments extend generally in a first direction along the main direction of the device, and two further line segments extend generally in an opposite direction. The dielectric body is divided into different portions having different effective dielectric values. A first body portion is located adjacent to a first pair of line segments extending in opposite directions, and a second body portion is located adjacent to a second pair of line segments likewise extending in opposite directions. In this way, even if the line segments have substantially the same length, it is possible to obtain a phase angle difference. Preferably, the feed line structure is configured as the letter “H” with four line segments of substantial equal length. 
   The difference in “effective dielectric value” may be obtained in different ways. The two body portions may be made of different materials having two different dielectric constants. Alternatively, or in addition thereto, the two body portions may have different geometrical cross-sections along at least a major part of their respective lengths, e.g. a difference in thickness. Preferably, as a further alternative, the two body portions may have mutually different geometrical irregularities making the effective dielectric values different. Such irregularities may comprise holes, e.g. extending in a transverse direction from the respective line segments to the ground plane. 
   Advantageously, the feed line structure may comprise strip line segments located between top and bottom walls of a closed elongated housing, the top and bottom walls serving as a ground plane. Then, each body portion may comprise upper and lower parts located above and below the strip line segments, respectively. 
   These and other features of the invention will become apparent from the detailed description below. 
   The invention will be explained more fully below with reference to the appended drawings illustrating some preferred embodiments. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows the device according to the invention in a perspective view; 
       FIG. 2  shows the device of  FIG. 1  in an end view; 
       FIG. 3  shows a longitudinal central section through the device of  FIG. 1 ; 
       FIG. 4  shows a planar view of the device of  FIG. 1  with a top wall of the housing being taken away; 
       FIG. 5  shows a cross section through the device of  FIG. 1 ; 
       FIG. 6  shows a cross section through a modified version of the device of  FIG. 1 , and 
       FIG. 7  shows a second embodiment of the device, including a different feed line structure. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   The device shown in  FIGS. 1 and 2  comprises an elongated box-like housing  10  consisting of an upper part  20 , a lower part  30 , end pieces  40 ,  50  and a feed line structure, generally denoted  100 , inside the housing  10 . 
   The housing  10  is of the general kind described in the separate Swedish patent application entitled “Shielded Housing” filed simultaneously by the same applicant. The disclosure of the “Shielded Housing” application is included herein by reference. 
   The upper part  20  of the housing includes a substantially planar top wall  21  and, integral therewith, two downwardly directed, longitudinally extending outer side flanges  22 ,  23 . The lower part  30  of the housing includes a substantially planar bottom wall  31  and, integral with the longitudinal edge portions of the bottom wall  31 , inner side flanges  32  and  33 . These inner side flanges  32 ,  33  are dimensioned to make contact, substantially over the entire external surface thereof, with the inside surfaces of the outer side flanges  22 ,  23 . As explained in the separate “Shielded Housing” application, such a surface contact is obtained irrespective of the exact dimensions of the upper and lower parts within certain limits maintained during manufacture of the device. The top and bottom walls  21  and  31  of the housing are held at a pre-determined, well-defined mutual distance defined by the respective end piece  40 ,  50  as explained in detail in the “Shielded Housing” application. 
   The housing  10  accommodates a feed line structure  100  and a movable dielectric body  111  serving as a device for adjusting the beam direction radiated from a stationary array of antenna elements (not shown), coupled to the device. 
   In the illustrated embodiment, the feed line structure  100  is configured like the letter “H” with a central source connection terminal  101 , first and second straight line segments  102 ,  103  extending in a first direction along the main direction A of the device and third and fourth straight line segments  104 ,  105  extending in a second direction being opposite to the first direction. Each feed line segment is connected to an associated feed connection terminal  102   a,    103   a,    104   a  and  105   a  respectively. See also FIG.  4 . 
   The source connection terminal  101  is connectable to a signal source by means of a feed conductor  106 , which extends centrally between the two line segments  104 ,  105  and is connected to a feed terminal  106   a.    
   In use, the feed terminal  106   a  is connected, e.g. via a coaxial cable, to transceiver circuits (not shown), e.g. included in a base station of a cellular mobile telephone system. The feed connection terminals  102   a,    103   a,    104   a,    105   a,  on the other hand, are connected, e.g. via four coaxial cables, to associated antenna elements or sub-arrays, e.g. pairs of antenna elements, arranged in a stationary array, normally a linear row, in an antenna, e.g. a base station antenna. Preferably, the transmission lines between the respective feed connection terminals and the associated antenna elements have such lengths that the phase shift, from the source connection terminal to the respective antenna element or sub-array, is generally different for each one of the four antenna elements or sub-arrays. Moreover, these differences can be adjusted by means of the feed line structure  100  with a displaceable dielectric body inside the housing  10 , as will be explained below. 
   Turning now to  FIGS. 3 and 4 , a microwave signal appearing at the feed terminal  106   a  will propagate along the central feed conductor  106  to the centrally located source connection terminal  101 . In order to gradually match the impedance to the impedance value at the junction point, the feed conductor  106  is widened stepwise towards the source connection terminal. Furthermore, adjacent to the terminal  101 , there are upper and lower stationary dielectric elements  109 ,  110 , serving to additionally match the impedance of the four feed line segments  102 ,  103 ,  104 ,  105  extending electrically in parallel from the source connection terminal  101  to the four feed connection terminals  102   a,    103   a,    104   a,    105   a.  Thanks to the dielectric elements  109 ,  110 , the impedance matching can be achieved without making the feed conductor  106  extremely wide adjacent to the source connection terminal  101 . Therefore, the width of the housing  10  can be relatively small so as to reduce the overall dimensions of the device. These dimensions will be reduced for other reasons as well, as will be explained further below. 
   The feed conductor  106  and the feed line segments  102 ,  103 ,  104 ,  105  are embodied as strip lines between the top and bottom walls  21  and  31 , the latter walls serving as ground planes. See also  FIGS. 5 and 6 . 
   As compared to microstrip embodiments, the strip line structure has a number of advantages. First, the device can be made shorter and less wide. The reduced width is obtained because the strip lines are generally narrower than corresponding microstrip lines (with the same impedance and ground plane distance), and the parallel line segments can be positioned closer to each other without mutual coupling, since the double ground plane configuration limits the coupling between neighbouring parallel conductors more effectively. Also, dielectric material can be disposed above and below each strip, so virtually all of the electrical field is influenced by the dielectric material. Therefore, for a given phase angle difference, the length in the longitudinal direction can be reduced. 
   Secondly, there will be no problems with spurious radiation, since the total structure is confined within a shielded box or housing  10 . Thirdly, the dielectric material above and below the strip can serve as spacing elements so as to keep the strip line in position. 
   In accordance with the present invention, a unitary body  111  of dielectric material is arranged between the housing walls  21 , 31  and the feed line segments  102 ,  103 ,  104 ,  105  so as to influence the propagation velocity and the phase shift of the signal components being transferred along the respective line segments. The dielectric body  111  is linearly displaceable along the longitudinal direction A of the device between two end positions, one of which is the fully drawn position in FIG.  4  and the other being the one indicated by dashed lines  111 ′ somewhat to the right. 
   The dielectric body  111  includes two longitudinal side portions connected by a transverse body portion  112 , namely a first body portion  113  located along the first and third line segments  102 ,  104  and a second body portion  114  located along the second and fourth line segments  103  and  105 . The overall length of the dielectric body  111  is somewhat greater than the distance between the end positions indicated in FIG.  4 . Also, the dimensions are such that each body portion  113 ,  114  is always located in a longitudinal region close to the source connection terminal  101 , so that its end portions  113 ′,  113 ″ and  114 ′,  114 ″, respectively, are situated adjacent to the oppositely extending line segments  102 ,  104  and  103 ,  105 , respectively. According to the invention, the two body portions  113 ,  114  have different effective dielectric values. In the illustrated embodiment, this is achieved in that the major part of the second body portion  114  is solid, whereas the first body portion  113  is provided with a row of through-going holes  115 , so that the retarding effect of the dielectric material is greater in the second body portion  114  than in the first body portion  113 . In the illustrated embodiment, each body portion  113 ,  114  has an upper part  113   a,    114   a,  and a lower part  113   b,    114   b  respectively (FIG.  5 ). These upper and lower parts also serve as spacing elements between the feed line segments and the upper and lower housing walls  21 ,  31 . 
   The longitudinal end portions  113 ′,  113 ″,  114 ′,  114 ″ of the two dielectric body portions  113 ,  114  are provided with recesses  116 ,  117  and holes  118 ,  119 , respectively, so as to provide an impedance transformation between the central parts containing dielectric material and the air-filled spaces on both longitudinal sides of the dielectric body  111 . 
   In a manner similar to that explained in the above-mentioned document WO 96/37922 (Allgon), the phase angle differences between the signal components at the feed connection terminals  102   a,    103   a,    104   a,    105   a  will depend on the particular position of the dielectric body  111 . When the dielectric body  111  is displaced a certain distance, all the phase shifts of the four signal components will be changed uniformly. Accordingly, the phase angle difference between the terminals associated with adjacent antenna elements (or sub-arrays) will always be mutually the same. Thus, the phase angle differences between the terminals  103   a  and  102   a,  between the terminals  102   a  and  104   a,  and between the terminals  104   a  and  105   a  will be equal to each other. Therefore, the composite beam from the four antenna elements coupled to these terminals will always have a wave front substantially in the form of a straight line, and the inclination of this wave front can be adjusted by displacing the dielectric body  111  to a different position in the longitudinal direction of the device. 
   In order to enable a controlled displacement of the dielectric body  111 , a movement transfer member  120  is secured to the dielectric body  111  and extends through a longitudinal slot  121  in the bottom wall  31  of the housing  10 . The member  120  is connected to a slide member  122 , which is longitudinally guided in profiled grooves  123  formed at the lower side of the bottom wall  31 . Of course, the slide member  122  can be mechanically activated as desired to adjust the inclinational angle of the beam from the antenna. 
   It will be appreciated that there are various ways to achieve a difference in effective dielectric value of the two body portions  113  and  114 . An alternative to holes is to make the thickness of the two portions  113 ,  114  different, as illustrated in  FIG. 6 , where the second body portion  214   a,    214   b  is much thinner than the first body portion  213   a,    213   b.    
   The illustrated embodiment with holes  115  in one of the body portions is advantageous for the reason that the two body portions  113 ,  114  have the same overall thickness and serve as effective spacing elements between the feed line segments and the housing walls. 
   Of course, other kinds of irregularities may be used instead of holes, such as recesses extending only partially through the material in a transverse direction. Longitudinal slots or the like are also possible. 
   Preferably, the dielectric material has a high dielectric constant. A suitable material is IXEF 1032 (manufactured by SOLVAY, Belgium) which has a dielectric constant of 4.5. Preferably, the dielectric constant of the dielectric material should be in the integral between 2 and 6. 
   Generally, low dielectric constant values make the whole structure longer, as the difference in electrical length is less between an air line and a line with dielectric material beneath and above. A too high dielectric constant value, on the other hand, makes the impedance difference so great that multiple transformation sections  113 ′, 113 ″, etc might be necessary to achieve a good impedance match, with associated increased length. A higher dielectric constant value also makes the design more sensitive to thickness tolerance induced air gaps between the strip line and the dielectric material. 
   The central source connection terminal may itself serve as a feed connection terminal for direct connection to an antenna element. Moreover, there may be more than four feed line segments extending in a star configuration from the central source connection terminal, e.g. three feed line segments in each opposite direction with associated dielectric body portions having mutually different effective dielectric values. 
   A modified embodiment of the feed line structure is shown in  FIG. 7 , where corresponding parts are denoted with numerals  201 , etc instead of  101 , etc. (FIGS.  3  and  4 ). The displaceable dielectric body  211 , with side portions  213 , 214 , covers (partially) only the four line segments  202 ,  203 ,  204 ,  205 , whereas the feed conductor  206  and a fifth line segment  207  extend freely inside the housing with air gaps to the top and bottom walls  21 , 31  (FIG.  2 ). 
   The fifth line segment  207  is connected to a centrally located antenna element. The phase angle of the signal component reaching this centrally located antenna element (not shown) or sub-array is independent of the particular position of the displaceable dielectric body  211 . The line segments  202 , 203  are connected, e.g. via coaxial cables, to two antenna elements or sub-arrays on one side of the central element, and the line segments  204 , 205  are connected to two antenna elements or sub-arrays on the other side of the central element.