Phase shifter

Abstract: A phase shifting element for controllably varying the phase of signals of desired frequencies or frequency range passing along an adjacent transmission line section associated with, for example, an antenna array to electrically down-tilt the array's electromagnetic wave pattern. The phase shifting element includes a planar dielectric member having at least three discrete co-planar interactive segments extending from an edge thereof and arranged to movably overlap or intersect or interleave the transmission line section to vary its effective electrical length. Optimum dimensions of each interactive segment and optimum widths of gaps defined by facing edges of adjacent segments are determined by a computer optimisation program, such that the phase shifting element achieves a wide operating frequency range and minimum aggregate signal reflection.

Description: Field of Invention This invention relates a method of obtaining a phase shifting element for antennas and in particular for an arrangement to electrically down-tilt the electromagnetic wave pattern associated with a transmit antenna array, or electrically re-orient the pattern of a receive antenna array. Background of Invention It is sometimes desirable to adjust the orientation of the electromagnetic wave pattern of a transmit antenna array, particularly a downward adjustment, typically 0 degrees to 15 degrees below horizontal, when the antenna is located at a higher altitude than other antennas that communicate with the transmit antenna array. The downward adjustment of the radiation pattern alters the coverage area and may enhance communication with mobile users situated in nearby or shadowed areas below the transmit antenna array. Besides actually mechanically tilting the entire antenna assembly, it is known to electrically down-tilt the radiation pattern by controllably varying the relative phase or phases between two or more radiating elements of the antenna array. One known method by which the relative phase or phases between two or more radiating elements can be changed is to change the relative lengths of respective transmission lines connecting the antenna's common feed point to each element of the antenna array. Typically, various predetermined lengths of jumper cable are provided which are selectively connected between the common feed and each element to obtain a desired down-tilt. The jumper cables include co-axial connectors to facilitate connection. Furthermore, if stripline is used in the connection between the common feed point and the respective elements of the antenna array, some form of transition means is required to couple the jumper cable's co-axial connections to the stripline. A disadvantage of this known method is that it is expensive, unreliable and susceptible to the generation of intermodulation products. Another known method by which the relative phase between two or more radiating elements can be changed is to change the propagation velocity of the transmission line connecting the common feed point to at least some of the elements of the antenna array. Typically, this latter method is achieved by selectively changing the dielectric constant of the transmission line dielectric. If the transmission line active conductor is in the form of a conductive strip, the propagation velocity may be changed by introducing a dielectric material between the strip and its associated ground plane. It is, however, well understood that the introduction of dielectric material under such a conductive strip causes the strip's normal impedance to be disturbed. For example, if a conductive strip having a certain width is spaced above a ground-plane at a certain distance such as to present a 50 ohm characteristic impedance, the introduction of a dielectric material between the conductive strip and the ground-plane will reduce the value of this impedance to a value that depends upon the effective dielectric constant of the dielectric material. The resulting impedance mismatch would cause a degradation of return-loss performance of the antenna. Australian Patent No. 664625 discloses an arrangement of an adjustable phase shifter comprising dielectric phase shifter elements moveably interposed between conductive strips that couple radiating elements, and a common ground-plane. The phase shifter elements are of a characteristic configuration which minimises the impedance disturbance over a particular but limited range of frequencies. This arrangement also has limitations to the range of phase shift produced, which consequently imposes limits to the range of tilt. Australian Patent Application No. 14278/99 discloses an arrangement of an adjustable phase shifter comprising a transmission line in the form of a printed circuit board supporting conductive members on one side thereof, and a ground-plane spaced below the other side thereof. A moveable dielectric element is arranged adjacent to the conductive members. The moveable dielectric element is provided with a plurality of teeth along opposite edges for selectively overlapping the conductive members. This arrangement provides an adjustable phase shifter having stable impedance characteristics and a relatively large phase shift as compared with the prior art. New radio frequency bands have been allocated to provide more channels for the rapidly increasing cellular mobile telephone usage. Instead of having separate base station antennas for different bands, it is desirable to provide multiple-band antennas. For example, it may be desirable to combine the 1710- 1880 MHz DCS band with the 1920- 2170 MHz UMTS band, with an overall bandwidth of 460 MHz, which is wider than previous systems. In order to electrically down-tilt the radiation pattern of such a multiple band antenna, a phase shifter is required that has a wider operating frequency range than the aforementioned prior art phase shifter arrangements can accomplish. Summary of Invention It is an object of the present invention to provide a method of obtaining a phase shifting element for use in an adjustable radio frequency phase shifter arrangement having a wider operating frequency range than prior art arrangements as set out in the appended claims. The present invention is based on the concept that, as phase shift is generally proportional to the length of added or inserted dielectric, if the total dielectric length required for one particular phase shift is broken up into a plurality of segments, then it is possible to optimise all of the lengths and spacings of those segments in such a way as to reduce the aggregate signal reflection at several frequencies, or over a range of frequencies. Thus for a specified frequency range and for one required electrical phase shift, computer optimisation is used to determine the optimum lengths of a plurality of dielectric segments and the spacings between them. The same procedure is repeated for other phase shift values to produce an optimum profile for the movable dielectric element. In order that the invention may be readily carried into effect, embodiments thereof will now be described in relation to figures of the accompanying drawings, in which: Brief Description of the Drawings Figure 1 is a plan view of a planar dielectric element provided with a plurality of characteristic spaced segments extending therefrom. Figure 2 is a side view of Figure 1. Figure 3 is a plan view of a phase shifter arrangement incorporating the dielectric element according to an embodiment of the present invention. Figure 4 is a side view of the arrangement shown in Figure 3. Figure 5 is a schematic layout of an antenna array incorporating the phase shifter arrangement shown in Figure 3. Figure 6 is a plan view of a further embodiment of a phase shifter arrangement incorporating the dielectric element according to an embodiment of the present invention. Figure 7 is a schematic layout of an antenna array incorporating the phase shifter arrangement shown in Figure 6. Referring to Figures 1 and 2 of the drawings there is shown a planar dielectric element 1 comprising a rectangular body section 2 and five segments, 3,4,5,6 and 7 extending from a major edge of body section 2. The segments are separated by four air gaps 8,9,10 and 11. The segments lie in the same plane as the body section. To improve the structural rigidity of the dielectric element, the air gaps may be replaced by a dielectric material of a different dielectric constant to that of the material of the dielectric element 1. A Iternately, the air gaps may be replaced by thinner portions of the same material as the dielectric element. As shown in Figures 3 and 4, dielectric element 1 is slidably mounted and adjacent to the top surface of a distribution element comprising a planar dielectric circuit board 12 supporting a conductive member 13 on a first surface 12a thereof. The conductive member and the dielectric circuit board form a transmission line whose distal ends terminate at respective terminals T and B. The distribution element is supported in a spaced relationship with a conductive ground plane 14. The dielectric circuit board's second surface 12b and the ground plane face one another. Alternately, the second surface of the circuit board and the ground plane can be contiguous (not shown). The movable dielectric element 1 is supported above the first surface 12a of circuit board 12 in a linearly slidable manner by two parallel rods 15,16 attached to the ground plane. It will be understood that the movable dielectric element will have the effect of varying the phase whether it is adjacent the first surface 12a or the second surface 12b of the distribution element, that is, not interposed or interposed between the conductive member 13 and the ground plane 14. The phase shift achieved by each such arrangement will be different, indeed the movable dielectric element will have a greater effect when interposed between the conductive member 13 and the ground plane 14. It will be further understood that if the conductive member 13 has sufficient mechanical rigidity, the planar dielectric circuit board would not be required to support it. The greater the number of segments extending from the body section of the element 1, the smaller it is possible to make the aggregate signal reflection caused by the dielectric element. However, a greater number of segments requires a greater overall length of the element. But the length of the element has to be taken into account when operatively associating the element with an antenna array, to avoid constructional problems. Therefore, the choice of the number of segments is a compromise between electrical performance and practical dimensions. If, as shown in Figure 1, the element 1 has five segments (and correspondingly four gaps) then an equivalent electrical circuit comprises nine equivalent transmission line sections in series. For the desired operating frequency range and one specific phase shift, the lengths of those nine elements are to be adjusted or optimised to simultaneously achieve the desired phase shift and minimise the aggregate signal reflection. Using as a basis a commercially available radio frequency analysis tool, an optimisation algorithm such as gradient search is implemented to adjust the lengths of the equivalent transmission lines to simultaneously achieve the desired phase shift and minimise the aggregate signal reflection. The lengths of the equivalent transmission lines then represent the optimum lengths of the dielectric segments, and the optimum width of the gaps, overlapping the physical transmission line conductor for that particular phase shift. The same process is then repeated again and again for different phase shift values desired. Then, to construct the means by which a linear transverse movement of the dielectric element is converted to a variable phase shift, the various lengths of dielectric segments and widths of gaps overlapping the transmission line are joined to produce the required profiles of the segments. From these profiles, information is derived in a known manner and used in suitable numerically controlled cutting equipment to produce the complex shaped segments of the dielectric element. Referring to Figure 6 of the drawings there is shown a second embodiment of the invention for use with a three section antenna array (see Figure 7). The planar dielectric element 17 is provided with segments that extend from opposite major edges of the dielectric element's body section. The movable dielectric element 17 is slidably mounted and adjacent to one surface of a planar distribution element 18 that supports two conductive members 19 and 20. The conductive members and the dielectric circuit board form a transmission line network for splitting a radio frequency signal applied to a signal input terminal I into three paths that terminate respectively in three terminals T, B and C for feeding the input signal to the Top (T), Bottom (B) and Centre (C) sections of a three section antenna array (Fig. 7). The distribution element 18 is supported in a spaced relationship with a conductive ground plane 20. The movable dielectric element 17 is supported in a linearly slidable manner by two parallel rods 21 and 22 attached to the ground plane 20. It will be understood that the arrangement by which the dielectric element can be selectively moved in relation to the transmission line to vary the phase of signals is not limited to the preferred arrangement of parallel rods used in the embodiments described in relation to Figures 1 and 6. Various known arrangements could be adapted, such as, for example, rotational arrangements. Further, remotely controlled servomechanisms could be adapted to move the dielectric element.

Claims: Method of obtaining a phase shifting element having a wide operating frequency range, said phase shifting element being adapted to cooperate with a transmission line including an active line element spaced from a ground element, the phase shifting element including a movable planar dielectric member of predetermined dielectric constant, said planar dielectric member being provided with three or more discrete dielectric segments extending from at least one edge thereof, the movement of said planar dielectric member inducing a variable phase shift by varying the extent to which said three or more discrete dielectric segments overlap said active line segment, said method being characterized in that it comprises the steps of: determining by computer optimisation means optimum dimensions of each said segment and optimum widths of gaps defined by facing edges of adjacent segments so as to correspond to a desired operating frequency range and a particular phase shift and to minimize the aggregate reflection of signals passing along said transmission line, the various dimensions of each said segment and widths of gaps being joined to produce the profile of segments and gaps. Method as claimed in claim 1 including the steps of: implementing a radio frequency analysis and optimisation computer program to calculate data; controlling cutting equipment means under the control of the computer data to produce said dielectric segments having optimum dimensions and optimum widths there between. Method as claimed in claims 1 or 2, wherein said gaps are air gaps. Method as claimed in claims 1 or 2, wherein one or more said gaps are at least partly filled by material whose dielectric constant is different from that of the said dielectric segments. Method as claimed in claims 1 or 2, wherein one or more said gaps are at least partly filled by the same material as that of the dielectric segments, and wherein the thicknesses of the fillings are less than the thickness of said segments. Method as claimed in any one of claims 1 to 5, wherein said computer optimisation means includes a radio frequency analysis and optimisation computer program to calculate data for controlling cutting equipment means to produce said dielectric segments having optimum dimensions and optimum widths there between. Method as claimed in any one of the preceding claims, wherein said planar dielectric member comprises a rectangular body section, said dielectric segments extending from a major side thereof. A method of obtaining a phase shifter arranged to controllably vary the effective electrical length of a section of transmission line including an active line element spaced from a ground element, comprising the steps of: obtaining a phase shifting element as set out in any one of claims 1 to 7, movably mounting said phase shifting element and orienting its segments such that they moveably overlap or interleave with the transmission line. A method of obtaining a phase shifter as claimed in claim 8, providing a further transmission line and inserting the phase shifting element between the transmission line elements. A method of obtaining a phase shifter as claimed in claim 8, wherein the phase shifting element is located on the remote side of the active line element from the ground element. A method of obtaining a phase shifter as claimed in any one of claims 8 to 10, wherein said transmission line is operatively associated with an antenna array. A method of obtaining a phase shifter as claimed in claim 8, wherein said transmission line section includes an active transmission line element of predetermined electrical length supported on a planar dielectric circuit board, said circuit board being mounted in a spaced relationship with a ground element. A method of obtaining a phase shifter as claimed in claim 12, wherein said movable phase shifting element is interposed between said dielectric circuit board and said ground element. A method of obtaining a phase shifter as claimed in claim 12, wherein said movable phase shifting element is disposed above said dielectric circuit board and remote from said ground element. A method of obtaining a phase shifter as claimed in any one of claims 8 to 14, including adjustment means for controllably moving said phase shifting element in relation to said section of transmission line such that said segments and said gaps movably overlap or interleave with said transmission line elements to vary the phase of signals passing along said transmission line section. A method of obtaining a phase shifter as claimed in claim 15, wherein said adjustment means includes a remotely controllable servomechanism.

Inventors: MCDONALD NOEL, WILLIAMS CHARLIE G, MCDONALD, NOEL, WILLIAMS, CHARLIE G.
Applicants: CIT ALCATEL, ALCATEL
Application year: 2001
Main classification: H01Q3
IPC codes: H01Q3, H01P1