EP1522118A1

EP1522118A1 - Phase shifter for antenna

Info

Publication number: EP1522118A1
Application number: EP03738320A
Authority: EP
Inventors: Fergal Joseph Lawlor; Niallo Donal Carrol; William Keung Piu Tang
Original assignee: Finglas Technologies Ltd
Current assignee: Finglas Technologies Ltd
Priority date: 2002-07-11
Filing date: 2003-07-08
Publication date: 2005-04-13
Also published as: AU2003244844A1; GB0216048D0; WO2004008568A1

Abstract

A phase shifter for a radio antenna has a feed terminal (10, 12) and a plurality of antenna terminals (28-37, 500), each of said antenna terminals being connected, in use, to a respective one or more antenna elements. The feed terminal and antenna terminals are connected together by means of a plurality of transmission line (14, 16) comprising a plurality of conductive tracks connecting the feed terminal (10) to the antenna terminals, grand conductor means and a solid dielectric substrate sandwiched between the tracks and the ground conductor means. The shifter includes a variable delay means movable relative to the tracks so as to vary the relative times taken for the signals to travel along the tracks between the feed and the antenna terminals.

Description

C902.01/S

Phase Shifter for Antenna

Field of the Invention

This invention relates to a phase shifter for a radio antenna having a plurality of elements.

Background to the Invention

In certain situations, it is desirable to adjust the orientation of the radiation pattern of a radio antenna. For example, the beam pattern for a base station of a cellular mobile telecommunications system might need to be tilted downwards so that the associated cell does not encroach on neighbouring cells. To that end it is known to alter the orientation of the antenna's dipole array, but this can have adverse side effects, such as undesirable movement of the antenna side lobe.

Another approach is to tilt the radiation pattern by altering the relative phase of signals in the elements of the dipole array. Increasingly, therefore, antennas are built incorporating variable phase shifters to allow variable tilt adjustment of the beam, which can be accomplished with either a manual control on the antenna or remotely through control signals from a controlling device such as a computer. In this way, the relative phases of the signals delivered to (or received by) radiating elements are controlled so that the main beam of the antenna can be steered to a desired direction based on the configuration of its dipole array.

Various forms of phase shifter exist. Some employ PIN diodes or other micro electromechanical switches which electronically switch the signals to pass through different lengths of transmission line, and thus vary the phase delay of the signals. Other devices use sliding contacts between transmission lines so that the relative lengths of the transmission paths of signals passing through the phase shifter are mechanically varied. However, there are many drawbacks associated with these techniques, including low power handling ability, high passive intermodulation generation, high manufacturing cost and poor long term reliability.

Another, more effective approach to controlling the relative phase of signals in the dipole array is to alter dielectric properties of the transmission paths through the phase shifter, an approach that utilises the fact that the velocity of propagation of a signal along a transmission line is affected by the permittivity of the medium through which the signal propagates.

One known example of such a phase shifter has a feed terminal, for connection to a transmitter and/or receiver, connected to a number of antenna terminals (each for connection to a respective antenna element) via a corresponding number of stripline transmission lines. The transmission lines take the form of a series of conductive tracks printed on either side of a support sheet which holds the tracks in position between two opposed ground planes spaced from the tracks. The air between the tracks and the ground planes forms part of the dielectric medium through which signals propagate between the feed and the antenna terminals. The medium through which the signals can propagate includes a pair of dielectric sliders disposed one on either side of the support sheet. The tracks are printed on the support sheet in a pair of opposed serpentine patterns which are so arranged that the distance travelled by a signal along any of the tracks through the dielectric medium is dependent upon the translational position of the slider and that the relationship between slider position and dielectric path length will vary from track to track. Consequently movement of the slider along the sheet correspondingly alters the relative phases of signals at the antenna terrninals.

However strip line technology suffers from a number of weaknesses, especially when a high performance compact phase shifter is required. For example, such phase shifters suffer from high insertion loss, need to be large (because of the volume of contained air between the ground planes) difficulty in maώtaining structural and electrical integrity and high manufacturing costs. In addition, the strip line transmission lines suffer from difficulties in keeping the tracks accurately positioned and in ensuring the same ground potential exists on both of the ground planes.

Summary of the Invention

According to the invention, there is provided a phase shifter for a radio antenna having a plurality of elements, the phase shifter comprising a feed terminal, a plurality of antenna terminals, each for connection to a respective one or more antenna elements, a plurality of transmission lines for conducting signals between the feed terminal and the antenna terminals, the transmission lines comprising a plurality of conductive tracks connecting the feed terminal to the antenna terminals, ground conductor means, a solid dielectric substrate sandwiched between the tracks and the ground connector means and variable delay means moveable relative to the tracks so as to vary the relative times taking the signals to travel along the tracks between the feed and the antenna terminals.

Thus the substrate, by virtue of being interposed between the ground conductor and the tracks, provides the dielectric medium to which signals propagate between the antenna terminals and the feed terminal. Because a solid dielectric can have a much higher permittivity in air, the volume of dielectric needed to convey these signals is considerably less than that required by a stripline device. Thus, a phase shifter in accordance with the invention can be of a relatively compact construction, with the tracks etched onto just one side of the substrate. Also, as there is only one ground plane, and the solid substrate is sandwiched between the plane and the tracks, the problems of potential matching of two ground planes, and of a variation in the positions of the conductive tracks, associated with a stripline device, are avoided.

The applicants have realised that the economic and practical advantages associated with the invention outweigh the relatively high potential costs of the substrate. The substrate may be of a composite structure, for example a laminated plate, that is preferably substantially homogenous.

Preferably, the substrate of a relative permattivity of at least 2.0 and a thickness of at least 0.6mm. Preferably the substrate has a relative permattivity of 2.5 and a thickness of .76mm. The substrate conveniently is a PTFE board.

Preferably, the ground conductor means comprises a common ground plane.

The transmission lines may therefore be microstrip transmission lines.

Preferably, the variable delay means comprises a dielectric member slideable over the tracks, the arrangement being such that the relative lengths of the paths between the feed and antenna terminals extending under the dielectric member are varied by said sliding movement of the member.

Preferably, the dielectric member is connected to an adjustment mechanism for moving the member along the tracks, the mechanism preferably comprising a worm drive. The worm drive is particularly useful for remote adjustment, as it can be connected to the rotary output of an electric motor.

The phase shifter may be one of two such phase shifters wherein the dielectric sliders are connected to a single common adjustment mechanism. In this case, and when the ground connectors comprise ground planes, the shifters are preferably contained within a common housing in which the substrates are planar and are parallel to each other, with the ground planes being adjacent to each other.

This prevents signals transmitted through one shifter causing interference in the other shifter.

Brief Description of the Drawings The invention will now be described, by way of example only with reference to the accompanying drawings in which:

Figure 1 is an isometric view of apparatus in accordance with the invention, the apparatus comprising a pair of phase sihfters;

Figures 2,3, and 4 are respective plan side and end elevational views of the two phase shifters;

Figure 5 is a partially exploded isometric view of the apparatus with its top plate removed;

Figure 6 is an isometric view of a series of transmission lines of one of the phase shifters of the apparatus;

Figure 7 is an isometric view of a dielectric slider for the phase shifter;

Figure 8 shows a mechanism for moving two dielectric sliders, one for each phase shifter; and

Figure 9 is a sectional side view of a part of one of the phase shifters.

Detailed Description

The apparatus shown in Figure 1 has a pair of phase shifters (one for each antenna polarisation), generally referenced 1, situated between two parallel copper plates, referenced 2 and 4 respectively. Two cable clamps, 6 and 8 are bolted to the plates 2 and 4 at the end regions of the plate, and are sandwiched between the two plates. Each of the phase shifters has an input feed (10 and 12respectively) and eleven output feeds such as the feed 34. Each of the output feeds one or more elements in a dipole antenna array (not shown), via a respective coaxial cable. These cables have been omitted from figure 1 for the sake of clarity but some are shown (from one end of the apparatus at 100-111) in figure 5.

In use the apparatus is mounted with the plates 2 and 4 vertical, and the inputs feeds 10 and 12 situated at the bottom of the device.

Each of the phase shifters includes a respective arrangement of transmission lines 14 and 16 which connect the input feeds 10 and 12 to the output feeds, and which are described in further detail below.

) Each phase shifter also includes a respective one of a pair of dielectric sliders 18 and 20 are connected to a common drive mechanism 22, also described in detail below.

With reference to Figures 2-5, the two arrangements of transmission lines, 14 and 16, are substantially identical, and only the arrangement of transmission lines 14 will, therefore, be described in detail.

The transmission lines 14 comprise a PTFE board 21 on the upper surface of which an arrangement of copper tracks 22 has been etched. The tracks define two generally serpentine paths 24 and 26 across the width of the board 14. The serpentine paths 22 and 24 are connected at one end to the input feed 10 and run from that end to the output feeds 28 and 12 respectively. The path 24 is connected to the input terminal 10 at a Wilkinson divider which also connects the terminal 10 to a through track 50. The track 50 leads to a Wilkinson divider 40 which connects the track to the path 26 and to a branch track extending to a reference output 29. The tracks include a number of intermediate branches, each connecting a respective one of the other output feeds (referenced 30-37 and 500) to the serpentine paths through a respective Wilkinson Power Divider (40-48). The through track 50 provides a route for a signal between feed 29 and terminal 10 which does not pass through any runs of the paths 24 and 26. The opposite face of the PTFE board 21 is coated with a continuous layer of copper 100 (Figure 9) to provide the ground plane for the transmission lines. The PTFE board has a thickness of ,76mm., low loss characteristics and a dielectric constant of 2.5. The sides of the board 21 include lateral steps 52 and 54 which act as guide tracks (with end stops) for two end strips 56 and 58 projecting downwardly from the ends of the dielectric slider 18 (Figure 7). The slider has a dielectric constant of 4.5 and a loss tangent of .002, and its position along the tracks 52 and 54 affects the length of the path taken by a signal from the feed terminal 10 to the output feeds other than output 29) which passes under the slider 18, and during which the speed of transmission of the signal is therefore modified by the slider 18.

It will be appreciated that the length of the transmission path for signal from the terminal 10 to the output 29 extending under the slider 18 will be the same for all positions of the slider 18 along the tracks 52 and 54. However, the total length of each of the serpentine paths 24 and 26 under the slider 18 will be related to the linear position of the slider. Furthermore, the length of the portions of the transmission path from the feed terminal 10 to any of the outputs 30,31,32,33,28 and 34,35,36,37,500 passing under the slider 18 (and hence the total electric lengths of the paths) will differ from path to path by virtue of the fact that the branches for the outputs are connected to the paths 24 and 26 at differing distances therealong.

The following table illustrates the effect on the relative phase of signals between the feed terminal and each of outputs bays 28, 29, 30, 31, 32, 33 (top half) and 34,35,36,37 and 500 (bottom half).

1 2 3 4

Output Slider moved (top) Centre Slider moved (bottom)

28 0°-5Δ° 0° 0°+5Δ

33 0°-4Δ 0° 0°+4Δ°

32 0°-3Δ 0° 0°+3Δ

31 0°-2Δ° 0° 0°+2Δ°

30 0°-Δ° 0° 0°+Δ°

29 (Fixed Ref) 0° 0° 0° 34 0°+Δ° 0°-Δ° 35 0°+2Δ° 0° 0°-2Δ° 36 0°+3Δ° 0° 0°-3Δ° 37 0°+4Δ° 0° 0°-4Δ° 500 0°+5Δ° 0° 0°-5Δ°

Column 1 shows the eleven outputs of the phase shifter. The phases at ten of these outputs are variable and that at output 29 is fixed. The other columns each show the phase differences of signals at the outputs after a respective position of the slider 18. Column 3 shows the phase differences when the slider 18 is equally covering the serpentine paths 24 and 26. Column 2 in the table indicates the relative changes caused by the movement of the slider 18 towards the top end of the board 14. This movement increases the portion of the paths from the input feed 10 to the top 26 (outputs 30,31,32,33,28) under the sliding dielectric 18. This correspondingly increases the time taken for a signal to travel from input feed 10 to those outputs, and thus introduces a phase delay (represented by -Δ°). The movement of the slider 18 towards the top of board 14 also causes the reduction in the portion of the serpentine path 24 under the slider 18. Therefore, there is an equal but opposite phase change at output 34 when compared to the output 30. The phase shift to each of the remaining outputs is shown in the above table. For any position of the dielectric slider 18 the phase shift between outputs 31 and 35 is double that of outputs 30 and 34 and so on. The fourth column shows the relative phase changes when the sliding dielectric 18 is moved towards the bottom of board 14. It can be seen that this has the opposite effect on the relative phases.

The phase shifter, like an antenna is of a reciprocal nature. Thus the shifter can convey signals to be transmitted by the antenna from the input 10 to the output bays or can output signals received by the antenna through the terminal 10. In either case, the phase shifter allows the radiation pattern of antenna to adjusted.

With reference to figure 9, the transmission lines 16 of the second phase shifter are mounted in such a way that the conductive tracks (22') face downwards so that the ground planes (100 and 100') of the two sets of transmission lines face each other. The boards of the transmission lines 14 and 16 are separated by an insulator board 15. The dielectric slider 20 is bolted to the end pieces 56 and 58 attached to the slider 18. Two mounting lugs 60 and 62 are attached to the upper surface of the slider 18 and extend through a pair of slots 64 and 68 in the upper plate 2.

These lugs are attached to a driving plate 70 of the mechanism 22 (figure 8).

The ends of the mechanism 22 include mounting brackets 72 and 74 by which the mechanism 22 is bolted to the upper plate 2 as shown at 75-78 in figure 1.

A pair of parallel guide rails 80 and 82 are fixed to the brackets 72, whilst the central externally screw threaded drive shaft 84 is rotationally mounted on the guide plates in a position in which it is parallel with, and interposed between, the rails 80 and 82.

The shaft 84 extends through a screw threaded bore 86 in the plate 70 so that rotation of the shaft 84 about its axis will cause the plate 70 to move linearly along the rails 80 and 82, and thus will cause corresponding movement of the sliders 18 and 20.

A summary of the features, and certain advantages of the described embodiment of phase shifter is set out below.

1. The variable phase shifter has one input and a transmission path to eleven outputs printed on the top side of a low loss PTFE substrate with a ground plane on the other side.

2. The required amplitude at each of the eleven outputs is achieved using Wilkinson Power Dividers. The Wilkinson Power Dividers have the property of being lossless when their output ports are matched and have very high isolation between ports. 3. An arrangement of ten U shaped signal conductors (constituting the serpentine paths 24 and 26) printed on the substrate form the transmission path between the adjacent outputs.

4. A piece of sliding dielectric of known dielectric constant is secured above the printed signal conductors. This piece of dielectric is allowed move in the X-axis only.

5. By sliding this dielectric over the signal conductors a known phase change is either added or subtracted to the phase at each of the 11 outputs. The phase of the reference output remains constant at all times.

6. The phase shift is a broadband variable phase shifter with excellent IMD

7. Performance, and can be simply scaled to suit any frequency band.

Microstrip circuits have major advantages over the traditional stripline circuits.

1. The microstrip phase shifter is a single rigid PTFE broad with tracks printed on the top side and a ground plane on the bottom side. The stripline circuit has its tracks suspended between top and bottom ground plates. This leads to difficulties in keeping the stripline tracks accurately suspended and also in ensuring the same ground potential exists on the top and bottom ground plates.

2. The microstrip signal conductors are less than half the width of the stripline tracks. This allows for a considerable saving to be made in the width of the phase shifter.

3. The spacing between the signal conductors on the microstrip phase shifter can be as low as 6mm while still maintaining good isolation between ports. This allows for more saving in width.

4. Due to the advantages in 2 & 3 twice the number of outputs can fit on a microstrip phase shifter. 5. This allows for the required number of outputs to fit on one microstrip board with no need for multi-layer configurations. This allows a microstrip phase shifter to be less than 1/3 the height of the stripline phase shifter.

6. With the microstrip phase shifter very simple lossless transitions are used and there is no need for lossy vertical transitions used to connect layers. This allows for the low loss phase shifter.

7. Due to the open structure of the microstrip phase shifter toning and adjustment of the circuit is made simple.

8. Also because of this open structure there is no chance of getting unwanted box resonance's.

9. Because of the very low component count and the ease of manufacturing high repeatable phase shifters can be easily achieved.

In use, the phase shifter is mounted in the vicinity of an antenna, with the output bays connected to the antennas' elements. The beam tilt of the antenna is adjusted by operating the mechanism 22 (either using a remotely controlled stopper motor connected to the shaft 84 or by means of a mechanical linkage connecting a manual control to the shaft 84. It will be appreciated that, due to the reciprocal nature of antennas, this effects the antenna when used in both transmission and reception modes.

Claims

C902.01/SClaims

1. A phase shifter for a radio antenna having a plurality of elements, the phase shifter comprising a feed terminal, a plurality of antenna terminals, each for connection to a respective one or more antenna elements, a plurality of transmission lines for conducting signals between the feed terminal and the antenna terminals, the transmission lines comprising a plurality of conductive tracks connecting the feed terminal to the antenna terminals, ground conductor means, a solid dielectric substrate sandwiched between the tracks and the ground conductor means and variable delay means moveable relative to the tracks so as to vary the relative times taken by the signals to travel along the tracks between the feed and the antenna terminals.

2. A phase shifter according to claim 1, in which the substrate is of a composite structure.

3. A phase shifter according to claim 1 or claim 2, in which the substrate has a relative permittivity of at least 2.0 and a thickness of at least 0.6 mm.

4. A phase shifter according to claim 3, in which the substrate has a relative permittivity of 2.5 and a thickness of 0.76 mm.

5. A phase shifter according to claim 3 or claim 4, in which the substrate is a PTFE board.

6. A phase shifter according to any of the preceding claims, in which the ground conductor means comprises a common ground plane.

7. A phase shifter according to any of the preceding claims, in which the transmission lines are microstrip transmission lines.

8. A phase shifter according to any of the preceding claims, in which the variable delay means comprises a dielectric member slideable over the tracks, the arrangement being such that the relative lengths of the paths between the feed and antenna terminals extending under the dielectric member are varied by said sliding movement of the member.

9. A phase shifter according to claim 8, in which the dielectric member is connected to an adjustment mechanism for moving the member along the tracks.

10. A phase shifter according to claim 9, in which the mechanism comprises a worm drive.

11. A phase shifter according to any of the preceding claims, in which the phase shifter is one of two such phase shifters wherein the dielectric sliders are connected to a single common adjustment mechanism.

12. A pair of phase shifters according to claim 11, in which the ground connectors comprise ground planes, and the shifters are contained within a common housing in which the substrates are planar and are parallel to each other, with the ground planes being adjacent to each other.