Variable Differential Phase Shifter Providing Phase Variation of Two Output Signals Relative to One Input Signal

This application claims benefit of international application PCT/NZ94/00107 filed Oct. 14, 1994.

The present invention relates to a variable differential phase shifter. The variable differential phase shifter of the invention allows the phase of two output signals to be continuously varied over a given range with respect to an input signal. The variable differential phase shifter of the invention is particularly suitable for use in tilting the beam of an antenna array.

Referring to FIG. 1 a prior art antenna array consisting of four elements 1-4 is shown. Feed-line 5 supplies a signal to drive the antenna elements 1-4. The signal from line 5 is equally divided between branches 6 and 7. Feed line 6 supplies the driving signal to antenna elements 1 and 2. The signal from branch 6 is further divided between branches 9 and 10. A phase shifter 11 is provided in branch 10 to shift the phase of the signal supplied to antenna element 2 by .beta. with respect to the phase of the signal driving antenna element 1. In branch 7 phase shifter 8 introduces a phase shift of 2.beta. with respect to the phase of the signal in branch 6. This phase shifted signal is divided between branches 12 and 13. Antenna element 3 thus receives a driving signal which is phase shifted by 2.beta.. A further phase shift element 14 is provided in branch 13 so that the signal driving antenna element 4 is phase shifted by 3.beta..

Accordingly, the antenna elements 1, 2, 3, 4 are phase shifted by an amount 0, 1.beta., 2.beta., 3.beta. respectively. In this way the beam of the antenna array can be tilted by a desired amount. Sometimes, to control side lobe levels and beam shape, other than progressive phase shift may be employed. Non-equal power division may also be employed.

In prior art systems phase shifters 8, 11 and 14 may be lengths of cable or active phase shifters. Commonly, active phase shifters using PIN diodes are employed which can be switched on or off to introduce phase shifts in a branch of the feed network. The phase shifters may include a number of PIN diodes to allow a number of delays of different magnitudes in be introduced into a feed path as required.

Such prior art phase shifters suffer from the disadvantage that they can usually only provide phase shifts between respective branches in a stepped manner and cannot usually provide continuous differential phase shifting between branches. Further, high power PIN diodes used in active systems are both expensive, particularly where a large number of antenna elements are employed and have higher losses than the present device. Active systems using PIN diodes also introduce non-linearities and intermodulation.

Other particular advantages of the present invention are as follows:

Because there are no sliding metal contacts, the phase shifter will require little maintenance. If a suitable dielectric is used (for example polytetrafluoroethylene) the sliding friction will be low. This in an advantage when designing mechanical drive mechanisms or selecting suitable electric motors. Because there are no sliding electrically conductive surfaces in contact, the phase shift variation speed can be maximised.

Also, for a required differential phase shift, the amount of mechanical movement is half that required by in-line phase shifters. This may result in a more compact structure. Finally, incorporating a matching section in the phase shifter structure reduces the manufacturing cost of a typical feed network.

It is an object of the present invention to provide a variable differential phase shifter which overcomes the above disadvantages or at least provides the public with a useful choice.

According to one aspect of the invention there is provided a variable differential phase shifter comprising:

a coaxial line comprising an inner conductive rod and an outer conductive tube coupled at ends thereof to first and second outputs;

an inner sleeve capacitively coupled to the inner conductive rod and slideable therealong; and

an outer sleeve capacitively coupled to the outer conductive tube and slideable therealong; the inner and outer sleeves being connected to an input and being slideable along said coaxial line in fixed relative relationship to vary the phase relationship of the signals output at the first and second outputs with respect to a signal supplied to the input.

Preferably a dielectric layer is provided between the inner conductive rod and inner sleeve and a further dielectric layer is provided between the outer conductive tube and the outer sleeve. The outputs are preferably transition cones which enable the phase shifter to be coupled directly to coaxial cables.

The input preferably comprises a rod perpendicular to the inner sleeve which slides within a slot in the outer conductive tube, the rod being coaxial with a tube perpendicular to the outer sleeve and held in fixed relation thereto by an intermediate dielectric, the ends of the rod and tube away from the sleeves being connected to a transition cone.

There is also provided an unequal power variable phase shifter having a dielectric tube provided around a length of the inner conductive rod adpated so that the power output at the first and second outputs is unequal.

The invention will now be described by way of example with reference to the accompanying drawings in which:

FIG. 1: shows schematically the feed network of a prior art antenna array.

FIG. 2: shows a sectional view of a variable differential phase shifter according to one aspect of the invention.

FIG. 3: shows of the outer conductive tube shown in FIG. 2 viewed in the direction of arrow A.

FIG. 4: shows an antenna array incorporating the phase shifters of the invention.

FIG. 5: shows a mechanism for adjusting the phase shifter shown in FIG. 2.

FIG. 6: shows the phase shifter of FIG. 2 incorporating a dielectric tube for unequal power division.

Referring to FIG. 2 an equal power dividing variable differential phase shifter according to one aspect of the invention in shown. All elements shown are circular in cross-section. In alternate embodiments other cross-sections may he used, such as square, rectangular or hexagonal cross sections.

A coaxial cable 21 supplies a signal to the phase shifter and the outputs of the phase shifter are output via coaxial cables 22 and 23. Central conductor 21a of coaxial cable 21 is electrically connected to feed rod 32 via conical section 34. Feed rod 32 is electrically connected to inner sleeve 38 which may slide along inner conductive rod 24. Inner conductive rod 24 is preferably provided with a thin dielectric coating 25 along its length so that inner conductive rod 24 and inner sleeve 38 art capacitively coupled. The ends of inner conductive rod 24 are coupled to inner conductors 22a and 23a via conical sections 30 and 28, respectively.

The outer conductor 21b of coaxial cable 21 is electrically connected to feed tube 33 via conical portion 35. Feed tube 33 is electrically connected to outer sleeve 37 which can slide along outer conductive tube 26. Outer conductive tube 26 is provided with a thin dielectric layer 27 along its length upon which outer sleeve 37 slides. The ends of outer conductor 26 are coupled to the outer conductors 22b and 23b via conical sections 31 and 29 respectively. Conical sections 28, 29, 30, 31, 34 and 35 assist to minimize the voltage standing wave ratio (VSWR) at the input 21.

The dielectric coatings 25 and 27 should be a radio frequency low loss material, and should preferably have a low coefficient of friction. A suitable material is polytetrafluorethylene.

Feed rod 32 is held in fixed relationship with feed tube 33 by dielectric block 36. Referring to FIG. 3 it will be seen that outer conductive tube 26 is provided with a slot 39 along its axis. Feed rod 32 can slide within slot 39 as the tee assembly (33, 37, 32, 38) slides to and fro along outer conductive tube 26. It will be appreciated that all components indicated, apart from dielectric materials 25, 27 and 36, will be formed of suitable conductive material, such as brass, copper etc.

The arrangement of inner conductive sleeve 38, dielectric layer 25 and inner conductive rod 24 forms a capacitive coupling. Likewise, the arrangement of outer sleeve 37, dielectric layer 27 and outer conductive tube 26 forms another capacitive coupling. At frequencies around 900 MHz or above the reactances of the capacitive coupling are so low that they constitute a direct coupling between sleeves 37 and 38 and outer conductive tube 26 and inner conductive rod 24 respectively.

A signal supplied to input cable 21 will divide between the two outputs (i.e. coaxial output cable 22 and 23) evenly. By sliding the tee section with respect to outer conductive tube 26 the phase of a signal supplied to output coaxial cable 22 and output coaxial cable 23 may be varied. For example, if the tee connection is shifted so that it is to the left of the centre of outer conductive tube 26 then the distance the signal must travel to reach output coaxial cable 22 is less than the distance the signal must travel to reach output coaxial cable 23, hence there is a phase delay of the signal output to coaxial cable 23 with respect to the phase of the signal output to coaxial cable 22. By sliding the tee section right or left along outer conductive tube 26 the desired phase difference between the outputs 22, 23 say be achieved. It will be appreciated that the phase shifter described allows continuous phase variation between the outputs 22, 23 within the a allowed range.

For the equal power dividing variable differential phase shifter shown in FIG. 2, Z.sub.1, Z.sub.2, and Z.sub.3 are the characteristic impedances of the sections shown and R.sub.L is the system impedance (in this case 50 ohms).

For equal power division:

When properly terminated the tapping point impedance Z.sub.T is equivalent to two R.sub.L loads in parallel (Z.sub.T =R.sub.L /2).

Thus, a matching section is required between line 21 and the tapping point. It is formed by feed rod 32, feed tube 33 and dielectric material 36. Feed rod 32 is preferably a quarter wavelength long and inner conductive sleeve 38 is preferably between one sixteenth to an eighth of a wavelength long.

If, for example, the system impedance is 50 ohms then

For an unequal power dividing variable differential phase shifter, Z.sub.1 does not equal Z.sub.2. One option is to let either to or Z.sub.1 or Z.sub.2 =R.sub.L so that the other characteristic impedance is less than R.sub.L, e.g:

Transformer Z.sub.3 could be constructed from two sections, one of Z.sub.3 ' and the other Z.sub.3 ". Alternatively, it could be made with a tapered characteristic impedance. It will be recognized by a person skilled in the art that these alternatives will increase the operating bandwidth of the device.

Referring now to FIG. 6, to adjust the impedance of section Z.sub.2 to the desired value a dielectric tube 40 may be secured to inner sleeve 38 which is slideable relative to inner conductive rod 24. It will however be appreciated that other means may be used to alter the impedance of section Z.sub.2.

It should also be appreciated that in other embodiments the phase shifter may be driven via coaxial cable 22 or 23. If the phase shifter is driven by coaxial cable 22 then the output at coaxial cable 23 stays in constant phase relationship with the input at coaxial cable 22. Only the output at coaxial cable 21 varies as the t-section slides to and fro. It will be appreciated that for such a configuration the characteristic impedances would have to be adjusted, using similar equations to those described above but with Z.sub.1 and Z.sub.3 interchanged. Dielectric tube 36 may be replaced by spacers at the ends thereof if less dielectric material is required.

Referring now to FIG. 4 an antenna array incorporating the phase shifter of the invention is show. The antenna array consists of antenna elements 40 to 43. Phase shifters 45 to 47 are of the form shown in FIG. 2. A signal supplied from feed line 44 is divided by phase shifter 45 between branches 48 and 49. Phase shifter 46 divides the signal from feedline 48 between antenna elements 40 and 41. Phase shifter 47 divides the signal supplied on feedline 49 between antenna elements 42 and 43.

If the tee of phase shifters 46 and 47 is moved up a distance d from their central positions and the tee of phase shifter 45 is moved up a distance to 2d from its central position then phase shifts of 0, .beta., 2.beta., 3.beta. will result for the antenna elements 40, 41, 42 and 43. It will thus be appreciated that the beam of the antenna may be tilted by any desired amount by shifting the phase shifters 46 and 47 a distance d from centre and phase shifter 45 a distance 2d.

In one embodiment a mechanical coupling may be provided so that the tees of phase shifters 46 and 47 are shifted in unison and the tee of phase shifter 45 is moved twice the distance of phase shifters 46 and 47. The tees of phase shifters 46 and 47 may be linked by a rigid member to ensure that they move in unison whilst the tee of phase shifter 45 may be linked to the member via a pivoted arm so that the tee of phase shifter 45 moves twice the distance of the tees of phase shifters 46 and 47.

A possible mechanism is shown in FIG. 5. Points 51 and 52 of member 50 may be linked to the tees of phase shifters 46 and 47 to ensure that they move in unison. Member 53 may be pivotally connected to member 50 at point 54. One and 55 of member 53 may be connected to a pivot point mounted to an antenna housing. The other end 56 may be connected to the tee of phase shifter 45. The length 58 between pivot point 54 and point 56 may be the same as the length 57 between pivot point 54 and pivot point 55. In this way the tee of phase shifter 45 moves twice the distance moved by the tees of phase shifters 46 and 47.

It will be appreciated that there are many other possible mechanisms that may be used to adjust the tees in the required manner. Length 57 may be greater than or less than length 58 if other than progressive phase shifting is required. Non-linear linkages may be employed where other than progressive phase shifting is required. The linkages may be manually adjusted or driven by suitably geared motors, stepper motors or the like.

The present invention thus provides a relatively inexpensive continuously variable differential phase shifter suitable for use in high power phase shifting applications. The phase shifter of the present invention may find particular application in high power antenna arrays.

Where in the foregoing description reference has keen made to integers or components having known equivalents then such equivalents are herein incorporated as if individually set forth.

Although this invention has been described by way of example it is to be appreciated that improvements and/or modifications may be made without departing from the scope or spirit of the invention.

The variable differential phase shifter of the present invention may find application in the construction and operation of antenna arrays wherein beam tilting or squinting is required, Such arrays are commonly found in telecommunications applications such as cellular networks. The variable differential phase shifter may also be substituted for PIN diodes in situations where a device is required for varying the phase of two output signals.