Abstract:
An antenna system for sharing of operation employs contiguous transmit frequencies. Transmit frequencies are separated into non-contiguous sub-groups isolated from one another by filters  158 (+) and  160 (−) associated with positive and negative polarisation. Received frequencies are filtered and split into five signals for input to base station receive ports. Non-contiguous transmit frequency sub-groups are combined by a quadrature hybrid  110  and pass with 90 degree relative phase shift to mutually orthogonal antenna stack ports P(+) and P(−) associated with orthogonally polarised sets of antenna elements AS(+) and AS(−): the ports P(+) and P(−) are isolated from one another by the hybrid  110 . The 90 degree phase shift results in one transmit subgroup being radiated with left hand circular polarisation and the other transmit subgroup being radiated with right hand circular polarisation. Changing the relative phase shift changes the radiated polarisation to linear or elliptical, and signal amplitude weighting provides control of antenna beam polarisation direction.

Description:
BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention relates to an antenna system and particularly, but not exclusively, to a phased array antenna system having a plurality of antenna elements arranged in at least two sub-arrays. The antenna system is suitable for use in many telecommunications systems but finds particular application in cellular mobile radio networks, commonly referred to as mobile telephone networks. More specifically, the antenna system of the present invention may be used with third generation (3G) mobile telephone networks and the Universal Mobile Telephone System (UMTS). 
     (2) Description of the Art 
     Operators of cellular mobile radio networks generally employ their own base-stations, each of which includes one or more antennas. In a cellular mobile radio network, the antennas are a factor in defining the desired coverage area which is generally divided into a number of overlapping cells, each associated with a respective antenna and base station. Each cell contains a fixed-location base station which communicates with mobile radios in that cell. The base stations themselves are interconnected by other means of communication, either radio links or fixed land-lines, and are arranged in a grid or meshed structure allowing mobile radios throughout the cell coverage area to communicate with each other as well as with the public telephone network outside the cellular mobile radio network. 
     The antennas used in such networks are often composite devices known as phased array antennas which comprise a plurality (usually eight or more) or array of individual antenna elements or dipoles. The direction of maximum sensitivity of the antenna, i.e. the vertical or horizontal direction of the main radiation beam or “boresight” of the antenna pattern, can be altered by adjusting the phase relationship between the elements. This has the effect of allowing the beam to be steered to modify the coverage area of the antenna. 
     In particular, operators of phased array antennas in cellular mobile radio networks have a requirement to adjust the vertical radiation pattern (VRP), also known as the “tilt”, of the antenna since this has a significant effect on the coverage area of the antenna. Adjustment of the coverage area may be required, for example, owing to changes in the network structure or the addition or removal of other base stations or antennas in the cell. 
     The adjustment of the angle of tilt of an antenna is known and is conventionally achieved by mechanical means, electrical means, or both, within the antenna itself. When tilt is adjusted mechanically, for example by mechanically moving the antenna elements themselves or by mechanically moving the housing for the elements, such an adjustment is often referred to as “adjustment of the angle of mechanical tilt”. The effect of adjusting the angle of mechanical tilt is to reposition the boresight such that it points either above or below the horizon. When tilt is adjusted electrically, by adjusting the phase of signals supplied to the antenna elements without physically moving either the housing for the elements, the antenna elements themselves or any other part of the antenna radome, such an adjustment is commonly referred to as “adjustment of the angle of electrical tilt”. The effect of adjusting the angle of electrical tilt is also to reposition the boresight so that it points either above or below the horizon but, in this case, is achieved by changing the time delay of signals fed to each element (or group of elements) in the array. 
     A disadvantage of mechanical adjustment of the angle of electrical tilt is that it must be carried out in situ by manual mechanical adjustment of the antenna. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an improved antenna which overcomes the aforementioned problem. 
     In the following description, the term “antenna system” is used in place of the previous term “antenna” to describe a system having an “antenna assembly”, that is an array of antenna elements, and control means for controlling signals supplied to the antenna elements in the antenna assembly. 
     According to one aspect of the present invention, therefore, there is provided an antenna system comprising: 
     an antenna assembly having an angle of electrical tilt and a plurality of antenna elements mounted upon an antenna carrier and arranged in at least two sub-arrays, each sub-array including one or more of said elements, 
     control means for controlling electrically the phase of signals supplied to at least one of said sub-arrays from a location remote from said antenna assembly, wherein said control means include phase adjustment means for connection through first and second input feeds to a respective one of said sub-arrays, thereby to adjust the phase of signals applied thereto, and
 
an additional mechanical phase adjustment arrangement for further adjusting the phase of signals supplied to each element of the antenna assembly.
 
     Advantageously, the antenna assembly may comprise first and second phase adjustment means, each of said first and second phase adjustment means being in connection with a respective one of said sub-arrays through the respective first or second input feed, thereby to adjust the phase of signals supplied to said respective one of said sub-arrays. 
     Typically, the antenna carrier may be a mast. 
     In a first embodiment, the control means may be located at a base of the antenna carrier, remote from the antenna assembly. In an alternative embodiment, the control means are arranged at a distant location from the base of the antenna carrier or mast, for example several kilometers away. 
     The control means may include a single port for receiving a single input signal and means for splitting said input signal into first and second split signals to be supplied to a respective one of said first and second phase adjustment means. 
     Advantageously, the system further comprises means for automatically controlling the phase of signals supplied to a first one of said arrays in dependence on the phase of signals supplied to a second one of said arrays. 
     In one preferred embodiment, said elements in said antenna assembly are arranged in first, second and third sub-arrays and said antenna system comprises: 
     first control means for controlling the phase of signals supplied to said first sub-array, and 
     third control means for controlling the phase of signals supplied to said third sub-array, and 
     second control means arranged to control automatically the phase of signals supplied to said second sub-array in dependence on a predetermined function of the phase of the signals supplied to said first and third sub-arrays. 
     Advantageously, said predetermined function is the vector sum of the phase of the signals supplied to said first and third sub-arrays. 
     Said second control means may preferably include a combiner unit for receiving a first input signal having the phase of the signals supplied to said first sub-array and a second input signal having the phase of signals supplied to said third sub-array, and for providing an output signal to the second sub-array in dependence on the predetermined function of the phase of the signals supplied to said first and third sub-arrays. 
     In one embodiment, the predetermined function is the vector sum of the phases of the signals supplied to said first and third sub-arrays. 
     In a further preferred embodiment, the second control means includes at least one quadrature combiner unit for receiving a first input signal having the phase of signals supplied to the first sub-array and a second input signal having the phase of signals supplied to the third sub-array and for providing a first output signal to one element of the second sub-array and a second output signal to a different element of the second sub-array, wherein said first and second output signals are dependent upon the predetermined function of the phase of the first and second input signals. 
     The quadrature combiner unit may be configured such that the phase of said output signals provided by the quadrature combiner unit is the average of the phase of said first and second input signals. 
     The first control means may be arranged to control and/or adjust the phase of said signals supplied to said first sub-array by a first predetermined amount and said second control means may be arranged to control and/or adjust the phase of said signals supplied to said second sub-array by a second predetermined amount, wherein the magnitude and/or polarity of said second predetermined amount is different to that of said first predetermined amount. 
     The antenna assembly is conveniently supplied with a maximum of two signal feeds from said first and second phase adjustment means. 
     The antenna assembly conveniently includes respective signal distribution means associated with each sub-array for splitting and distributing signals across the elements of the associated sub-array. Preferably, each of said signal distribution means includes a splitter arrangement for distributing signals to one or more of said sub-arrays. Conveniently, the splitter arrangement is arranged to distribute signal strength of said signals to said sub-arrays in a substantially uniform distribution, thereby to increase antenna boresight gain. 
     In one embodiment, at least one output signal from said distribution means associated with a first sub-array is spatially combined or overlapped with at least one output signal from said distribution means associated with a third sub-array, thereby to provide first and second combined output signals to first and second elements of a second sub-array. The combining of signals may be achieved simply in air, and provides the further advantage that higher boresight gain and lower sidelobe levels may be achieved, particularly when the system is electrically tilted. 
     The additional mechanical phase adjustment arrangement may include an array of moveable dielectric elements. The signal path to each array element may provided with an associated dielectric element, unique to that element, or may share a dielectric element with the signal path to another of the array elements. 
     Each element has an associated input transmission line and, in one embodiment, each of the dielectric elements is arranged for linear movement relative to the associated transmission line to vary the further phase shift of signals supplied to said element through said transmission line. 
     Alternatively, each of the dielectric elements is arranged for rotary movement relative to the associated transmission line to vary the further phase shift of signals supplied to said element through said transmission line. 
     The additional mechanical phase adjustment arrangement may therefore include either rotary or linear actuation means for moving the dielectric elements. Each additional mechanical phase adjustment arrangement may be identical, so as to provide a substantially equal amount of further phase adjustment to signals supplied to each array element upon linear or rotary actuation of the dielectric elements. Alternatively, each additional mechanical phase adjustment arrangement may be different such that linear or rotary actuation generates a different amount of further phase adjustment to signals to each element. 
     According to another aspect of the present invention, there is provided an antenna system comprising: 
     an antenna assembly having a plurality of elements arranged in at least two sub-arrays, each sub-array comprising one or more of said elements; 
     first control means for controlling the phase of signals supplied to a first one of said arrays; and 
     second control means for automatically controlling the phase of signals supplied to another of said sub-arrays in dependence on the phase of said signals supplied to said first one of said sub-arrays. 
     Preferably, said elements in said antenna assembly are arranged in first, second and third sub-arrays and said assembly includes: 
     first control means for controlling the phase of signals supplied to said first sub-array; and 
     third control means for controlling the phase of signals supplied to said third sub-array; 
     and wherein said second control means is arranged to control automatically the phase of said signals supplied to said second sub-array in dependence on a predetermined function of the phase of said signals supplied to said first and third sub-arrays. 
     Advantageously, the predetermined function is the vector sum of the phase of the signals supplied to said first and third sub-arrays. 
     It will be appreciated that the features described as optional and/or alternatives of the first aspect of the invention may also be applicable to the further aspects of the invention. 
     According to yet a further aspect of the present invention, there is provided an antenna system comprising: 
     an antenna assembly having a plurality of elements arranged into at least first, second and third sub-arrays, each array comprising one or more of said elements; and 
     control means for controlling the phase of signals supplied to each of said sub-arrays; 
     wherein said antenna assembly is supplied with a maximum of two signal feeds. 
     The systems of the invention as described in the preceding paragraphs provide several advantages over existing systems. In particular, control and/or adjustment of the phases of signals supplied to each sub-array in the antenna assembly can be achieved simply and quickly and from a location remote from the antenna assembly. It is known to adjust the angle of tilt of an antenna by manual mechanical adjustment of the antenna elements and/or the antenna housing mounted on the antenna carrier or mast itself. Such an adjustment process is inconvenient and labour intensive. The present invention provides the advantage that the angle of tilt can be adjusted from a location remote from the antenna mast by electrical means, for example from a base station or control centre at the base of the antenna mast, or a base station situated several kilometers from the mast. Moreover, the system is appropriate for multi-user (i.e. multi operator) applications, by providing each user with independently operable control means, and by combining the user signals in a frequency selective combiner device. 
     The invention also provides the advantage that the distribution of the phase and amplitude of the signals fed to each antenna element is controlled so as to provide improved control of the antenna gain and side lobe level, particularly when the system is electrically tilted. The provision of the mechanical phase adjustment means, for example, for further adjusting the phase of signals supplied to each element of the array, provides the user with a means of fine tuning the vertical radiation pattern, to permit further optimisation of the boresight gain and sidelobe levels. 
     This aspect of the invention also provides an advantage over other known techniques in that a reduction in the number of components required to adjust the electrical tilt of the antenna assembly may be achieved with a corresponding reduction in system complexity and cost. 
     For the purpose of this specification, it will be appreciated that the phrase “user” is intended to mean the user of the system of the invention (i.e. a system operator), and not the user of the telephone handset for receipt/transmission of signals to/from the system. 
     The present invention will now be described, by way of example only, with reference to the accompanying drawings in which: 
    
    
     
       DESCRIPTION OF THE FIGURES 
         FIG. 1  illustrates the vertical radiation pattern (VRP) of a known phased array antenna assembly; 
         FIG. 2  is a schematic block diagram of a known antenna assembly incorporating mechanical means for adjusting the angle of electrical tilt; 
         FIG. 3  is a schematic block diagram of a first embodiment of a dual sub-array antenna system according to the invention, 
         FIG. 4  is a schematic block diagram of a practical implementation of the antenna system in  FIG. 3 , 
         FIG. 5  is a schematic block diagram of a triple sub-array antenna system of an alternative embodiment, using spatial over-lapping of sub-arrays, 
         FIG. 6  shows a schematic block diagram of an alternative triple sub-array antenna system to that shown in  FIG. 5 , 
         FIG. 7  shows a schematic block diagram of a practical implementation of the antenna system in  FIG. 6 , 
         FIG. 8  shows a schematic block diagram of a quintuple sub-array antenna system of a further alternative embodiment, 
         FIG. 9  illustrates one embodiment of the mechanical phase adjustment arrangement forming part of the system in  FIGS. 3 to 8 , 
         FIG. 10  illustrates an alternative mechanical phase adjustment arrangement to that shown in  FIG. 9 , 
         FIG. 11  is a further alternative embodiment of a triple sub-array antenna system, to show details of the mechanical phase adjustment arrangement in  FIG. 10 , 
         FIG. 12  is a further alternative embodiment of a triple sub-array antenna system, to show details of the mechanical phase adjustment arrangement in  FIG. 9 , and 
         FIG. 13  is a schematic block diagram of an alternative form of system according to the invention, incorporating a dual polarity antenna assembly. 
     
    
    
     In the drawings, like reference numerals are used to denote similar parts. In the following description, the invention is described in the context of an antenna system suitable for use in a cellular mobile radio network and particularly the Universal Mobile Telephone System (UTMS). However, it will be appreciated that the invention is not confined to such use and may be equally applicable to other communications systems. 
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows the vertical radiation pattern (VRP) of a conventional phased array antenna assembly. The drawing is shown in side view and the antenna assembly is represented by the point  1 . 
     The VRP of the antenna assembly  1  consists of a main lobe or “boresight”  2  which diverges in a vertical plane as it extends from the antenna assembly and represents the region of maximum radiation intensity of the beam radiated by the antenna assembly. The VRP of the antenna assembly also includes a number of side lobes  4 , representing regions of much lower radiation intensity, which extend from the antenna assembly in directions which are approximately equiangularly spaced about the antenna assembly in a vertical plane. The lobes  3  immediately adjacent the boresight  2  are termed the first upper and first lower side lobes respectively. 
     The angle of tilt of the antenna assembly, when adjusted mechanically by physically moving the antenna elements and/or their housing or casing, is known as the angle of “mechanical tilt” and is conventionally achieved by repositioning the boresight so that it points either above or below the horizon. When adjusted electrically, the tilt of the antenna assembly is known as “electrical tilt” and moves the boresight line up or down by changing the time delay or phase of signals supplied to groups of elements in the antenna, rather than by mechanical movement of the elements themselves. The time delay may be achieved by changing the phase of the radio frequency carrier. Providing that the phase delay is proportional to frequency across the band of interest, and has zero intercept, then the phase delay produces a time delay. Phase shift and time delay are thus synonymous. 
     It will benefit the reader&#39;s understanding of the following description to note that both “electrical tilt” and “mechanical tilt” may be controlled and/or adjusted either by electrical means, or by mechanical means, or both means, such that, for example, mechanical movement of parts may be used to implement electrical phase adjustment in which the antenna elements themselves are not physically moved to adjust the position of the boresight. 
     In  FIG. 2 , the antenna assembly of a known antenna system incorporating a mechanical means of adjusting the angle of electrical tilt is shown in schematic block form generally at  10 . The antenna assembly is a phased array antenna consisting of an array of twelve elements or dipoles E 1 –E 12  which are arranged into three sub-arrays labelled A, B and C. 
     Each sub-array A, B, C includes four elements, mutually connected in parallel, and is coupled to the output of respective first, second and third delay devices  12 ,  14 ,  16 . The delay devices  12 ,  14 ,  16  comprise conventional mechanical phase adjustment mechanisms of the type shown in  FIGS. 9 and 10  and described in further detail below. A radio frequency (RF) signal to be transmitted by the antenna is supplied to each of the delay devices  12 ,  14 ,  16  from a common RF port or feeder  18 . 
     The function of the delay devices  12 ,  14 ,  16  is to adjust the phase of the RF signal supplied to the respective sub-array A, B, C by a predetermined amount. The second delay device  14 , connected to the centre sub-array B, is a fixed delay device, arranged to shift the phase of the signal supplied to sub-array B by a fixed amount. On the other hand, the first and third delay devices  12 ,  16 , connected to sub-arrays A and C respectively, are variable delay devices, each of which is operable to shift the phase of the RF signals supplied to sub-arrays A and C respectively, by a variable amount. 
     The first and third delay devices  12 ,  16  can apply phase shifts of, typically, between 0 and ±45° to the RF signal supplied to sub-arrays A and C and are adjustable by means of a mechanical arrangement  20  such as that shown in  FIGS. 6 and 7 . The mechanical arrangement  20  includes means, shown representatively at  22 , for reversing the direction of the phase shift applied to the signal by the third delay device  16  compared with that applied by the first delay device  12 . Thus, the phase shift applied to the RF signals by the first and third delay devices  12 ,  16  is equal in magnitude but opposite in polarity. In other words, if the first delay device  12  shifts the phase of the signal supplied to sub-array A by +45°, then the third delay device  16  shifts the phase of the signal supplied to sub-array C by −45°. As the second delay device  14  is a fixed delay device, in practice a phase shift is applied to the signal supplied to sub-array B which is the median of the shifts applied by the first and third delay devices  12 ,  16 . 
     The angle of electrical tilt of such an antenna assembly typically varies by ±5° for ±45° of phase shift per sub-array. This gives a tilt sensitivity of approximately 18° of phase shift per degree of electrical tilt. In this example, therefore, since the RF signals supplied to sub-arrays A and C differ by 90°, the electrical tilt of the antenna assembly is approximately 5°. The direction of electrical tilt of the antenna assembly depends on the polarity of the phase shift applied to the signals supplied to the sub-arrays. Where the signal to the upper sub-array (in this case sub-array A) has a positive phase and the lower sub-array (in this case sub-array C) has a negative phase shift, the angle of electrical tilt will be positive, i.e. above the normal boresight line. For phase shifts of opposite polarity the angle of electrical tilt will be negative. 
     The antenna assembly of  FIG. 2  suffers from a number of disadvantages. In particular, manual adjustment of the mechanical arrangement  20  is required to adjust the phase shift applied by the first and third delay devices  12 ,  16  in order to vary the angle of electrical tilt of the antenna assembly. Moreover, owing to the provision of a common mechanical adjustment arrangement  20 , the magnitude of the phase shifts applied by the first and third delay devices  12 ,  16  is always equal in magnitude and opposite in direction (polarity), thereby limiting the tilt of the antenna assembly. In addition, the side lobe level is increased relative to that of the boresight. As a consequence, the gain of the antenna assembly is disadvantageously reduced. 
     In  FIG. 3 , a preferred form of antenna system according to the invention is shown in block form generally as  100 . In this embodiment, the antenna system  100  comprises an antenna assembly shown at  102  and a control unit  104 . The antenna assembly  102  comprises a phased array antenna having an array of eight elements E 1  to E 8  mounted upon an antenna carrier or mast (not shown). The elements E 1  to E 8  are arranged into two sub-arrays: an upper sub-array  100 A, comprising elements E 1  to P 4 , and a lower sub-array  100 B comprising elements E 5  to E 8 . The elements in each of the sub-arrays  100 A,  100 B, are connected in parallel to respective signal distribution means in the form of distribution networks  151 N 1 ,  151 N 2 . The distribution networks  151 N 1  and  151 N 2  are fed through carrier lines  120 ,  122  respectively, and will be described in further detail hereinafter. 
     The antenna assembly  102  includes two input ports represented by squares  112 ,  114 , each of which is connected to the respective distribution network  151 N 1 ,  151 N 2  via the respective input carrier line  120 ,  122 . The control unit  104  also includes an input splitter/combiner unit  125 , the common port to which is connected to the output of a single RF port  126 . The input splitter/combiner unit  125  has two ports which are connected, via first and second splitter lines  128 ,  130 , to first and second phase adjusters  132 ,  134  respectively. The first phase adjuster  132  is connected at its output to input port  112  via a first input feeder line  136  whilst the second phase adjuster  134  is connected to input port  114  via a second input feeder line  138 . The antenna assembly  102  is therefore provided with signals from the control unit  104  through dual feeder lines. 
     In addition to the phase adjustment implemented by the first and second phase adjusters  132 ,  134 , additional phase adjustment means  150 E 1 – 150 E 8  are provided in the signal path to each element of the assembly, each additional phase adjustment means  150 E 1 – 150 E 8  taking the form of a mechanical phase adjustment arrangement of the type described in further detail below with reference to either  FIG. 9  or  10 . Each of the mechanical phase adjustment arrangements  150 E 1 – 150 E 8  serves to further adjust the phase of signals supplied to the individual elements in each sub-array  100 A,  100 B, as is controlled by means of a servo motor  101  under the control of a servo controller  103 . The servo controller  103  controls the servo motor  101  through a control cable  206 , which may be of sufficient length so that the servo controller  103  may form part of the control unit  104  located remotely from the antenna assembly  100 . 
     The distribution networks  151 N 1 ,  151 N 2  are shown in further detail in  FIG. 4 . The first distribution network  151 N 1  associated with the upper sub-array  100 A includes first, second and third splitter/combiner units  116 A,  116 B,  116 C respectively. The input carrier line  120  supplies a signal to the second splitter/combiner unit  116 B, which supplies first and second output signals of substantially equal strength to a respective one of the first and third splitter/combiner units  116 A,  116 C. The first and third splitter units  116 A,  116 C, further divide the signal so that each provides first and second output signals of substantially equal strength to a respective one of the phase adjustment means  150 E 1  to  150 E 4 . The second distribution network  151 N 2  for the lower sub-array  100 B includes an identical arrangement of splitter units  118 A,  118 B,  118 C. The arrangement of splitter/combiner units  116 A– 116 C,  118 A– 118 C ensures an equal power distribution to each element E 1  to E 8  of the array, and thus also ensures maximum boresight gain and that the beam pattern is the same in both transmit and receive modes. 
     Referring to  FIGS. 3 and 4 , in operation a signal to be transmitted by the antenna system is fed from the RF port  126  to the input of the input splitter unit  125 . The input splitter unit  125  splits the signal into two output signals of equal strength and supplies one split signal to each of the first and second phase adjusters  132 ,  134  respectively. The first and second phase adjusters  132 ,  134  are operable to adjust the phase of the signal supplied thereto within a range of ±60°. Each phase adjuster  132 ,  134  is controlled such that, if the first phase adjuster  132  is arranged to apply a positive phase shift to the RF signal, the second phase adjuster  134  is arranged to apply a negative phase shift to the signal, and vice versa. However, each phase adjuster is arranged to adjust the phase of the signal supplied thereto independently so that the magnitude of the phase shift applied by each phase adjuster may be different. 
     The phase shifted signal from the first phase adjuster  132  is supplied to the input port  112  on the antenna assembly  102  via the first feeder line  136 . Similarly, the phase adjusted signal from the second phase adjuster  134  is supplied to the input port  114  via the second feeder line  138 . In practice, the first and second feeder lines  136  and  138  can be made as long as desired so that the control means  104  for adjusting the angle of electrical tilt of the antenna assembly  102  can be situated in a location remote from the antenna assembly itself. 
     The phase shifted signals supplied to input ports  112 ,  114  are supplied as signals Sa and Sb, on the input carrier lines  120 ,  122 , to the first and second primary splitter units  116 B,  118 B respectively. The first primary splitter unit  116 B serves to split the signal Sa and supplies the split signal from its two outputs to the elements in sub-array  100 A via the upper sub-array splitter units  116 A,  116 C and the associated phase adjustment arrangements  151 E 1  to  150 E 4 . 
     Similarly, the second primary sub-array splitter unit  118 B serves to split signal Sb and supplies the split signal from its two outputs to the elements in sub-array  100 C via the lower sub-array splitter units  118 A,  118 C and the associated phase adjustment arrangements  151 E 5  to  150 E 8 . 
     The manner in which the signals Sa, Sb are split and distributed to the elements in the antenna assembly will immediately be appreciated by those skilled in the art from the way in which the splitter units are interconnected. That is, the signal strength of each of the two signal outputs for a splitter unit will be substantially half that of the input signal strength. Thus, the signal strength of the signal supplied to each element E 1  to E 8  is substantially the same. 
       FIG. 5  is an alternative embodiment to that shown in  FIGS. 3 and 4 , in which the antenna assembly  102  includes eight antenna elements E 1  to E 8  arranged in three sub-arrays; an upper sub-array  100 A including antenna elements E 1  to E 3 , a centre sub-array  100 B including E 4  and E 5  and a lower sub-array  100 C including antenna elements E 6  to E 8 . Each of the elements E 1  to E 4  (i.e. three elements of the upper sub-array  100 A and one element of the centre sub-array  100 B) is fed by the distribution network  151 N 1 , and is provided with an additional phase adjustment means  150 E 1 – 150 E 4  respectively. Each of the elements E 5  to E 8  (i.e. the three elements of the lower sub-array  100 C and one of the elements of the centre sub-array  100 B) is fed by the other distribution network  151 N 2 , and is provided with an associated additional phase adjustment means  150 E 5 – 150 E 8  respectively. The phase adjusted signals to the centre sub-array elements,  150 E 4  and  150 E 5 , are derived by spatially combining, in air at  160 , one of the output signals from the first distribution network  151 N 1  and one of the output signals from the second distribution network  151 N 2 . Air combining of the two signals to derive the inputs to the centre sub-array  100 B occurs after the output signals from the distribution networks  151 N 1 ,  151 N 2  have passed through the associated phase adjustment arrangement  150 E 4 ,  150 E 5 . 
     The distribution networks  151 N 1 ,  151 N 2  in  FIG. 5  may include substantially identical splitter arrangements to those shown in  FIG. 4 . Thus, the outputs from the first splitter unit  116 A of the first distribution network  151 N 1  feed elements E 1  and E 2 , and one of the outputs from the third splitter unit  116 C feeds element E 3 . As the feeds to elements E 4  and E 5  are interchanged in the  FIG. 5  embodiment, the second output from the third splitter unit  116 C of the first distribution network  151 N 1  feeds element E 5 . Similarly, the two outputs from the third splitter unit  118 C of the second distribution network  151 N 2  feed elements E 7  and E 8 , and one of the outputs from the first splitter unit  118 C feeds element E 6 . As the feeds to elements E 4  and E 5  are interchanged, one of the outputs from the first splitter unit  118 A of the second distribution network  151 N 2  feeds element E 4 . 
     An advantage is obtained by spatially overlapping two of the elements from the upper and lower sub-arrays  100 A,  100 C to derive the inputs to the centre sub-array  100 C, in that the phase distribution across the array elements is a closer approximation to a linear distribution. Higher boresight gain and lower side-lobe levels can therefore be achieved, particularly when the antenna is electrically tilted. 
       FIG. 6  shows a further alternative embodiment of the antenna assembly, in the form of a triple-sub group, variable electrical tilt assembly. The antenna assembly  102  includes twelve elements, E 1  to E 12 , divided into three sub-arrays  100 A,  100 B,  100 C such that each sub-array includes four elements, E 1  to E 4 , E 5  to E 8  and E 9  to E 12  respectively. Similar parts to those shown in the embodiments of  FIGS. 3 to 5  are indicated with like reference numerals, and will not be described in further detail. The input carrier lines  120 ,  122  feed respective signals Sa and Sb to primary splitter units  140 A,  140 B respectively, each of which provides two outputs of equal strength. The first output of the first primary splitter unit  140 A is connected to a first output carrier line  106  and the second output of the first primary splitter unit  140 A is connected to a first input of a combiner unit  124 . The first output of the second primary splitter unit  140 B is connected to a second output carrier line  110 , while the second output of the second primary splitter unit  140 B is connected to a second input of the combiner unit  124 . 
     The combiner unit  124  is operable to output the vector sum of the two signals on an output carrier line  108 . As the signal strength of each of the signals input to the combiner unit  124  is half that of the signals Sa, Sb, having been halved by the first and second primary splitter units  140 ,  140 B respectively, in combining the signals output from the first and second primary splitter units  140 A,  140 B, the signal output by the combiner unit  124  has the same signal strength as either of the signals Sa, Sb. In addition, since the combiner  124  unit generates the vector sum of the two signals Sa, Sb, and since the phase of the signals Sa, Sb has been adjusted differentially (i.e. at opposite polarities), the phase of the signals output by the combiner unit  124  along line  108  is the median of the phases of Sa and Sb. Furthermore, the combiner unit  124  provides the median of the phases of signals Sa and Sb without any loss of the signal power to sub-group  100 B. 
     The combiner unit  124  provides the vector-summed signal on the carrier line  108  to the second distribution network  151 N 2 , which in turn provides signals to each of the elements E 5  to E 8  through the associated phase adjustment means  150 E 5  to  150 E 8 . This configuration provides a further improvement in phase linearity, as the output from the combiner unit  124  is the average phase of the signal on the input carrier lines  120 ,  122 . Thus, the total power fed to the elements of the centre sub-array  100 B (elements E 5  to E 8 ) remains substantially constant with phase difference between the carrier lines  120 ,  122 . 
       FIG. 7  shows a practical embodiment of the triple sub-group antenna assembly in  FIG. 6 , to show the distribution networks  151 N 1 ,  151 N 2 ,  151 N 3  in more detail. The first and second splitter units  140 A,  140 B are fed by a respective one of the input carrier lines  120 ,  122 , and each of the splitter units  140 A,  140 B produces two output signals. The first output signal from the first splitter unit  140 A is supplied to a phase shift unit  170 A of the first distribution network  151 N 1  to apply an additional phase shift, typically between −45 and −60 degrees, to the signal from the primary splitter unit  140 A. The phase shifted output signal is provided to a splitter unit  116 B forming part of a splitter arrangement  116 A,  116 B,  116 C of the kind shown in  FIG. 4 . The splitter arrangement  116 A,  116 B,  116 C provides output signals to the phase adjustment means  150 E 1 – 150 E 4  respectively, so that each element receives a signal of substantially equal strength. 
     A second output from the splitter unit  140 A is provided to a further splitter unit  172 A forming part of the second distribution network  151 N 2 , which splits the input it receives into a first output signal which is provided to one input (A) of a first quadrature hybrid combiner unit  174 A and a second output signal which is provided to an input (A) of a second quadrature combiner unit  174 B. 
     The second splitter unit  140 B provides a first output signal to a further splitter unit  172 B forming part of the second distribution network  151 N 2 . 
     The further splitter unit  172 B provides an output signal to a second input (B) of the first quadrature combiner unit  174 A and to a second input (B) of the second quadrature combiner unit  174 B. 
     Each of the first and second quadrature combiner units  174 A,  174 B provides first and second output signals to two elements of the centre sub-array  100 B: the first quadrature combiner unit  174 A provides signals to elements E 5  and E 6  and the second quadrature combiner unit  174 B provides signals to elements E 7  and E 8 . The first and second quadrature combiner units  174 A,  174 B ensure the phase of signals provided to elements E 5  to E 8  is the average of the phase of the signals on the input carrier lines  120 ,  122 . For example, as the power fed to element E 5  decreases, the power fed to element E 6  increases so that the total power fed to the elements E 5 , E 6  remains substantially constant. 
     A second output signal from the second splitter unit  140 B is passed through a second phase shift unit  170 B forming part of the third distribution network  151 N 3 . The second phase shift unit  170 B applies a phase shift of +45 degrees (i.e. opposite polarity to phase shift unit  170 A) to a splitter unit  118 B. The splitter unit  118 B forms part of a splitter arrangement  118 A,  118 B,  118 C, of the kind shown in  FIG. 4 , and provides output signals to the phase adjustment means  150 E 9 – 150 E 12  respectively of the elements E 9  to E 12  of the lower sub-array  100 C. 
       FIG. 8  is an alternative embodiment of the invention, in which the antenna assembly includes five sub-arrays  100 A– 100 E (i.e. a quintuple sub-array assembly), wherein the third and fourth sub-arrays  100 B,  100 D are obtained by spatially overlapping elements of a three sub-array assembly, such as that shown in  FIG. 6 , similar parts to those shown in  FIG. 6  being indicated with like reference numerals. The input carrier lines  120 ,  122  supply signals Sa, Sb respectively to first and second primary splitter units  140 A,  140 B respectively. The first splitter unit  140 A provides a first output signal to a first distribution network  151 N 1  along output carrier line  106  and a second output signal to a combiner unit  124 . The second splitter unit  140 B provides a first output signal to a third distribution network  151 N 3  along output carrier line  110 , and a second output signal to a combiner unit  124 . The combiner unit  124  provides an output signal along output carrier line  108  to the second distribution network  151 N 2 . 
     Each distribution network  151 N 1 ,  151 N 2 ,  151 N 3  provides four output signals, each one of which is provided, through an associated phase adjustment arrangement  150 E 1 – 150 E 12 , to an element of the array. One of the output signals  180 A from the first distribution network  151 N 1  is spatially overlapped with one of the output signals  180 B from the second distribution network  151 N 2  by combining the signals in air, to provide the signals to the elements, E 4  and E 5 , of the sub-array  100 B. Similarly, one of the output signals  180 C from the second distribution network  151 N 2  is spatially overlapped with one of the output signals  180 D from the third distribution network  151 N 3  by combining in air, to provide the signals to the elements, E 8  and E 9 , of the sub-array  100 D. The configuration in  FIG. 8  provides a further improvement in phase linearity across the elements E 1 –E 12 , and further improves boresight gain and side-lobe suppression when the assembly is tilted electrically. 
     In practice, the distribution network  151 N 1  in  FIG. 8  may include the splitter arrangement  116 A,  116 B,  116 C and the phase shit unit  170 A of the  FIG. 7  embodiment, and the third distribution network  151 N 3  may include the splitter arrangement  118 A,  118 B,  118 C and the phase shit unit  170 B of the  FIG. 7  embodiment. The combiner unit  24  and the second distribution network  151 N 2  may include the first and second splitter units  172 A,  172 B and the first and second quadrature combiner units  174 A,  174 B, as described previously with reference to  FIG. 7 . 
       FIGS. 9 and 10  illustrate known apparatus for the mechanical adjustment of the phase of the signal fed to each element in the antenna assembly. Either or both of these methods may be used in the antenna assemblies of  FIGS. 3 to 8  as the phase adjustment arrangements  150 E 1 – 150 En (where n=number of elements in the antenna assembly). 
     In  FIG. 9 , mechanical adjustment of the phase of a signal on a transmission line is achieved by linear movement of an element of dielectric material beneath the transmission line. The mechanical adjustment arrangement  601  includes a base plate  602 , across which the transmission line T to the antenna element runs, and a generally planar plate of dielectric material  604  disposed between the base plate  602  and the transmission line T. The plate of dielectric material  604 , commonly termed a “wedge”, is generally rectangular with a triangular or V-shaped segment  606  cut away from one longitudinal edge thereof. The wedge  604  is movable relative to the base plate  602  and the transmission line T in a direction, shown by arrow A, generally transverse to the transmission line T. Owing to its shape, linear movement of the wedge  604  causes a greater or lesser amount of dielectric material to be interposed between the transmission line and the base plate  602 , thereby causing the propagation velocity and, hence, the phase of any signal on the transmission line T to be shifted by an amount dependent on the linear position of the wedge. Such linear movement is usually effected by a linear actuator in the form of a servo or other motion transducer. 
     The amount of phase shift applied to the signal on the transmission line T is set by the position of the wedge  604  beneath the transmission line T and the “wedge angle”, the internal angle of the V-shape cut into the wedge. 
       FIG. 10  shows a mechanical phase adjustment arrangement, referred to generally as  701 , which is operable to shift the transmission time delay, and hence the phase, of a signal on a transmission line by means of the rotary movement of a movable length of transmission line capacitively coupled to a fixed line length. The arrangement  701  includes a base plate  702  on top of which is a layer of dielectric material  704 . A fixed length of transmission line T forms a transmission line with the base plate  702  and the dielectric layer  704 . The transmission line is discontinuous to form two portions of transmission line T 1 , T 2 , the first portion T 1  extending across the dielectric layer  704  to form a circumferential quadrant of a circle having radius R and the second portion T 2  extending across the dielectric layer  704  to form a circumferential quadrant of a circle having radius, r. 
     A planar disc of dielectric material  706  is disposed over the transmission line T and is rotatable relative thereto about an axis coaxial with the centre of the circle defined by the first and second portions of the transmission line T 1 , T 2 . The dielectric disc  706  carries a U-shaped length of transmission line U having a first arm, U 1 , defining a circumferential quadrant of a circle having radius R and a second arm, U 2 , defining a circumferential quadrant of a circle having radius r. 
     The transmission lines T, U are coupled together via the dielectric disc  706  and phase adjustment of a signal on the transmission line T can be effected by rotating the dielectric disc  706  to adjust the position of the transmission line U relative to the transmission line T. As the disc is rotated through 90°, the coupling between the two transmission lines, and thereby the effective length of the transmission line to the antenna element, varies to shift the phase of a signal carried by the transmission line. 
     Although not shown in  FIG. 10 , it is possible to use the apparatus of  FIG. 10  to control the phase of more than one antenna element. For example, for such a device to control the phase of signals on two separate transmission lines, a second arrangement of transmission lines T, U could be arranged on the opposite quadrant of the dielectric disc  706 . The phase shift applied to each antenna element, or each sub group of elements, can be set either by the radius of the transmission line T, U on each disc, the mechanical coupling between the transmission lines or by both means. 
       FIG. 11  illustrates an alternative embodiment of the invention, in which the arrangement of splitter units is a so-called “family tree” configuration, which allows signals of equal strength to be supplied to each of the elements in the assembly. Such a configuration is appropriate where the phase adjustment of individual antenna elements is present, since a cosine squared voltage distribution is not necessary to maximise boresight gain. 
     In this particular embodiment, the antenna assembly consists of eight elements E 1  to E 8 ; upper sub-array  101 A comprising elements E 1 –E 3 , centre sub-array  100 B comprising elements E 4  and E 5  and lower sub-array  100 C comprising elements E 6  to E 8  (i.e. a triple sub-array system). Remote adjustment of the angle of electrical tilt of the antenna assembly is achieved by means of servo control of the mechanical phase adjustment apparatus, in combination with differential phase shift applied by electrical means to the signals supplied to the antenna elements. 
     The base-station control unit  104 , comprising the input splitter/combiner unit  125 , the RF port  126  and the first and second phase adjusters  132 ,  134  (none of which are shown), supplies the first and second phase shifted signals Sa, Sb to the input ports  112 ,  114  via the first and second feeder lines  136 ,  138  respectively. The input ports  112 ,  114  apply the signals to the input carrier lines  120 ,  122  respectively. The phase shifted signals Sa and Sb, on the input carrier lines  120 ,  122 , are supplied to the first and second primary splitter units  116 ,  118  respectively. The splitter units are arranged such that each output of the first and second primary splitter units  116 ,  118  is connected to the input of a respective splitter unit in a second row of splitter units  116 A,  116 B,  118 A,  118 B. 
     The two outputs of the splitter unit  116 A are connected to the antenna elements E 1  and E 2  respectively via a first phase adjustment arrangement D 1  similar to that shown in  FIG. 10 . The first output of the splitter unit  116 B is connected to the antenna element E 3  via a second phase adjustment arrangement D 2 . The second output of the splitter unit  116 B is connected to a first input of the combiner unit  124 , as is the first output of the splitter unit  118 A. The combiner unit  124  has two outputs, each of which is connected to the elements E 4  and E 5  via the second and third phase adjustment arrangements D 2 , D 3 , respectively. The second output of the splitter unit  118 A is connected to the element E 6  via the third phase adjustment arrangement D 3  while both outputs of the splitter unit  118 B are connected to the elements E 7 , E 8  respectively via a fourth phase adjustment arrangement D 4 . 
     In  FIG. 11 , rotation of the discs in the phase adjustment arrangements D 1  to D 4  is achieved by linear movement of an actuating arm  200  pivotally and eccentrically mounted to each of the rotating discs  706  of the mechanical phase adjustment arrangements  701 . Linear movement of the actuating arm  200  may be achieved, for example, by the servo motor  101  which is controlled by means of the servo controller  103 . The control cable  206  can be of any desired length, enabling the servo motor  103  to be controlled from a location remote from the antenna assembly  100 . The phase adjustment arrangements D 1  to D 4  may be configured such that movement of the respective discs through the single control point results in a substantially equal degree of rotation for each disc. However, different amounts of phase shift may be applied to the signals to each antenna element depending on the coupling between the transmission lines T, U in each of the phase adjustment mechanisms. 
       FIG. 12  illustrates a triple sub-array embodiment of the antenna system in which the mechanical phase adjustment arrangement  601  connected to each antenna element E 1  to E 8  is a mechanism similar to that shown in  FIG. 9 , and in which an increased number of mechanical adjustment arrangements are required to implement individual mechanical tilt to each element E 1  to E 8 . In other words, the  FIG. 12  embodiment differs from that in  FIG. 11  in that there is an independent and separate moveable dielectric element associated with each element E 1  to E 8 . A servo motor  101  and a servo controller  103  are provided, as described previously, and, again, remote adjustment of the angle of electrical tilt of the antenna assembly  100  is achieved by means of servo control of the mechanical phase adjustment arrangements  601  through the control cable  206 , in combination with differential phase shift applied to the signals Sa, Sb, supplied to the antenna elements E 1  to E 8 . 
     The phase of signals supplied to each element E 1  to E 8  is controlled by the linear movement of the dielectric wedge in each mechanism, each of which is connected to an actuating arm  200 . It will be noted that the phase adjustment arrangements connected to the lower four elements E 5 –E 8  are reversed compared to those connected to the upper four elements E 1  to E 4 . Consequently, an increase in delay (a negative phase shift) applied to the signals supplied to the elements E 1  to E 4  will cause a decrease in delay (a positive phase shift) to be applied to the signals supplied to the elements E 5  to E 8 . 
     In order to retain maximum boresight gain and control of the side lobe levels when the angle of electrical tilt of the antenna assembly is changed, each antenna element may require a different amount of delay for a given movement of the actuating arm  200 . In the linear mechanical phase adjustment arrangement, this may be achieved by changing the angle of the V-shaped segment  606  of the wedge  604  (as shown in  FIG. 9 ). 
     It will be appreciated that the rotary mechanical phase adjustment arrangement of  FIG. 10  may be used in place of the linear mechanical phase adjustment arrangements  600  in  FIG. 12 . Using the rotary mechanical phase adjustment arrangements of  FIG. 10 , a different amount of delay for a given movement of the actuating arm  200  may be achieved by using a different radius for the transmission line mounted on each rotatable disc. 
     Although the arrangement of the splitter units  116 A– 116 C,  118 A– 118 C and combiner unit  124  in  FIG. 12  is different from that described previously, it will be apparent from the foregoing description how this arrangement distributes the signal strength across the elements E 1  to E 8 . 
       FIG. 13  shows yet a further embodiment and illustrates how the system of the present invention can be used with a dual-polarity antenna assembly. The use of dual polarity antenna assembly is well known and common in telecommunication systems. In this embodiment, the antenna assembly includes a stack of four crossed dipole elements C 1  to C 4 , arranged in a first array of four elements angled at +45° to the vertical and a second array of four elements angled at −45° to the vertical. The first and second arrays are effectively electrically separate with individual RF feeders  1110 ,  1112  being supplied to each array. The first and second arrays share the common feature that the mechanical phase adjustment/splitter arrangements (referred to generally as  1114  and  1116 ) to each individual element (where present) are adjusted by means of a common servo mechanism so that both the first and second array have the same angle of electrical tilt. Once again, the servo motor  101  is controlled by a servo controller  103  which communicates with the servo motor  101  via a control cable  206 . 
     It will be appreciated that the means by which the actuating arm  200  for the mechanical phase adjustment arrangements,  601 ,  701 ,  1114 ,  1116 , is moved need not take the form of a servo control arrangement  101 ,  103 , but may the form of an alternative arrangement which is operable from a location remote from the actuating arm  200 . 
     It will also be appreciated that the present invention provides an effective way of remotely adjusting the electrical tilt of a phased array antenna. For example, it is possible to control and/or adjust the electrical tilt from a base station located at the base of the antenna mast upon which the antenna elements are mounted or from a location several miles from the antenna mast, as there is no requirement for manual adjustment of the antenna elements themselves. Moreover, the invention allows the independent phase shifting of signals to individual sub-arrays within the antenna assembly and automatic differential phase adjustment of signals to the centre sub-array to permit the use of only two RF inputs. Furthermore, signals to the upper and lower sub-array can be phase shifted by varying degrees which are not necessarily equal in magnitude. The vector summing of the signals supplied to the outer sub-arrays by the combiner unit  124  allows the signals supplied to the centre sub-array always to be shifted to the median value thereof, if required. 
     The combined mechanical and electrical control of the electrical tilt of the antenna system allows an optimum beam pattern for the antenna system to be generated with maximum boresight gain and lower side lobe levels and, moreover, such control is achievable from a location remote from the antenna assembly, for example several kilometers from the base of the antenna mast. The performance of such an antenna system is substantially improved compared with existing systems. 
     It will be appreciated that although different embodiments of the invention are shown and described as having a different number of antenna elements (for example E 1  to E 8  in  FIG. 5 , E 1  to E 12  in  FIG. 6 ), any of the embodiments may be adapted to include more or less antenna elements, sub-grouped into an appropriate arrangement of more or less sub-arrays than those shown, in a manner which would be readily apparent to a skilled person from the description hereinbefore, whilst still maintaining the aforementioned advantages. 
     Although the servo control mechanism  103  for the additional mechanical phase adjustment arrangements  150 E 1 – 150 En is shown as forming part of the control unit  104 , this need not be the case. The servo controller  103  may also be located remotely from the antenna assembly  100 , as is the control unit  104 , but it need not be located in the same place. 
     Throughout the specification, a reference to “electrical tilt” shall be taken to mean adjustment of the radiation pattern transmitted and/or received from the antenna assembly without physically moving the antenna radome, or the antenna elements, but instead implemented by adjusting the phase of signals supplied to one or more of the antenna elements. It will be appreciated, however, that electrical tilt may be adjusted by an arrangement having both mechanical and electrical adjustment elements, as shown for example in  FIG. 11 .