GB2410838A - Antenna with adjustable beam direction - Google Patents

Abstract

An antenna having an adjustable beam tilt is described. The antenna comprises an array of antenna elements and phase-shifting apparatus to adjust the phase of the signal supplied to each antenna element 12, 14, 16, 18 in the array such that the beam is caused to tilt by a desired amount. The phase shifting apparatus comprises a microstrip line feed network connected to input terminals, output terminals and one or more stator pads. Disposed around the stator pad is a transmission line arc. A rotatable conductor element connects the stator pad, and the feed network to the transmission line arc, the position on the arc of the rotatable conductor element determining the phase-shift introduced in the signal. The antennas described comprise a plurality of phase shifters, the rotatable conductive elements of which are operated by a single actuator rod. The antenna also comprises novel shielding 20 between respective antenna elements in the array that reduce cross-modulation products and that stabilize the radio beam in the desired direction.

Description

24 1 0838 - 1 -

ANTENNA WITH ADJUSTABLE BEAM DIRECTION

This application relates to an antenna with adjustable beam direction.

Base station antennas for mobile radio base stations typically comprise a linear array of radiating elements; each element comprises one or more dipole, patch or slot radiators configured to create the required azimuth beamwidth, while the number of elements used in the elevation plane is chosen to produce the required gain and elevation beamwidth. Typical base station antennas use one, two or three elements in each tier and between 4 and 16 tiers in elevation.

The elevation beamwidth of a typical base station antenna is typically between 5 and 15 . In order to control the coverage footprint of each base station and to reduce mutual interference between base stations, the direction of maximum radiation in the elevation plane is typically depressed by a few degrees below the horizontal plane (an angle known as the beamtilt). As a network of stations evolves the optimum beamtilt for each base station antenna changes. In order to adjust the beamtilt it is currently necessary to physically move the antenna and a significant cost is involved in sending personnel to climb the antenna structure to carry out this adjustment.

As an alternative to physical movement of the antenna it is possible to change the effective beamtilt by introducing a progressive phase shift between the elements of the antenna array. In order to tilt the beam down (increasing the beamtilt) it is necessary to advance the phase of the currents in the upper elements relative to those of the lower elements. In order to preserve the shape of the beam and to obtain the highest gain from the array it is desirable that the phase shifts introduced are 2 - as linear as possible so that equal phase shifts are introduced between elements in a progressive manner.

Figure 1 illustrates the principle. The currents radiated by each element sum in the direction indicated by the arrows, resulting in a phase difference of between the elements and a beam angled below the horizontal.

The design of a suitable system of phase shifters is constrained by the operating power level required (typically a total continuous effective power in the range 100W-500W) and the very high linearity required to allow a single antenna to provide for the simultaneous transmission and reception of multiple radio carriers.

These constraints imply the use of simple passive components which must be designed to provide extremely low levels of passive intermodulation products (IMPs).

A high-frequency phase shifter is described in International Patent Application WO 01/13459. The phase shifter comprises two stripline sections arranged concentrically in relation to each other. A connecting line connects a section of the strip-line to an input line.

An antenna control system employing phase shifter apparatus which allows remote variation of antenna beam tilt is also described in International Patent Application We have appreciated that there is a need for an antenna having means to adjust the antenna beam tilt easily and reliably.

SUMMARY OF INVENTION

The invention is defined in the independent claims to which reference should now be made. Advantageous features of the invention are defined in the appendant claims.

BRIEF DESCRIPTION OF DRAWINGS

Preferred embodiments of the invention will now be described, by way of example, with reference to the drawings in which: Figure 1 is a diagram illustrating how a phase shift introduced between successive elements of an antenna array produces a downward tilt in the antenna beam; Figure 2 is an illustration of the antenna assembly according to the first embodiment of the invention; Figure 3 is an illustration of the nine element antenna array of the first embodiment of the invention, shown in the cut-away section of Figure 2; Figures 4a, 4b & 4c show different views of a single element array section of the antenna array; Figures 5a, 5b & 5c show different views of a double element array section of the antenna array; Figure 6 shows a known phase shifting apparatus for introducing a phase shift in signals received by different elements in an antenna array; Figures 7a, 7b and 7c shows the phase shifting apparatus for use in the first embodiment of the invention; Figure 8 is a schematic illustration of the arrangement of phase shifters and antenna elements employed in a second preferred embodiment of the invention; Figure 9 illustrates the phase shifting apparatus employed in the second embodiment of the invention; and Figure 10 illustrates an antenna element employed in the second embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS _. _

The first preferred embodiment of the invention is illustrated in Figure 2 to which reference should now be - 4 - made. Figure 2 shows a base station antenna 2 having an exterior housing, or radome 4 housing an antenna array 6.

The antenna array is visible in Figure 2 through a cut- away section of radome 4. The near end of the base station antenna 2, as shown in Figure 2, is provided with controls 7 for adjusting the phase shift of the signal sent to different elements in the array, thereby adjusting the beamtilt of the antenna. The controls employed in the embodiment shown in Figure 2 may consist of a plunger for manually adjusting the phase shift of the elements in the array. Alternatively, controls may be provided such that the phase shift can be adjusted remotely by means of a control signal. Cable attachments 8 and 9 receive coaxial cables (not shown) carrying input and output signals. In the embodiment shown, each attachment is associated with signals having one of two orthogonal polarizations, typically +45 and -45 linear polarizations.

The array 6 of antenna elements is shown in more detail in Figure 3 to which reference should now be made.

In the first embodiment the array consists of nine broadband stackedpatch dual-polar radiating elements 10.

The array is divided into seven array sections, containing either one or two antenna elements 10. The two antenna elements 10 at each end of the antenna array 6 are grouped together into single respective antenna array sections 12a and 12b. As will be appreciated later, the two sections 12a and 12b are linked such that they receive an antenna signal with a phase shift that is equal in magnitude but opposite in polarity. Working inwards, the next two antenna sections 14a and 14b each contain only a single antenna element 10, and are also linked such that they receive an antenna signal that has been phase shifted by the same amount but in opposition directions. The same is true for the next two array sections 16a and 16b. The central array section 18 also comprises a single antenna - 5 - element, but receives an antenna signal with no phase shift.

In the first embodiment described, the antenna array comprises 9 antenna elements. The number of elements is determined by the required gain of the antenna, and alternative embodiments of the invention could therefore comprise any number of elements necessary to give the required gain, taking into account any physical limitations such as overall size of the antenna.

The amplitude of signal sent to each of the array sections is caused to taper off according to the position of the array section in the array. The greatest taper amplitude being applied to the signal sent to the end two terminal array sections. This is well known as a method to reduce the magnitude of side lobes in the radiation pattern of the antenna.

The phase shift introduced in the signal sent to the elements in each of the array sections 12 to 18 is required to produce the desired shaping of the elevation pattern, that is to modify the beamtilt. However, mutual coupling between the elements is likely to perturb the required radiating currents and will therefore lead to errors, such as increased side lobe levels, unwanted nulls, and a reduction in the gain of the array.

In order to reduce the co- and cross-polar mutual impedances between the elements, a rectangular conducting fence 20 is placed around each radiating element. The lateral dimensions of the fences are determined by the required inter-element spacing and their height is chosen to obtain the required azimuth beam width. Preferably, the fence is made from metal or electroplated injection- moulded thermoplastic, typically ABS (Acrylonitrile- butadiene-styrene).

In the first embodiment, for a maximum operating frequency of 2170MHz, an inter-element spacing of 130mm for an 8 beamtilt has been found to avoid excess grating - 6 - lobes. The first embodiment further provides less than - 30db coupling for both co- and cross-polar arrangements.

Figure 4 shows in more detail a single element of the array 6. Figure 4a is a top view of the array section, Figure 4b is an isometric view of the same section, and Figure 4c is an exploded isometric view of the section showing the component parts that make up the single section.

In Figure 4, the conducting fence 20 can be seen to be part of a five sided box housing 21. The inside of the conducting fence is provided with a flange 22 which retains the base plate or printed circuit board 23 of the radiating element in place within the conducting fence, as well as providing capacitive coupling between the ground side of the printed circuit board and the rest of the antenna element. In the first embodiment the radiating element is a stacked-patch dual-polar element, having upper patch 24 and lower patch 25. The two patches 24 and are both separated from each other and mounted on the base plate 23 by non- conductive plugs 26. Base plate 23 is a printed circuit laminate which bears driving circuit network 28. Driving network 28 comprises a driving circuit part 30 of substantially identical shape to upper and lower patch elements 24 and 25. Upper and lower patch elements and driving circuit part 30 are substantially square in shape (upper antenna element 24 is shown in the drawings as having two truncated opposite corners) and are disposed at 45 on base plate 23. Circuit limbs extend at right angles to each other from respective sides of driving circuit part 30 to form cross- polar input ports 31 and 32. Away from the driving circuit part 30, the input ports bend around to extend towards each other and parallel to the side of base plate 23. Cross polar input ports 31 and 32 end in input terminals 33 and 34 which receive cross polar input signals via connections 37 and 38. The signals are phase shifted or not phase shifted according to the position of the section in the array. - 7 -

In order to increase isolation between the cross polar input ports of each element unit, a capacitive coupling strip 35 is placed closed to the patch, spanning the input port terminals 33 and 34. The shape of the coupling strip is such that enough energy is induced in the strip to compensate the natural coupling which occurs in the edge-fed patch element.

In use, the array sections are mounted within the radome 4 such that when the radome is positioned vertically, the array sections are also positioned vertically and the antenna patch elements 24 and 25 are orientated at 45 to the vertical with the capacitive coupling strip vertically above the elements. The direction of maximum radiation of the antenna element 10 lies in front of the patch elements perpendicular to the plane in which the patch elements are situated. However, the direction of maximum radiation of a simple edge-fed patch is found to show some instability as the operating frequency is changed. To compensate for this, the beam is accurately centred in the wanted direction by small planar conductive elements 36 positioned at the two lower corners of the element enclosure or conducting fence 20. The conductive elements are orientated in a plane perpendicular to the plane in which the patch elements are orientated and such that they are substantially parallel to the sides of the patch element. In practice this means that they are angled at 45 to the conductive fence around the array section. The conductive elements 36 are dimensioned to provide the most accurate mean beam direction across the operating frequency band.

Figure 5 shows a two element antenna array section.

Figure 5a is a top view of the array section, Figure 5b is an isometric view of the same section and Figure 5c is an exploded isometric view. The elements in the two element section array similarly have an upper element 44a and 44b, and a lower element 45a and 45b separated from one another, and mounted on printed circuit laminate base - 8 - section 43, by plugs 46. The base section 43 is retained within the conducting fence 40 by flange 42 on the inside of the five-sided box housing 41. Base section 43 provides a driving circuit network 48 comprising circuit driving parts 50a and 50b situated under respective patch elements, cross-polar input limbs 51a, 51b, 52a and 52b, and input terminals 53 and 54. In order to reduce cross- polar coupling of input limbs 51 and 52 coupling strips 55a and 55b are provided across sections of input limbs 51 and 52 which loop back on each other close to patch elements 44 and 45. In the first embodiment, the two element array sections are not provided with conductive elements 36 located in the corner of the conductive fence 40, as they were found to be unnecessary in practice.

The amount of phase shift required for each element is determined by the length of the array and the angle over which it is desired to change the beamtilt.

Typical high-power phase shifters comprise either a length of radio frequency transmission line whose length can be physically varied or whose effective electrical length can be varied by the introduction of dielectric materials whose function is to change the relative propagation velocity of radio waves in a section of the transmission line. An alternative design, provides a three-terminal device, the relative phases of two outputs being controlled by varying the effective physical position at which the input is coupled to a fixed length of transmission line. Both forms of phase shifter can be realised either in a co-axial line or in a stripline/microstrip format.

In order to provide a beamtilt angle which is independent of the operating frequency it is necessary that the phase shifts provided by the feed network have values directly proportional to the operating frequency.

The transmission line phase shifters described above have this desirable property. - 9 -

The phase shifts required to provided adjustable beamtilt have the characteristic that there is a small phase shift between adjacent elements, but as these are cumulative the phase shifts between distant elements are much larger. Any feed system must ensure that these relative phase shifts remain correct for all values of beamtilt.

Figure 6 shows a schematic illustration of the phase shifting apparatus like that employed in the first embodiment of the invention. Only three phase shifters 60, 62 and 64, are required to supply the necessary phase shifted signals to each element in the nine element antenna array 6.

As has been describe above, the array 6 is divided into seven groups or sections comprising 2, 1, 1, 1, 1, 1, and 2 elements respectively. Each section is fed as part of a symmetrical pair of sections from the outputs of a three-terminal phase-shifter. Thus the two element sections 12a and 12b at either end of the array 6 are both fed from the respective outputs of phase shifter 60; similarly, single element sections 14a and 14b are fed by the respective outputs of phase shifter 62; and single element sections 16a and 16b are fed from the respective outputs of phase shifter 64.

The other terminal of each of the phase shifters 60, 62 and 64 are connected to power divider 66, which receives the antenna input signal on input line 68.

Since there is an odd number of elements in the array 6, the signal sent to the central single element section 18, does not require phase shifting. Section 18 is therefore connected straight to power divider 66, and receives the un-phase shifted antenna signal received on input line 68. The other section pairs, 16a, 16b, 14a, 14b, 12a and 12b, are fed with phases which differ from that of the central section by approximately +n degrees, where n is the number of wavelengths between the centre of the array section and the centre of the array, and is the phase shift per wavelength necessary to create the desired beamtilt.

When an even number of elements is used, the array centre falls between elements but the same formula applies although no element is to be fed directly with an unphase- shifted signal.

As phase shifters are expensive, the arrangement described above is desirable, as it allows a small number of phase shifters to be used an therefore affords a reduction in production costs. The arrangement of the phase shifters however is such that a good approximation to a linear phase shift across the aperture of the antenna is still provided.

Figures 7a, 7b and 7c to which reference should now be made, show the phase shifter apparatus employed in the first embodiment.

Referring to Figure 7a, it can be seen that the first embodiment comprises three pairs of phase shifters 70a, 70b, 72a, 72b and 74a and 74b. Since the preferred element design is a cross-polar patch design, with two input ports, three phase shifters are required to phase shift the signal being input into the element by one input port, and three more required in respect of the signal being input via the other input port.

The three phase shifter pairs 70, 72 and 74 are mounted on a printed circuit board 76 carrying conductive feed networks 78a and 78b formed from microstrip line.

Each network has a first branching line section 79a, 79b, which carries input terminals 80a, 80b for receiving an input antenna signal, and output terminals 82a and 82b which connect to the input ports of central antenna section 18 via cables (not shown). Referring to Figure 7b, the branching line sections 79a and 79b terminate in the central stator pads 84a, 84b, 86a, 86b, 88a, and 88b, of the phase shifters 70a, 70b, 72a, 72b, 74a, and 74b respectively. Where the network is connected to the stator - 11 pads, vestigial bands 90a, 90b, 91a, 91b, and 92a and 92b are provided on the network. By adjusting the dimensions of these bands and the width of the microstrip line forming that network section, an accurate impedance match is obtained between the phase shifters and the feed networks.

Each network 78a and 78b also has three transmission line arcs 94a, 94b, 96a, 96b, 98a and 98b, separate from the first branching line sections. The transmission line arcs encircle the respective central stator pads 84a, 84b, 86a, 86b, 88a, and 88b, and terminate in output terminals 99. These terminals carry the phase shifted signal from each phase shifter device to a respective antenna array section and element, as will be explained later.

A rotor 100, carrying a section of transmission line, bridges the gap between the central pads and the transmission line arc for each phase shifter to close the path between the input terminals 80a and 80b and the output terminals 99. By altering the orientation of the rotor around the central stator pad, the position of contact on the transmission line arc, and therefore the length of the transmission line is changed producing a phase shift in the signal. The rotor 100 comprises a conductive element 101 which forms the transmission line and a non-conductive rotor guide arm 102 which sits above the conductive element and which has teeth for engaging the teeth on a corresponding plunger rack or ratchet 104.

It is preferred if the material from which the non- conductive rotor guide arm is made has a low dielectric loss factor. The rotor 100 is substantially the same for each of the phase shifters.

The plunger rack 104 is fixed on plunger or actuator rod 105 which extends from underneath the circuit board to the adjustment controls 7 provided on the exterior of the antenna assembly 2. In the first embodiment, the controls are manual and may be operated simply by pushing or pulling the plunger in line with its axis. Doing so causes - 12 the teeth of plunger rack 104 to act against the teeth on each of the six rotor guide arms and cause the rotors 101 to turn around the central pads.

The plunger may be implemented in other ways. In an alternative embodiment for example the plunger have a thread which engages with the teeth such that when the plunger is turned around its longitudinal axis the thread acts on the teeth to turn the rotor 101.

Both the rotor conductive element 101 and the rotor guide arm are fixed in place on their respective central pad by a retaining pin. The rotor conductive element 101 is held in place underneath rotor guide arm 102, by virtue of a lip 103 cut into the conductive element itself. When assembling the rotor 100, the lip is bent upwards to engage with a corresponding lip on the underside of the non-conductive rotor guide arm 102. Thus, when the rotor guide arm is caused to turn by the action of the plunger 105, the conductive element forming the transmission line of the rotor 100 is caused to turn with it.

The feed network, central pad, and transmission line arcs are dimensioned to give a characteristic impedance equal to that of the transmission lines connected to the input and output terminals, and to provide the required Voltage Standing Wave Ratio (VWSR) over the operating frequency band.

The phase shifters of the first embodiment have a contactless design in order to provide very low IMP levels and high reliability in operation. An insulating membrane 108a, 108b, llOa, llOb and 112a, and 112b sits on the circuit board above the transmission line arc and under the rotor conductive element 101, and serves to provide capacitive coupling between the transmission line of the rotor and the transmission line arc, while avoiding any direct connection which could give rise to noisy contact or the generation of intermodulation products. Preferably, the membrane is made out of polyimide as this provides the necessary mechanical characteristics of stability, - 13 toughness and wear resistance, while having low electrical loss, low water absorption and relatively high dielectric constant.

Each of the phase shifter pairs is contained within a durable plastic housing 114, 116 and 118. The housing has a specially contoured interior upper surface comprising ridges that are concentric with transmission line arcs 94a, 99b, 96a, 96b, and 98a and 98b, around centre pads 84a, 84b, 86a, 86b, 88a, and 88b. Each rotor guide arm 102 is provided with a retaining spring assembly 106 in its top surface which engages with a ridge such that the spring assembly is kept in compression. The compression force in the spring acts on the rotor guide arm 102 and therefore the conductive element 101 to ensure that it stays in close contact with the membrane, and the transmission line arc. Thus, a constant capacitive contact is maintained.

As described with reference to Figure 6, each of the phase shifter pairs 70, 72 and 74 serves a pair of array sections; each individual phase shifter in the pair providing one of the cross-polar element signals. Thus, it will be understood with reference to Figure 7, that the two output terminals 99a on the transmission arc of each phase shifter 70a, 72a and 74a, connect to the corresponding input terminals 33, 51a and 52a of pairs of corresponding array sections, and the two output terminals 99b on the transmission arc of each phase shifter 70b, 72b and 74b, connect to the corresponding input terminals 34, sib, and 52b of pairs of the same corresponding array sections. The input and output terminals may be connected by co-axial cable or be connected directly to other microstrip lines or components.

The correct relative phase shifts between each connected pair of elements is achieved by the choice of length of the rotor arm and its associated transmission line arc, together with the radius of the associated gear arc of the rotor guide arm. - 14

The phase shifter providing the largest phase shift to the signal is phase shifter 70a and 70b. The phase shifter is arranged such that the radius of transmission arc 94 is large, and the gearing of the teeth on the gear arc of rotor guide arm 102 is such that a small change in the longitudinal position of the plunger causes a relatively large angular displacement and therefore a relatively large change in the length of the transmission path. Large phase shifts can therefore be introduced into the signal. The outputs of each phase shifter 70a and 70b are connected to an input terminal of both two-element section arrays 12a and 12b.

The phase shifter providing the next largest phase difference is phase shifter 72. The gearing arrangement is the same as that for phase shifter 70, but the radius of the transmission line arc 96 is smaller. The outputs of each phase shifter 72a and 72b are connected to the inputs of both array sections 14a and 14b.

The phase shifter providing the smallest phase shift is 74. The outputs of phase shifter 74 are connected to the inputs of array sections 16a and 16b. Although the radius of the transmission line arc 98 is greater than that for the transmission line arc 96, the gearing of the rotor guide arm is much higher than that of the rotor guide arms for phase shifters 70 and 72, and as a result the same push or pull from plunger 105 produces a much smaller angular displacement than for the other two phase shifters. The outputs of each phase shifter 74a and 74b are connected to the inputs of both array sections 16a and 16b.

In the first embodiment a dual-polar antenna element antenna is employed, and the pairs of phase shifters can be operated by a single actuating mechanism. It will be appreciated however that, the embodiment described could easily be adapted to other configurations of antenna and is not limited to dual-polar antennae. -

A second preferred embodiment, using a different arrangement of phase shifters, will next be described.

The second embodiment of the invention preferably comprises 10 antenna elements 120, like those shown in Figure 4a, 4b and 4c, but unlike the first embodiment has only four individual phase shifters. As before, the antenna is used in a dual polarization transmission system with two phase shifters 122, 124 being used to provide the desired phase shifts for each polarization. The system for a single signal polarization is schematically illustrated in Figure 8.

Each phase shifter output is connected to a pair of antenna elements 120, leaving one central pair of antenna elements to receive an unphaseshifted signal from a signal divider 126 connected to signal input. The input signal received at input 128 is divided by signal divider 126 and is passed to the inputs of both of the phase shifters 122 and 124.

As only two phase shifters are required per polarization, the phase shifting apparatus can be made more compact than the larger network of phase shifters employed in the first embodiment. This means that the transmission lines connecting the phase shifters with the signal input or with the antenna elements can be made shorter, leading to smaller transmission line losses.

Conversely, fewer phase shifters means that the phase of the signal across the antennae aperture cannot be made as linear as it can in the first embodiment. However, the improvement in gain exhibited by a three phase shifter system over a two phase shifter arrangement has been found to be smaller than the corresponding improvement in transmission line losses when two phase shifters are used rather than three. This means that in most applications the performance of the second preferred embodiment will provide the most advantages.

Although, ten individual separate antenna elements are illustrated in Figure 8, it will be understood that five of the double antenna elements shown in Figure 4b, or any other appropriate combination of single ordouble antenna elements could also be used with the arrangement of four phase shifters shown.

Figure 9 shows the implementation of phase shifters used in the second embodiment of the invention.

The phase shifters are of substantially the same construction as those used in the first embodiment; however the location of the phase shifter corresponding to a particular phase shift differs. The printed circuit board on which the phase shifters are formed comprises two sections 130 and 132. Phase shifter pairs 134 and 136 are mounted on each printed circuit board 130 and 132 respectively. Each pair provides the required phase shifts for a respective one of the two signal polarizations; each pair can thus be seen to comprise the two phase shifters 122 and 124 shown schematically in Figure 8. It will be understood that the two phase shifters 122 and 124, receive the same input signal and provide phase shifts of different amounts to achieve the required change in beam direction across the aperture of the antenna. Each of the two outputs from each phase shifter branches in two to provide two output connections 138 to respective antenna elements 120.

The arrangement differs from that of the first embodiment in that the phase shifters for a single signal polarization are grouped together, rather than phase shifters that provide the same amount of phase shift being grouped together. This results in improved isolation between the two signal polarizations.

It is not necessary to mount the two phase shifter pairs 134 and 136 on separate printed circuit boards, as a single board could clearly be used. Using separate boards does however mean that the pairs of phase shifters can be easily located far apart from one another within the antenna housing, thereby improving the isolation further. - 17

The amount of phase shift required is determined by the maximum amount of beamLilt which is needed from the array. This is typically at least equal to the -3db elevation beamwidth of the array, that is the main beam is tilted down until the first null lies approximately in the horizontal plane. This consideration determines the dimensioning of the phase shifters.

Both of the first and second embodiments have been described as comprising multi-layer patch elements. In WIDEBAND antenna applications that cover the standard frequency band of 1710MHz to 1880 MHz and the new 3G band l900MHz to 2170MHz, the patch elements are not appropriate. Instead, the first and second embodiments are used in a modified form employing a pair of printed circuit dipole elements 140 and 142 contained in the same conducting guard fence 144 as before. This is illustrated in Figure 10. The dipoles are fed using coaxial cable and a crossed Pawsey-stub balun 145 is provided. The cross- connections at the feed point are printed on a double sided printed circuit board 146 to ensure a high level of consistency in the associated stray capacitances and inductances at this point. To physically shorten the baluns (which would otherwise be one-quarter-wavelength long, a dielectric rod 148 is slid over the four balun conductors, increasing the velocity ratio at this point, and consequently shortening the physical length needed. - 18

The control system that has been described is a manual system. However, the antenna is also intended to be operable remotely via lap-top computer, connected via a modem, or connected directly to a connection at the base of the antenna station for example. Alternatively, the antenna could be permanently connected via a dedicated cable. A co-axial cable could be used to carry both the transmission signal and a control signal to the antenna.

Such connections allows the antenna to be integrated with a Monitoring and Control system. The beam tilt of the antenna could then be adjusted by sending control signals to a servo-motor connected to the actuator rod 105. - 19

Claims (4)

1. A base station antenna comprising an array of antenna elements for transmitting and/or receiving radio signals, a mounting plate on which the array of antenna elements is mounted and a feed cable for carrying signals between the array of antenna elements and signal processing circuitry, wherein the mounting plate also comprises a conductive fence separately encircling one or more of the antenna elements such that the antenna elements are substantially shielded against inductance currents induced by the other antenna elements in the array.

2. A base station antenna according to claim 1 comprising one or more insulated conductive elements mounted on the mounting plate normal to the direction of the radio beam transmitted and/or received from the antenna elements such that the cross-section and directionability of the antenna elements is increased.

3. A base station antenna according to claim 2 wherein the antenna elements are patch elements and the conductive fence encircling them is substantially square, the patch elements being orientated at substantially 45 to the conductive fence, and the insulated conductive elements being located parallel to the sides of the patch element in the corner of the conductive fence.

4. A base station antenna according to any of claims 1 t 3 wherein the array of antenna elements comprises at least five antenna elements, the two antenna elements at either end of the array being disposed together inside the same respective conductive fence, the intermediate antenna elements each being disposed inside its own respective conductive fence.