GB2424765A

GB2424765A - Dipole antenna with an impedance matching arrangement

Info

Publication number: GB2424765A
Application number: GB0506389A
Authority: GB
Inventors: Jimmy Ho; Andrew Phillip Hyde
Original assignee: CSA Ltd
Current assignee: CSA Ltd
Priority date: 2005-03-29
Filing date: 2005-03-29
Publication date: 2006-10-04
Anticipated expiration: 2025-03-29
Also published as: GB2424765B; GB0506389D0

Abstract

A dipole antenna 10 comprises two dipole halves 12, 14 which extend away from each other and are parallel to an axis 13, where the dipole halves 12, 14 are spaced apart in a direction perpendicular to the said axis 13 such that the input impedance of the dipole 10 matches that of a feed line 34 connected to the dipole. The dipole antenna 10 maybe used in a crossed dipole formation where the dipole halves 12, 14 each comprise an L-shaped planar member projecting from a conductive base plane. Each of the dipole halves 12, 14 may have a hole in it and a stub projecting from its distal end. A feed line 34 may be directly electrically connected to and integrated with a dipole halve 12. The dipole halves 12, 14 may have a space between them in the range of 0.0015 g to 0.02 g with a length of g /4 where g is the mean operating wavelength of the dipole antenna. The dipole arrangement may be used in an antenna array which includes isolation means between crossed dipole elements. The antenna array may have antenna elements which operate at different frequencies and these antenna elements may have different spacing between other elements according to their intended operational frequency. The antenna may be used in mobile telecommunication systems to provide a good standing wave ratio performance over a wide bandwidth with good signal isolation.

Description

A DIPOLE ANTENNA

Background of the Invention

This invention relates to a dipole antenna, a crossed dipole antenna and a dipole antenna array for operating at mobile telecommunications frequencies.

A mobile telecommunications signal can travel to its destination along multiple paths because the signal is reflected from buildings and other obstructions. This is called multi-path propagation. The reflected and direct components of the signal can interfere with each other, which leads to signal fading.

Signal fading can be reduced by using one of the known diversity techniques. These techniques require at least two signal paths that carry the same information.

One diversity technique is spatial diversity where two antennas are used which are spaced widely apart, typically by around 20 times the operating wavelength (at frequencies in the region of 500 to 1000MHz this is about 3m to 6m). Spatial diversity systems require large base station towers to support these spaced-apart antennas and communities are concerned with the visual impact of these towers.

Another diversity technique is polarisation diversity. A typical polarisation diversity system uses two antennas in the same casing, each antenna having a different polarisation sense or direction. The polarisations are orthogonal and typically one antenna has +450 linear polarisation (where 0 corresponds to vertical) and the other antenna has -45 linear polarisation. Polarisation diversity antennas produce uncorrelated signal paths and can thus reduce fading. These antennas can be incorporated into a single unit in a cruciform or crossed dipole configuration. This type of antenna is called a dual-polarisation antenna.

This type of arrangement is also effective in compensating for polarisation mismatch, which is caused by the random orientations of mobile phone handsets.

Polarisation diversity antennas have the advantage over space diversity antennas of requiring less space.

Mobile communication operators require triple band antennas, with adjustable electrical tilt (AET) capability on all bands, which allow a mobile communication operator to use the antenna in a single band, dual band or triple band mode. Mobile communication operators require good voltage standing wave ratio (VSWR) (the VSWR of an individual radiating element is usually required to be less than 1.3:1), radiation pattern performance, and gain. These requirements may also need to be achieved over a range of electrical tilts (typically 0 to 100 in the mobile communication sector). Furthermore, antennas should display good performance over a wide band as well as good port-to-port isolation (a typical requirement for any mobile telecommunications dipole antenna array is a port-to-port isolation better than 30dB). Good performance should also be repeatable, that is, it should be easy to mass-produce antennas with consistent good performance. Other requirements can sometimes be imposed on antenna design. For example, environmental planning pressures can force mobile communication operators to reuse existing antenna sites, and antennas should be low weight to make installation easier.

A known dipole antenna configuration called the "airline" antenna configuration is shown schematically in Figure 1. This "airline" antenna I has coplanar radiating elements 2 and 4 forming a substantially T shape and with one radiating element constituting one arm of the head of the T and one half of the tail of the T, thus forming an inverted L shape. A slot 6 with width W extends through the head 7 of the T and part way through the tail 9 of the T. At the open end of the slot the sides of the slot in the head of the T are cut away or flared as shown. A microstrip hook or inverted U shape feed line 8 is located with its base across the slot 6 in a plane parallel to but slightly spaced from the T shape by a distance S. The distance from the bottom of the slot to the bottom of the feed line, where the feed line 8 crosses the slot, is H. The feed line 8 comprises one section extending along one side of the slot 6, crossing over the slot 6 at the head 7 of the T below the cut away portion and extending along the tail 9 of the T on the other side of the T by a distance L, part way down the tail 9, and by a distance less than H and less far than the other side, which extends a distance greater than H. The feed line 8 makes an electromagnetic (EM) coupling to the radiating elements 2 and 4. The radiating elements 2 and 4 are usually formed from one flat piece of metal or etched into a printed circuit board. The magnitude of the currents in the radiating elements 2 and 4 are determined by the length L of the feed line 8. The length L is typically a quarter of the mean or centre operating wavelength.

Usually, two of these dipole antennas are used in a crossed dipole configuration to form a dual-polarisation antenna such as in US Patent No. 6,034,649, which discloses radiating dipole elements made from massproduced, low cost parts. Each dipole includes two half dipoles which have a generally inverted L-shape profile. A microstrip hook is attached to and spaced from each of the dipoles by clips. The microstrip hook electromagnetically couples each dipole to the feed network.

To efficiently connect an antenna to a feed network, the impedance of the antenna should match the impedance of the connecting line, which is usually a coaxial cable, that is an unbalanced line.

This type of antenna uses a balun to attempt to transform an unbalanced line to a balanced line, to make a matched system. A balun provides both a conversion from an unbalanced signal to a balanced signal together with an impedance transformation. Referring to Figure 1, the known approach makes use of the coupling between the EM coupled feed line 8 and the radiating elements 2 and 4 to form a balun. However, this creates a reactance and, for a balun to operate efficiently, the input impedance to the balun should ideally be purely resistfve. This results in the bandwidth of the antenna I being limited.

Furthermore, it is difficult to achieve good repeatable VSWR performance with good port-to-port isolation with this type of configuration for many well-known reasons. Referring to Figure 1, the electro-magnetic coupling between the feed line 8 and the radiating elements 2 and 4 is very sensitive to the spacing S between the feed line 8 and the radiating elements 2. In particular, if the spacing S is large the coupling is reduced and impedance matching is difficult. The width W of the slot 6 and the spacing S are also very critical as the slightest change in these dimensions has a large impact on VSWR and the distance H between the edge of the slot 6 to the point where the parasitic feed 8 crosses the slot 6 is also very critical for good performance. All these parameters are extremely sensitive and tight tolerances are required. Hence, an airline configuration is very difficult to manufacture economically and repeatably in volume.

Individual antennas are often assembled in arrays comprising a plurality of crossed dipole antennas mounted in the same housing. This improves gain, however, good isolation is required between antennas.

US Patent No. 5,952,983 and 6,072,439 both disclose antenna arrangements to improve isolation between antennas in an array.

US Patent No. 5,952,983 discloses a plurality of metallic parasitic elements placed between some of the crossed dipole antennas in an antenna array to improve isolation. The parasitic elements of this arrangement are very heavy.

US Patent No. 6,072,439 discloses a bow-tie shape parasitic element located between some of the crossed dipole antennas in an antenna array to improve isolation of the antenna array. This configuration adds to the weight of the antenna array and time consuming (and costly) fine-tuning is required to optimise the isolation.

Summary of the Invention

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

A preferred embodiment of the invention is described in more detail below and takes the form of a dipole antenna for operating at mobile telecommunications frequencies comprising a dipole having two half dipoles which extend away from each other substantially parallel to an axis. The two half dipoles are spaced apart in a direction substantially perpendicular to the axis, such that the impedance of the dipole substantially matches that of a feed line electrically connected to the dipole. In this way, good voltage standing wave ratio performance is achieved over a wide bandwidth and with good isolation.

Brief Description of the Drawings

The invention will be described in more detail by way of example with reference to the accompanying drawings, in which: Figure 1 (described above) is a perspective view of a known antenna; Figure 2 is a perspective view from the side of a dipole antenna embodying the present invention; Figure 3 is a perspective view from above of the dipole antenna of Figure 2; Figure 4 is a perspective view from the side of another dipole antenna embodying the present invention; Figure 5 is a detailed view of a portion of the dipole antenna of Figure 4; Figure 6 is a perspective view from above of the dipole antenna of Figures 4 and 5; Figure 7 is a perspective view from the side of a crossed dipole antenna embodying an aspect of the present invention; Figure 8 is a perspective view from above of the crossed dipole antenna of Figure 7; Figure 9 is a graph of VSWR against frequency of the crossed dipole antenna of Figures 7 and 8; Figure 10 is a graph of isolation against frequency of the crossed dipole antenna of Figures 7 and 8; Figure 11 is a perspective view from one side of another crossed dipole antenna embodying an aspect of the present invention; Figure 12 is a perspective view of the crossed dipole antenna of Figure 11 viewed from approximately perpendicular to the view of Figure 11; Figure 13 is a view from above of the crossed dipole antenna of Figures 11 and 12; Figure 14 is a plan view from above of the crossed dipole antenna of Figures 11 to 13; Figure 15 is a plan view from the side of the crossed dipole antenna of Figures 11 to 14; Figure 16 is a plan view of the crossed dipole antenna of Figures 11 to 15 viewed perpendicular to the view of Figure 15; and Figure 17 is a perspective view from above of an array of dipole antennas embodying an aspect of the present invention.

Detailed Description of the Preferred Embodiment

A preferred embodiment of a mobile telecommunications dipole antenna 10 will now be described with reference to Figures 2 and 3.

The antenna 10 comprises a first half dipole member 12 and a second half dipole member 14. The two half dipole members 12 and 14 are both substantially L-shape thin planar sheets, the L being inverted in the orientation shown. In use, it will not normally be in that orientation but it is convenient to describe it so. The two L-shapes project upwardly from a base 16 with their legs parallel to each other and to an axis 18, and are arranged to form together a substantially T-shape dipole antenna 10, with the leg 20 of the T projecting upwardly from the base 16 and the head 22 of the T spaced from the base 16. The head is constituted by the arms of the two L-shapes which extend away from each other parallel to an axis 13 which is spaced from and substantially parallel to the planar base 16.

The half dipole members 12 and 14 are spaced apart slightly in a direction perpendicular to the axis 13 and parallel to the base 16. A feed line 34 is electrically connected directly to one 12 of the half dipole members.

The L-shape of first half dipole member 12 comprises a leg portion 24 that has substantially straight parallel sides and a flared arm portion 26, wider than the leg portion 24, which has a lower edge 28 that projects perpendicular to the leg portion 24 and an upper edge 30 that slopes at an angle (in this example, by 45 to the leg portion) before projecting parallel to the other edge 28. The free end 32 of the arm portion 26 has smoothed or rounded corners.

A feed line 34 is electrically connected to the first half dipole member 12. The feed line 34 is also an L-shape thin planar sheet. It lies in the same plane as the first half dipole member 12 and is integral with it.

The feed line 34 has a leg portion 36 that extends substantially parallel to the leg portion 24 of the first half dipole member 12 and an arm portion 38, of similar width to the leg portion 36, which has one edge 40 that projects perpendicular to the leg portion 30 and another edge 42, which slopes at an angle (in this example, by 45 to the leg portion), before projecting parallel to the other edge 40. The end of the arm portion 38 is electrically connected to the flared arm portion 26 of the first half dipole member 12 where one edge slopes at an angle from the leg portion 24, that is about at the middle of the height of the main portion of the first half dipole member. The leg portion 34 forms a balun for the antenna 10.

The second half dipole member 14 is the same shape as the first half dipole member 12, but it does not have a feed line.

Thus, the leg portions 20 of the first and the second half dipole members 12 and 14 project upwardly, in the same direction, from the base 16. The flared arm portion 26 of the first half dipole member 12 extends outwardly in the opposite direction to the flared arm portion 26 of the second half dipole member 14, to form the substantially 1-shape dipole antenna 10 when seen from the direction orthogonal to both the axes 13 and 18.

As seen from the foregoing, the first half dipole member 12 lies in a plane spaced from and parallel to the plane of the second half dipole member 14. The plane in which the first half dipole member 12 lies is typically spaced from the second half dipole member 14 by between 0.0015 and 0.02 times the mean or centre operating wavelength of the antenna 10 for a 50ohm input impedance, which at a typical operating wavelength of about 0.3m, is between 0.5mm and 6.7mm. In this example, and most preferably, the plane in which the first half dipole member 12 lies is typically spaced from the second half dipole member by 0.006 times the mean or centre operating wavelength for a 5Oohm input impedance, which in this case is 2mm. However, the spacing between the first half dipole 12 and second half dipole 14 can have alternative spacings. For example, the spacing can be between 0.004 and 0.01 times the mean or centre operating wavelength (typically 1.3mm to 3.4mm) or between 0.005 and 0.007 times the mean or centre operating wavelength (typically 1.7mm to 2.3mm).

The leg portions 20 of the first and the second half dipole members 12 and 14 are both supported by and electrically connected to the base 16, which is itself electrically conductive, by sleeves 42 which project outwardly from the base 16. The free end of each leg portion 20 is housed in one of the projecting sleeves 42.

The feed line 34 feeding the first dipole member 12 is electrically connected to a coaxial cable 44. The inner conductor 46 of the coaxial cable 44 is electrically connected to the feed line 34 by solder. In the example of Figure 2, the inner conductor 46 is shown electrically connected to the free end 48 of the feed line 34. However, it could be electrically connected at any location along the feed line 34. The outer conductor 50 of the coaxial cable 44 is electrically connected to a projecting housing 52 in the base 16 by solder. Suitable means other than solder could be used for electrically connecting the inner conductor 46 and outer conductor 50 of the coaxial cable 44 to the antenna 10, for example, the inner conductor 46 and/or outer conductor 50 could be connected to the antenna 10 by clips.

The base 16 comprises a through hole 54 complementary to the coaxial cable 44, through which the coaxial cable 44 passes. This through hole 54 has a plurality of projections 56 projecting inwardly and upwardly from its circumference and forming the projecting housing 52, which support the coaxial cable 44 and provide a good electrical connection between the outer conductor 50 of the coaxial cable 44 and the base 16. Further through holes 57 are provided through which clips are passed to mechanically connect the base 16 to a suitable mount (not shown).

The half dipole members 12 and 14 are held apart by an electrically insulating member. The electrically insulating member shown in Figures 2 and 3 is an electrically insulating disc 60 that has through holes 62 through it. It is located between the base 16 and the arm portions 26. The faces 64 of the disc 60 face along the axis 18 of the leg portions 24 of the dipole members 12 and 14. The through holes 62 each have a rectangular cross section which is complementary to the cross section of the leg portions 24 of the first and second half dipole members 12 and 14 and the feed line 34 that pass through them. The through holes through which the first and second dipole members 12 and 14 pass are spaced apart by the required distance between the first and second dipole members 12 and 14 (in this example 0.006 times the mean or centre operating wavelength, which in this case is 2mm).

In other words, the half dipole arms or members 12 and 14 are displaced laterally and transversely forming a two-conductor open feed transmission line and one of the half dipole arms or members 12 and 14 is provided with a planar feed line 34. Figure 3 shows the gap 66 between the first half dipole member 12 and the second half dipole member 14 which lie in parallel planes.

In this example, the electrically conducting elements of the antenna 10 are made from aluminium because it is low weight. However, other materials such as brass could be used. This configuration can be easily pressed from a metal sheet. Alternatively, the antenna could be made using printed circuit board, with the two elements being on opposed sides of the board.

These configurations provide high mechanical stability.

In use, the inner conductor 46 and outer conductor 50 of the coaxial cable 44 are electrically connected to the feed network of an antenna array (not shown). The antenna 10, when transmitting, can receive signals from the feed network, which are fed along the coaxial cable 44 and through the feed line 34. The feed line 34 then uses the first half dipole member 12 as the ground plane to guide RE signals to excite the first and second half dipole members 12 and 14 in order to transmit signals from the feed network into the surrounding space. In receive mode, the antenna 10 can receive signals from the surrounding space, in which case, the opposite process occurs.

The dimensions of the antenna 10 of Figures 2 and 3 are as follows. Sides a and b of the half dipole members 12 and 14 project outwardly from their legs 20 by 6.0cm. The height c of the arm portions 26 is 2.5cm (1 inch). The width d of each leg 20 is 1.0cm. The length e of each leg 20 from the base 16 to the bottom of the arm 26 is 6.0cm. The width f of the feed line 36 is 0.4cm and its length g is 7.3cm. The gap h between the leg 20 of the first half dipole member 12 and the feed line 36 is 1cm. The thickness i of the sheet making up the antenna 10 is 2mm throughout. The lengths a and e of the inner, shorter sides of the L- shape are equal and correspond to one quarter of the mean operating wavelength of the antenna 10. The length of the sides a and b are also the same. That is, the arm portions 26 and the legs 20 have a projecting length of one quarter of the mean or centre operating wavelength.

An alternative embodiment is shown in Figures 4 to 6. The dipole antenna 110 of Figures 4 to 6 is similar to that of Figures 2 and 3 and like components have been given the same reference numerals.

Each of the half dipole members 12 and 14 has a through hole 112 through the planar surface of its arm portion 26. Each of the through holes 112 has a rectangular cross section that extends towards the periphery of the arm portion 26 it goes through.

Each of the arm portions 26 has a securing through hole 114 at the junction of the flared arm portion 26 and the leg portion 20. The securing through hole 114 in the first half dipole member 12 has a circular cross-section and the securing through hole 114 in the second half dipole member 14 has a square cross-section.

The arm portion 26 of each of the half dipole members 12,14 has a stub 116 projecting outwardly from the middle of the free end 118 of the arm portion 26 in the plane of the arm portion 26. These optional stubs 116 are used to improve the match of the antenna 110.

In this embodiment, the feed line 34 is a separate component to the half dipole members 12,14. The feed line 34 comprises a substantially L-shape bracket 124 (shown best in Figure 5) having a planar leg portion 122 and a planar arm portion 120 with a spacing portion 126 in between them. The leg portion 122 of the L-shape bracket 124 extends from its free end 128 and widens to a tag 130, which projects outwardly in the plane of the leg portion 122 and which has a through hole 132 through it. Beyond the tag 130, the outer side 134 of the leg portion 122 slopes towards the spacing portion 126 (in this example, by 450 to the leg portion) and the inner side 136 of the leg portion 122 projects away from the tag 130 to the spacing portion 126. The spacing portion 126 projects at an angle outwardly from the plane of the leg portion 122 of the feed line 34 (in this example, by 45 to the planar leg portion) to the planar arm portion 120. The planar arm portion 120 has a tag 138 at its free end, which has a through hole 140 through it. The planar leg portion 122 lies on the surface of the leg portion 20 of the second dipole member 14 and is mechanically connected to it by a first clip 142 (first clip 142 shown in Figure 6) that passes through the through hole 132 in the tag 130. The planar arm portion 120 lies on the surface of the leg portion 20 of the first dipole member 12 and is electrically connected to it by the tag 138 and mechanically connected to it by a second clip 144 (second clip 144 shown in Figure 6) that passes through the through hole 140 in the tag 138.

The first clip 142 comprises a head portion 146 having two resilient prongs 148, projecting outwardly from it, which have barbed ends 150.

The resilient prongs 148 have a circumference smaller than the circumference of the through hole 132. The circumference of the barbed ends 150 is larger than the circumference of the through hole 132.

The second clip 144 comprises two portions: an inner portion 154 and an outer portion 152. The inner portion 154 comprises a head portion 156 having two resilient prongs 158 projecting outwardly from it.

The resilient prongs 158 have flat outer edges 160 and curved inner edges (not shown). The outer portion 152 comprises a head portion 162 having a narrower shaft 164 projecting outwardly from it, which has a barbed free end 166. The end of the shaft 164 nearest the head portion 162 is wider than the rest of the shaft 164. The shaft 164 has a circular cross-section.

In use, the first clip 142 is located through the securing through hole 114 in the first half dipole member 12 and the through hole 140 in the planar arm portion 124 of the L-shape bracket 120 in order to secure the L-shape bracket 120 and the first dipole member 12 together between the head portion 146 and the barbed ends 150 of the resilient prongs 148 of the first clip 142. An electrical connection is also made between the Lshape bracket 120 and the first half dipole member 12. The inner portion 154 of the second clip 144 is located through the securing through hole 114 in the second dipole member 14. The head portion 156 of the inner portion 154 is located between the second half dipole 14 and the L- shape bracket 120. The thickness of the head portion 156 forms a spacer that is the required distance between the first and second dipole members 12 and 14 (in this example 0.006 times the mean or centre operating wavelength, which is 2mm). The outer portion 152 of the second clip 144 extends through the inner portion 154 of the second clip 144 and the head 162 of the outer portion 152 projects beyond the surface of the second half dipole member 14 and the barbed free end 166 projects beyond the end of the inner portion 154 of the second clip 144 and the second half dipole member 14 is mechanically connected to the L-shape bracket 120, leaving a 2mm gap between them.

A coaxial cable 44 extends through the base 16, and through a supporting tube 168, which projects from the base 16 along the leg portion 20 of the second half dipole member 14. The supporting tube 168 can either be an electrical conductor or an electrical insulator. In the example shown, the supporting tube 168 is an electrical conductor of moulded metal. The inner conductor 46 of the coaxial cable 44 is electrically connected, by solder, to the free end of the planar leg portion 122 of the L-shape bracket 120. The outer conductor 50 of the coaxial cable 44 is electrically connected to a lug 170, which is electrically connected to the second half dipole member 14. The lug 170 projects outwardly from the surface of the second half dipole member 14. The lug 170 has a curved outer surface 172 forming a recess that is complementary to the outer conductor 50 of the coaxial cable 44. The coaxial cable 44 lies in the lug's 170 recess and is supported by it.

The base 16 is disc shape and has a central square cross-section through hole 174 for locating the base 16 to an antenna support (not shown). The base 16 has through holes 176 distributed around its circumference (three through holes in this example), through which screws can be passed for securing the base to the antenna support.

The base 16 has a recess 178 for supporting a dipole, when required, as discussed below. A similar recess supports the supporting tube 168 of the dipole arrangement described above.

In other respects the device of Figures 2 and 3 is the same as that ofFigures4to6.

The embodiments of Figures 2 to 6 show single polarisation embodiments in which the two half dipole members 12,14 are on different planes. This allows the feed line 34 to connect directly to one half dipole member 12. Because the displacement gap 66 is much less than the wavelength at base station frequencies, the radiation pattern of the antenna 10 is not affected, let alone degraded, by the gap 66. The radiation pattern is highly symmetrical with good cross polarisation characteristics. Furthermore, because the half dipole members 12,14 are fed directly, the problems of known balun structures described above are eliminated.

Figures 7 and 8 show another embodiment of a dipole antenna 210 according to the present invention. The antenna of Figures 7 and 8 is similar to that of Figures 2 and 3 save that it comprises two antennas of the type shown in Figures 2 and 3, and like components have been given the same reference numerals.

The antenna 210 of Figures 7 and 8 is a dual polarisation embodiment in which the radiating elements include a first dipole 212 and a second dipole 214 each comprising a first and a second half dipole member 12,14 as described above. The two dipoles 212 and 214 are configured mutually at right angles in a cruciform shape.

In most respects each of the dipoles 212 and 214 of Figures 7 and 8 is the same as that of Figures 2 and 3. However, there are some modifications.

As shown in Figures 7 and 8, the arm portion 38 of the feed line 20 of the second dipole 214 projects outwardly further than the arm portion 38 of the first dipole 212. The arm portion 38 of the second dipole 214 that projects furthest extends over the arm portion 38 of the first dipole 212. A feed line 34 of the type described above is electrically connected directly to one of the half dipole members 12,14 of each dipole 212,214.

The antenna comprises two coaxial cables 44, one for feeding each dipole 212,214. The inner conductor 46 of one coaxial cable 44 is electricallyconnected to the free end of one of the feed lines 34 and the inner conductor 46 of the other coaxial cable 44 is electrically connected to the other feed line 34.

Each coaxial cable 44 passes through a complementary through hole 54 in the base 16. Each through hole 54 has a plurality of projections 56 projecting from its circumference, which support the coaxial cable 44 that passes through it, and provides a good electrical connection between the outer conductor 50 of the coaxial cable 44 and the base 16.

The feed line 34 electrically connected to the first half dipole member 12 of the first dipole 212 uses the first dipole 212 as the ground plane to guide the RF signals to excite dipole members 12 and 14 of the first dipole 212. The other feed line 34 uses the first half dipole member 12 of the second dipole 214 as the ground plane to guide the RF signals to excite half dipole members 12 and 14 of the second dipole 214.

This configuration is also easily pressed from metal sheets and provides high mechanical stability. Other details of the construction will be apparent by reference to Figures 2 and 3.

Figure 9 shows a graph of VSWR for the dual polarised crossed antenna 210 described above, and illustrated in Figures 7 and 8, for a frequency range of 806MHz to 960Mhz (which corresponds roughly to GSM900) and Figure 10 shows a graph of isolation for the same antenna 210 at the same frequency range. Figures 9 and 10 indicate that the antenna 210 produces two orthogonal polarised signals with wideband VSWR performance and good isolation and it easily achieves 30dB of isolation with good VSWR across a very wide bandwidth.

Figures 11 to 16 show another embodiment of a dipole antenna 310 according to the present invention. The antenna 310 of Figures 11 to 16 is similar to that of Figures 4 to 6 save that it comprises two antennas of the type shown in Figures 4 to 6, and like components have been given the same reference numerals.

The antenna 310 of Figures 11 to 16 is a dual polarisation embodiment in which the radiating elements include a pair of dipoles 312,314, each comprising a first half dipole member 12 and a second half dipole member 14 as described above in relation to Figures 4 to 6, which are configured mutually at right angles in a cruciform shape. As shown in Figures 11 and 12, the feed lines 34 are separate components to the half dipole members 12 and 14. Each feed line 34 comprises a substantially L- shape bracket 120 as described above in relation to the embodiment of Figures 4 to 6.

The planar arm portion 120 of the L-shape bracket 124 of the first dipole 312 projects outwardly further than the planar arm portion 120 of the Lshape bracket 124 of the second dipole 314. The planar arm portion 120 that projects furthest extends over the other planar arm portion 124. A feed line 34 of the type described above is electrically connected directly to one of the half dipole members 12,14 of each dipole 312,314. The antenna 310 comprises two coaxial cables 44, one for feeding each dipole 312, 314. Each coaxial cable 44 is electrically connected to the dipole 312, 314 it feeds in the same way as in the single dipole antenna 110 embodiment described above. That is, the inner conductor 34 of each coaxial cable 44 is electrically connected, by solder, to the free end of the planar leg portion 122 of one of the L-shape brackets 120. The outer conductor 50 of each coaxial cable 44 is electrically connected to a lug 170, which is electrically connected to one of the second half dipole members 14.

The arm portion 26 of each of the half dipole members 12,14 of the first dipole 312 has a stub 116 projecting outwardly from the middle of the free end 118 of each of the arm portions 26 in the plane of the arm portion 26. The half dipole members of the first dipole do not have stubs.

The second dipole member's stubs are optional; they are used to improve the match of the antenna 310.

In other respects the antenna of Figures 11 to 16 is the same as that of Figures 4 to 6.

For some applications it is desirable that a single physical antenna unit is able to support simultaneous operation on two or more different frequency bands, for example GSM900 (870 to 960MHz), GSM1800 (1710 to 1880MHz) and UMTS (1900 to 2170MHz). This functionality can be provided by the dipole antenna array 400 shown in Figure 17.

The dipole antenna array 400 comprises a plurality of columns 402,404,406 of antennas 210 of the dual polarisation type described above and illustrated in Figures 7 and 8. However, any of the antennas 10, 110, 310 described above could be used. The columns 402,404,406 of antennas 210 are spaced apart transversely along the base 408. The antennas 210 are mounted to a planar, rectangular cross section base 408 and project outwardly from it. Each column 402,404,406 comprises a plurality of identical antennas 210, spaced apart longitudinally along the base 408. Antennas 210 in the same column 402,404,406 operate at the same frequency. The distance along the column between antennas 210 in the same column is inversely proportional to their operating frequency.

Antennas 210 in different columns 402,404,406 operate at different frequencies. In the example shown in Figure 17, there are three columns 402,404,406 of antennas 210. The outer columns 402,406 of antennas 210 are dimensioned for operation on a higher frequency band than the antennas 210 along the middle column. The column 402 of antennas 210 along one edge operates in the GSM1800 band, the middle column 404 of antennas 210 operates in the GSM900 band and the column 406 of antennas 210 along the other edge operates in the UMTS band.

Alternatively, patch radiators or other suitable forms of radiating element could be used for some of the columns 402,404,406 of antennas 210. In this example, the antennas 210 operate in pairs in which one antenna 210 acts to receive signals and the other acts to transmit signals. However, the same antenna 210 could be used to both transmit and receive signals.

The antennas 210 are shown oriented for 45 linear polarisation.

Each of the higher frequency antennas 210 along an outer column 402,406 is mounted in the centre of a rectangular electrically conductive open-top box 410. The sides of each of the boxes 210 extend less far outwardly from the base 408 than the antenna 210 it extends around.

The function of each box 210 is to control the azimuth beamwidth and the back-to-front ratio of the antenna 210 it surrounds. The boxes 210 are optional.

The columns 402,406 of higher frequency antennas 210 may form a single array, that is two antennas 210 wide by N antennas 210 long (where N is an integer), or the columns 402,406 may comprise two independent arrays one antenna 210 wide by N antennas 210 long.

When the antennas form a single array, the antennas are fed from the same source and thus, the greater the number of elements width-wise, the narrower the azimuth beam width and the greater the gain of the array. When there are two independent arrays, an operator has the choice of using one or both of the arrays at the same time. For example, the operator could choose to operate both arrays in the PCN band or one in the PCN band and one in the UMTS band. One array could be used to transmit and one can be used to receive or both could be used to transmit or to receive.

Antennas have been described which offer wideband VSWR performance. Embodiments of the antenna can achieve a VSWR of 1.3:1 over a 30% bandwidth. This is larger than the largest bandwidth required for the mobile base station antenna industry. Indeed in the examples, the system requirements are achieved with relative ease over a much larger frequency operating band than required. Furthermore, the example antennas provide high isolation, low intermodulation distortion, and high repeatability. They also have good radiation pattern characteristics, including radiation pattern purity, across a wide operating frequency band.

The example antennas are easy to construct because they are tolerant to manufacturing errors as they do not have the dimensional sensitivity of the airline dipole design illustrated in Figure 1. The lack of dimensional sensitivity is because the slot is eliminated and instead of the feed line being coupled to the dipole electromagnetically, the dipole is fed directly.

Hence, the antenna is ideal for mass production in the antenna base station industry.

The dual polarisation antenna is suitable for applications in base station antennas or as an element in single band or triple band configurations.

Embodiments of the present invention have been described with particular reference to the examples illustrated. However, it will be appreciated that variations and modifications may be made to the examples described within the scope of the present invention. For example, although the invention has been described as operating at frequencies in the range of GSM900 (870 to 960MHz), GSMI800 (1710 to 1880MHz) and UMTS (1900 to 2170MHz), these are the most preferable frequency ranges. With appropriate scaling, the present invention can operate at other frequencies ranges in the high frequency spectrum and microwave spectrum (3MHz to 300GHz) and preferably 870MHz to 2170MHz, which correspond to wavelengths of 95m to 9.5 x 10m and 0.33m to O.13m respectively.

Claims

1. A dipole antenna for operating at mobile telecommunications frequencies comprising: a dipole having two half dipoles which extend away from each other substantially parallel to an axis; wherein the two half dipoles are spaced apart in a direction substantially perpendicular to the axis, such that the impedance of the dipole substantially matches that of a feed line electrically connected to the dipole.

2. A dipole antenna according to claim 1, wherein the half dipoles are planar.

3. A dipole antenna according to claim 2, wherein the plane of each of the half dipoles lies perpendicular to the planar base surface.

4. A dipole antenna according to claim 2 or 3, wherein each half dipole has a through hole through its planar surfaces.

5. A dipole antenna according to any preceding claim, wherein the planar base surface is an electrically conductive base to which the half dipoles are fixed.

6. A dipole antenna according to claim 5, further comprising an electrically insulating portion between the electrically conductive base and the half dipoles.

7. A dipole antenna according to any preceding claim, wherein the feed line is electrically connected to a coaxial cable.

8. A dipole antenna according to any preceding claim, wherein the two half dipoles are spaced apart in a direction substantially perpendicular to the axis and substantially parallel to the planar base surface by between 0.0015A and 0.02A where A is the mean operating wavelength of the dipole antenna.

9. A dipole antenna according to claim 8, wherein the two half dipoles are spaced apart in a direction substantially perpendicular to the axis and substantially parallel to the planar base surface by between 0.004A and 0.0 IA.

10. A dipole antenna according to claim 9, wherein the two half dipoles are spaced apart in a direction substantially perpendicular to the axis and substantially parallel to the planar base surface by between 0.005A and 0.007A.

11. A dipole antenna according to claim 10, wherein the two half dipoles are spaced apart in a direction substantially perpendicular to the axis and substantially parallel to the planar base surface by 0.006A.

12. A dipole antenna according to any preceding claim, wherein the half dipoles are spaced from the planar base surface by substantially 0.25A where A is the mean operating wavelength of the dipole antenna.

13. A dipole antenna according to any preceding claim, wherein the half dipoles each extend along the axis by substantially 0.25A where A is the mean operating wavelength of the dipole antenna.

14. A dipole antenna according to any preceding claim, wherein the two half dipoles are substantially L-shape.

15. A dipole antenna according to any preceding claim, wherein the feed line is electrically connected to one of the half dipoles and lies in the same plane as the half dipole to which it is electrically connected.

16. A dipole antenna according to claim 15, wherein the feed line is integral with the half dipole to which it is electrically connected.

17. A dipole antenna according to claim 16, wherein the feed line is substantially L-shape, having a leg portion and an arm portion substantially perpendicular to each other, the leg portion extending substantially parallel to one side of the half dipole member that it is integral with and the arm portion being arranged to form the electrical connection between the half dipole that it is integral with and the feed line.

18. A crossed dipole antenna for operating at mobile telecommunications frequencies, the crossed dipole antenna comprising: two dipole antennas according to any preceding claim, wherein the half dipoles of one dipole antenna are substantially perpendicular to the half dipoles of the other dipole antenna.

19. A dipole antenna array for operating at mobile telecommunications frequencies, the dipole antenna array comprising: at least one column of dipole antennas or crossed dipole antennas according to any preceding claim; wherein the dipole antennas or crossed dipole antennas are each dimensioned substantially the same for operating at one mean operating wavelength.

20. A dipole antenna array according to claim 19, wherein the dipole antennas or crossed dipole antennas are spaced apart along the at least one column by a distance inversely proportional to their mean operating wavelength.

21. A dipole antenna array according to claim 19 or 20, wherein at least one of the dipole antennas or crossed dipole antennas are mounted in rectangular electrically conductive open-top box boxes.

22. A mobile telecommunications dipole antenna comprising: a dipole having a first half dipole and a second half dipole, said first half dipole comprising a first substantially L-shape member having a first arm member which extends substantially parallel to a first axis and a first leg member which extends substantially parallel to a second axis, said first axis being perpendicular to said second axis, said second half dipole comprising a second substantially L-shape member having a second arm member which extends substantially parallel to said first axis and a second leg member which extends substantially parallel to said second axis, said first arm member projecting from said first leg member by substantially one quarter of a mean operating wavelength of said mobile telecommunications dipole antenna; said second arm member projecting from said second leg member by substantially one quarter of said mean operating wavelength of said mobile telecommunications dipole antenna; and said first substantially L-shape member and said second substantially L-shape member being arranged in a substantially 1-shape; wherein said first half dipole and said second half dipole are spaced apart in a direction substantially perpendicular to said first axis and said second axis, such that, an impedance of said dipole substantially matches that of a feed line electrically connected to said dipole.

23. A mobile telecommunications dipole antenna comprising: a dipole having a first half dipole and a second half dipole; said first half dipole comprising a first substantially planar L-shape member, which lies in a plane, said first half dipole having a first arm member which extends substantially parallel to a first axis and a first leg member which extends substantially parallel to a second axis, said first axis being substantially perpendicular to said second axis, said second half dipole comprising a second substantially L-shape planar member having a second leg member which extends substantially parallel to said first axis and a second arm member which extends substantially parallel to said second axis, said first arm member projecting from said first leg member by substantially one quarter of a mean operating wavelength of said mobile telecommunications dipole antenna; said second arm member projecting from said second leg member by substantially one quarter of said mean operating wavelength of said mobile telecommunications dipole antenna; said first substantially L-shape member and said second substantially L-shape planar member being arranged in a substantially T- shape; said first half dipole and said second half dipole being spaced apart in a direction substantially perpendicular to said first axis and said second axis such that an impedance of said dipole substantially matches that of a feed line; wherein said feed line is electrically connected directly to said first substantially L-shape planar member; and said feed line is substantially planar and lies substantially in said plane, said feed line being substantially L-shape having a leg portion substantially perpendicular to an arm portion, said leg portion extending substantially parallel to said first leg member and said arm portion being arranged to form an electrical connection between said first substantially L-shape planar member and said feed line.

24. A dipole antenna as substantially hereinbefore described with reference to and as illustrated by the accompanying drawings of Figures 2 to 17.

25. A crossed dipole antenna as substantially hereinbefore described with reference to and as illustrated by the accompanying drawings of Figures 2 to 17.

26. A dipole antenna array as substantially hereinbefore described with reference to and as illustrated by the accompanying drawings of Figures 2 to 17.