GB2382928A

GB2382928A - Antenna Assembly

Info

Publication number: GB2382928A
Application number: GB0226314A
Authority: GB
Inventors: Louis David Thomas
Original assignee: Qinetiq Ltd
Current assignee: Qinetiq Ltd
Priority date: 2001-11-23
Filing date: 2002-11-12
Publication date: 2003-06-11
Anticipated expiration: 2022-11-12
Also published as: GB0128132D0; GB0226314D0; GB2382928B

Abstract

An antenna assembly 70 has multiple antennas with non-convergent boresight headings e.g. 76. No antenna is located nearer to a scene to which another antenna transmits or from which it receives. The antennas are closely spaced with respect to one another. Antennas are incorporated in radomes A1 to A15, with adjacent radomes e.g. A7 and A8 having an intervening gap Dr only sufficient to accommodate relative movement between the radomes due to changing environmental conditions experienced in normal use. The gap Dr may be less than one tenth of any antenna operating wavelength in free space, preferably less than 0.033 of such wavelength. The radomes A1 to A15 have support struts 72 extending backwardly away from their respective transmission directions and disposed generally symmetrically about respective lines through antenna centres perpendicular to antenna planes.

Description

Antenna Assembly This invention relates to an antenna assembly in which antennas are closely spaced while retaining an acceptably low degree of mutual interference therebetween.

In recent years the number of cellular radio systems and operators has increased sufficiently to require addition of further antenna radomes to existing antenna installations and also to increase the number of antenna sites. Both of these developments have led to public concern and difficulties in obtaining planning consent. Moreover, adding further antenna radomes to an existing installation increases the scope for àntenna beam deformation and mutual interference between different antenna radomes. In the United Kingdom there is a potential need to co-locate antenna radomes for up to nine operators consisting of two using GSM900, two using GSM1800, and five using UMTS: here GSM means Global System for Mobile communications operating at 900 or 1800 MHz and UMTS is Universal Mobile Telephone System operating at 2000 MHz.

It is currently the practice to provide additional antenna radomes to existing installations either by increasing the size of an existing antenna gantry, or by adding further gantries to a mast. This may mean increasing the size of an existing gantry and increasing the height and strength of a mast. The disadvantage of this approach is that it results in an antenna system with increased size, wind loading, cost, visibility and obtrusiveness.

Existing installations utilise gantries that are physically close to an antenna radome which increases the potential for antenna beam deformation and mutual interference through the mechanism of conduction and reflection of radio frequency energy. There is also an increased probability of generation of unwanted passive inter-modulation products formed between signals at different frequencies. If antenna separation is decreased in order to reduce visual impact, beam deformation and mutual interference between antennas can result.

Furthermore, it may be necessary to change an antenna's boresight heading, either to optimise a cell plan, or to track a population of users whose location is varying. This increases the possibility that an unknown level of beam deformation and mutual interference may be introduced into antenna radomes of other GSM or UMTS operators. In

this event it is not practical to re-measure the cell coverage and performance of antenna radomes and systems used by other operators.

The invention has the objective of producing a compact antenna assembly while achieving acceptable degrees of visual impact, beam deformation, inter-modulation products, interference, mast size and wind loading.

The present invention provides an antenna assembly having a plurality of antennas with non-convergent boresight headings characterised in that a) no antenna is located nearer to a scene than an antenna plane of another antenna arranged to transmit to or receive from the scene, b) the antennas are closely spaced with respect to one another, c) the antennas are incorporated in radomes, and d) adjacent radomes have an intervening gap which is only sufficient to accommodate relative movement between and thermal expansion of the radomes due to changing environmental conditions experienced in normal use.

The invention provides the advantage of providing compact antenna assembly because the antennas are closely spaced, so visual impact, mast size and wind loading are comparatively low. Moreover, beam deformation, inter-modulation products and interference are low because non-convergent boresight headings and antenna location with respect to the plane of another antenna reduce the scope for one antenna to receive from another.

The antennas may be incorporated in radomes: adjacent radomes may have an intervening gap which is only sufficient to accommodate relative movement between and thermal expansion of the radomes due to changing environmental conditions experienced in normal use. The gap may be less than one tenth of any antenna operating wavelength in free space, e. g. less than 0.033 of such wavelength. It may be less than 1 cm, e. g. in the range 3mm to 5mm.

Antennas may have supports extending in backwardly away from their respective transmission directions. The supports may be disposed generally symmetrically about respective lines extending through radome centres perpendicular to antenna planes. They may have axes offset by not more than 1cm (e. g. less than 0. 25cm) from respective lines extending through radome centres perpendicular to antenna planes.

The antenna assembly may include at least one antenna having a plurality of selectable boresight angles. It may include a plurality of antenna radomes having the same boresight heading and a common antenna plane. It may include fifteen antennas in three groups of five antennas, each group being arranged to communicate with a respective sector and containing three antenna radomes having the same boresight heading and antenna plane.

Each group may be arranged to communicate with a respective sector and contain three antenna radomes capable of having the same boresight heading and antenna plane.

In order that the invention might be more fully understood, embodiments thereof will now be described, by way of example only, with reference to the accompany drawings, in which: Figure 1 schematically illustrates a prior art cellular radio antenna installation in elevation ; Figure 2 is a horizontal section on lines 11-11 in Figure 1; Figure 3 is a drawing of a horizontal radiation pattern of an antenna; Figures 4 and 5 show antenna configurations of the invention having two and three antenna radomes respectively; Figure 6 shows an antenna configuration with one antenna having a dual boresight heading requirement and not in accordance with the invention; Figure 7 shows a modification to the antenna configuration of Figure 6 to produce conformity with the invention; Figure 8 illustrates an antenna arrangement with one antenna having a dual boresight heading requirement and configured in accordance with the invention; Figure 9 shows an antenna installation of the invention with fifteen antenna radomes enabling each of five operators to cover three sectors; and Figure 10 shows an alternative antenna installation of the invention with fifteen radomes.

Referring to Figure 1, a conventional prior art antenna installation 10 is shown in elevation for a multi-band cellular radio antenna system. The installation 10 comprises a tower 12 on which upper and lower gantries 14U and 14L (collectively 14) are mounted. The

gantries 14 each support three pairs of antenna radomes, of which a respective pair 14UA -14UA2, 14LA,-14LA2, is shown for each gantry 14U, 14L : each of these radomes is of conventional construction and contains an antenna (not shown) consisting an array of antenna dipoles configured as a phased array. It also contains and radio frequency (RF) transmit/receive circuitry (not shown) for the antenna.

Referring now also to Figure 2, the installation 10 is shown in horizontal section on lines 11- 11 in Figure 1 through the upper gantry 14U. The gantry 14U is triangular and has pairs of radomes 14UA1/14UA2, 14UBi/14UB2 and 14UC,/14UC2 covering three 1200 sectors A, B and C respectively. The upper and lower gantries 14U and 14L are of like construction.

The gantries 14 collectively provide for a total of four operators to transmit to and receive from three 1200 sectors A, B and C indicated by chain lines 16 ; two operators are associated with each gantry and each operator has three radomes (e. g. 14UAi, 14UB, and 14UC,). To ensure that interference between antennas is low, pairs of radomes (e. g.

14UA, and 14UA2) covering individual sectors (e. g. A) are widely separated, e. g. by one or more metres or more than six wavelengths at 2GHz : this increases the size, cost and obtrusiveness of the tower 10.

In a multi-antenna assembly, that is to say an assembly of two or more antenna radomes, each antenna may both create interference to be received by other antennas and itself be a victim of interference. To minimise interference, both transmission and reception by every antenna in an installation must be considered. For the purposes of this specification, three expressions will now be defined, (a) antenna boresight or antenna boresight heading : the direction in which a signal transmitted from an antenna is a maximum, or in reception the direction in which antenna sensitivity is a maximum ; (b) antenna plane : a plane in the antenna's transmitting/receiving surface or that of its radome if present ; (c) boresight angle : angle between a central antenna boresight direction and an actual antenna boresight direction to which the antenna boresight has been steered ; such steering may be by rotating the antenna, or, in a phased array of antennas, by applying a phase change to the antennas which varies linearly with antenna position across the array.

Figure 3 shows a typical horizontal radiation pattern (HRP) 20 for an antenna consisting of a dipole array (not shown) in a radome 21 for cellular radio. The pattern 20 has a large forward or main radiation lobe 22 and a small backward radiation lobe 23. It has an antenna boresight heading 24 at which its sensitivity is a maximum 24A (boresight level),

and this heading extends to the right in the drawing defining a forward direction for the antenna. An opposite direction to this heading extends at 23A through the backward radiation lobe 23, and defines a backward direction for the antenna and directions 25A and 25B in which its sensitivity is 3dB below that along the antenna boresight 24. There is an angle 26 of 65 degrees between the directions 25A and 25B which is the antenna beamwidth. An antenna plane 27 lies in a forward surface 28 of the radome 21.

An antenna for cellular radio also has a vertical radiation pattern (VRP, not shown), but this pattern has a much smaller beamwidth than the pattern 20. The VRP has a typical beamwidth of 6. 5 degrees.

It has been discovered in accordance with the invention that there are three rules for mounting antenna radomes in a multi-antenna assembly to minimise interference while preserving the ability to pack the radomes closely together. These rules are as follows : Rule #1 : Allowable Range of Boresight Angle For the purposes of this rule, a connecting line is drawn between two antenna radomes located adjacent to one another and a respective normal (perpendicular) to the connecting line is drawn through each antenna; the allowable range of boresight heading angle for each antenna is an angle subtended between the respective normal to the connecting line on one side of that antenna to the normal to the connecting line on the other side. When there are more than two antenna radomes connecting lines and associated normals are drawn for each antenna and its neighbours; each antenna now has a first connecting line and normal with respect to a first neighbouring antenna on one side and a second connecting line and normal with respect to a second neighbouring antenna on another side: these two normals delimit the respective antenna's allowable range of boresight angle.

Rule #2 : Allowable Change in Boresight Angle When it is necessary to change the boresight angle of an antenna, or to have two or more boresight angles to accommodate a changing user environment, then a second boresight angle should be within the allowable range of boresight angle for that antenna. If the second boresight angle is outside the allowable range, then the overall multi-antenna configuration should be changed, or a further antenna should be added to accommodate the new boresight angle.

Rule #3 : Allowable Position for Antenna Mounting Structure The structure used to support an antenna should be located immediately behind the antenna and on, or near too, the boresight line extended in a backward direction from the antenna: here"behind"means on a side of an antenna opposite to that of a scene from which the antenna receives or to which it transmits, and"backward direction"means a direction opposite to an antenna's transmission direction.

For antenna radomes in a multi-antenna configuration, the consequences of Rule #1 are: 1) all antenna boresight headings extending forwards or outwardly from the antenna configuration are either parallel or divergent with respect to one another-i. e. convergent boresight headings are not allowed ; 2) no antenna is located further forward with respect to the plane of any other antenna: here "forward" means towards a scene to which an antenna transmits or from which it receives: I. e. where a first antenna is arranged to transmit to or receive from a scene, no second antenna is located nearer to that scene than the first antenna 3) if a plurality of antenna radomes have the same boresight heading then they can only have a common antenna plane ; 4) no two ranges of allowable boresight angle overlap ; and 5) the sum of all ranges of allowable boresight angles is 360 degrees if full all round coverage is required.

If has been found that if these rules are obeyed, it is possible to minimise antenna separation, e. g. adjacent antennas or at least their radomes can be very close together.

Referring to Figure 4, two antenna radomes 31 and 32 are shown which are configured to obey Rule #1. They have a connecting line 34 whose length is their separation distance; normals (perpendiculars) to the connecting line 34 through respective radomes 31 and 32 are indicated by chain lines 31 N and 32N. Each radome 31/32 contains a single antenna (not shown) comprising a phased array of antenna dipoles. The radomes 31 and 32 have respective allowable ranges of boresight angle 31 R and 32R which are both equal to 180 degrees. They are shown steered to boresight headings 31 P and 32P offset from central boresight headings 31 C and 32C respectively.

In Figure 5, three antenna radomes 41,42 and 43 equivalent to those described earlier are shown which are also configured to obey Rule #1. Radome pairs 41/42,41/43 and 42/43 have respective connecting lines 4412, 4413 and 4423 giving separation distances; pairs of normals to the connecting lines 4412, 4413 and 4423 through radomes 41,42 and 43 are indicated by chain lines 41 N1i41 N13, 42N12/42N23 and 43Nis/43N23 respectively. The radomes 41,42 and 43 have respective allowable ranges of boresight angle 41 R, 42R and 43R all equal to 120 degrees between pairs of normals e. g. 41 N12/41 N13 at each antenna radome e. g. 41. They are shown steered to central boresight headings 41 C, 42C and 43C respectively. Compared to the two-radome configuration in Figure 3, the Figure 4 configuration has more limited allowable boresight angle range per radome, i. e. 120 degrees as opposed to 180 degrees.

Figure 6 is similar to Figure 5, except that a requirement has been introduced for one radome to have an additional boresight heading which is outside an allowable boresight heading range. Parts equivalent to those described earlier are like referenced. The second radome 42 has a central boresight heading 42C within an allowed range between a pair of normals 41 N12/41 N13 : it also has a second boresight heading 42H outside the allowable boresight heading range between normals 41 N12/41 N13, and this heading will give rise to interference between antenna radomes 42 and 43. This is an example of a dual boresight heading requirement where one boresight heading is outside of the allowable range, and Rule #1 is not satisfied.

If radome 42 is changed from boresight heading 42C to boresight heading 42H, then antenna radome 43 is located in further forward with respect to a plane (not shown) in a front surface 42F of radome 42, for which it is therefore a potential source of beam deformation. Moreover, if radome 43 is steered to its maximum allowable anti-clockwise angle along normal 43N23, then boresight headings 42C and 43N23 would be convergent contrary to Rule #1 and creating a potential source of interference.

Figure 7 is similar to Figure 6, except that a modification has been introduced to accommodate the additional boresight heading within an allowable boresight heading range. Parts equivalent to those described earlier are like referenced. The third radome 43 has been moved to the left a distance D (see Figure 6) so that its distance from the first radome 41 is half that in Figure 6. Rule #1 now provides for an allowable boresight heading range 43R for the third radome 43 which is bounded by chain lines 43N13 and 43N23: these chain lines are extensions of lines 44Nia and 44N23 between radome pairs

41/43 and 42/43 respectively. The additional boresight heading 42H is parallel to chain line 43N13 so Rule #1 is satisfied.

Referring now to Figure 8, there is shown an antenna radome configuration 60 in which Rule #2 has been implemented to accommodate a requirement for a second boresight heading of the kind not allowable at 42H in the Figure 6 arrangement. Four antenna radomes 61 to 64 are shown, of which 61 and 64 are equivalent to antenna radomes 41 and 43 in Figure 5 except that antenna radome 64 has a more restricted allowable range of boresight headings than antenna radome 43.

Radome 42 has been replaced by two radomes 63 and 64 both obeying Rule #1.

Radomes 63 and 64 have boresight heading ranges between pairs of normals (chain lines) 62N12/62N23 and 63N2 : J63N34 respectively. Radome 63 has an allowable boresight heading 63H along normal 63N34 which does not contravene Rule #1.

An antenna base station 66 is connected to the antenna radomes 62 and 63 via a single pole double throw switch 68: the switch 68 allows one or other of the antenna radomes 62 and 63 to be selected, together with a corresponding boresight heading 62H or 63H.

When two or more antenna radomes are c-located according to Rule #1, Rule #2 and Rule #3 then: 1. separation distances between antenna radomes can be greatly reduced; 2. antenna assembly size, weight, wind loading, feeder lengths and cost can all be reduced; 3. visual impact of the antenna assembly is reduced; and 4. boresight headings can be changed with a minimum change in mutual interference.

Referring now to Figure 9, an antenna assembly 70 is shown intended for five operators each to cover three sectors. It has a total of fifteen radomes A 1 to A 15 equivalent to those described earlier : radomes A 1 to A15 mounted upon respective support pillars such as 72 extending from a common metal mast 74 of equilateral triangle section. The struts 72 are non-metallic : they locate the radomes A 1 to A15 a distance from the mast 74 so that the mast has little effect on antenna radiation patterns. Each strut 72 has a length of substantially one free space wavelength at the operating frequency or not more than 5%

more or less than this, e. g. 150. 75cm at 2000 MHz: it is cylindrical with 5cm diameter and coaxial with a line through the radome centre perpendicular to the antenna plane-its axis may be offset by up to 1 em from this line but preferably the distance between the two is not more than 0.25cm. Each support 72 is of smaller diameter (or lesser width) than its associated radome. It is located at the rear of and (for unsteered radomes such as A2) extending backwards with respect to the antenna: here"backwards"means along the respective antenna boresight line in the opposite direction to the boresight heading in each case (see e. g. arrow 76 which is the boresight heading for radome A4). Such location of the supports 72 reduces their effect on antenna radiation patterns.

Radomes A1 to A5 cover a Sector X, radomes A6 to A10 cover a Sector Y, and radomes A11 to A15 cover a Sector Z. In each Sector X, Y or Z, boresight headings such as 76 for radome A4 are the same for all operators, ignoring the fact that a minority of the radomes have a range of allowable boresight heading angles: hence as drawn all five antenna radomes (e. g. A1 to A5) covering each sector (e. g. Sector X) share a common antenna plane Px, Py or Pz (chain lines) extending perpendicular to the plane of the drawing.

Each of the three rows of five radomes A 1 to A5, A6 to A10 and AH to A15 has a respective set of three inner radomes A2 to A4, A7 to A9 and A12 to A14, and a pair of outer radomes A1/A5, A6/A10 and A11/A15. Radomes in each inner set of three such as A2 to A4 are unsteered, i. e. they have fixed boresight headings parallel to one another: were this not to be the case at least some boresight headings would be convergent. Radomes in each outer pair such as A1/A5 have a small allowable range of boresight heading variation indicated in each case by arrows such as 78. This is in accordance with Rule #1.

In each of the three rows of radomes A 1 to A5, A6 to A10 and A11 to A15, the radomes are close together. Radomes A7 and A8 have respective centre lines A7C and A8C (also boresight headings) separated by a distance Dc, and these radomes have an intervening gap of width Dr. If Rule #1 is observed, the distance Dc between the antenna centre lines A7C and A8C can be reduced until the radome gap width Dr is zero. In practice the radome separation distance (Dr) is given a small value, not more than 1 cm, e. g. 3-5mm, which is just sufficient to allow for thermal expansion and small movements of radomes relative to one another with wind loading and flexure of the mast 74. For operation at 900MHz or 1800 MHz and 2000 MHz, the free space wavelengths are 33 cm, 17cm and 15 cm respectively : 1 cm is therefore 0.03, 0.06 and 0.07 of a wavelength respectively. In

addition, 3-5mm corresponds to 0.01-0. 015,0. 018-0.03 and 0.02-0. 033 of a wavelength respectively. All these values of radome separation distance (Dr) are less than one tenth of a wavelength, preferably less than or equal to 0.033 of a wavelength.

Referring now to Figure 10, an antenna assembly 80 is shown intended for five operators each to cover three 120 degree Sectors U, V and W with three different boresight headings per sector. The three Sectors U, V and W are equivalent, and so description will be confined to Sector U.

Sector U has five radomes A21 to A25 mounted upon supports such as 82 extending from a common mast 84. As drawn, radomes A23 to A25 share a common antenna plane Qu extending perpendicular to the plane of the drawing, but radomes A23 and A24 are steerable out of this plane to give them boresight headings diverging from that of antenna radome A24. Radome A24 is unsteered, i. e. it has a fixed boresight heading. Radome A22 can have a boresight heading parallel to that of radome A21 or A23 depending on degree of adjustment of all three radomes. Radomes A21 to A23 and A25 have a small allowable range of boresight heading variation indicated in each case by arrows such as 86. This is in accordance with Rule #1.

Claims

1. An antenna assembly having a plurality of antennas with non-convergent boresight headings characterised in that a) no antenna is located nearer to a scene than an antenna plane of another antenna arranged to transmit to or receive from the scene, b) the antennas are closely spaced with respect to one another, c) the antennas are incorporated in radomes, and d) adjacent radomes have an intervening gap which is only sufficient to accommodate relative movement between and thermal expansion of the radomes due to changing environmental conditions experienced in normal use.

2. An antenna assembly according to Claim 1 characterised in that adjacent radomes have an intervening gap which is less than one tenth of any antenna operating wavelength in free space.

3. An antenna assembly according to Claim 1 characterised in that adjacent radomes have an intervening gap which is less than 0.033 of any antenna operating wavelength in free space.

4. An antenna assembly according to Claim 1 characterised in that adjacent radomes have an intervening gap which is less than 1 cm.

5. An antenna assembly according to Claim 1 characterised in that adjacent radomes have an intervening gap in the range 3mm to 5mm.

6. An antenna assembly according to Claim 1 characterised in that antennas have supports extending in backwardly away from their respective transmission directions.

7. An antenna assembly according to Claim 1 characterised in that antennas have supports disposed generally symmetrically about respective lines extending through radome centres perpendicular to antenna planes.

8. An antenna assembly according to Claim 1 characterised in that antennas have supports with axes offset by not more than 1 cm from respective lines extending through radome centres perpendicular to antenna planes.

9. An antenna assembly according to Claim 8 characterised in that antenna support axes are offset by not more than 0. 25cm from respective lines extending through radome centres perpendicular to antenna planes.

10. An antenna assembly according to Claim 1 characterised in that it includes at least one antenna having a plurality of selectable boresight angles.

11. An antenna assembly according to Claim 1 characterised in that it includes a plurality of antenna radomes having the same boresight heading and a common antenna plane ;

12. An antenna assembly according to Claim 1 characterised in that it includes fifteen antennas in three groups of five antennas, each group being arranged to communicate with a respective sector and containing three antenna radomes having or capable of having the same boresight heading and antenna plane.