AU2011236032A1 - Method of antenna manufacture - Google Patents

Abstract

This invention relates to a method of joining an elongate tubular antenna element to an elongate tubular 5 antenna boom. An antenna element has a uniform circular cross-section having a wall thickness and outer diameter thereby defining a ratio of outer diameter to wall thickness. An element is located through apertures formed in the wall of the boom, such that the element and boom are substantially perpendicular to each other. The element is then restrained at locations which are symmetric about the boom, the unsupported length of element between the restrained locations defining a free element length. An axial 0 compression load is then applied to the element between the restrained locations, such that the element is deformed in locations to form expanded portions, which interfere with the wall of the boom, such that a portion of the element is retained in the boom between the expanded portions, thereby fixing the element and boom relative to each other. For the element, a ratio of free element length to outer diameter and the ratio of outer diameter to wall thickness are both predetermined such that the element deforms to form 5 expanded portions. 0 c~) 'S P P. "5' 5* ,~P ,''~ ', * P 'C,. 'S '' S S.' S'S. S " *55 S 5 *" 55 *S Ii 7~~'' S - S. I '#, 5, 5,, 5,, 5, S. / SSSS\.~ ~, S 5 5%~ '~S ~SSS S~5 ''5 'at.) '~'~ ~s' '~. 5", )

Description

Regulation 3.2 AUSTRALIA PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT ORIGINAL Name of Applicant: Hills Holdings Limited Actual Inventors: Barry Seymour Griggs Andrew Richard John Thomas Address for Service: C/- MADDERNS, GPO Box 2752, Adelaide, South Australia, Australia Invention title: METHOD OF ANTENNA MANUFACTURE The following statement is a full description of this invention, including the best method of performing it known to us.

FIELD OF THE INVENTION This invention relates to the field of antenna manufacture and specifically to a method of joining an antenna element to an antenna boom. 5 In a particular form, this invention has application to the joining of conductive antenna elements to an antenna boom where the antenna may be of a type used to receive over the air broadcast television signals in the UHF and VHF frequency domain, such as a Yagi antenna, although the invention need not be necessarily limited to this application. 0 INCORPORATION BY REFERENCE This patent application claims priority from: Australian Provisional Patent Application 2010904597, entitled "Method of Antenna Manufacture", and filed on 14 October 2010. The entire content of this application is hereby incorporated by reference. 5 BACKGROUND OF THE INVENTION In the following, an antenna element refers to a part of an antenna that is designed to receive or radiate radio waves; an antenna boom refers to a part of an antenna designed to support and locate antenna elements. In some arrangements, the antenna boom may serve as a transmission line, conducting the radio 0 energy from the element to another part of the antenna. There are several joining techniques known in the field to join antenna elements to an antenna boom. For example, the element may be fastened directly to the boom or to an interface bracket by screws or other fastening elements. This technique requires multiple parts (for example screws, nuts, washers and .5 brackets) and adds complexity to the manufacturing and assembly processes for the complete antenna product. The elements may also be welded to the boom by a variety of techniques known in the art. The strength of a weld depends on the force and temperature that has been applied and on the cleanliness of the electrodes 30 and metal. These factors may not be constant when welding manually and as such the reliability of the connection may vary. An automated welding machine will likely add expense to the manufacturing process. A continuous or even a partial weld is an involved process multiplied many times for the multiple elements which form an antenna once connected to the boom. A welding process will increase the cost of the antenna and additionally requires a cleaning process to remove the spatter from each of the 35 welds so formed. Other joining techniques may include elements that are pivotally connected to the boom by way of complicated connections usually requiring parts additional to the elements and boom. The more parts 2 required for the connection will increase or multiply the opportunities or likelihood for failure due to corrosion, structural forces, or out of tolerance performance. There is therefore a need for a useful alternative to known methods of joining boom and element 5 members for simplified antenna manufacture. The method described herein prevents sliding and rotation and is a simple alternative which minimises parts, cost and manufacturing effort. The method requires no additional material apart from the element and boom and generates no waste material. An element and boom joined according to the method 0 described herein is also arguably aesthetically pleasing. A further advantage of the present invention is that an antenna joint arrangement formed according to the method simplifies computer simulation of the antenna's radio frequency (RF) performance and makes it more accurate. Further still, the joint arrangement minimises unwanted secondary RF modes that can be generated by more complex joints. 5 The method of the present invention provides a manufacturing process that is repeatable, reliable and suitable for a large-scale automated production process and produces an antenna joint and an antenna which is an alternative to those previously available. Other advantages of the present invention will become apparent from the following description, taken in .0 connection with the accompanying drawings, wherein, by way of illustration and example, an embodiment of the present invention is disclosed. It will be understood that the term "comprise" and any of its derivatives (e.g. comprises, comprising) as used in this specification is to be taken to be inclusive of features to which it refers, and is not meant to !5 exclude the presence of any additional features unless otherwise stated or implied. The reference to any prior art in this specification is not, and should not be taken as, an acknowledgment or any form of suggestion that such prior art forms part of the common general knowledge of the technical field. 30 SUMMARY OF THE INVENTION In one aspect of the invention there is provided a method of joining an elongate tubular antenna element to an elongate tubular antenna boom; the element having a uniform circular cross-section having a wall thickness and outer diameter thereby defining a ratio of outer diameter to wall thickness, the method 35 including the steps of: locating an element through apertures formed in the wall of the boom, such that the element and boom are substantially perpendicular to each other; 3 restraining the element at two locations which are symmetric about the boom, the unsupported length of element between the restrained locations defining a free element length; and applying an axial compression load to the element between the restrained locations, such that the element is deformed in locations to form expanded portions, which interfere with the wall of the boom, 5 such that a portion of the element is retained in the boom between the deformations, thereby fixing the element and boom relative to each other; wherein for the element, a ratio of free element length to outer diameter and the ratio of outer diameter to wall thickness are both predetermined such that the element deforms to form expanded portions. D In one form, the step of restraining the element is achieved by gripping the element. Preferably, the element is restrained against outward radial movement and sliding. In one form, a physical compression stop is provided such that when an axial compression load is applied 5 to the element, the stop defines a limit to the compression of the element. In one form, for an element that is 6061 -T5 aluminium alloy, the predetermined ratio of outer diameter to wall thickness of the element is in the range 6-15, and the predetermined ratio of free element length to outer diameter of the element is in the range 2-4. 0 In a preferred form, for an element that is 6061-T5 aluminium alloy, the predetermined ratio of outer diameter to wall thickness of the element is in the range 8.9-10, and the predetermined ratio of free element length to outer diameter of the element is in the range 2.3-2.4. .5 In an exemplary form, for an element that is 6061-T5 aluminium alloy, the predetermined ratio of outer diameter to wall thickness of the element is 8.9, and the predetermined ratio of free element length to outer diameter of the element is 2.4. In a further aspect of the invention, there is provided an antenna joint arrangement formed between an 30 elongate tubular antenna element and an elongate tubular antenna boom, including: a boom having a wall and two opposing wall apertures; and an element having a uniform circular cross-section, the element located through the opposing wall apertures of the boom, the element deformed in locations by compressive loading so as to have expanded portions, wherein each of at least two deformations of the element interfere with the wall of the 35 boom, such that a portion of the element is retained in the boom between said deformations, thereby fixing the element and boom relative to each other. 4 In one form, the wall of the boom has an outer wall surface and the deformations of the element interfere with at least a portion of the outer wall surface of the boom. In one form, the opposing wall apertures of the boom each define a surface in the wall of the boom, and 5 deformations of the element interfere with a least a portion of the surface defined by each opposing wall aperture. In a further aspect of the invention there is provided an antenna, including: a boom; and 0 at least one element that is joined to the boom according to the method described herein. In still another aspect of the invention there is provided a method ofjoining an elongate tubular antenna element to an elongate tubular antenna boom, including the steps of: locating an element through apertures formed in the wall of the boom, such that the element and 5 boom are substantially perpendicular to each other; and deforming the element in two locations so as to have expanded portions, wherein each deformation of the element interferes with the wall of the boom, such that a portion of the element is retained in the boom between the deformations, thereby fixing the element and boom relative to each other. 0 BRIEF DESCRIPTION OF THE DRAWINGS Various aspects of the present invention will be described in detail with reference to the following drawings in which: .5 FIGURE 1 shows a perspective view of an antenna manufactured according to the method of the present invention; FIGURE 2 shows a flow diagram illustrating the preferred method of the present invention; FIGURE 3 shows a perspective view of the lower and upper tools used in the manufacture of the antenna; FIGURE 4 shows a perspective view of the lower and upper tools including an antenna element and boom 30 setup in the lower tool before axial compression; FIGURES 5a and 5b depict top views of the upper and lower tools respectively; FIGURES 6a-6e depict a sequential series of cross-sectional views of an antenna element being axially compressed; FIGURES 7a and 7b depict front and top views of an antenna manufactured according to an embodiment 35 of the present invention; FIGURES 8a and 8b depict front and top cross-sectional views of an antenna manufactured according to an embodiment of the present invention; FIGURE 9 is an enlarged cross-sectional view of the joint arrangement depicted in figure 8b; 5 FIGURES 1 Oa and I Ob depict a cross-sectional and side view of the antenna element inserted through the boom before axial compression; and FIGURE I I shows a classification chart for the theoretical collapse modes of aluminium alloy tubes. 5 In the following description, like reference characters designate like or corresponding parts throughout the several views of the drawings. DETAILED DESCRIPTION OF THE INVENTION Referring to Figure 1 of the accompanying drawings, there is shown an antenna 30 manufactured 0 according to a preferred embodiment of the present invention. The antenna 30 includes a plurality of elongate tubular antenna elements 20 joined to an elongate tubular antenna boom 10. Each antenna joint arrangement includes deformations 25 (forming expanded portions, folds or bulges) in an element 20 that interfere with the wall of the boom 10, such that a portion of the element 20 is retained in the boom 10 between the deformations 25, thereby fixing the element 20 and boom 10 relative to each other. Each 5 antenna joint arrangement has two deformations 25 that are formed on opposing sides of the boom 10 that lock the element 20 to the boom 10 by pressure. The formation of the deformations 25 is achieved by applying an axial compression load to the element 20 that initiates a buckling failure mode of the tube resulting in the formation of repeatable folds at specific locations which interfere with the wall of the boom 10. Throughout this specification, axial compression refers to compression of the antenna element 0 20 along its longitudinal axis (refer to axis Y-Y in Figure 6a). The theoretical background of the present invention is based upon an understanding of the collapse failure modes of cylindrical ductile tubes under axial compression. When such a tube is loaded in axial compression, different failure modes may be observed depending primarily upon tube material and .5 particular geometric parameters. For a tube loaded in axial compression between two supports (or restraints), the unsupported length of tube between the supports is defined as the free element length of the tube L. In other words the free element length is the length of the tube that is subjected to axial compression. The tube supports may be located at the tube ends (i.e. providing an end restraint or end fixity), however practically and for the purposes of this invention, the tube may be supported at 30 intermediate locations along its length. Defining the tube outer diameter as D and wall thickness as t, it has been found that the collapse mode will depend primarily upon the ratio of D/t and L/D. For a given material and restraint, it is these ratios that determine the collapse mode and not the absolute values of D, t and L. 35 For a long and slender tube, Euler or Column buckling is the most likely failure mode, whereby the tube bends transversely between supports. The collapse modes relevant to the present invention however, are known as concertina or diamond buckling. 6 Concertina buckling is characterised by axisymmetric and sequential folding starting at near one of the tube supports. The first fold forms with the application of a maximum axial load, and subsequent folds are progressively formed in a concertina manner upon application of a mean axial load. 5 Diamond buckling is characterised by non-axisymmetric but sequential folding, whereby each collapsed fold is formed with several lobes. The term 'diamond' refers to the lobes on two adjacent folds that form the top and bottom points of a diamond. The lobes are separated by a straight fold line, which makes up the horizontal diagonal of the diamond. 0 A classification chart for the collapse modes of aluminium alloy tubes (as shown in Figure 11) has been provided in the paper "Classification of the axial collapse of cylindrical tubes under quasi-static loading"; K.R.F Andrews, G.L England and E. Ghani; Int. J. Mech. Sci. vol. 25, No.9-10, pp.687-69 6 , 1983. This chart illustrates the mode of collapse expected for various combinations of L/D and t/D ratios and illustrates the significance of these parameters. It should be noted that the horizontal axis in Figure 1I is 5 the t/D ratio, but the corresponding D/t ratio has been superposed above for consistency of terminology used throughout the specification. The present invention is concerned with controlling the formation of the first two folds when the tube begins to collapse under axial compression and buckle in the concertina or diamond mode. From the .0 description of the theoretical failure modes provided, it will be appreciated that in order to initiate the concertina or diamond buckling failure modes, the tube must have suitable ratios of D/t and L/D for a chosen tube material as illustrated by a chart such as that shown in Figure 11. Predetermining a suitable ratio of D/t and L/D may be achieved in several ways. One method is simply by experimentation, without reference to any charts from the literature. Another method is to consider a chart such as that shown in !5 Figure 11 which provides a range of possible D/t and L/D ratios which would are predicted to lead to the desired failure mode. For the present application, the range of applicable D/t and L/D ratios will be even further limited by antenna design constraints (e.g. performance) and the commercial availability of stock sizes of element material and possibly boom material. 30 Table 1 provides an example of how a useful subset of predetermined ratios of D/t and L/D may be calculated for a useful and commercially available set of tube parameters (D and t). Table I also shows the expected failure mode based on the chart of Figure 11. It should be noted that for Table 1, the 'Free Length' L is the same as the free element length L referred to throughout the specification. 7 Element Element wall Element Boom Gripper Free predicted Mean Initial diameter thickness mass/m width clearance length D/t LD Predicted failure mode number o axial load. axial load, D min, max (AI) min. max mm L (Andrews) bulges FA F mm t kg mm mm mm kg kg mm 3 0.r 0.027 10 3 1 10.0 2.7 concertina 3.4 141 161 F 1 0 0042 13 1 23 60 3 8 c3-hing & lilimp of trbe axis 34 304 268 0 7 043 12 4 114 2.5 cnceif 3-4 206 250 8 1 0 0 059 17 6 9 8 0 36 24lobediamond 4-5 351 357 10 0 0.062 16 5 25 12.5 2.6 concertina 34 281 357 10 1 2 0.090 20 8 3 8 3 3 6 2-lcbe diamond 4.5 516 536 12 I 8 0.076 19 6 ml 15.0 26 concertina 3.5 308 429 1 12 0110 25 10 15 10.0 3.8 5cnceulipa & 1r o lobe 3; ' d 4-6 565 643 Table I - Example showing how the ratios of D/t and L/D may be predetermined The information shown in Table I is used to infer that a suitable ratio of outer diameter to wall thickness 5 of the element is in the range 6-15, and a suitable ratio of free element length to outer diameter of the element is in the range 2-4. The above calculations are based on a tube material of 6061-T5 aluminium alloy. The ranges presented above are not exhaustive of all the values which may enable the invention and is presented merely to illustrate how suitable ratios of L/D and D/t may be predetermined to obtain the desired failure mode of the tube in axial compression. 0 If the material of the tube is different, e.g another grade of aluminium or a grade of steel, or tube restraints are varied, then these ratios may change. Referring now to Figure 2 there is shown a flow diagram illustrating the preferred method of the present 5 invention. A step 40 in the method of manufacture is to locate the antenna element 20 through apertures II (see Figure 1Ob) formed in the antenna boom 10. The apertures 11, which are circular or very nearly circular, are formed through opposing sides of the wall 12 (outer wall surface 14 and inner wall surface 15) of the boom 10 either sequentially or simultaneously. 20 The apertures are located such that each aperture 11 shares a common through axis, which is perpendicular or substantially perpendicular to the lengthwise direction of the boom 10. This ensures that when the antenna element 20 is inserted through the apertures 11, the element 20 will be perpendicular or substantially perpendicular to the antenna boom 10. The apertures 11 are formed as clearance holes relative to the outer diameter of the antenna element 20. The clearance used will depend upon the outer 25 tube diameter of the tube and support spacing. For an 8mm outer diameter tube having a support spacing of 21mm, a clearance of 0.1 -0.5mm is appropriate. The preferred technique for forming the apertures I 1 is to use a punch and approach the antenna boom 10 sequentially from opposing sides. The force applied by the punch may locally deform the material of the boom 10 around the apertures 1 but this has been found to have negligible effect in the overall manufacturing process on the reliability of the end product. 30 It will be appreciated that the apertures 11 may be formed in a variety of ways other than punching, including electrical and manual drilling. 8 As the apertures I I formed in the boom 10 are clearance holes, the antenna element 20 should easily pass through the apertures 11 unhindered as long as the movement is always along a path substantially perpendicular to the lengthwise axis of the boom 10. In this step 40, the antenna element 20 is positioned 3 as close as possible to a desired final position relative to the boom 10. Preferably, the antenna element 20 is located symmetrically about the lengthwise central axis of the antenna boom 10. In this way, the length of the antenna element 20 protruding from the wall 12 of the antenna boom 10 will be substantially equal on opposing sides of the wall 12. 3 A further step 42 in the method of manufacture is to restrain the antenna element 20 at two locations which are symmetric about the boom. The length of element 20 between the restrained locations defines the free element length L (or 'unsupported length' of the element 20). In a preferred form, the element 20 is restrained by gripping or clamping the element 20 at the respective locations. As shown in Figures 3, 4, 5a and 5b, there are gripper blocks (51, 61) that grip the antenna element 20 on opposing sides of the 5 boom 10. The separation between the gripper blocks (51, 61) (i.e. the free element length L) is a process parameter based on the predetermined ratio L/D. The gripper blocks (51,61) provide the restraints (or supports) for the antenna element 20 and provide an interface through which an axial compressive load is applied to the antenna element 20. A tube may be restrained or supported at its ends if the element length is suitable, however, in a preferred form, the element 20 is gripped at intermediate locations (i.e. not at the 0 tube ends) such that the element 20 is restrained against outwards radial movement and sliding relative to the gripper blocks (51,61). This allows the elements to have different lengths as will be required for the design of some antennas. An illustrative embodiment of one form of the gripper blocks (51,61) is shown in Figures 3, 4, 5a and 5b. .5 Figures 3, 4, 5a and 5b show a lower tool 50 and upper tool 60 designed to accommodate the gripping blocks (51,61) and the unfixed boom 10 and element 20 arrangement. The lower and upper tools (50,60) include lower gripper blocks 51, one for each side of the element 20, and upper gripper blocks 61, one for each side of the element 20 respectively, each with hemispherical grooves (58,68) having diameters substantially the same as the outer diameter of the antenna element 20. The grooves (58,68) are, if 30 anything, sized to maintain an interference fit with the element 20 which may vary slightly in outer diameter because of manufacturing tolerances. When the lower and upper tools (50,60) are mated by bringing the lower tool 50 and upper tool 60 together, the hemispherical grooves (58,68) are aligned and act to grip the antenna element 20 enclosed in 35 the cavity therein. The antenna element 20 is subsequently fixed in position such that the sections of the antenna element 20 secured in the gripper blocks (51,61) are restrained against outward radial movement. The antenna boom 10 is located in grooves 55 formed by boom locating blocks 52 adjacent the lower gripper blocks 51. The grooves 55 in the boom locating blocks 52 have a width which provides an 9 interference fit when the boom 10 is inserted into the lower tool 50. It will be appreciated that the above arrangement is just one embodiment of how the antenna element 20 and boom 10 may be aligned and gripped. i A still further step 44 in the method of manufacture is to apply an axial compression load to the antenna element 20. The load is applied through the gripper blocks (51,61) such that the free element length of the antenna element 20 is axially compressed. The lower gripper blocks 51 are able to slidably move toward each other, and similarly the upper gripper blocks 61 are able to do the same, which enables the antenna element 20 to be compressed. The compressive axial load may be generated in a variety of ways. In one embodiment, a mechanical press is used to generate the load. When the press is actuated, the upper tool 60 travels down and engages with the lower tool 50. The element locating pins 64 situated in the upper gripper blocks 61 of the upper tool 60 are received in holes 54 in the lower tool 50, which ensures that the antenna element 20 is located 5 correctly within the tool. The element locating pins 64 are slightly tapered which provide a level of tolerance for when the element locating pins 64 and holes 54 are slightly misaligned. Once the antenna element 20 has been located, the tapered pins 63 situated in upper gripper blocks 61 of the upper tool 60, enter and engage with the tapered bushes 53 situated in the lower gripper blocks 51 of the lower tool 50. A cam mechanism (not shown) in the upper gripper blocks 61 then translates the vertical force of the '0 press into a horizontal side ways force applied through the gripper blocks (51,61) to the antenna element 20. The force of the cam is only applied to the upper tool 60, but the engagement of the tapered pins 63 and tapered bushes 53 transfers sideways force from the upper tool 60 to the lower tool 50, to achieve symmetric axial compression of the antenna element 20. An axial compressive load is accordingly applied to the antenna element 20 restrained in the gripper blocks (51,61), such that the wall of the antenna .5 element 20 is deformed in locations to form expanded portions (or bulges), which interfere with the wall of the boom 10, such that a portion of the element 20 is retained in the boom 10 between the deformations 25, thereby fixing the element 20 and boom 10 relative to each other. In one embodiment, for a 6061 -T5 Aluminium Alloy tube with D/t=8.9 and L/D=2.4, (e.g. outer diameter 30 8mm, wall thickness 0.9mm and free element length (i.e. unsupported length) 21.5mm), a load of 6 tonnes is used to form the joint arrangement. However, 5 tonnes is required to overcome a gas spring (not shown) of the upper tool 60 such that only I tonne is actually used to axially compress the element 20 and produce the desired deformations 25. The axial compression is physically limited by a compression stop 56 (shown in Figure 5b) located between the lower gripper blocks 51. The compression stop 56 is slightly 35 wider than the width of the boom 10 to ensure that the boom 10 will not be crushed when the antenna element 20 is compressed and the lower gripper blocks 51 move toward each other. An equivalent compression stop 66 is located similarly for the upper tool 60 between the upper gripper blocks 61 as shown in Figure 5a. The compression stop may be a physical element as shown, although the 10 compression of the element may be limited in other ways including the use of position sensing techniques or movement controlled cams. Shown in Figures 6a-6e is a sequential series of cross-sectional views of the antenna element 20 being i axially compressed. Figure 6a shows an antenna element 20 located through opposing apertures in the wall 12 of the antenna boom 10 and restrained by gripper blocks (51,61). The separation between the gripper blocks (51,61) (i.e. the support spacing) defines the free element length L (i.e. the unsupported length of antenna element 20). Axis Y-Y defines the central longitudinal axis of the boom 10 and is the axis along which axial compression occurs. Figure 6b depicts the element being axially compressed and 3 shows the initial formation of the first deformation (or bulge) 25 which interferes with the boom wall 12. Upon further loading, a second deformation 25 begins to form (Figure 6c) and interfere with the opposing wall 12 of the boom 10. It is possible that a series of internal deformations 26 of the antenna element 20 may also form within the boom 10 (Figure 6d), said internal deformations 26 characterised by having expanded portions that are substantially smaller than the expanded portions of the deformations 25 that 5 are used to lock the element 20 to the boom 10. Figure 6e depicts the final formation of the deformations 25 and shows the final antenna joint arrangement. Figure 6e also depicts the final separation or spacing L' between the gripper blocks (51,61). In an exemplary embodiment of the invention, the outer diameter D of the antenna element is 8mm, the 0 wall thickness t of the antenna element is 0.9mm and the free element length L (i.e. the unsupported length of the antenna element 20) is 21.5mm. These parameters provide a ratio of D/t=-8.9 and L/D=2.4. For antenna joints manufactured using these parameters, the final separation L' between the gripper blocks (51,61) will be approximately 18.6mm. These exemplary parameters are used for an antenna element made from 6061-T5 aluminium alloy. In developmental experiments, the following parameters .5 have also been used with success: D=Omm, t=1.Omm, L=23mm (D/t=10 and L/D=2.3); D=8mm, t=1.05mm, L=19mm (D/t=10 and L/D=2.4) and D=IOmm, t=1.Imm, L=23mm (D/t=9.1 and L/D=2.3). Figures 7a, 7b, 8a, 8b and 9 show views of a typical antenna resulting from application of the method of manufacture described herein. Each antenna element 20 is fixed to the antenna boom 10 by deformations 30 25 that retain a portion of the element 20 in the boom 10. Each antenna joint arrangement has two deformations 25 that are formed on opposing sides of the boom 10 that lock the element 20 to the boom 10 by pressure. The deformations 25 have expanded portions which will typically be radially expanded portions such that the cross-sectional diameter of the antenna element 20 is enlarged. The deformations 25 may interfere with the outer wall surface 14 of the boom 10 or the aperture surface 13 in the wall 12 of 35 the boom 10 (refer to Figure 10a for depiction of cylindrical aperture surface 13). In practice, the joint arrangement is likely to be a result of the combination of the interference fit between deformations 25 and at least a portion of the outer wall surface 14 and the aperture surface 13 (as most clearly shown in Figure 9). IIl It is also shown in Figure 9 that the further deformations 26 that can form within the boom 10 may in some instances provide further interference between the antenna element 20 and the boom 10. The further deformations 26 may interfere with the inner wall surface 15 of the boom 10 thereby assisting the i deformations 25 in fixing the element 20 and boom 10 relative to each other. Although an illustrative embodiment of the present invention has been described in the foregoing detailed description, it will be understood that the invention is not limited to the embodiment disclosed, but is capable of numerous rearrangements, modifications and substitutions without departing from the scope of D the invention as set forth and defined by the following claims. 12

Claims (17)

1. A method of joining an elongate tubular antenna element to an elongate tubular antenna boom; the element having a uniform circular cross-section having a wall thickness and outer diameter 5 thereby defining a ratio of outer diameter to wall thickness, the method including the steps of: locating an element through apertures formed in the wall of the boom, such that the element and boom are substantially perpendicular to each other; restraining the element at two locations which are symmetric about the boom, the unsupported length of element between the restrained locations defining a free element length; 0 and applying an axial compression load to the element between the restrained locations, such that the element is deformed in locations to form expanded portions, which interfere with the wall of the boom, such that a portion of the element is retained in the boom between the deformations, thereby fixing the element and boom relative to each other; 5 wherein for the element, a ratio of free element length to outer diameter and the ratio of outer diameter to wall thickness are both predetermined such that the element deforms to form expanded portions.

2. The method as claimed in claim 1, wherein the step of restraining the element is achieved by 0 gripping the element.

3. The method as claimed in claims I or 2 wherein the element is restrained against from outward radial movement and sliding relative to the boom. ,5 4. The method as claimed in any of the preceding claims, further including: providing a physical compression stop, such that when an axial compression load is applied to the element, the stop defines a limit to the compression of the element.

5. The method as claimed in any one of the preceding claims in any of the preceding claims, 30 wherein for an element that is 6061-T5 aluminium alloy, the predetermined ratio of outer diameter to wall thickness of the element is in the range 6-15, and the predetermined ratio of free element length to outer diameter of the element is in the range 2-4.

6. The method as claimed in any of claims 1-4, wherein for an element that is 6061-T5 aluminium 35 alloy, the predetermined ratio of outer diameter to wall thickness of the element is in the range

8.9-10, and the predetermined ratio of free element length to outer diameter of the element is in the range 2.3-2.4. 13 7. The method as claimed in any of claims 1-4, wherein for an element that is 6061-T5 aluminium alloy, the predetermined ratio of outer diameter to wall thickness of the element is 8.9, and the predetermined ratio of free element length to outer diameter of the element is 2.4. 5 8. An antenna joint arrangement formed between an elongate tubular antenna element and an elongate tubular antenna boom resulting from the method as claimed in any one of the preceding claims.

9. An antenna joint arrangement formed between an elongate tubular antenna element and an D elongate tubular antenna boom, including: a boom having a wall and two opposing wall apertures; and an element having a uniform circular cross-section, the element located through the opposing wall apertures of the boom, the element deformed in locations by compressive loading so as to have expanded portions, wherein each of at least two deformations of the element 5 interfere with the wall of the boom, such that a portion of the element is retained in the boom between said deformations, thereby fixing the element and boom relative to each other.

10. The antenna joint arrangement as claimed in claim 9, wherein the wall of the boom has an outer wall surface and the deformations of the element interfere with a least a portion of the outer wall .0 surface of the boom.

11. The antenna joint arrangement as claimed in claim 10, wherein the opposing wall apertures of the boom each define a surface in the wall of the boom, and deformations of the element interfere with at least a portion of the surface defined by each opposing wall aperture. 15 12. The antenna joint arrangement as claimed in any one of the preceding claims, wherein the element and boom are substantially perpendicular to each other.

13. The antennajoint arrangement as claimed in any one of the preceding claims, wherein the boom 30 has a rectangular cross-section.

14. The antenna joint arrangement as claimed in any one of the preceding claims, wherein the boom is aluminium alloy tube. 35 15. The antenna joint arrangement as claimed in any one of the preceding claims, wherein the element is aluminium alloy tube. 14

16. The antenna joint arrangement as claimed in any one of the preceding claims, wherein the element is 6061-T5 aluminium alloy.

17. An antenna, including: 5 a boom; and at least one element that is joined to the boom according to the method claimed in any of claims I to 7.

18. An antenna, including: 0 a boom; and at least one element that is joined to the boom according to the joint arrangement of any of claims 8-16.

19. A method of joining an elongate tubular antenna element to an elongate tubular antenna boom, 5 including the steps of: locating an element through apertures formed in the wall of the boom, such that the element and boom are substantially perpendicular to each other; and deforming the element in two locations so as to have expanded portions, wherein each deformation of the element interferes with the wall of the boom, such that a portion of the .0 element is retained in the boom between the deformations, thereby fixing the element and boom relative to each other.

20. A method ofjoining an elongate tubular antenna element to an elongate tubular antenna boom substantially in accordance with any one of the embodiments of the invention described herein 5 and illustrated in the accompanying drawings.

21. An antenna joint arrangement substantially in accordance with any one of the embodiments of the invention described herein and illustrated in the accompanying drawings. 30 22. An antenna substantially in accordance with any one of the embodiments of the invention described herein and illustrated in the accompanying drawings. 15