PART

The present invention generally relates to a method of interconnecting aluminum parts of an antenna for radio frequency (RF) applications and the antenna part of these interconnected aluminum parts.

Antenna for radio frequency applications often comprise aluminum parts because of the high conductivity, the light weight and the corrosion resistance of the material, for example as a housing, a shielding or another RF conducting component. The aluminum parts may conduct a RF current. The antennas may notably operate with frequency division duplexing. A key performance indicator for the aluminum parts is a low passive intermodulation (PIM) level. An antenna part is considered PIM free by industry standards if the PIM level is lower than -150 dBc (decibels relative to carrier) when operating the antenna. Aluminum antenna parts are often manufactured by extrusion from a piece of aluminum.

Thus a continuous aluminum part is produced. The continuous aluminum part may be PIM free as it does not comprise any internal metal-to-metal connections, which would be prone to increase the PIM level. However, this manufacturing method entails certain disadvantages, such as design limitations, e.g. a fixed cross section. The fixed cross section can make it difficult to realize additional features inside or outside the cross section. The method is also limited in its possibility to produce big parts with thin walls, as the thickness of the walls is influenced by the length of the part and the shape of the cross section. As parts with thin walls function for high frequency applications, making walls thinner would allow reducing the weight and the material cost of the aluminum parts.

Another widely used manufacturing method for aluminum parts of antennas is milling.

This method is flexible in shaping the aluminum parts, but long hollow parts are difficult to manufacture. The milled aluminum parts mostly comprise at least two parts which are connected by metal-to-metal connections. These connections may comprise solder joints, screws or capacitive couplings. They all have some limitations. For solder joints, which are PIM free, the aluminum components (which cannot be soldered) need to be plated with a solder affine plating, and this is expensive. Screws in combination with a specific design of the contact area can create a high contact pressure over the whole contact area between the components, resulting in a PIM free galvanic connection. However, these screw connections have a PIM level that is not stable in the long term due to aging effects such as relaxation of the pressure, oxidation and microscopic movement by vibration or shocks. Finally, with capacitive coupling, there is no galvanic connection between the parts and hence no PIM. However, connections by capacitive coupling require a coupling area that is sufficiently big and this limits the design options to some extent.

It is an object is to provide a simple and effective method of manufacturing aluminum antenna parts that are PIM free, i.e. free of passive intermodulation, and do not have the aforementioned design limitations.

According to a first aspect, a method of interconnecting a first aluminum part and a second aluminum part of an antenna for radio frequency applications to produce an electrical and mechanical connection is proposed. The method comprises connecting the first aluminum part and the second aluminum part directly to each other by laser welding. The laser welding comprises moving a point of incidence of a laser beam on at least one of the first aluminum part and the second aluminum part, wherein the movement of the point of incidence is a superposition of a substantially rectilineal movement and wobbling. Thus a PIM free connection between the first aluminum part and the second aluminum part can be achieved.

The first aluminum part and the second aluminum part can be joined at their shared contact areas by wobble mode laser welding. This technique will normally not produce any irregular or spurious connections between the two aluminum parts and will instead produce a regular, homogenenous contact zone that is substantially PIM free. Substantially PIM free means that the connection has a very low PIM level, e.g. lower than -150 dBc. The wobbling movement reduces or eliminates welding burs, which would be a PIM source. To benefit from the PIM free connection between the first aluminum part and the second aluminum part, the two aluminum parts should be PIM free at their own. This laser welding technique allows the manufacturing of complex geometries. Aluminum parts can be manufactured by different manufacturing methods and can then be connected in a PIM free manner.

The movement of the point of incidence of the laser beam comprises two components: a substantially rectilineal movement and wobbling (i.e. a wobble movement). “Substantially rectilineal” in this context means that the rectilineal movement has (at any point in time) a curvature that is small compared to a curvature of the wobble movement (e.g. the curvature of the rectilineal movement is less than 10 % of the curvature of the wobble movement). The substantially rectilineal movement may in particular be straight, i.e. a movement along a straight line (in which case the curvature is zero). The wobbling may be a fast short-range cyclic movement of the point of incidence. “Shortrange” in this context means that the point of incidence, assuming that the substantially rectilineal movement is zero, is confined to move within a relatively small wobble zone. “Cyclic movement” in this context means a sequence of cycles, each cycle corresponding to one closed trajectory of the point of incidence (again assuming that the substantially rectilineal movement is zero). “Fast” in this context means that the wobbling movement is faster (i.e. has a higher speed) than the substantially rectilineal movement. The substantially rectilineal movement moves the wobble zone along a welding path. The wobble movement may notably be a periodic elliptic (preferably circular) movement, preferably with a constant speed. In this case, the wobble movement is performed in a sequence of periods (“wobble periods”), wherein the point of incidence of the laser beam describes one full ellipse (or circle) in each period.

In one embodiment, the laser welding comprises applying the laser beam in a back-and- forth stroke along a welding path. Thus welding is performed twice at every point of the welding path. Thus a PIM free connection can be reliably produced, even if the penetration depth of the laser beam is not sufficient at the onset point of the laser welding (e.g. if possible oxide layers or other residues on the first or second aluminum part prevent proper penetration of the laser beam during the first application of the laser beam). This is particularly important at the onset point of the laser welding, since the laser beam may hit an unbroken oxide layer at this point and the penetration depth might be not deep enough when activating the laser beam. Preferably, the laser welding starts at a first end of the welding path, is carried out to an opposite second end of the welding path and back to the first end. This way the back-and- forth stroke can be easily implemented and an even welding seam is formed.

In one embodiment, the wobbling is or comprises an elliptical movement, preferably a circular movement. The circular movement results in a relatively uniform laser power distribution within a stripe region (referred to herein as the welding stripe) that includes the welding path and extends to both sides of the welding path. This is favorable for achieving a PIM free connection between the first aluminum part and the second aluminum part.

Preferably, the substantially rectilineal movement is slower than the wobbling movement. In other words, the wobbling has a higher speed than the substantially rectilineal movement. In a particular preferred embodiment, a speed of the substantially rectilineal movement (referred to as the rectilineal speed) is in the range of 10 to 200 mm/s while a speed of the wobbling (referred to as the wobble speed) is in the range of 200 to 1000 mm/s. In this manner a laser power distribution favorable for producing a PIM free welding connection can be achieved in the welding stripe (i.e. a stripe region which includes the welding path and extends to both sides of the welding path).

In a preferred embodiment, a diameter of the laser beam is smaller than 10 pm and a power of the laser beam is between 250 W and 4000 W. These parameter ranges have been found suitable for achieving a appropriate penetration depth effectively, resulting in a smooth welding seem and hence a PIM free connection between the first aluminum part and the second aluminum part.

In an advantageous embodiment, the first aluminum part is welded to a contacting surface of the second aluminum part, wherein the laser welding is conducted from a welding surface of the second aluminum part, which is opposite and parallel to the contacting surface of the second aluminum part, wherein, preferably, the distance between the welding surface and the contacting surface in the direction of the laser beam is between 0.5 to 3.5 mm. The advantage of this embodiment is that no sparks are generated on the contacting surface, which would lead to particles at the connection between the first aluminum part and the second aluminum part and further to a possible PIM source. The mentioned distance range is advantageous as the laser welding is energy efficient and the contact area between the first aluminum part and the second aluminum part is welded completely.

The present disclosure also provides an antenna part for radio frequency applications comprising a first aluminum part and a second aluminum part, wherein the first aluminum part and the second aluminum part are interconnected by the welding method described above. The resulting connection has a low PIM level (it may be PIM free).

Preferably, the first aluminum part and the second aluminum part are made of an aluminum alloy selected from a group consisting of AI5052, AIMg3, AIMgl.5, A16063, AISi9, ADC12 or of pure AI. These materials are suitable for antenna parts and are particularly well interconnected with the inventive method.

In a particularly advantageous embodiment, the first aluminum part and the second aluminum part are metal sheet parts. The use of metal sheet parts allows wide design possibilities at low manufacturing costs. Further, additional features, which are not necessarily used for the connection between the two aluminum parts, can be formed cost effectively by bending and/or cutting.

In a preferred embodiment, a metal sheet edge of a feature of the first aluminum part is welded to a metal sheet surface of a feature of the second aluminum part, wherein an adjacent metal sheet surface of the metal sheet edge of the feature of the first aluminum part is perpendicular to the metal sheet surface of the second aluminum part and wherein the first aluminum part and the second aluminum part are welded together from an opposite surface of the metal sheet surface of the feature of the second aluminum part. This design of the connection allows an easy manufacturing and the metal sheet surface of the feature of the first aluminum is protected from sparks. It also creates a smooth transition between the surfaces of the feature of the first aluminum part and the surface of the feature of the second aluminum part.

In another embodiment, a metal sheet edge of a feature of the first aluminum part is welded to a metal sheet edge of a feature of the second aluminum part, wherein an adjacent metal sheet surface of the metal sheet edge of the feature of the first aluminum part is not in contact to an adjacent metal sheet surface of the metal sheet edge of the feature of the second aluminum part and wherein the feature of the first aluminum part and the feature of the second aluminum part are welded together from a position perpendicular to the adjacent metal sheet surface of the metal sheet edge of the feature of the first aluminum part and to the adjacent metal sheet surface of the metal sheet edge of the feature of the second aluminum part. This embodiment adds to a particularly compact design.

In another embodiment, a metal sheet surface of a feature of the first aluminum part is welded to a metal sheet surface of a feature of the second aluminum part, wherein the first aluminum part and the second aluminum part are welded together from an opposite surface of the metal sheet surface of the feature of the second aluminum part. This allows easy and fast alignment and welding of the parts to be connected.

In an advantageous embodiment, at least two connections are formed between the first aluminum part and the second aluminum part, wherein each connection has a separate welding path, which can be part of one or more major welding lines, wherein the distance between the separate welding paths belonging to one major welding line is smaller than a fifth of a wavelength of a RF signal transmitted by the antenna part. The separation of the major welding lines in separate welding paths makes the manufacturing of the antenna part less energy intensive and due to the limited distance between separate welding paths of one major welding line, the connection of the first aluminum part and the second aluminum part along this major welding line does not impair the RF function. Preferably, a length of the separate welding paths along the major welding lines is in a range between 1 and 30 mm. At the lower end of the given range, the length of the separate welding line allows an adequate use of the wobble mode laser technique during manufacturing and at the higher end of the given range, the length of the separate welding line limits the energy extensiveness of the manufacturing process. Preferably, the antenna part is a housing, a shielding or another RF conducting component of a radiating element, of a reflector, of a phase shifter, of a fdter, of a combiner or divider, of a distribution network, of a wave guide, of a suspended stripline, or of other transmission line types. These are antenna parts, which are made of aluminum and benefit the most from the inventive design. In an advantageous embodiment, the antenna part is a housing for a suspended stripline module, wherein the first aluminum part, in which the suspended stripline module is placed, is three-dimensional-shaped from a raw metal sheet by cutting and/or bending and the second aluminum part is, at least in the area of the laser welding, flat. This design allows the housing to be manufactured cost effective and PIM free.

Preferably, the first aluminum part comprises at least two contact tabs, which are bent away from a plane of the metal sheet of the first aluminum part in a way that end faces of the contact tabs are welded to the second aluminum part in a single plane. The use of contact tabs makes the manufacturing process less energy intensive.

In a preferred embodiment, the housing comprises at least one resonance breaker, wherein the resonance breaker is a strip or tab, which is cut out and bent away from a plane of the metal sheet of the first aluminum part such that it protrudes towards an inner cavity between opposite sides of the housing and is welded to the second aluminum part. As the inner cavity of the housing allows forming of resonant RF modes, if a ratio between the width of the cavity and the RF wavelength exceeds a certain value, the resonance breaker is used as a countermeasure. In this embodiment, the resonance breaker is manufactured cost effectively.

In a particular embodiment, the first aluminum part and the second aluminum part form a first housing for a suspended stripline module, wherein a third aluminum part, which is a metal sheet part, is welded to the second aluminum part according to the method of one of the claims 1 to 4 and wherein the second aluminum part and the third aluminum part form a second housing for a suspended stripline module. The use of the second aluminum part as a shared part of two separate housings allows wider design possibilities for the antenna part, eases the assembly process, allows a precise alignment of the two housings and is cost efficient.

In an embodiment, at least one of the aluminum parts comprises at least one reference mark for optical recognition, which is used for in-line image recognition prior to the welding process in order to align the aluminum parts of the antenna part to each other, wherein at least one of the aluminum parts has at least one opening, through which at least one reference mark of an underlying other aluminum part can be recognized by image recognition. The reference mark allows a very precise manufacturing of the antenna part within low tolerances.

The invention further comprises a method for a bottom-up assembly process of a housing for a suspended stripline module according to one of the claims 11 to 14, wherein the method at least comprises the following assembly steps in the given order: a) providing a first aluminum part, wherein the first aluminum part is a three- dimensional shaped sheet metal part; b) placing a suspended stripline module inside the first aluminum part; c) placing a second aluminum part onto the first aluminum part, wherein the second aluminum part is flat at least in the areas contacting the first aluminum part; d) joining the second aluminum part to the first aluminum part by wobble mode laser welding according to the method claimed in one of the claims 1 to 4, wherein each step is performed without moving the previously placed parts until the laser welding step is finished. The method is a cost effective way of manufacturing housings for a suspend stripline module, which are PIM free.

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings, wherein:

Figures la, b show a partial view of of an example of an antenna part according to a first embodiment of the invention diagonally from above, wherein Fig. la shows an exploded view and Fig. lb shows the aluminum parts in their designated position,

Figures 2a, b show a detailed view of the antenna part from Figures la and lb, wherein Fig. 2a shows a lateral side of the detailed view and Fig. 2b shows the top side of the detailed view,

Figure 3 shows an exploded view of the antenna part, which may further include a suspended stripline module, and Figures 4a, b show a partial view of an antenna part according to the invention diagonally from above, which comprises two housings, wherein Fig. 4a shows an exploded view and Fig. 4b shows the aluminum parts in their designated position.

For the following explanations, the same parts are designated by the same reference signs. If a Figure contains reference signs that are not described in more detail in the corresponding description of the Figure, reference is made to preceding or subsequent figures.

Figures la and lb show a partial view of a first embodiment of an antenna part 1 diagonally from above. The antenna part 1 in this first embodiment is a housing, for example, for a suspended stripline module. The antenna part 1 comprises a first aluminum part 2 and a second aluminum part 3. The first aluminum part 2 and the second aluminum part 3 are metal sheet parts, i.e. parts made from aluminum sheet metal. The aluminum sheet metal (and hence the metal sheet parts 2 and 3) may be made of an aluminum alloy. For example, the aluminum alloy may be selected from the group consisting of A15052, AlMg3, AlMgl.5, A16063, AiSi9, ADC12, orpure Al. The first aluminum part 2 has been shaped into a three-dimensional form, e.g. by bending a piece that was cut from a raw metal sheet. In the shown example, the first aluminum part 2 comprises contact taps 4 extending along two parallel edges of the aluminium part 2. The contact taps 4 are bent away from a principal plane 8 of the metal sheet from which the first aluminum part 2 has been made. The contact taps 4 comprise end faces 5 located in a single, common plane. The end faces 5 are in contact with the second aluminum part 3. The second aluminum part 3 is generally flat and forms an inner cavity together with the first aluminum part 2. The inner cavity may accommodate one or more modules, e.g. a suspended stripline module. Both aluminum parts 2, 3 may have additional features 26, which may be formed by cutting and/or bending as shown for example in Fig. 4a. The first aluminum part 2 and the second aluminum part 3 are welded together along the end faces 5 of the contact taps 4 and the contacting surface 14 of the second aluminum part 3 using laser welding. The welding is done separately for each contact tab 4 along two parallel welding paths 6. The two welding paths 6 produce two major welding lines 7. In order to have a PIM free connection between the two aluminum parts 2, 3, the laser welding has to be done according to the method described in more detail below with regard to Figs. 2a and 2b. Figures 2a and 2b schematically illustrate a manufacturing process in which the first aluminum part 2 and the second aluminum part 3 are joined together by laser welding to form the antenna part 1.Figure 2a and figure 2b are a schematic side view and a schematic top view, respectively. The welding process comprises applying a laser beam 9 to each of the end faces 5 and moving a point of incidence of the laser beam 9 along the welding path 6 in a wobble mode 11. The point of incidence is the point where the laser beam 9 strikes the respective target, i.e. where it strikes the respective end face 5. The wobble mode 11 is a fast periodic motion of small amplitude (e.g. back-and-forth or cyclic or elliptic) which is added to (i.e. superposed with) the substantially rectilineal movement of the laser beam along the welding path 6. The wobble movement may be generated at the laser source, e.g. by continuously pivoting the laser source.. By this laser welding technique, the end faces 5 of the first aluminum part 2, which are shown in figure 2b in dotted lines, are welded to the second aluminum part 3. The resulting connection between the two aluminum parts 2, 3 is substantially PIM free, which means that the PIM level is below - 150 dBc. Certain techniques are implemented to achieve a PIM level as low as possible at the welding connections. The welding with the wobble mode 11 starts at a first end 12 of the welding path 6, is carried out to a opposite second end 13 of the welding path 6 and back to the first end 12. This movement is called a back-and-forth stroke 10 and causes the welding to be performed twice at each point of the welding path 6. This is preferred, as the penetration depth of the laser may be too small at the beginning of the laser welding process and the back-and-forth stroke 10 can achieve complete welding of the connection at every point of the welding path 6. The speed of the back-and-forth stroke 10 may be in a range of 10 to 40 mm/s and the speed of the wobbling movement 11 may be in the range of 300 to 700 mm/s, for example. By moving the laser beam 9 with a diameter smaller than 7 pm at this speed, the applied power, which should be in the range of 500 W to 4000 W, of the laser beam 9 is distributed advantageously. Further, the welding is performed from a welding surface 15 of the second aluminum part 3 opposite to the contacting surface 14 of the second aluminum part 3, which is in contact with the end face 5 of the first aluminum part. The distance between these two parallel surfaces 14, 15 is given by the thickness 16 of the second aluminum part 3 and is in the range of 0.5 to 3.5mm. Welding the end faces 4 and the contacting surface 14 indirectly together from the welding surface 15, has the advantage, that no sparks are generated at the first aluminum part 2 and the contacting surface 14 of the second aluminum part 1. Further sensible parts like suspended stripline modules can be placed between the first aluminum part 3 and the contacting surface 14 before the interconnection of the aluminum parts 2, 3 without being damaged by the interconnection process. As the distance between the tabs 17 is dictated by the enclosed wave length of the antenna part and must be smaller than a fifth of the enclosed wavelength, the length 18, the height 19 and the width 20 can be chosen to form a mechanically stable but energy-efficient connection between the two aluminum parts 2, 3. The length of the taps 18, which corresponds to the length of the separate welding paths 6, is therefore in the range between 1 to 30 mm.

Figure 3 shows an exploded view of the embodiment of antenna part 1, which is shown in the figures la, lb, 2a and 2b. In addition to the first aluminum part 2 and the second aluminum part 3, figure 3 shows the suspended stripline module 21 and a spacer 22. To completely assemble the antenna part 1, which is a housing for the suspended stripline module, the first aluminum part 2 is placed with the end faces 5 of the contact taps 4 facing upwards. Then the spacer 22 is placed onto the general plane 8 of the first aluminum part 2 in between the two rows of contact taps 4. The spacer 22 fixes the position of the suspended stripline module 21 inside the antenna part 1. The suspended stripline module 21 is then placed onto the spacer 21 without moving the already placed parts. After that, the flat second aluminum part 3 is placed on top of the end faces 4 of the contact taps 5 of the first aluminum part 2. In a last step, the first aluminum part 2 and the second aluminum part 3 are welded together by the method according to the invention as explained above in more detail.

Figures 4a and 4b show a partial view of an embodiment of the antenna part 1 diagonally from above, which is a second embodiment of the invention. The shown antenna part 1 comprises two housings for suspended stripline modules. The first aluminum part 2 and the second aluminum part 3 together form a first housing 23. A third aluminum part 24, which is a metal sheet part and formed like the first aluminum part 2, and the same second aluminum part 3 together form a second housing 25. The first aluminum part 2 and the third aluminum part 24 are interconnected to the second aluminum part 3 according to the method by the invention. As the first aluminum part 2 and the third aluminum part 24 both comprises contact taps 4, which are spaced apart from each other, the suspended stripline modules inside the two housings 23, 25 may be interconnected by a stripline extending through an opening between the tabs. In addition to the contact taps 5, the first aluminum part 2 and the third aluminum part 24 comprise resonance breakers 26, which are tabs that are cut out and bent away from a general plane 8 of the first aluminum part 2 and the third aluminum part 24, such that the end faces of the resonance breakers 26 contact the second aluminum part 3 in the same plane as the end faces 5 of the contact taps 4. The end faces of the resonance breakers 26 are welded to the second aluminum part 3 by the same method as the end faces 5 of the contact taps 4 of the first aluminum part 2 and third aluminum part 24. The resonance breakers 26 are used to prevent the forming of resonant modes, which may occur, if a ratio between the width of the cavity of the housings 23, 25 and the encapsulated RF wavelength exceeds a certain value. The first aluminum part 2 and the third aluminum part 24 further comprise reference marks 27 in the form of one contact tap. The reference marks 27 are aligned to the openings 28 in the second aluminum part 3 by using an automatic alignments system and automatic image recognition system. This way the aluminum parts can be aligned to each other and the laser welding can be precisely performed indirectly.

List of reference signs

1. Antenna part

2. first aluminum part

3. second aluminum part

4. contact tabs

5. end faces

6. welding path

7. major welding line

8. general plane

9. laser beam

10. back-and-forth stroke

11. wobble mode

12. first end of the welding path

13. second end of the welding path

14. contacting surface

15. welding surface 16. thickness of second antenna part

17. distance between the separate welding paths

18. length of the separate welding paths

19. height of taps 20. width of taps

21. suspended stripline module

22. spacer

23. first housing

24. third aluminum part 25. second housing

26. resonance breaker

27. reference mark

28. opening