EP3249741B1

EP3249741B1 - Device for the connection between a strip line and a coaxial cable

Info

Publication number: EP3249741B1
Application number: EP16305603.9A
Authority: EP
Inventors: Aurélien Hilary; Patrick Lecam; Jean-Pierre Harel; Zied Charaabi
Original assignee: Nokia Shanghai Bell Co Ltd
Current assignee: Nokia Shanghai Bell Co Ltd
Priority date: 2016-05-24
Filing date: 2016-05-24
Publication date: 2020-02-26
Anticipated expiration: 2036-05-24
Also published as: EP3249741A1

Description

FIELD OF INVENTION

The present subject matter relates to a device for the connection between a strip line and a coaxial cable. The strip line is realized on a printed circuit board. This device allows the transition between a coaxial cable and a suspended stripline and is also known as junction.

BACKGROUND

Base station antennas are built with arrays of several radiating elements, connected to distribution networks (power dividers, phase shifters) with transmission lines. It is also known the US patent applications references US2003/218515 and US2012/244727 .
The market trends to require more and more complex antennas for example dual polarization antenna, multiband band antenna, MIMO arrays. In these very complex antennas the radiating elements are almost always connected to the distribution network using coaxial cables because it is impossible to interleave several network lines with technologies like strip lines, or PCB micro strip lines with acceptable performances.
However the most efficient distribution network technology is strip line, which is a fully shielded transmission line and preferably air strip line, as air is the minimum loss and cheapest dielectric available. Patent application US2003218515 relates to a radio frequency circuit module, formed by mounting circuit elements on a circuit substrate having dielectric layers for a module and a communication apparatus using the radio frequency circuit. Patent application US2012244727 relates to a device for the mechanical and electric connection between a coaxial cable conveying a high frequency signal and a circuit comprising multilayer microstrip or stripline lines. Patent US3587004 relates to contradirectional couplers and, more particularly, to transmission line power dividers employing contradirectional couplers. Patent US8362856 relates to a radio frequency (RF) transition for a three dimensional molded RF structure. More specifically, the invention relates to an RF transition from a microstrip transmission line to a suspended substrate transmission line used in conjunction with the RF structure.
The problem, solved by the preset subject matter, is to design an efficient junction between the strip line distribution network sub-assemblies and the coaxial cables driving the signals to the radiating elements of the antenna array.
The device proposed in this divulgation will have the following advantages:

to ensure low Passive Inter Modulation performances (PIM) for the junction
to propose a very low cost junction, because in such complex antennas there is a lot of these connections (for example, in a penta band antenna there is hundreds of these connections) .

SUMMARY

An apparatus and a method for the connection between a strip line and a coaxial cable are disclosed as defined in the appended claims.

BRIEF DESCRIPTION OF THE FIGURES

The detailed description is given with reference to the accompanying figures. In the figures, the left-most digit(s) of a reference number identifies the figure in which the reference number first appears. The same numbers are used throughout the figures to reference like features and components. Some embodiments of system and/or methods in accordance with embodiments of the present subject matter are now described, by way of example only, and with reference to the accompanying figures, in which:

Figure 1 presents a device for the connection between a strip line and a coaxial cable
Figures 2-a to 2-d present in another way the device for the connection between a strip line and a coaxial cable

In the present document, the word "exemplary" is used herein to mean "serving as an example, instance, or illustration." Any embodiment or implementation of the present subject matter described herein as "exemplary" is not necessarily to be construed as preferred or advantageous over other embodiments.

DESCRIPTION OF EMBODIMENTS

In an prior art solution the junction of coaxial cables to strip lines is realized by using metal plates on which the cable braid can be soldered directly for example be brass, copper. Provided that a good soldering process is used then the PIM performances are good, but the main problem regarding this solution is material cost. Huge brass or copper plates are necessary, and these materials are expensive (three times aluminum cost, and the trend is regular increase of these copper alloys cost).
In another prior art solution the junction of coaxial cables to strip lines is realized by using metal plates on which the cable cannot be soldered, for example aluminum. In this case, the two ways of doing the junction are:

The cable braid has a direct contact with the 2 ground plates of the strip line via a metal grounding interface soldered on the coaxial cable, and this interface is tightened between the 2 ground plates of the strip line with screws or studs.
The cable braid is soldered on a grounding interface, and this grounding interface is virtually in contact with the two ground plates of the strip line by capacitive coupling.

The main problem with these 2 solutions is the cost, because the quality of the parts and of the assembly must be very high to avoid PIM generation. Indeed from these conventional methods of connection, either expensive material strip line ground plates associated with critical soldering process, either interfaces with several parts with significant assembly time are used.
Moreover, for both solutions, the assembly process is not optimal: either soldering operation on brass plates is rather long, either assembling several interface parts together takes also some time. The consequence is that the production line capacity is lowered.
Figure 1 presents an illustrative example of a device for the connection between a strip line 101, realized on a first part of a printed circuit board 102, and a coaxial cable 103. The coaxial cable 103 includes an inner conductor 104 and an outer conductor 105. The device comprises a cover top 106 and a cover bottom 107 respectively located over and under the printed circuit board. Within this device a second part of the printed circuit board 102 is inductively coupled with the cover top 106 and the cover bottom 107. The inner conductor 104 is electrically connected to the strip line 101 and the outer conductor 105 is inductively or electrically connected to the second part of the printed circuit board. Regarding the connection of the outer conductor 105 to the second part of the printed circuit board the best results are achieved by using a inductive connection.
The device of this illustrative example allows cheap material for the strip line ground plates with no solder on these plates. This illustrative example also avoids adding extra parts to reduce cost and PIM performances. This illustrative example reduces assembly time to improve production line capacity.
In an embodiment of the device the second part of the printed circuit board is cover by a metal layer on both side of the printed circuit board and

the two metal layer are connected via a plurality of via and
the two metal layer are covered by a insulating layer and
the cover top comprises a flat part in contact with the insulating layer in one side of the printed circuit board and
the cover bottom comprises a flat part in contact with the insulating layer in the other side of the printed circuit board.

In an embodiment of the device at least one of the insulating layers is constituted of a insulating varnish.
In an embodiment the second part is located around the strip line.
The figures 2-a to 2-d present an embodiment according to the invention. As presented by these figures a coaxial cable is connected to a strip line as. The whole assembly comprises the coaxial cable, the strip line conductor, the top and bottom covers. The strip line conductor is printed on a thin PCB board. The top and bottom covers are made with bended plates of low cost material, for example aluminum. The top cover is capacitively coupled trough the PCB top varnish layer to the PCB metal coupling area (top side). This PCB metal coupling area is connected to the PCB bottom coupling area by PCB metalized via. This PCB bottom coupling area is also capacitively coupled through a PCB varnish layer to the bottom cover. The link to the coaxial cable is made by soldering the braid on the PCB top metal coupling area. The inner conductor of the cable is soldered on the strip line conductor. A window is shaped in the top cover in front of the cable to avoid random contact.
In an embodiment the top and bottom covers and the printed circuit board are be maintained together by very low cost plastic rivets (not represented on the figures). As the plates use capacitive coupling, the contact pressure required is much less critical than direct contact for good PIM performances.
Another embodiment of the present subject matter is an antenna comprising one of the connection devices previously described.
In an embodiment of the antenna at least one connection device is used to connect radiating elements and/or feeding networks and/or splitters and/or phase shifters and/or filters and/or active boards and/or any other RF devices implemented within the antenna.
The connection device of the present subject matter can also be used for any kind of device requiring a coaxial cable to stripline connection, for example filters, amplifiers, etc..

Claims

A device for connection between a strip line and coaxial cable, comprising;
a printed circuit board (102) comprising a strip line (101), a top coupling area and a bottom coupling area,
a coaxial cable (103), including an inner conductor (104) and an outer conductor (105), the inner conductor electrically connected to the stripline and the outer conductor formed by a braid soldered to the top coupling area,
a cover top and a cover bottom respectively located over and under the printed circuit board,
wherein the cover top is capacitively coupled through a first insulating layer to the top coupling area and the cover bottom is capacitively coupled through a second insulating layer to the bottom coupling area and the cover top is in contact with the first insulating layer on one side of the printed circuit board and the cover bottom is in contact with the second insulating layer on the other side of the printed circuit board.
A device according to claim 1 wherein the top coupling area is connected to the bottom coupling area by metalized vias.
A device according to any preceding claim wherein, the top and bottom covers and the printed circuit board are maintained together by plastic rivets.
A device according to any preceding claim wherein the top and bottom cover are bended plates of aluminium.
An antenna comprising the device of any of the preceding claims.
An antenna according to claim 5 further comprising any one or more of a filter, splitter, phase shifter, active board or RF device.
A method for connecting a stripline and a coaxial cable comprising,
locating a cover top (106) and a cover bottom (107) over and under a printed circuit board (102) respectively and placing a flat part of the cover top on a insulating layer and placing a flat part of the cover bottom on a insulating layer of said printed circuit board,
electrically connecting an inner conductor (104) of the coaxial cable to the strip line (101) on a first part of the printed circuit board, soldering an outer conductor (105) of the coaxial cable formed by a braid to a top coupling area on a second part of the printed circuit board,
maintaining together the cover top, the cover bottom and the printed circuit board by plastic rivets.