WO2005091431A1

WO2005091431A1 - Matching element for mobile antenna

Info

Publication number: WO2005091431A1
Application number: PCT/GB2005/001117
Authority: WO
Inventors: Andrew Jesman; Christopher Jesman
Original assignee: Andrew Jesman; Christopher Jesman
Priority date: 2004-03-16
Filing date: 2005-03-16
Publication date: 2005-09-29
Also published as: GB0405865D0

Abstract

A matching element for a mobile antenna includes a coupling box having a coaxial connector, a circuit board provided within the coupling box and a central conductor provided within a coaxial screen for shielding the central conductor. The central conductor extends between the circuit board and the coaxial connector. The coaxial screen is electrically connected at one end to an outer shield of the coaxial connector and thence to the coupling box, and the other end terminates adjacent the circuit board. The central conductor with the coaxial screen are led along an indirect route within the coupling box, the route being configured in relation to conductive patterns of the circuit board so as to minimise interaction between electrical fields that, in use, are radiated within the coupling box, thus minimising signal losses.

Description

MATCHING ELEMENT FOR MOBILE ANTENNA

This invention relates to a matching element for a mobile antenna, more especially an antenna of the kind intended to be attached to a glass screen of a motor vehicle.

Antennas of the kind mentioned above are well known and generally comprise a radiating antenna element adhesively secured to the exterior of a glass screen, and a coupling pad adhesively secured to the interior of the screen in facing relationship to the antenna element. Capacitative coupling between the external antenna and the internal pad is provided by overlapping conductive plates arranged externally and internally of the glass screen. The internal coupling pad should also incorporate an impedance matching circuit for matching the antenna to the conventional 50 ohm coaxial cable.

In a particular simple manner, the impedance matching circuit can be provided by conductive patterns on a double sided circuit board that also provides the capacitative plate to be arranged internally of the glass screen. Overlapping conductive patterns on the two sides of the circuit board have the effect of providing both inductance and capacitance of the required matching circuit. Careful configuration of the conductive patterns on the two sides of the board is effective in providing the required circuit, and signals for coupling to the mobile antenna may be coupled thereto by means of a coaxial cable the conductors of which are either directly soldered to the board, or connected thereto via a coaxial connector provided in an external housing and coupled to the board internally of the housing.

In US-A-4882592, for example, the circuit board is arranged within a conductive shell providing a ground plane. The conductive shell is provided with a coaxial connector for a signal cable, and the connector provides coupling between a central conductor of the cable, and the circuit board, by means of an elongated conductor extending within the conductive shield in spaced relation to the circuit board. The elongated conductor is not shielded, and therefore forms part of the impedance matching circuit so that dimensions and positioning of the conductor relative to the circuit board is critical.

In EP-A-0456350, the double sided circuit board is contained within a non- electrically conductive housing and a coaxial signal cable is directly connected to the circuit board without the need for an intermediate connector. The configuration of the circuit board is such that the conductors of the coaxial cable have a minimal influence upon the performance of the matching circuit, which is essentially defined by the shape and arrangement of the conductive patterns of the two sides of the circuit board.

The present invention concerns an improvement in a matching element of the kind described in EP-A-0456350. Which is intended to enable the use of such a circuit board at higher frequencies.

As the frequencies applied to such a matching circuit increase, the performance of the circuit becomes increasingly influenced by the electric fields radiated from the circuit boards, to the extent that shielding of the circuit board by means of an electrically conductive housing becomes essential. However, when the circuit board is enclosed within an electrically conductive shield, there is a problem in achieving an electrical connection between a signal cable and the conductive patterns of the circuit board, since at the very high frequencies involved, for example 1900 to 2170 MHz it is much more difficult to avoid interaction between electrical fields within the shielded housing and the central conductor of the electrical cable than in the case of hitherto known designs that were intended to operate at lower frequencies such as the 800 to 900 MHz signal band. In an ideal arrangement, it would be possible to avoid interaction between radiated electrically fields and the central cable conductor by routing the central cable conductor to its connection point on the circuit board along a path that is essentially perpendicular to the plane of the circuit board. The difficulty is, however, that such an arrangement would not be acceptable for a matching circuit to be connected to the screen of a motor vehicle, as aesthetic considerations require that the coaxial cable extend laterally from the connector box in a direction parallel to the vehicle screen.

Accordingly, it is an object of the present invention to provide a solution to the above-mentioned problem.

According to the present invention there is provided a matching element for a mobile antenna, including: a coupling box having a coaxial connector; a circuit board provided within the coupling box; and a central conductor provided within a coaxial screen for shielding the central conductor, the central conductor extending between the circuit board and the coaxial connector, the coaxial screen being electrically connected at one end to an outer shield of the coaxial connector and thence to the coupling box, and the other end of the coaxial screen terminating at a free end adjacent the circuit board, wherein the central conductor together with the coaxial screen are led along an indirect route within the coupling box, the indirect route being so configured in relation to conductive patterns of the circuit board so as to minimise interaction with electrical fields that, in use, are radiated within the coupling box.

Preferably, the central conductor and the coaxial screen are guided to follow the indirect route by means of a former of dielectric material mounted between the circuit board and the coupling box, the former defining a required path for the central conductor and the coaxial screen.

Preferably, the indirect route includes two substantially right-angled turns.

Preferably, the central conductor and the coaxial screen are guided from the coaxial connector and then turned through a right angle towards an upstanding part-cylindrical portion of the coupling box and then turned through a further right angle towards the circuit board, as the indirect route. Preferably, the former has a channel that closely surrounds the coaxial screen between the coaxial connector and the upstanding part-cylindrical portion of the coupling box.

Preferably, the former further has an integral boss shaped so as to engage around the upstanding part-cylindrical portion of the coupling box and the boss has a bore through which the central conductor can pass through to the circuit board.

The invention is illustrated by way of example in the accompanying drawings, in which;

Figure 1 is an exploded view of the components of a matching element in accordance with one embodiment of the invention, and

Figure 2 is a view showing the two opposite sides of a two layer printed circuit board and the arrangement of electrically conductive patterns thereon.

Referring to Fig. 1 of the drawings, a matching element for a mobile antenna in accordance with one embodiment of the invention comprises a coupling box 1 formed as a diecasting from suitable electrically conductive material, in one side wall of which is provided a coaxial connector 2 which may, for example, be of so called SMA type, providing an outer shield that is electrically connected to the coupling box 1 and a central conductor that is housed within and electrically insulated from the outer shield. In known manner, the central conductor is connected to receive a coaxial electrical connector of a signal cable for the conduction of signals to be transmitted to the mobile antenna. The coupling box 1 has integrally formed therein a pair of bosses 3 having screw threaded bores 3A for receiving mounting screws 4. A printed circuit board 5 comprises a substrate of suitable materials such as glass fibre bearing on each of its two opposite surfaces electrically conductive copper tracks to be referred to in more detail below. The circuit board 5 has a pair of mounting holes 6 for receiving the mounting screws 4 and by means of which the circuit board 5 can be secured firmly against contact surfaces provided by the bosses 3. In addition, the edges of the circuit board are located by means of abutments 7 formed integrally spaced around the internal periphery of the coupling box 1. The circuit board 5 is thus supported in a position with the upper surface as viewed in Fig. 1 flush with the peripheral margin of the coupling box 1. In known manner, the upper surface of the circuit board 5 is covered by means of a double sided self adhesive pad 8 the upper surface of which as viewed in Fig. 1 is provided with a peelable film covering the upper self-adhesive layer.

The circuit board 5 has, at a precisely defined point determined in accordance with the design of its conductor tracks, an aperture 9 providing a solderable connection for a central conductor 10 or a coaxial cable 11 providing coupling between the circuit board the coaxial connector 2. The coaxial cable 11 is of semi-rigid construction and comprises the central conductor 10 that is spaced from a coaxial screen (not illustrated), by means of an insulating core 12 and an outer electrically insulating sheath that covers the coaxial screen in the conventional manner.

The central conductor and the coaxial screen of the cable 11 are electrically connected to the corresponding central conductor and sheath of the electrical connector 2 and, whereas the central conductor 10 is electrically connected to the circuit board 5 by soldering at the point 9, the coaxial screen terminates short of the circuit board and remains unconnected thereto. Electrical connection between the coaxial screen and the circuit board 5 is provided via the connector 2 and the body of the coupling box 1 , the bosses 3 of which are held in engagement with electrically conducted tracks of the circuit board 5, by means of the mounting screws 4.

In order to minimise interaction between the central conductor 10 of the coaxial cable 11 and electrical fields that are radiated from the circuit board 5 in use, the coaxial cable 11 is routed along a precisely determined indirect path, as illustrated in Fig. 1 , the cable extending between the connector 2 and an upstanding part-cylindrical portion 13 formed integrally with the coupling box 1. As can been seen from Fig. 1 the cable 11 passes from an outlet of the connector 2 in a linear direction and is then bent through a right angle towards the portion 13 of the box 1. The intermediate portion of the cable between the connector 2 and the portion 13 is maintained as close as possible to the base of the coupling box 1 before being turned through a further right angle so that it meets the circuit board 5 in a direction perpendicular thereto. It has been found, in practice, that with the preselected patterns of the conductor tracks on the circuit board 5, the illustrated position of the coaxial cable 11 is routed as far as possible through dead space that exists within the coupling box when in use, so that the coaxial cable 11 is effectively invisible to the coupling circuit provided by the circuit board 5, thus minimising signal losses due to interaction between radiated electrical fields within the coupling box 1 and the central conductor 10 of the cable.

In order to ensure that a predefined path of the electrical cable 11 is maintained in use, there is provided between the circuit board 5 and the base of the coupling box 1 a former 14 formed of dielectric material such as a polyamide. The underside of the former 14 has a channel, not illustrated in the drawing, which closely surrounds the circumference of the cable 11 and extends along the predefined path between the connector 2 and the portion 13 of the coupling box. The former 14 further has a recess 14A at one end of the channel for engaging the portion of the cable 11 that passes downwardly from the connector 2 towards the base of the coupling box 1 , and has at the other end of the channel an integral boss 14B shaped to engage around the portion 13 of the coupling box and providing a bore 14C through which the central conductor 10 can pass through to the circuit board 5. The former 14A further comprises two semi-circular recesses 14D and 14E shaped to fit around the peripheries of the bosses 3, and an upstanding peg 14F for engagement with the underside of the circuit board 5. The former 14 is thus held securely in place within the box 1 and clamped between the underside of the circuit board 5 and the base of the coupling box 1 so that the coaxial cable 11 is held securely in its desired position. The circuit board 5 and the electrically conductive tracks thereon are illustrated in more detail in Fig. 2. The circuit board is formed of a glass fibre substrate 1.6mm in thickness, and is of rectangular shape having a dimension A of 45mm and a dimension B of 45.5mm. The conductor tracks are shown as drawn to scale, and conductor tracks on the two sides of the board are electrically interconnected by means of plated through holes 15, 16 and 17. Thus a conductor track 18 on the upper surface of the circuit board is electrically connected to the central conductor 10 and forms a capacitor plate for coupling a signal through the glass of a vehicle windscreen to an externally positioned mobile antenna in known manner. Such an antenna is electrically connected to a capacitor plate that is positioned externally on the glass screen so as to overlap at least partially with the conductor track 18, the outline of the external capacitor plate being shown in broken lines at 30. The remaining conductor tracks 19 and 20 on the upper side of the board and 21 on the lower side of the board are electrically connected with the coupling box 1 and thence to the outer screen of a signal cable. Suffice it to say that the dimensions and the shaping of the respective conductor tracks are effective to provide distributed inductance and capacitance that, when in use, provides an appropriate coupling circuit for coupling the external antenna to the 50 ohm coaxial signal cable.

The illustrated arrangement has the advantage that the coupling box is effective to provide coupling to an external antenna, not only over the frequency band of 800 to 900 MHz but also over the higher, so called 3G, frequency band of 1900-2170 MHz.

Although the above described matching element provides one example of an embodiment of the invention, it will be appreciated that the invention is not limited to detailed features of the preferred embodiment, and variations and modifications may be made by one skilled in the without departing from the scope of the invention as will be defined in the claims appended hereto.

Claims

CLAIMS:

1. A matching element for a mobile antenna, comprising: a coupling box having a coaxial connector; a circuit board provided within the coupling box; and a central conductor provided within a coaxial screen for shielding the central conductor, the central conductor extending between the circuit board and the coaxial connector, the coaxial screen being electrically connected at one end to an outer shield of the coaxial connector and thence to the coupling box, and the other end of the coaxial screen terminating at a free end adjacent the circuit board, wherein the central conductor together with the coaxial screen are led along an indirect route within the coupling box, the indirect route being so configured in relation to conductive patterns of the circuit board so as to minimise interaction with electrical fields that, in use, are radiated within the coupling box.

2. A matching element for a mobile antenna according to claim 1 , wherein the central conductor and the coaxial screen are guided to follow the indirect route by means of a former of dielectric material mounted between the circuit board and the coupling box, the former defining a required path for the central conductor and the coaxial screen.

3. A matching element for a mobile antenna according to claim 1 or 2, wherein the indirect route comprises two substantially right-angled turns.

4. A matching element for a mobile antenna according to claim 1 , 2 or 3, wherein the central conductor and the coaxial screen are guided from the coaxial connector and then turned through a right angle towards an upstanding part-cylindrical portion of the coupling box and then turned through a further right angle towards the circuit board, as the indirect route.

5. A matching element for a mobile antenna according to claim 2 or either of claims 3 or 4 when dependent on claim 2, wherein the former has a channel that closely surrounds the coaxial screen between the coaxial connector and the upstanding part-cylindrical portion of the coupling box.

6. A matching element for a mobile antenna according to claim 5, wherein the former further has an integral boss shaped so as to engage around the upstanding part-cylindrical portion of the coupling box and the boss has a bore through which the central conductor can pass through to the circuit board.