EP0358529A1

EP0358529A1 - Antenna for mounting on a non-conductive surface, such as a window of a vehicle

Info

Publication number: EP0358529A1
Application number: EP89309161A
Authority: EP
Inventors: Kevin Davies
Original assignee: BANTEX Ltd
Current assignee: BANTEX Ltd
Priority date: 1988-09-09
Filing date: 1989-09-08
Publication date: 1990-03-14
Also published as: GB8821171D0; WO1990003048A1; AU4205389A

Abstract

A communications antenna for mounting on a non-conductive surface, such as a window (20), of a vehicle, in which signals are passed between a transmission line (10) and the radiator element (21) of the antenna via a capacitive element (C3) comprising two conductive coupling elements (16, 17) disposed on opposite surfaces (18, 19) of the window (20). The transmission line (10) is coupled to the conductive coupling member (16) on the interior surface of the window (20) by a tuned circuit (L1 C1) and a further capacitative element (C2) which match the impedance of the transmission line (10) and maximises the transmission of signals between the antenna and the transmission line.

Description

The present invention relates to an antenna and particularly to a communications antenna for mounting on a non-conductive surface such as a window of a vehicle.
Communications antennae for vehicles are now well known and various constructions have been proposed. Nowadays, for motor vehicles, it is frequently preferred to mount a communications antenna on a window and utilize capacitive coupling of the signal through the glass of the window rather than mount the antenna on a body panel which requires a hole to be drilled through the panel.
Despite the number of different constructions which have been proposed, there still exists a need for an antenna whose impedance can be readily matched to the impedance of the transmission line and which gives maximum coupling of the signal through the glass or other non-conductive surface on which the antenna is mounted. In the past, the physical length of the antenna was an important consideration due to the wavelength of the signals to be received or transmitted. This resulted in the use of electrically shortened radiators. This in turn meant that an impedance matching circuit had to be introduced in order that the impedance of the radiator would be matched to the impedance of the transmission line to which it was connected.
One such approach is disclosed in US 4,238,799 where a 1/2 λ inductively loaded radiator is is matched to a 50Ω transmission line by means of a parallel LC circuit. The parallel LC circuit is disclosed as matching to the impedance of 100,000 Ω to a 50 Ω transmission line which of necessity introduced losses. One further point to be noted in the disclosure of US 4,238,799 is that it is an on-glass antenna but in this case, the impedance of the capacitance formed by inner and outer transfer members fixed to the interior and exterior respectively of a window is used as an integral part of the radiator design and is adjustable in order that the radiator turning can be varied. It is further clearly stated that the LC circuit should be permitted to radiate in order for the antenna to function properly.
The dual function of the LC circuit results in a degree of compromise in a practical design with a resulting degradation in performance.
It is an object of the present invention to provide a communications antenna for mounting on a non-conductive surface in which the transmission of a signal between the antenna and a transmission line is maximised.
The present invention provides a communications antenna for use on a vehicle comprising a tuned circuit arrangement to be interposed between the conductors of a transmission line and a first conductive coupling member arranged to be mounted on one surface of a non-conductive member, on the other surface of which a second conductive coupling member is arranged to be mounted, the tuned circuit comprising an LC circuit arranged to match the impedance of the transmission line, and a further capacitive member for maximising the coupling between the conductive coupling members mounted on the surfaces of the non-conductive member.
The further capacitive member is dimensioned to provide a signal at a point adjacent the first coupling member which is in phase with the signal on the first conductive coupling member whereby to reinforce the field in the region of the coupling members.
The radiator element is connected to the second coupling member and has a length preferably both physically and electrically 1/4λ or an odd integer multiple thereof. Such an arrangement can be used at 900 MHz due to the small wavelength at that frequency.
In order that the present invention be more readily understood, an embodiment thereof will now be described by way of example with reference to the accompanying drawings in which:-

Figure 1 shows an equivalent circuit diagram of an antenna according to the present invention;
Figure 2 shows diagrammatically the constructions of the antenna shown in figure 1; and
Figure 3 shows one way of arranging capacitive elements forming part of the antenna shown in Figure 2.

An equivalent circuit diagram of an antenna arrangement according to the present invention is shown in Figure 1. A co-axial transmission cable 10 comprises an outer, earthed conductor 11 and an inner signal carrying conductor 12. An impedance matching circuit is connected to the transmission cable 10 and comprises an inductance L1 one end 14 of which is earthed and the other end 15 of which is connected to one plate 16 of a coupling capacitance C3. A capacitance C1 is connected in parallel with the inductance L1. A further capacitance C2 is provided between the plate 16 of the coupling capacitance C3 and earth in order to optimize the value of the coupling capacitance C3 to provide in phase signals for maximising signal transfer through the coupling capacitance C3. The need to optimise the coupling capacitance comes about due to the fact that the area of the plate 16 is determined by the capacitance required to provide the correct matching impedance for connection to the transmission line. Further, the thickness of the glass, i.e. the separation of the plates constituting the coupling capacitance C3 is predetermined. Unfortunately, if one were to simply attach an outer coupling member of the same or larger area than the area of the inner coupling member, the capacitance would not be correct for efficient signal coupling.
The inner conductor 12 is connected to the inductance L1 at a position intermediate the ends of the inductance L1 so as to match the impedance at the base of a radiator 21 to the impedance of the transmission cable 10 at the desired resonant frequency of the antenna.
Referring now to Figure 2, this shows diagrammatically how to implement the circuit diagram shown in Figure 1. For simplicity the same reference numerals have been used to represent the same parts.
Reference numeral 20 indicates the glass on one surface 18 of which is attached a planar member. At least the surface of the member adjacent the glass 20 is conductive and forms the plate 16 of the capitance C3 whose other plate 17 is affixed to the external surface 19 of the glass 20. The radiator 21 is attached to the plate 17.
The capacitance C1 is formed by the conductive member constitutes the p late 16 and by a further planar member 22 at least one surface of which is conductive.
The inductance L1 is formed by a wire extending between the plate 16 and member 22.
The capitance C2 is formed by the conductive surface of the planar member forming the plate 16 and by a conductive foil 24 extending from one edge of the member 22 towards the edge of the plate 16 but separated therefrom by a distance suitable to establish the correct value for the capitance C2. The foil 24 has a portion 25 which extends in the plane 16. The length, breadth and thickness of the foil 24 are chosen to provide not only the correct value for the capitance C2 but also the correct electrical length of the path from the connection point 26 between the carrier conductor 12 and the inductor L1 along the member 22 and through the foil 24.
The extent of the portion 25 is not critical for co-phasing signals at the edges of the plate 16 and foil 24 but is designed to ensure that the tuned circuit exhibits the required high impedance to signals attempting to flow back to the outer conductor 11 of the transmission line.
This arrangement of tuned circuit will support a radiator 21 which is both physically and electrically 1/4 λ in length or the usual radiator combinations which are an odd integer multiple thereof in order to achieve the desired gain. Because the radiator 21 is 1/4 λ the bottom of the radiator is current fed and is at a low impedance, typically less than 50Ω.
By way of example, for 900 MHz the wire forming the inductor L1 may be a 6mm length of 18 SWG tinned copper wire, the plate 16 and member 22 may be single sided printed circuit boards of size 35 x 20 mm and the end portion 25 of the foil 24 may extend from the edge of the plate 16 by a distance of 10 mm.
Figure 3 shows a housing 29 for the circuit and comprises a surface 30 having a first opening 31 for the plate 16 and a further opening 32 for the portion 25 of the foil 24. The two openings are separated by a wall which stabilizes the cap between the edge of the plate 16 and the foil 24. In use, the surface 30 is provided with an adhesive layer and is attached to the inner surface of a window of a vehicle.
The above arrangement thus has the advantage that the antenna is matched to the transmission line and the transmission of the signal through the glass is maximised. Further, because it is not a ground plane type antenna the amount of restriction to rear view or distraction to the driver is reduced to a minimum. Ground plane antenna either have short stubs which are unsightly and restrict vision or else ground mats in the forms of adhesive strips of foil stuck to the glass which not only restrict vision but also render the antenna difficult to install.

Claims

1. A communications antenna for use on a vehicle, comprising:
a foil conductive coupling member (16) arranged to be mounted on one surface (18) of a non-conductive member (20);
a second conductive coupling member (17), arranged to be mounted on another surface (19) of the non-conductive member (20) opposite the first conductive coupling member (16);
a radiator element (21) connected to the second coupling member;
a tuned circuit (L1 C1), arranged to be intersposed between the conductors (11, 12) of a transmission line (10) and the first conductive coupling member, the tuned circuit comprising an inductance-capacitance circuit (L1 C1) arranged to match the impedance of the transmission line; and
a further capacitative member (C2), for maximising the coupling between the first and second conductive coupling members.

2. An antenna according to claim 1, wherein:
the further capacitative member (C2) is dimensioned to provide a signal at a point adjacent the first conductive coupling member (16) which is in phase with the signal on the first conductive coupling member, whereby to reinforce the electric field in the region of the first and second conductive coupling members (16, 17).

3. An antenna according to claim 1 or 2, wherein:
the first conductive coupling member (16) is connected to the junction of the inductance (L1) with the capacitance (C1) in the tuned circuit.

4. An antenna according to any of t he preceding claims, wherein:
the further capacitative member (C2) is connected in parallel with the tuned circuit (L1 C1).

5. An antenna according to any of the preceding claims, wherein:
the capacitor (C1) of the tuned circuit comprises a first conductive member (22), spaced apart from the surface (18) of the non-conductive member on which the first conductive coupling member (16) is mounted, the first conductive coupling member, and the medium disposed therebetween.

6. An antenna according to claim 5, wherein:
the further capacitative member (C2) comprises a second conductive member (24), extending between the first conductive member (22) and the non-conductive member (20), the first conductive coupling member (16), and the medium disposed therebetween.

7. An antenna according to any of the preceding claims, wherein:
the second conductive (24) member has an edge portion (25) of pre-determined length, at the edge remote from the first conductive member (22), which is substantially parallel to the first conductive coupling member (16) and spaced at a predetermined distance therefrom.

8. An antenna according to any of the preceding claims, wherein:
the first conductive member (22) comprises a printed circuit board (PCB), the PCB having a conductive layer on at least one surface thereof.

9. An antenna according to any of the preceding claims, wherein:
the first conductive coupling member (16) comprises a printed circuit board (PCB), the PCB having a conductive layer on at least one surface thereof.

10. An antenna according to any of the preceding claims, wherein:
the inductor (L1) of the tuned circuit comprises a length of tinned copper wire.

11. An antenna according to claim 9, wherein:
the signal carrying conductor (12) of the transmission line is connected to the inductor at a pre-determined point (26) intermediate the ends (14, 15) of the wire.

12. An antenna according to any of claims 7 to 11, wherein:
the second conductive member (24) comprises a conductive sheet of pre-determined thickness.

13. An antenna according to claim 13, wherein:
the conductive sheet comprises metal foil.