Non-contact coupling through a dielectric

Abstract

 The present invention relates to a system for transmitting radio frequency energy through a dielectric (5), for example through a plastics wall of an equipment containing box, and to an antenna assembly including the system for transmitting radio frequency energy through the dielectric.
A non-contact connection through a dielectric (5) comprising a pair of counter-wound conductive spirals (1,2) provided on opposite sides of the dielectric (5), is characterised in that a pair of ground planes (8,9) sandwich the conductive spirals (1,2) and the dielectric (5).

Description

[0001] The present invention relates to a system for transmitting radio frequency energy through a dielectric, for example through a plastics wall of an equipment containing box, and to an antenna assembly including the system for transmitting radio frequency energy through the dielectric.

[0002] RF electrical equipment is often contained in a hermetically sealed equipment box. To pass RF signals to and from the equipment, it is known to provide a dedicated connector which passes through the dielectric wall of the equipment box. This is undesirable, firstly as the dedicated connector can be expensive to provide, therefore giving an unacceptable additional cost to the equipment, and secondly as the connector constitutes a weak point in the hermetic seal around the equipment.

[0003] US-A-4,238,799 discloses a non-contact connection using capacitive coupling to allow transmission of signals to and from an in-car mobile telephone antenna. In this case, the capacitive coupler is combined with an inductive susceptance to create a well matched system and thereby ensures a low level of reflected energy. The coupling of the connection is optimised by selection of an appropriate inductor.

[0004] US-A-5,105,201 discloses a glass mounted antenna for a car radio having a conductive spiral coil connected to the antenna for coupling with a second conductive spiral coil on the inside of the glass which is connected to a receiver.

[0005] According to a first aspect of the present invention, a non-contact connection through a dielectric comprising a pair of counter-wound conductive spirals provided on opposite sides of the dielectric, is characterised in that a pair of ground planes sandwich the conductive spirals and the dielectric.

[0006] With this arrangement, the counter-wound conductive spirals are inductively coupled to each other to allow transmission of signals through the dielectric. The ground planes create a capacitative coupling effect and constrain the fields in the device, resulting in a low profile device with reduced stray radiation and therefore high efficiency. The ground planes also allow greater control over the environment in which the device operates, allowing greater control of the impedances within the device so that the bandwidth may be optimised.

[0007] It is preferred that a capacitor is provided at the input of each of the spirals. This gives a low reflection coefficient which ensures low transmission losses. In this case, the connection can be matched by adjusting the capacitance of the capacitor. It is possible to provide a variable capacitance to allow the coupling to be optimised for any given dielectric substrate through which the signals are to be transmitted, and depending on the frequency of the signals to be transmitted. However, due to the large bandwidth for which the system is preferably designed, the system is relatively insensitive to the dielectric properties of the dielectric, and accordingly a standard capacitance is acceptable.

[0008] Advantageously, the two counter-wound conductive spirals have the same configuration. The conductive spirals advantageously have either a rectangular, square or circular configuration.

[0009] The counter-wound conductive spirals may be formed from wire, however it is preferred that the spirals are formed onto a substrate, for example by a conventional deposition or etching method. Preferably the groound planes are formed on the rear of the substrate. Where a capacitor is provided to optimise the connection, the capacitor is advantageously printed or otherwise formed on the substrate with the conductive spiral. This is beneficial as it allows the conductive spiral and capacitive element to be encapsulated to help prevent ingress of water or other fluid which may damage the device, and results in a more compact structure.

[0010] It is preferred that the two counter-wound conductive spirals are rotatably provided with respect to each other. This allows the coupling to be formed as a rotatable coupling through which signals can be transmitted.

[0011] The radio frequency energy to be transmitted by the coupling according to the present invention preferable has a frequency of at least 50MHz, and preferably has a maximum frequency of 2,000MHz.

[0012] According to a second aspect of the present invention, an antenna assembly comprises a non-contact connection according to the first aspect of the present invention, and an antenna connected to one of the conductive spirals of the non-contact junction.

[0013] It is preferred that the antenna assembly includes a matching network to match the antenna and the non-contact coupling.

[0014] The present invention will now be described in accordance with the accompanying drawings, in which:

Figure 1 shows a coupling;

Figure 2 shows a view of the back and front of a dielectric plate used in the coupling of Figure 1;

Figure 3 shows an antenna assembly including a loop antenna and a coupling as shown in Figure 1;

Figure 4 shows an antenna assembly including a monopole antenna and a coupling as shown in Figure 1; and,

Figure 5 shows a rotatable coupling.

[0015] As shown in Figure 1, the coupling includes two counter-wound spirals (1,2) formed of a conductive material. These spirals (1,2) are printed onto respective dielectric substrates (3,4) on the opposite sides of which are conductive ground planes (8,9). The spirals (1,2) may alternatively be formed on the dielectric substrates (3,4) by etching, or may be formed separately and provided on the dielectric substrates (3,4) subsequently. A capacitor (10) is connected in series with each of the conductive spirals (1,2). The capacitors are printed on the dielectric substrates (3,4) together with the conductive spirals (1,2), and are formed, for example by trimming, to optimise the transmission characteristics of the coupling for the particular dielectric through which the coupling is to be made and for the particular frequency range of the signals to be transmitted through the dielectric.

[0016] Each spiral (1,2), dielectric substrate (3,4) and ground plane (8,9) is provided against the opposite faces of a dielectric layer (5), forming a multi-layer sandwich with the dielectric layer (5) in the centre and the ground planes (8,9) on the outer edges.

[0017] In use, an RF signal to be transmitted through the dielectric layer (5) is supplied through a capacitor to one of the conductive spirals (1,2). Due to the inductive coupling through the dielectric layer (5), RF signals are induced in the other conductive spiral (1,2) from where the signals are output through the second capacitor.

[0018] Although not shown in the drawings, the dielectric substrate (5) may be the plastics wall of an equipment box. In this case, electrical equipment is provided within the box, and is connected to one of the conductive spirals (1,2) which is provided on the inner wall of the box. The other conductive spiral (1,2) is provided on the outer surface of the box, and forms the non-contact inductive coupling between the inside and outside of the box through which signals are transmitted to and from the equipment within the box.

[0019] A specific example of a coupling according to the present invention is shown in Figure 2. This is designed to work at a frequency of 184MHz through a lmm dielectric wall of polycarbonate. The dielectric substrates (3,4) on which the conductive spirals (1,2) are formed are also polycarbonate substrates having a thickness of 2.4mm, and being approximately 40mm square. The ground plane (8,9) is formed on the rear of the dielectric substrate (3,4), and a 10pf capacitor (10) is provided on the same side of the substrate (3,4) as the ground plane (8,9). Both the conductive spirals (1,2) and both the ground planes (8,9) are formed from copper. The capacitor (10) is connected to the outer end of the spiral (1,2) and the inner end of the spiral (1,2) is connected to the ground plane (8,9) via plated through holes (11) extending through the substrate (3,4). This allows the surface on which the spiral (1,2) is formed to remain flat. As shown in Figures 3 and 4, the coupling may be used in conjunction with an antenna (6,7). In this case the antenna (6,7) is connected directly to one of the conductive spirals (1,2). Depending on the particular antenna used, it may or may not be necessary to include a capacitor to optimise the non-contact coupling, in particular it is not necessary to include a capacitance when a monopole antenna is used. An RF signal is applied to the other conductive spiral (1,2) via an input (12) from within a housing (13), and this is coupled through the non-contact inductive coupling to the other conductive spiral (1,2) and to the antenna (6,7) to transmit the signal. This system can also be used for reception of the signals by the antenna (6,7). Figure 3 shows a loop antenna (6) used in conjunction with the non-contact coupling, and Figure 4 shows a monopole antenna (7) having a helical design, although other antennas may be used.

[0020] As shown in Figure 5, the non-contact coupling can be formed as a rotatable coupling. As shown in the Figure, the dielectric is an air gap. In this case, one of the ground plates (3) and the associated conductive spiral (1) are rotatably mounted with respect to the other ground plate (4) and spiral (2). Due to the spiral design of the conductive coupling, the relative angular position of the substrates (3,4) and conductors (1,2) will not affect the coupling.

Claims

1. A non-contact connection through a dielectric (5) comprising a pair of counter-wound conductive spirals (1,2) provided on opposite sides of the dielectric (5), characterised in that a pair of ground planes (8,9) sandwich the conductive spirals (1,2) and the dielectric (5).

2. A non-contact connection according to claim 1, further comprising a capacitor (10) provided at the input of each of the spirals (1,2).

3. A non-contact connection according to claim 2, in which the connection can be matched by adjusting the capacitance of the capacitor.

4. A non-contact connection according to claim 2 or 3, in which the capacitor is a variable capacitor.

5. A non-contact connection according to any one of the preceding claims, in which the conductive spirals (1,2) have a rectangular, square or circular configuration.

6. A non-contact connection according to any one of the preceding claims, in which the spirals (1,2) are formed onto a substrate (3,4).

7. A non-contact connection according to claim 6, in which the ground planes (8,9) are formed on the rear of the substrate (3,4).

8. A non-contact connection according to claims 6 or 7, in which a capacitor is printed or otherwise formed on the substrate (3,4) with the conductive spiral (1,2).

9. A non-contact connection according to any one of the preceding claims, in which the two counter-wound conductive spirals (1,2) are rotatably provided with respect to each other.

10. An antenna assembly comprising a non-contact connection according to any one of the preceding claims, and an antenna connected to one of the conductive spirals (1,2).

Drawing