GB2350938A

GB2350938A - Radio frequency rotary joints

Info

Publication number: GB2350938A
Application number: GB0003308A
Authority: GB
Inventors: Mark Shopland
Original assignee: APPLIED SATELLITE TECHNOLOGY L
Current assignee: APPLIED SATELLITE TECHNOLOGY L
Priority date: 1999-02-23
Filing date: 2000-02-15
Publication date: 2000-12-13
Also published as: GB0003308D0; GB9903983D0

Abstract

An RF rotary joint has a stator disc and rotor disc (20 and 22 respectively, see figure 6) each carrying strip line conductors centred around the rotation axis. The respective strip line earth and signal tracks are electrically coupled, either capacitively with a dielectric varnish layer between the discs, or by direct contact dc coupling. Preferably the stator disc has two semicircular tracks 6 fed by two signals of the same relative phase and terminated by impedance matching resistors 7. The rotor disc signal track may comprise a Tchebychett matching section (11, see figure 5). The joints may be stacked in sets, with each set handling one particular channel of an input signal (see figure 6).

Description

2350938 Radio Frequency Rotai Joints This invention relates to an RF

(radio frequency) rotary joint.

There are requirements in various fields of application for RF frequency rotary joints. One example is the coupling of an antenna tracking an orbiting satellite to stationery signal processing equipment. If the range of possible rotation is not too great, e.g. not exceeding about 180 degrees, a simple joint can be formed by a wrapping and unwrapping coaxial cable. However, sometimes the range of rotation required for the joint can be much greater than 180 degrees, sometimes substantially in excess of one complete rotation, and in this case the use of a wrappinglunwrapping coaxial cable is not feasible.

A special form of printed circuit board known as Strip Line can also be used to carry an RF signal. Strip Line is a length of circuit board printed with an earth track over the entirety of one face and a signal track printed on the other face. By selection of the board thickness, its dielectric constant, and the dimensions and thickness of the printed regions, Strip Line can provide the direct electrical equivalent of a coaxial cable of any desired RF electrical characteristics.

According to the present invention, there is provided an RF rotary joint comprising a stator and a rotor each comprising a disc of printed circuit board around which conductive material is printed in the form of Strip Line centred on the axis of the disc, the two discs being coupled coaxially together for relative rotary movement about their common axes with their respective earth and signal tracks electrically coupled.

The earth and signal tracks may be in direct contact, so as to slide around one another during relative rotation, whereby to provide d.c. coupling between the discs, but more preferably the facing surfaces of the two discs are coated with a dielectric material, e.g. a suitable insulating varnish, whereby to provide a.c., i.e. capacitative, coupling between the two discs.

Terminal connections may be made to the earth and signal tracks of the two discs, at one or more suitable points around each disc.

The above-described RF rotary joint can provide reliable RF coupling through an angle of rotation of the two discs. However, a single pair of discs can only handle one signal channel.

In the case, for example of a satellite tracking antenna on board a moving object such as a ship or oil rig, the antenna may receive up to four signals, typically high and low frequency bands with vertical and horizontal polarisations. In accordance with another aspect of the invention, however, the RF coupling of the invention can be stacked in multiple sets almost without Emit, with the rotor discs ganged together, the discs in each set being designed to handle one particular channel.

Two topologies of construction have been utilised, one which uses a resonant structure and one a travelling wave structure. The former exhibits low loss but a limited bandwidth whilst the latter has a much broader bandwidth but an increased loss figure.

In the accompanying drawings:- Figure 1 shows the well known form of Strip Line around which this design is based. The electrical performance of such a component is governed by the distance apart of the two ground planes 1, 1, the dielectric constant of the material separating them 2, and the dimensions of the centre conductor 3; and Figure 2 shows how this structure has been modified by dividing it in two across the centre conductor to provide a moving interface. It is applicable to both topologies.

In Figure 2, the line of the interface is shown at 4. A further development is to extend the ground plane around the printed circuit board in order to provide a bearing surface and screen the radiation of extraneous signals, as shown at 5.

An example of the travelling wave form of this invention is described with reference to the further accompanying drawings, in which:- Figure 3 is a view of the inner face of the joint's stator component; Figure 4 shows the layout of a Wilkinson power divider; Figure 5 is a view of the inner face of the joint's rotor component; and Figures 6 and 6A show a practical arrangement incorporating the invention for a satellite tracking antenna for use on board a ship.

Referring to Figure 3, the stator component has two semi-circular tracks 6, fed by two signals of the same relative phase and both terminated with resistors 7 of the same characteristic impedance as that of the system. These two signals are derived from a Wilkinson power divider similar to that shown in Figure 4.

The power divider has the ability to divide the input signal into two parts which share the same relative phase. The outputs 8 of this divider are directly coupled to the inputs of the rotor. Under normal operation the resistor 9, which has a value of twice the characteristic impedance of the system, does not dissipate any power. It is only when power is reflected back from the load that it would then -4be dissipated giving isolation between the output ports.

The rotor component shown in Figure 5 has two elements. The first element 12 is the coupler element which couples indirectly to the semi-circular tracks of the stator component. These two sections are isolated from one another by the use of a varnish coating, the plating thickness on the outer groundplane tracks being adjusted to accommodate this. The second element 11 is a Tchebycheff matching section which has a wide bandwidth and matches the input source to the coupler element.

The above-described RF rotary joint can provide reliable RF coupling throughout an unlimited number of complete relative rotations of the stator and rotor components.

For example, in the case of a satellite tracking antenna on board a ship (or other moving object), an antenna may receive a number of signals, typically in high and low frequency bands with differing polarisations. In this case a stack of stator/rotor pairs may be stacked on the same axis, with the number of channels limited only by available space. Figure 6 shows an example, wherein reference 20 denotes a stator disc (shown in plan view in Figure 6A) and reference 22 denotes a rotor disc. The discs are stacked on a column 24, the rotor discs 22 for rotation and the stator discs 24 fixed. The tongue 20A on a stator disc (see Figure 6A) can be used to fix the stator against rotation. The pairs of discs 20, 22 are axially clamped on the column 24, separated by spacers 26 at least one of each being to some extent compressible to ensure good electrical contact between each stator and its corresponding rotor.

Claims

1. An RF rotary joint comprising a stator and a rotor each comprising a disc of printed circuit board around which conductive material is printed in the form of Strip Line centred on the axis of the disc, the two discs being coupled coaxially together for relative rotary movement about their common axes with their respective earth and signal tracks electrically coupled.

2. A joint according to claim 1, wherein the earth and signal tracks are in direct contact, so as to slide around one another during relative rotation, whereby to provide d.c. coupling between the discs.

3. A joint according to claim 1, wherein the facing surfaces of the two discs are coated with a dielectric material, e.g. a suitable insulating varnish, whereby to provide a.c., i.e. capacitative, coupling between the two discs.

4. A joint according to any of claims I to 3, wherein terminal connections are made to the earth and signal tracks of the two discs, at one or more suitable points around each disc.

5. A joint according to any of claims I to 4, wherein the stator disc has two semi-circular tracks respectively fed by two signals of the same relative phase and terminated by resistors of the same characteristic impedance as any system in which the disc is incorporated.

6. A joint according to claim 5, wherein the two signals are derived from a Wilkinson power divider.

7. A joint according to any of claims I to 6, wherein the rotor component -6includes a coupler element which couples with tracks on the stator component.

8. A joint according to claim 7, wherein the rotor component also includes a Tchebychett matching section matching the input source to the joint.

9. A joint according to any of claims 1 to 8, having a multiplicity of stator/rotor components stacked in multiple sets, with the rotor discs ganged together, the discs in each set being designed to handle one particular channel of an input signal.

10. An RF rotary joint substantially as herein described with reference to the accompanying drawings.