EP0281712A1

EP0281712A1 - High frequency circulator

Info

Publication number: EP0281712A1
Application number: EP87310880A
Authority: EP
Inventors: Ann Catherine Gravina
Original assignee: Marconi Electronic Devices Ltd
Current assignee: Marconi Electronic Devices Ltd
Priority date: 1987-02-12
Filing date: 1987-12-10
Publication date: 1988-09-14
Also published as: GB8703207D0; GB2201044A; GB2201044B

Abstract

A high frequency waveguide circulator has a ferrite body (15) positioned centrally at the junction of three equally angularly spaced waveguide ports (11, 12, 13). The ferrite, which is in the shape of one or more short cylindrical rods, is centrally positioned by means of a sleeve or tube of dielectric material (16). The inner surface of the sleeve fits closely over the ferrite, and the outer surface is a snug fit in the waveguide walls. The length of the sleeve is chosen to give the required electrical performance, and the use of the sleeve gives a very positional accuracy for the ferrite in an extremely simple manner allowing high electrical performance to be achieved.

Description

This invention relates to a high frequency circulator in which three signal ports angularly offset by 120° from each other are linked at a common point at which a ferrite body is present in order that an electromagnetic wave input at one port is output at a different predetermined port. An external magnetic field is applied to the circulator such that the field is aligned with the axis of the ferrite body and when all the ferrite junction and magnetic parameters are correctly adjusted for a given frequency, a signal entering at a first port is transmitted to a second port whilst the third port is isolated. Similarly, a signal entering at the second port is transmitted to the third port whilst the first port is isolated, and finally a signal entering at the third port is transmitted to the first port with the second port being isolated. Such devices are conveniently used for separating or combining electromagnetic high frequency signals.
It is generally desired to operate the circulator over a reasonably wide frequency band rather than at a single spot frequency and this can be achieved by adapting the design, but a high degree of geometrical symmetry with respect to all three ports is found to be essential for proper operation. It can be difficult and time consuming to manufacture high frequency circulators having a sufficiently good wideband operating performance particularly as for some kinds of circulator the degree of symmetry is very critical. Also some form of impedance matching is required, commonly a metallic platform to reduce the height of the junction or some kind of dielectric loading. The present invention arose in an endeavour to solve the problem of positioning the ferrite body accurately to achieve the required symmetry.
According to this invention a high frequency circulator includes a ferrite body located at the junction point of three equally angularly spaced waveguide ports which are capable of supporting propagation of an electromagnetic wave, the ferrite body being in the form of a cylinder which is closely surrounded by dielectric material, the outer surface of which engages with three wall regions, each wall region being associated with respective pairs of ports, the ferrite body being thereby located centrally with respect to the three ports.
It will be appreciated that by employing the invention the ferrite body can be very accurately located by the dielectric material thereby overcoming the aforementioned problem. Furthermore, rather than adversely affecting the performance of the circulator it has been found that the dielectric material can serve to match the impedance of the ferrite body to the waveguide.
The dielectric material is preferably in the form of a preformed sleeve or tube which is placed around the cylindrical ferrite body.
The electric vector of the electromagnetic energy is in the direction of the short wall of a rectangular waveguide channel. The invention is particularly suitable for use with an E-plane circulator: that is to say, a circulator in which the direction of the electric vector in the waveguide is normal to the axis of the ferrite cylinder.
The height of the waveguide will be determined primarily by the operating frequency of the circulator and it is unlikely to coincide with the required length of ferrite body. Accordingly, the ferrite body is preferably of shorter length than the height of each port. Coveniently, two short ferrite rod portions which are axially aligned with each other are placed in short dielectric tubes which are each then attached to opposite faces of the waveguides. Alternatively, a common dielectric tube can be used to locate the two ferrite rod portions. In each instance the thickness of the wall of the dielectric tube is chosen so as to bridge the space between ferrite rod and the junction points of each pair or ports so as to firmly and securely locate the ferrite rods in their required position of symmetry. The length and diameter of the ferrite material, and of course the lengths of the dielectric can then be adjusted so as to give the correct electrical and magnetic properties.
The invention is further described by way of example with reference to the accompanying drawings, in which:

Figure 1 shows a sectional plan view of an E-plane ferrite circulator,
Figure 2 shows a sectional view of the circulator on the line XX', and
Figures 3 and 4 show sectional views of alternative embodiments of the port of the circulator.

Referring to Figure 1, there is shown therein a sectional view through the central plane of an E-plane circulator. In an E-plane circulator, the axis of symmetry of the ferrite is shown in the direction of the arrow 1 shown in Figure 2 and this is aligned with the maximum dimension of a rectangular waveguide channel 2. An external field is applied to the ferrite and the direction of the field is aligned with the arrow 1. The polarity of the external field determines the direction of circulation of the electromagnetic energy.
Conveniently, such a waveguide channel 2 is machined from a pair of conductive metallic blocks 3 and 4, the two blocks mating together exactly so as to constitute the required waveguide structure. It will be seen that there are three ports 11, 12 and 13 which have a junction region at the centre of which is positioned a circular ferrite body 15. It is located in place by a preformed sleeve or tube 16 of dielectric material having a wall thickness which is such that the inner diameter of the tube closely surrounds the ferrite body 15 and the outer surface of the tube fitting closely within the three contact points 17, 18 and 19 at the junction regions of respective pairs of ports 11, 12 and 13. Although in Figure 1, each junction region is in the form of just a line contact with sleeve 16, this need not always be so, and will depend on the geometry of the structure.
One possible configuration of the ferrite body 15 and the dielectric tube is illustrated in Figure 3, in which two relatively thick walled dielectric tubes 20 and 21 surround separate short ferrite rods 22 and 23. Although the tubes do not fit tightly on the rods, they are a close fit so that there is negligible clearance between them. Similarly the outer surfaces of the tubes 20 and 21 co-operate with the contact points 17, 18, 19 so that the rods 22 and 23 are aligned with each other and are retained precisely at the centre of the waveguide portion. The rods and tubes are retained firmly in contact with the walls 24 of the waveguide by means of a low loss adhesive such as one known as Silastic RTV.
In this construction, the tubes 20 and 21 together with the length and diameter of the ferrite rods 22 and 23 enable the bandwidth of operation of the cirulator to be tailored to particular requirements, and they permit the assembly to be quickly and accurately lcoated without the need for setting jigs, machined locating recesses, or the use of a microscope.
In this example the rods 22, 23 are made from a material known as L3 Lithium Titanium Zinc Iron Spinel, available from Marconi Electronic Devices Limited, although other materials are suitable. Impedance matching is provided by the sleeve or tubes 20,21 each having an external diameter of 4.95mm, made from "Rexolite 1422", a dielectric made by Oak Materials Group Inc, USA. The dimensions of the E-plane waveguide channel are 10.67 x 4.32mm approximately.
Figure 4 shows an alternative embodiment in which the ferrite rods 25, 26, are located by a common sleeve or tube 27 of dielectric material.

Claims

1. A high frequency circulator including a ferrite body located at the junction of three angularly spaced waveguide ports which are capable of supporting propagation of an electromagnetic wave, characterised in that the ferrite body (15) is closely surrounded by a dielectric material (16), the outer surface of which engages three wall regions (17,18,19), associated with respective pairs of ports (11,12,13), the ferrite body being thereby located centrally with respect to the three ports (11,12,13).

2. A circulator as claimed in claim 1 characterised in that the dielectric body is in the form of a preformed tube or sleeve (16).

3. A circulator as claimed in claim 1 or 2 characterised in that it is configured as an E-plane circulator.

4. A circulator as claimed in claim 1, 2 or 3 characterised in that the ferrite body (22,23,25,26) does not extend over the full height of the waveguide ports.

5. A circulator as claimed in claim 1, 2, 3 or 4 characterised in that the ferrite body is in the form of two cylinders (22,23 or 25,26) mounted coaxially, with the remote ends of the cylinders in contact with opposite surfaces of a waveguide.

6. A circulator comprising a body defining three or more waveguide ports (11,12,13) opening onto a cavity and a ferrite resonator (15) positioned in the cavity, characterised by a dielectric member (16) which serves to match the impedence of the resonator (15) to each waveguide (2) and which engages the said body (3,4) at positions (17,18,19) between the ports (11,12,13) so as to locate the resonator (15) correctly in the cavity.