MICROWAVE CIRCULATOR

Abstract: An waveguide circulator including at least three waveguides and a centre section. The waveguides are cross-coupled in the centre section. In the centre section a ferrite element is arranged. The circulator has means for magnetising the element. One of the waveguides has at least one constriction element in its interior. The constriction element is arranged to reduce leakage signals arising from imperfections in the circulator.

Description: MICROWAVE CIRCULATOR Technical Field of the Invention The present invention relates to a waveguide circulator including at least three waveguides, a centre section in which the waveguides are cross-coupled, a ferrite element arranged in said centre section and means for magnetising said ferrite element. The present invention also relates to a method for optimising the performance of said circulator. Background Art A microwave circulator is a component with three or more ports which, ideally, has perfect transmission of energy from a first port to a second port while a third port is isolated from the flow of energy. A typical waveguide circulator is a metallic structure consisting of three waveguides, which are cross-coupled in the centre of the structure. A ferrite element is arranged in the centre and is magnetised by at least one magnet. Interaction between the microwave energy and the statically magnetised ferrite element leads to circulation so that energy entering one port is directed towards the nearest port clockwise or counter-clockwise depending on the direction of the magnetic field. One common application for circulators is to couple a transmitter and receiver to a common antenna. The performance of the circulator is considered good when .the coupling from the transmitter to the antenna and from the antenna to the receiver occurs with as small losses as possible and with as high isolation as possible between the transmitter and receiver. Most often, it is also desired that the circulator maintains its performance over a broad frequency range for transmitted/received signals . In order to improve the performance of circulators with respect to impedance matching, a variety of matching elements made of metallic or dielectric material have been used in the prior art . These matching elements make the manufacturing of circulators expensive and complex, especially if one circulator requires several matching elements . Furthermore, due to mounting imperfections and tolerances in included components, prior art uses post- production trimming by placing additional trimming elements in the circulator. This of course making manufacture of the circulators more expensive. Summary of the Invention An object of the present invention is to minimise at least one of the above mentioned drawbacks with prior art circulators. Another object is to provide a circulator with improved performance and which is inexpensive to manufacture . These objects are achieved by a waveguide circulator and a method for optimising the performance of a waveguide circulator having the preambles and the features as defined by the appended claims. The present invention is based on the idea that by narrowing a waveguide connected to the central section, signal reflections will occur when energy is received by this waveguide via the central section from an energy originator waveguide. These signal reflections will destructively interfere with leakage signals which are received by another waveguide than the intended due to imperfections (which in turn are due to manufacturing errors and tolerances) of the circulator. Thus, the isolation between the waveguide via which the energy originates and this another waveguide, not being intended as a receiving waveguide, will be greatly improved. Thus, e.g., when coupling, counter-clockwise, one waveguide to a transmitter, one waveguide to a common antenna, and one waveguide to a receiver, and when magnetising the circulator to achieve a counter-clockwise flow of energy in the circulator, the present invention will provide high isolation between transmitter and receiver when transmitting energy from the transmitter via the antenna. The high isolation is achieved due to destructive interference in the receiver waveguide between reflected signals and leakage signals, the reflected signals being received from a narrowed antenna waveguide and the leakage signals from the transmitter waveguide . Of course, if desired, the present invention may also be used in such way that the waveguide of the receiver is narrowed, thereby improving the isolation between receiver and transmitter when the receiver receives energy via the antenna. In general, the present invention provides a circulator which improves the isolation between a first and a second port of the circulator when coupling energy between the first and a third port. Moreover, simulations show that the circulator of the present invention achieves this for high operating frequencies and over a broad frequency range. By designing the circulator to include a constriction element in one of the circulator waveguides, leakage signals due to imperfections of the circulator are reduced, thereby improving the isolation between the waveguides where zero transmission is desired. The constriction element is easy to manufacture and can be of the same material as the waveguide. Thus, the element can be cast in the waveguide. Alternatively, the element and the waveguide are of different material. In the latter case the element is attached to the waveguide in an appropriate manner, i.e. by soldering. According to an aspect of the invention, the performance of the circulator is optimised by introducing a constriction element of certain dimensions at a certain location. This is a further contributory cause to that the circulator of the present invention does not require any post-production trimming. Thus, when designing and manufacturing the circulator of the present invention, impedance matching elements or trimming elements are not required. Instead a constriction element is positioned in one of the waveguides. All in all, this makes the manufacture of the circulator more simple and less expensive. According to one embodiment of the invention, the ferrite element is displaced from the centre of the centre section towards the constriction element. Surprisingly, simulations show that this further improves the isolation between two given waveguides. Alternatively, or in addition, shaping the ferrite element asymmetrically also has the effect of improving the isolation between two given waveguides. By such optimisation of the circulator, the need for post- production trimming is eliminated even further. Brief Description of the Drawings The invention will now be described in detail with reference to following drawings, in which: FIG. 1 is a perspective view of an exemplifying embodiment of the present invention; and FIG. 2 is a perspective view of the embodiment of FIG. 1 showing the flow of energy; and FIG. 3 is a graph showing the isolation and transmission between various waveguides in the embodiment of FIG. 1. Detailed Description of an Embodiment of the Invention A waveguide circulator 1 in accordance with an exemplifying embodiment of the invention is shown in FIG. 1. It consists of three equiangularly spaced waveguides 2, 3, 4 and a central section 5, in which a ferrite element 6 is arranged. One magnet is located on each side of said element for magnetisation of the ferrite. These magnets are not shown in FIG. 1. A person skilled in the art realises that the magnetisation can be performed with one magnet. A reversal of the magnetic field applied to the ferrite element will result in a reversal of the direction of circulation. In one of the waveguides 3 , a constriction element 7 is arranged. The element has a height H extending perpendicular from the inner wall on which it is located. It also has a length L extending in the longitudinal direction of the waveguide 3. The element 7 has a width extending from one of the inner walls to the opposite inner wall . The constriction element 7 is further located on a distance D from the ferrite element. The constriction element is made of conductive material and is either cast in the waveguide or mounted by means of, for example, soldering. The ferrite element 6 is slightly displaced from the centre of the centre section 5 towards the constriction element 7 in order to further improve the isolation between waveguides 2 and 4. A person skilled in the art may without undue burden experiment with the symmetry of the ferrite element and come to the conclusion that an asymmetrically shaped ferrite element solely, or in addition to the displacement, as mentioned earlier, also improves said isolation. When circulation occurs in counter-clockwise direction, energy applied to waveguide 2 will be coupled to waveguide 3 while, ideally, waveguide 4 is isolated from waveguide 2. A common application for three-port circulators is to have a transmitter connected to waveguide 2, an antenna to waveguide 3 and a receiver to waveguide 4. The flow of energy for such an application, or any other application utilising the circulator of the present invention, is illustrated with reference to FIG. 2. In FIG. 2, energy 8 is applied to waveguide 2 from a transmitter (not shown) . Most part of the energy 8 will be coupled as energy 9 to an antenna (not shown) via waveguide 3, i.e. the direction of circulation is in this case counter-clockwise. Because of imperfections in the circulator, a portion 10 of the transmitter energy 8 fed to the circulator will be coupled to a receiver (not shown) through waveguide 4. This portion 10 of the energy 8 is called the leakage signal. By optimising the constriction element 7, concerning its dimensions and its distance to the ferrite element 6, it is possible to cause an appropriate reflection signal 11, as indicated in FIG. 2. The reflection signal 11 occurs when a small part of the energy 9 entering waveguide 2 is reflected against the constriction element 7. Thus, the major part of energy 9 will be fed to the antenna via waveguide 3, while a small part of the energy 9, i.e. the reflection signal 11, will be guided towards waveguide 4. The energy loss due to this reflection is insignificant with respect to the resulting energy that will be transmitted by the antenna. The reflection signal 11 will destructively interfere with the leakage signal 10. Since the constriction element 7 is optimised such that the amount of energy in the reflection signal 11 and in the leakage signal 10 is the same, and furthermore being out of phase, the two signals 10 and 11 will cancel each other out, or, leakage signal 10 will at least be reduced by way of destructive interference from reflection signal 11. The dimensions and location of the constriction element 7 are prepared in several steps. First, the centre section and its ferrite element 6 are dimensioned for a certain mode and operating frequency of the circulator. Using computer simulations, i.e. field simulators or EM-CAD (ElectroMagnetic Computer Aided Design) adjustments are then made and the performance optimised. Preferably, the constriction element is modelled as a transmission line in the interior of the waveguide, which transmission line has a certain length, impedance and distance from the ferrite element 6. At least one, but preferably all, of these three parameters are varied with the help of a circuit simulator until the leakage signal 10 has been cancelled out or reduced as much as possible. This will result in a given set of parameters for the transmission line. Based on this set of parameters, a preliminary estimation is done of the length L, height H and location D of the constriction element 7 within the waveguide. With the assistance of a field simulator, the shape of the constriction element is corrected so the reflection from the constriction element corresponds with the reflection from the transmission line . The swept frequency response for the embodiment of FIGS. 1 and 2 is shown in FIG. 3. The isolation is measured between waveguides 2 and 4. Also depicted in FIG. 2 is transmission between waveguides 2 and 3 as well as the reflection 11 resulting from the constriction element 7. It can be seen that the isolation is 20 dB over a frequency range of 74.8 to 80.5 GHz. Thus a commercially accepted isolation is obtained between waveguides 2 and 4 at a high operating frequency and within a wide frequency band. The values of the operating frequency and frequency band both constitute considerable improvements when compared with prior art circulators.

Claims: Claims 1. A waveguide circulator including at least three waveguides, a centre section in which the waveguides are cross-coupled, a ferrite element arranged in said centre section and means for magnetising said ferrite element, c h a r a c t e r i z e d in, that one of the waveguides has at least one constriction element in its interior, which constriction element at least essentially extends between two opposite inner walls of the waveguide, and which constriction element is arranged to reduce leakage signals arising from imperfections in the circulator. 2. A waveguide circulator according to claim 1, wherein said constriction element is located on a first inner wall of the waveguide, said constriction element having a height extending perpendicular from the first inner wall, a length extending in the longitudinal direction of the waveguide, and a width extending perpendicular to the longitudinal direction of the waveguide and at least essentially from a second inner wall to a third inner wall, the second and third inner walls both being adjacent to the first inner wall. 3. A waveguide circulator according to claim 1 or 2 , wherein the ferrite element is displaced from the centre of the centre section towards the constriction element. 4. A waveguide circulator according to any one of claims 1 - 3, wherein the ferrite element is asymmetrically shaped. 5. A method for optimising the performance of a waveguide circulator, the circulator including at least three waveguides, a centre section in which the waveguides are cross-coupled, a ferrite element arranged in said centre section and means for magnetising said ferrite element, c h a r a c t e r i z e d in, that the method includes the steps of : introducing a constriction element in the interior of one of said waveguides, the constriction element having certain dimensions and a certain location, the dimensions including a width which at least essentially extends between two opposite inner walls of the waveguide; and determining said dimensions and said location based on an amount of reduction achieved for a leakage signal within the circulator. 6. A method according to claim 5, wherein said step of introducing said constriction element includes positioning the constriction element on a first inner wall of the waveguide, wherein the said dimensions include a height extending perpendicular from the first inner wall, a length extending in the longitudinal direction of the waveguide, and a width extending perpendicular to the longitudinal direction of the waveguide and at least essentially from a second inner wall to a third inner wall, the second and third inner walls both being adjacent to the first inner wall. 7. A method according to claim 5 or 6, wherein said determining step includes: modelling the constriction element as a transmission line, the transmission line having three parameters in the form of a length, an impedance and a distance to the ferrite element; varying at least one of the transmission line parameters until the leakage signal within the circulator is reduced; and determining said dimensions and said location of said constriction element based on the result of said varying step . 8. A method according to any one of claims 5 - 7, wherein said method includes displacing said ferrite element from the centre of the centre section towards the constriction element, wherein the position of the ferrite element is determined by varying the distance between the ferrite element and said location of said constriction element until said leakage signal is reduced even further. 9. A method according to any one of claims 5 - 8, wherein said method includes the step of introducing an asymmetrically shaped ferrite element, thereby reducing said leakage signal even further.

Inventors: MATTSSON LENNART, MATTSSON, LENNART
Applicants: MATTSSON LENNART, SAAB AB, MATTSSON, LENNART
Application year: 2001
Main classification: H01P1
IPC codes: H01P1