DIRECTIONAL COUPLER

Abstract: The present invention relates to a waveguide directional coupler, which includes a first and a second waveguide, wherein the waveguide share a common dividing wall. In said wall a first hole and a second hole is provided for coupling power between the first and the second waveguide. The wall has a thickness of (n+1) lambda g/4 and the distance between the holes is (2m+3) lambda g/4, wherein lambda g is the mean operating wavelength of said first and second waveguide, n is any positive integer (0, 1, 2, ...), and m is any positive integer (0, 1, 2, ...). The invention also relates to a wall for a directional coupler and a method for constructing a directional coupler.

Description: DIRECTIONAL COUPLER Technical field of the invention The present invention relates to a directional coupler for coupling power between two waveguides. Background art Directional couplers are commonly used for branching off a certain amount of power form a primary waveguide to a secondary waveguide. There are different approaches for coupling power between two waveguides. For instance, the waveguides can be arranged with their broad sides in contact with each other, or with their short sides. Also, a first waveguide may be arranged with its short side in contact with the broad side of a second waveguide. The waveguides do not need to run in parallel, but can e.g. cross each other. Directional couplers often include a wall dividing the waveguides. The wall is provided with holes or windows through which the waves can propagate or leak, thereby achieving the power coupling between the waveguides. A typical prior art waveguide directional coupler is schematically illustrated in the accompanied Fig. 1. A primary waveguide 2 is arranged in parallel and adjacent to a secondary waveguide 4. The waveguides are separated by a common wall 6 which is provided with two holes HI and H2. Four ports P1-P4 of the waveguides are indicated in the figure. The first port PI is the input port of the primary waveguide 2, whereas the second port P2 is the output port thereof. Ports P3 and P4 are output ports of the secondary waveguide 4. The distance L between the two holes HI and H2 , centre to centre, is a quarter of the operating wavelength or odd multiples thereof: L = (2n+l)*λ g /4, wherein n is a positive integer. This means that power coupled from the primai'y waveguide 2 through the first hole HI to the secondary waveguide 4 and heading towards port P3 will travel a half wavelength less than power coupled through the second hole H2 and heading towards port P3. The effect or resultant of the half wavelength difference is a cancellation of power. In contrast, power heading towards port P4 travels the same distance regardless of which hole it is coupled through, thus resulting in added powers. At relatively low frequencies, e.g. 10 GHz (i.e. a wavelength of 3 cm) , a wall thickness of 1 mm can be used. However, at extremely high frequencies (EHF) , i.e. the wavelength being in the millimetre region, a much thinner wall is needed for good coupling results. For instance, for a frequency of 77GHz (i.e. a waveguide wavelength of 5 mm) , a wall thickness of less than 0.1 mm would be needed in order to achieve an equally good coupling. Thus, in terms of wavelength, the wall thickness would for high frequencies need to be less than λ g /50. For high frequency waves it is clearly a problem to produce a waveguide directional coupler which has just as good coupling efficiency as in the case of waves with relatively low frequencies. The required thin walls are not only difficult to manufacture with good accuracy and stability, but also very expensive. Summary of the invention An object of the present invention is to provide a waveguide directional coupler which is suitable for high frequency waves . Another object of the present invention is to provide a waveguide directional coupler which is easy and cheep to manufacture. These and other objects which will become apparent in the following are achieved by a directional coupler and a method as defined in the accompanied claims. Thus, according to one aspect of the invention a waveguide directional coupler- is provided. The directional coupler includes : a first waveguide; a second waveguide sharing a common dividing wall with the first waveguide; a first hole and a second hole provided in said wall for coupling power between the first waveguide and the second waveguide ,- wherein said wall has a thickness of („ + 1 )^ and the distance (centre to centre) between the holes is wherein: λ g is the mean operating wavelength of said first and second waveguide, n is any positive integer (0, 1, 2,...) , m is any positive integer (0, 1, 2,...) . According to a second aspect of the invention a wall for a waveguide directional coupler is provided. The wall includes : a first hole and a second hole for coupling power between a first waveguide on one side of the wall and a second waveguide on the other side of the wall; wherein the wall has a thickness of and the distance (centre-to-centre) between the holes is wherein: λ g is the mean operating wavelength of said first and second waveguide, n is any positive integer (0, 1, 2,...) , m is any positive integer (0, 1, 2,...) . According to a third aspect of the invention a method for constructing a waveguide directional coupler is provided. The method includes the steps of: providing a primary waveguide and a secondary waveguide; providing a wall according to the second aspect of the invention; arranging said wall as a common dividing wall between the primary waveguide and the secondary waveguide, wherein the holes in the wall couple power between the primary waveguide and the secondary waveguide . This means that for high frequency waves, i.e. wavelengths of the order of one or several millimetres, the wall of the directional coupler according to the present invention is quite thick. Comparing with the numbers presented in the background art paragraph, for a wavelength of 5 mm, a wall according to the invention could be 1.25 mm thick. In prior art a wall thickness of less than 0.1 mm was required. This great difference in thickness results in considerable simplification and cost reduction for the manufacturing of a directional coupler according to the present invention. Thus, the invention is based on the insight that it is possible to achieve good power coupling even by increasing the wall thickness. This is obviously a solution which goes completely against the traditional technique in prior art, in which skilled persons noχ-mally, to obtain a good coupling, attempt to make thin walls, which are costly and difficult to manuf cture. By making the wall provided with holes a quarter of a wavelength thick (λ g /4), or multiples thereof, the same power coupling efficiency can be obtained for high frequency waves, as with a normal thin wall. Using a thick wall according to the invention, it is preferable to configure the wall holes relatively large in order to avoid cut-off or attenuation of power. For instance, the width of each hole is suitably dimensioned to substantially correspond to the width of a waveguide. The person skilled in the art will, however, appreciate that smaller as well as larger holes are possible depending on the application and desired coupling result. The holes can be designed with different shapes. A preferred shape is rectangular, however, others are possible, such as rounded shapes without sharp edges. Different shapes facilitate different manufacturing methods . With large wall holes it is preferable to have them interspaced by more than a quarter of a wavelength, from centre to centre. Thus, according to the invention a number of half wave lengths are added to the typical prior art distance of a quarter of a wavelength. By adding a multiple of half wavelengths, the relative difference in path length, between the powers coupled through the two holes, is unchanged. Thus, the powers will be added in one direction and cancelled in the other so that high directivity and low isolation is obtained. Naturally, the wall of the waveguide directional coupler according to the invention may be provided with more than two holes. Additional holes are interspaced in the same way as the two first ones. The coupling factor between the waveguides depends on the number of holes and the size (e.g. the height) of the holes. More and larger holes achieve better performance. Furthermore, it is to be understood that the common dividing wall may be comprised of two partial walls, i.e., one partial wall belonging or being associated to the first waveguide and the other partial wall belonging or being associated to the second waveguide. When the waveguides are joined, the two partial walls will together constitute said common dividing wall . This joining of two sub-assemblies into one unit may be advantageous from a manufacturing point of view. In one embodiment of the invention, two walls that are characteristic of the invention are utilised for bleeding power to both sides of a primary waveguide. Accordingly, the primary waveguide is on one side connected to a secondary waveguide with a separating wall in between them, as previously described, and on the opposite side the primary waveguide is connected to another secondary waveguide with a wall in between them. The power from the primary waveguide is thus coupled to two parallel secondary waveguides. As previously mentioned, a great advantage of the waveguide directional coupler and the thick wall according to the invention is that they are easy to manufacture. Some examples of different possible manufacturing processes are milling, cutting, moulding, casting and pressing. Brief description of the drawings Fig. 1 illustrates schematically a typical prior art two-hole directional coupler. Fig. 2 illustrates a waveguide directional coupler according to one embodiment of the invention. Detailed description of the drawings Fig. 1 illustrates schematically a typical prior art two-hole directional coupler as has been described previously in detail in connection with the background art disclosure . Fig. 2 illustrates a waveguide directional coupler 10 according to one embodiment of the invention. The waveguide directional coupler 10 includes two waveguides: a primary waveguide 20 and a secondary waveguide 30, each having the shape of a box or a rectangular parallelepiped, and thus have a rectangular cross section. The waveguides 20, 30 have broad sides 22a and 32a, respectively, and short sides 22b and 32b respectively. As can be seen in the figure, the waveguides 20, 30 are arranged with their short sides adjacent to each other, with their respective longitudinal axis extending in parallel . The waveguides share a common dividing wall 40, which also extends in the direction of said longitudinal axes. The wall 40 in this embodiment is provided with two holes 42, 44 bringing the waveguides 20, 30 in communication with each other so that power can be coupled from the primary waveguide 20 to the secondary waveguide 30. The holes 42, 44 are rectangular, i.e. the channels formed by the holes between the waveguides have a rectangular cross section. The wall thickness t is a quarter of the mean operating wavelength λ g of said first and second waveguide, or multiples thereof. In order to avoid power attenuation the holes are made large, e.g. with a width corresponding to the width of a waveguide. The two holes are separated by a distance = 3λ g /4, measured from centre to centre. In other words, a half wavelength has been added between the holes in comparison to the distance according to the prior art directional coupler in Fig. 1. Naturally, multiples of half a wavelength can be added to that distance L . Thus, in the present invention the difference in travelled path length for power coupled through the different holes 42, 44 provides a difference of a half wavelength, or rather a multiple thereof, in a corresponding way as in the prior art λ/ -distance, resulting in cancellation in one direction of the secondary waveguide and addition of powers in the other direction. It is to be understood that even though one specific embodiment has been pointed out, it is only an elucidative example for the ease of understanding. Obviously, many others are conceivable. For instance, instead of arranging the waveguides with their short sides adjacent to each other, an alternative would be to arrange them with their broad sides adjacent to each other. Furthermore, the cross-section of the waveguides may be other than rectangular, e.g. the corners may be rounded and the walls do not have to be in parallel. Accordingly, it should be noted that numerous modifications and variations can be made without departing from the scope of the present invention defined in the accompanied claims .

Claims: CLAIMS 1. A waveguide directional coupler, including: a first waveguide; a second waveguide sharing a common dividing wall with the first waveguide; a first hole and a second hole provided in said wall for coupling power between the first waveguide and the second waveguide; wherein said wall has a thickness of and the distance (centre to centre) between the holes is wherein: λ g is the mean operating wavelength of said first and second waveguide, n is any positive integer (0, 1, 2,...), m is any positive integer (0, 1, 2,...) . 2. The waveguide directional coupler as claimed in claim 1, wherein the coupler is configured for operating wavelengths of the order of millimetre (s) . 3. The waveguide directional coupler as claimed in any one of claims 1 - 2, wherein the width of each hole corresponds substantially to the width of a waveguide. 4. The waveguide directional coupler as claimed in any one of claims 1 - 3, wherein the holes are substantially rectangular. 5. The waveguide directional coupler as claimed in any one of claims 1 - 4, wherein at least one additional hole is provided in the wall at a distance (centre to centre) of (2,„ + 3)- from a neighbouring hole . 6. The waveguide directional coupler as claimed in any one of claims 1 - 5, wherein said common dividing wall is comprised of a first and a second partial wall being associated to the first and the second waveguide, respectively. 7. A wall for a waveguide directional coupler, including: a first hole and a second hole for coupling power between a first (rectangular) waveguide on one side of the wall and a second (rectangular) waveguide on the other side of the wall; wherein the wall has a thickness of and the distance (centre-to-centre) between the holes is (2 + 3)-^ 4 wherein: λ g is the mean operating wavelength of said first and second waveguide, n is any positive integer (0, 1, 2,...), m is any positive integer (0, 1, 2,...) . 8. The wall as claimed in claim 7, wherein the wall is configured for operating wavelengths of the order of millimetre (s) . 9. The wall as claimed in any one of claims 7 - 8, wherein the width of each hole, corresponds substantially to the width of a waveguide, with which the wall is to be associated. 10. The wall as claimed in any one of claims 7 - 9, wherein the holes are substantially rectangular. 11. The wall as claimed in any one of claims 7 - 10, wherein at least one additional hole is provided in the wall at a distance (centre to centre) of (2m + 3)-^ 4 from a neighbouring hole. 12. The wall as claimed in any one of claims 7 - 11, wherein it is comprised of a first and a second partial wall being associated to the first and the second waveguide, respectively. 13. A method for constructing a waveguide directional coupler, including the steps of: providing a primary waveguide and a first secondary waveguide; providing a first wall according to any one of claims 7 - 12; arranging said first wall as a common dividing wall between the primary waveguide and the first secondary waveguide, wherein the holes in the first wall couple power between the primary waveguide and the first secondary waveguide. 14. The method as claimed in claim 13, further including the steps of : providing a second secondary waveguide; providing a second wall according to any one of claims 7 - 12 ; arranging said second wall, opposite to said first wall, as a common dividing wall between the primary waveguide and the second secondary waveguide, wherein the holes in the second wall couple power between the primary waveguide and the second secondary waveguide .

Inventors: GRABS JAN, MATTSSON LENNART, GRABS, JAN, MATTSSON, LENNART
Applicants: GRABS JAN, MATTSSON LENNART, SAABTECH ELECTRONICS AB, GRABS, JAN, MATTSSON, LENNART
Application year: 2001
Main classification: H01P5
IPC codes: H01P5