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(11) | EP 0 524 000 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Multi-port microwave coupler |
| (57) A multi-port microwave coupler has eight input ports In 1, In 2, In 3, In 4, In 5,
In 6, In 7 and In 8 and eight output ports Out 1, Out 2, Out 3, Out 4, Out 5, Out
6, Out 7 and Out 8 and is synthesised from twelve 2 X 2 hybrid couplers which are
arranged as three sets, that is a first set A, B, C and D, a second set E, F, G and
H and a third set I, J, K and L. A first group of transmission lines 11, 12, 13, 14,
15, 16, 17 and 18 interconnect the first and second sets of hybrid couplers, and a
second group of transmission lines 21, 22, 23, 24, 25, 26, 27, and 28 interconnect
the second and third sets of hybrid couplers. The first set of hybrid couplers are
arranged substantially in a row in the sequence first, second, fourth and third, that
is A, B, D and C with one half of the second and third sets E, G, I and J arranged
on one side of the row and with the other half of the second and third sets F, H,
K and L on the other side of the row. In this manner all cross-overs in the first
and second groups of transmission lines are avoided thereby providing a planar realisation
of a 8 X 8 multi-port microwave coupler, Instead of the first set of hybrid couplers
being arranged in a row, it is also possible to achieve a similar result by arranging
the second set of hybrid couplers in a row in the same sequence. The invention is
of application to higher order n x n multi-port microwave couplers where n = 2(2+p) and p is a whole number. |
[0001] This invention relates to a multi-port microwave coupler particularly, but not exclusively, to be used as a part of a beam-forming network for a multi-beam antenna carried by a satellite.
[0002] Such multi-port microwave couplers are well-known in the art of microwave frequency transmission and typically comprise a hybrid coupler having four ports, that is two input ports and two output ports. Such hybrid couplers are commonly referred to as 2 x 2 hybrid couplers and have the following characteristics:-
1. When a microwave signal is applied to one of the input ports, the complex voltages appearing at both output ports are equal in amplitude, and no power appears at the other input port.
2. When equal-amplitude microwave signals are applied to both of the input ports, all of the power can be made to appear at only one of the output ports by appropriately selecting the relative phases of the two input signals.
[0003] However there is a requirement for higher-order couplers in certain applications, for example in beam-forming networks for multi-beam antennas. Such higher-order couplers have equal numbers of input ports and output ports, and a coupler with 2n ports is commonly referred to as an n x n coupler. In the case where the hybrid order n is a power of 2, such higher-order couplers can be synthesised from combinations of 2 x 2 hybrid couplers interconnected by transmission lines.
[0004] In synthesising higher-order couplers from 2 x 2 hybrid couplers, the transmission lines interconnecting the 2 x 2 hybrid couplers essentially cross one another. With the simplest higher-order coupler, the hybrid order n is the second power of 2 and only four 2 x 2 hybrid couplers are necessary to provide a 4 x 4 coupler. This arrangement only incurs one cross-over between the transmission lines and it is known to rearrange the positions of the four 2 x 2 hybrid couplers to avoid this single "cross-over".
[0005] Multi-port couplers of even higher orders can be synthesised from 2 x 2 hybrid couplers to give an n x n coupler where n = 2(2+p) and p is a whole number. Thus, when p = 1 an 8 x 8 coupler can be achieved, when p = 2 a 16 x 16 coupler, when p = 3 a 32 x 32 coupler, and so on. Existing 8 x 8 couplers involve many cross-overs with the result that the transmission lines become a complex multi-layer structure.
[0006] Such cross-over in the transmission lines may be implemented in various ways. For example, in stripline, microstrip and similar realisations, the 2 x 2 hybrid couplers can be fitted with connectors and external semi-rigid cables can be used for the transmission lines. In microstrip realisations, bridges of wire, foil or cable can be used. In square-ax realisations, bridging devices can be used. In waveguide realisations, combinations of waveguide bends can be used. Also multi-layer microstrip or stripline devices could be desired.
[0007] In all of the above realisations, the requirement for cross-overs incurs penalties in the mass, size, mechanical complexity and electrical accuracy of any synthesised multi-port couplers in which n = 2(2+p), and such penalties are problematic in satellite applications where lightness, smallness and simplicity are important.
[0008] It is an object of the present invention to reduce the number of cross-overs in multi-port microwave couplers where n = 2(2+p) and p is a whole number.
[0009] According to the invention a multi-port microwave coupler having n input ports and n output ports, when n = 2(2+P) and p is a whole number, comprises a matrix formed from several sets of hybrid couplers, each hybrid coupler having two inputs and two outputs, a first of said sets comprising n/₄ pairs of the 2 x 2 hybrid couplers of which the inputs define said input ports, a second of said sets also comprising n/₄ pairs of the 2 x 2 hybrid couplers, a third of said sets comprises n/₂ of the 2 x 2 hybrid couplers, the outputs from each pair of 2 X 2 hybrid couplers from the first set being connected by a first group of transmission lines to the appropriate inputs of an associated pair of 2 x 2 hybrid couplers from the second set without any cross-over connections, the outputs from the second set being connected by a second group of transmission lines to the appropriate inputs of said third set with less than six cross-over connections.
[0010] In the case of an 8 x 8 microwave coupler, p would of course be 1 thereby providing 8 input ports and 8 output ports, and wherein there would be only 3 sets of the 2 x 2 hybrid couplers whereby the outputs of the third set define said output ports, and said second group of transmission lines do not cross each other. In this case, the first and second groups of transmission lines lie in the same plane.
[0011] In a preferred multi-port microwave coupler, according to the invention, one of said sets of 2 X 2 hybrid couplers are arranged substantially in a row, one half of another of said sets of 2 X 2 hybrid couplers are arranged to one side of said row and the other half of said another set are arranged to the other side of said row, and the first group of transmission lines lie in the same plane.
[0012] The first set of 2 X 2 hybrid couplers may be arranged substantially in a row, one half of the second set of 2 X 2 hybrid couplers being arranged to one side of said row, the other half of the second set of 2 X 2 hybrid couplers being arranged to the other side of said row, and the first group of transmission lines lying in the same plane. In this case, the four 2 X 2 hybrid couplers of the first set are preferably arranged in the sequence, first, second, fourth and third. Also the second group of transmission lines preferably connect one half of third set of 2 X 2 hybrid couplers to said one half of the second set of 2 X 2 hybrid couplers and the other half of the third set of 2 X 2 hybrid couplers to said other half of the second set of 2 X 2 hybrid couplers, and the second group of transmission lines lying in the said plane.
[0013] Alternatively, the second set of 2 X 2 hybrid couplers may be arranged substantially in a row, one half of each of the first and second sets of 2 X 2 hybrid couplers being arranged to one side of said row, the other half of each of the first and second sets of 2 X 2 hybrid couplers being arranged to the other side of said row, and the first group of transmission line lying the same plane. In this case the four 2 X 2 hybrid couplers of the second set are preferably arranged in the sequence, first, second, fourth and third.
[0014] In addition to the provision of a multi-port microwave coupler, the invention also extends to a beam-forming network for a multi-beam antenna incorporating such multi-port microwave coupler.
[0015] The invention will now be described, by way of example only, with reference to the accompanying drawings, in which:-
Figure 1 is a diagram of a known 2 x 2 hybrid coupler illustrating its operation;
Figure 2 is a diagram of a known 4 x 4 coupler synthesised from four 2 x 2 hybrid couplers;
Figure 3 illustrates a known reorganisation of the 4 x 4 coupler illustrated in Figure 2;
Figure 4 is a diagram of a known 8 x 8 coupler synthesised from twelve 2 x 2 hybrid couplers;
Figure 5 illustrates the reorganisation of the 8 x 8 coupler of Figure 4 to avoid any cross-over connections, and
Figure 6 illustrates an alternative reorganisation of the 8 X 8 coupler of Figure 4 to avoid any cross-over connections.
[0016] With reference to Figure 1 a 2 x 2 3db hybrid coupler A is shown in each of its two operative modes. In the upper part of this figure, a microwave signal applied to input port 1 produces amplitude equal signals in phase quadrature at both of the output ports 3 and 4, but with no power appearing at the other input port 2. In the lower part of this figure, equal microwave signals applied to the input ports 1 and 2, but with a 90° phase separation, causes the resultant signals to cancel each other out at output port 3, whilst the signals combine at output port 4.
[0017] In figure 2, four 2 x 2 3db hybrid couplers A, B, E and F have been synthesised in known manner to provide a 4 x 4 multi-port coupler having four input ports, In 1, In 2, In 3 and In 4 and four outputs Out 1 , Out 2, Out 3 and Out 4. It will be noted that the hybrid coupler A is connected by transmission lines 5 and 6 respectively to the one inlets of hybrid couplers E and F, whilst the hybrid coupler B is connected by transmission lines 7 and 8 to the other inlets of hybrid couplers E and F. As a consequence the transmission lines 6 and 7 "cross-over" as indicated by arrow 9.
[0018] Figure 3 illustrates a known manner of reorganising the hybrid couplers A, B, E and F of Figure 2 so that their transmission lines 5, 6, 7 and 8 do not cross-over each other. This enables the transmission lines 5, 6, 7 and 8 to be arranged in the same plane and gives a truly planar implementation of a 4 x 4 hybrid coupler. This planar realisation has the following advantages:-
1. Lower insertion loss from input to output ports because features, such as connectors, cables, bridges, etc. all of which would add to the basic loss of the device, are avoided.
2. Better return loss and isolation because reflections caused by connectors, bridges, and other discontinuities are absent.
3. Reduced size because the height is limited to that of the basic planar transmission lines structure, and the extra length often required to accommodate cross-overs is avoided.
4. Lower mass as a result of the smaller size.
5. Better reproducability between examples of the device is possible, either as simple printed or machine structures, without any need for hand-made interconnections.
6. Lower cost and higher reliability because the structure is simpler, and the extra parts and connections required for cross-overs are avoided.
7. Less likelihood of passive intermodulation product generation and multipaction breakdown because internal discontinuities are avoided. This is particularly important in high power, multi-carrier application.
8. Better amplitude and phase balance and tracking between output ports, as the electrical lengths within the network are better controlled due to the inherent symmetry.
[0020] When one considers the synthesis of n x n multi-port microwave couplers from 2 x 2 hybrid couplers where n = 2(2+p), the incidence of cross-over connections is much higher as illustrated in Figure 4. From this figure it will be noted that an 8 x 8 microwave coupler is formed from twelve 2 x 2 hybrid couplers which are arranged in three sets of four, the first set A, B, C and D defining the eight input ports In 1, In 2, In 3, In 4, In 5, In 6, In 7 and In 8 and the third set I, J, K and L defining the eight output ports Out 1, Out 2, Out 3, Out 4, Out 5, Out 6, Out 7 and Out 8. This 8 x 8 microwave coupler is known as a Butler matrix and it will be noted from Figure 4 that there are two cross-overs in the first group of transmission lines 11, 12, 13, 14, 15, 16, 17 and 18 between the first set A, B, C and D the second set E, F, G and H, and six cross-overs in the second group of transmission lines 21, 22, 23, 24, 25, 26, 27 and 28 between the second set E, F, G and H and the third set I, J, K, and L, thereby incurring a total of eight cross-overs.
[0021] The portion of the matrix provided by the couplers A, B, E and F is the same as shown in Figure 2 and is also the same as the arrangement of the 2 X 2 hybrid couplers C, D, G and H. It might be considered that two of the cross-overs in the 8 X 8 multi-port coupler of Figure 4 could be avoided by reorganising the 2 X 2 hybrid couplers A, B, E and F as is know from the arrangement of Figure 3, and similarly by reorganising the 2 X 2 hybrid couplers C, D, G and H. However, this does not simplify the remaining six cross-overs and adds further complications as the outputs Out 1 and Out 2 in Figure 3 are on the opposite side of the coupler to the outputs Out 3 and Out 4 - indeed this introduces two more cross-overs so that one is left with eight cross-overs and no improvement except that all cross-overs are limited to the second group of transmission lines 21, 22, 23, 24, 25, 26, 27 and 28.
[0022] Figures 5 and 6 illustrate alternative reorganisations of the 8 X 8 multi-port coupler of Figure 4 to eliminate all cross-overs. As the components and connections are identical to Figure 4, the same reference letters and numerals have been used to indicate equivalent components.
[0023] Referring specifically to Figure 5, it will be noted that the first set A, B, C and D are arranged in line but in the sequence A, B, D and C - that is the first, second, fourth and third hybrid couplers of the first set. It will be noted that one half of the second set of hybrid couplers, F and H is arranged to one side of the first set A, B, D and C whereas the other half of the second set of hybrid couplers E, G is arranged on the other side, the first and second sets being connected by the first group of transmission lines 11, 12, 13, 14, 15, 16, 17 and 18 as before. It will also be seen that transmission lines 21, 22, 25 and 26 of the second group connect one half of the third set of hybrid couplers, J and I, to the hybrid couplers E, G of the second set lying to one side of the line of couplers A, B, D and C. Similarly, coupling lines 23, 24, 27 and 28 of the second group connect the hybrid couplers K and L of the third set to hybrid couplers F and H of the second set lying on the opposite side of the line of hybrid couplers A, B, D and C. In this manner all of the transmission lines are arranged to achieve the same connections as indicated in Figure 4, but without incurring any cross-overs. Figure 5 therefore teaches one way of synthesising an 8 X 8 multi-port microwave coupler from 2 X 2 hybrid couplers without any cross-over connections, thereby enabling all of the first and second groups of transmission lines to lie in one plane to give a planar realisation with all the attendant advantages already listed above in relation to the planar realisation of the 4 X 4 multi-port coupler of Figure 3. Although it is convenient to arrange the first set of hybrid couplers A, B, D and C in line as shown in Figure 5, it is not essential to do so provided they are arranged substantially in a row extending between the second set of 2 X 2 hybrid couplers with the hybrid couplers E and G on one side of the row and the hybrid couplers F and H on the other side. If desired, the input ports In 1, In 2, In 3 and In 4 could be rotated into the centre of the group A, F, B and E so that the first four input ports are located close to each other. Similarly, the input ports In 5, In 6, In 7 and In 8 could be rotated into the centre group D, H, C and G. Also the outlet ports Out 1 and Out 2 could be rotated upwards to localise the first four output ports Out 1, Out 2, Out 3 and Out 4 between the hybrid couplers J and I, and similarly the output ports Out 5 and Out 6 could be rotated downwards to locate the remaining output ports Out 5, Out 6, Out 7 and Out 8 between the hybrid couplers K and L.
[0024] Referring finally to Figure 6, it will be noted that the second set of 2 X 2 hybrid couplers E, F, G and H are arranged in line but in the sequence E, F, H and G - that is the first, second, fourth and third couplers in the second sequence. Half of the first set of 2 X 2 hybrid couplers A and C and one half of the third set of 2 X 2 hybrid couplers K and I are arranged to one side of the line of couplers E, F, H and G whilst the other half of the first and third sets, B, D, L and J are arranged to the other side of the line of couplers E, F, H and G. In this manner the first and second groups of transmission lines are also positioned so that there are no cross-overs and again a planar realisation of the 8 X 8 multi-port microwave coupler is achieved. As with Figure 5, the inlet and outlet ports may be moved so that they are grouped together in sets of four.
[0025] Whilst the invention has been specifically described with reference to a multi-port microwave coupler having n input ports and n output ports where n = 2(2+p) and p = 1, it is believed that the teaching of Figures 5 and 6 may be usefully applied to higher orders of multi-port microwave couplers. The next order of multi-port coupler (that is where p = 2) would require twice the input and output ports of an 8 X 8 multi-port coupler. Clearly the first three sets of 2 X 2 hybrid couplers could be split into two 8 X 8 multi-port couplers as shown in either Figure 5 or Figure 6, thereby enabling a considerable simplification of the 16 X 16 multi-port coupler. At the present time we have not studied the complete circuitry for a 16 X 16 multi-port coupler and have not established whether all cross-overs could be eliminated by utilising the manipulations taught herein. However, it is quite clear that the total number of cross-overs could be greatly reduced by utilising the teaching of the present invention.
[0026] As previously stated, the input ports and output ports in both Figures 5 and 6 can be rotated to form groups of four and this opens the possibility of arranging higher-order multi-port microwave couplers as a series of circuits (such as Figure 5 or Figure 6) with the eight outputs of one circuit being connected to the eight inputs of the next circuit.
1. A multi-port microwave coupler, having n input ports (In 1 to In 8) and n output ports
(Out 1 to Out 8), where n = 2(2+p) and p is a whole number, comprising a matrix formed from several sets of hybrid couplers
(A to L), each hybrid coupler (A to L) having two inputs and two outputs, a first
of said sets (A, B, C, D) comprising n/₄ pairs of the the 2 x 2 hybrid couplers of which the inputs define said input ports,
a second of said sets (E, F, G, H) also comprising n/₄ pairs of the 2 x 2 couplers, a third of said sets (I, J, K, L) comprising n/₂ of the 2 x 2 hybrid couplers, the outputs from each pair of 2 X 2 hybrid couplers
from the first set being connected by a first group of transmission lines (11 to 18)
to the appropriate inputs of an associated pair of 2 X 2 hybrid couplers from the
second set without any cross-over connections, and the outputs from the second set
being connected by a second group of transmission lines (21 to 28) to the appropriate
inputs of said third set with less than six cross-over connections.
2. A microwave coupler, according to Claim 1, in which p is 1 thereby providing 8 input
ports (In 1 to In 8) and 8 output ports (Out 1 to Out 8), and wherein there are only
three sets of 2 x 2 hybrid couplers (A, B, C, D, E, F, G, H, I, J, K, L) whereby the
outputs of the third set (I, J, K, L) defines said output ports, and said second group
(21 to 28) of transmission lines do not cross each other.
3. A microwave coupler, according to Claim 2, in which the first and second groups of
transmission lines (11 to 18 and 21 to 28) lie in the same plane.
4. A multi-port microwave coupler, according to claim 1, in which one of said sets of
2 X 2 hybrid couplers (A, B, C, D) are arranged substantially in a row, one half (F,
H) of another of said sets of 2 X 2 hybrid couplers (E, F, G, H) are arranged to one
side of said row and the other half (F, G) of said another set (E, F, G, H) are arranged
to the other side of said row, and the first group of transmission lines (11 to 18)
lie in the same plane.
5. A multi-port microwave coupler, according to claim 4, in which the first set of 2
X 2 hybrid couplers (A, B, C, D) are arranged substantially in a row, one half (F,
H) of the second set of 2 X 2 hybrid couplers (E, F, G, H) are arranged to one side
of said row, the other half (E, G) of the second set of 2 X 2 hybrid couplers (E,
F, G, H) are arranged to the other side of said row.
6. A multi-bit microwave coupler, according to Claim 5, in which the four 2 X 2 hybrid
couplers of the first set (A, B, C, D) are arranged in the sequence, first, second,
fourth and third (A, B, D, C).
7. A multi-bit microwave coupler, according to Claim 5 or 6, in which the second group
of transmission lines (21 to 28) connect one half (K, L) of the third set of 2 X 2
hybrid couplers (I, J, K, L) to said one half (F, H) of the second set of 2 X 2 hybrid
couplers (E, F, G, H) and the other half (I, J) of the third set of 2 X 2 hybrid couplers
(I, J, K, L) to said other half (E, G) of the second set of 2 x 2 hybrid couplers
(E, F, G, H), and the second group of transmission lines (21 to 28) lie in the same
plane.
8. A multi-port microwave coupler, according to Claim 4, in which the second set of 2
X 2 hybrid couplers (E, F, G, H) are arranged substantially in a row, one half of
each of the first and third sets (B, D and L, J) of 2 X 2 hybrid couplers are arranged
to one side of said row, of the first and third sets (A, C and I, K) of 2 X 2 hybrid
couplers are arranged to the other side of said row.
9. A multi-bit microwave coupler, according to Claim 8, in which the four 2 X 2 hybrid
couplers of the second set (E, F, G, H) are arranged in the sequence, first, second,
fourth and third (E, F, H, G).
10. A beam-forming network for a multi-beam antenna incorporating a multi-bit microwave
coupler in accordance with any preceding claim.