Lossless Orthogonal Beam Forming Network

1. Field of the Invention

This invention relates generally to phased array antenna systems and more particularly to an orthogonal beam forming network for simultaneously providing multiple beams from a single aperture.

2. Description of the Prior Art

In radar systems, multiple beams allow parallel operation and a higher data rate than can be obtained from a single beam. In communications systems they allow for reuse of frequency and data transmission to different areas. Moreover, if these beams are comprised of sin x/x beams, they can be formed independently and losslessly. If there is provided an N element array with element spacing d, then N (sin x/x) beams can be formed which are spaced .lambda./N.times.d in space and where .lambda. is the operating wavelength. As is well known, the expression for the angle .alpha. of the progressive phase fronts required to form a set of orthogonal sin x/x beams can be stated as: ##EQU1## where m is the beam number and N is the number of elements in the array. Also as is well known, sin x/x beams have relatively high sidelobe levels but they can be combined to form beams with much lower sidelobe levels by providing an amplitude taper.

Until recently, the only network utilized for forming a lossless set of orthogonal sin x/x beams was the well known Butler matrix which has a binary number (2.sup.n) of input ports and output ports coupled to a binary number of antenna elements notwithstanding the fact that an article entitled, "Multiple Beams From Linear Arrays", J. P. Shelton and K. S. Kelleher, IRE Transactions On Antennas And Propagation, Mar., 1961, pp. 154-161, showed that other numbers of elements are theoretically possible using all values of 3.sup.m .times.2.sup.n through the use of a six port junction. One such six port network has furthermore been disclosed, in U.S. Pat. No. 4,231,040, entitled, "Simultaneous Multiple Beam Antenna Array Matrix And Method Thereof", S. Walker, Oct. 28, 1980. Such an arrangement, however, comprises a narrow bandwidth device since both the amplitude and output phase is far from being constant as a function of frequency.

Accordingly, it is an object of the present invention to provide an improvement in orthogonal beam forming networks coupled to a plurality of phased array antenna elements.

Another object of the invention is to provide an improvement in non-binary matrices for forming multiple antenna beams.

Still another object of the invention is to provide an improved six port junction for forming orthogonal antenna beams in a phased array.

Briefly, the foregoing and other objects of the invention are provided in accordance with an orthogonal beam forming network comprising at least one six port junction having three input ports and three output ports and including a pair of first type (3-dB) quadrature couplers and one second type (4.8-dB) quadrature coupler, each having two input ports and two output ports, with one of the input ports of the 4.8-dB coupler and both input ports of one of the pair of 3-dB couplers forming the three input ports of the six port junction, with one of the output ports of one 3-dB coupler being directly coupled to the other input port of the 4.8-dB coupler, first fixed phase shift means (-90.degree.) coupling the other output port of said one first type coupler to one of the input ports of the other or second 3-dB coupler, with one of the output ports of the 4.8-dB coupler being directly connected to the other input port of the second 3-dB coupler, second fixed phase shift means (-240.degree.) coupling the other output port of the 4.8-dB coupler to an output port forming one of the output ports of the six port junction, third fixed phase shift means (-30.degree.) coupling one output port of the second 3-dB coupler to another output port forming the second of the three output ports of the six port junction and the other output port of the second 3-dB coupler functioning as and forming the third output port of the six port junction, whereby a non-binary phased array matrix is formed providing a 120.degree. progressive phase front.

FIG. 1 is an electrical block diagram of a nonlinear matrix illustrating the preferred embodiment of the invention; and

FIGS. 2 through 4 electrical block diagrams illustrative of various types of expanded orthogonal beam forming networks utilizing the embodiment shown in FIG. 1.

Referring now to the drawings and more particularly to FIG. 1, disclosed thereat is an embodiment of an improved six port junction in accordance with the subject invention and one defining a 3.times.3 non-binary signal combiner or beamformer matrix 10 having three output ports a, b and c respectively coupled to three phased array antenna elements 12.sub.1, 12.sub.2 and 12.sub.3. The input ports are designated A, B and C and with the configuration shown, provide respective progressive phase front angles .alpha. of -120.degree., 0.degree., and +120.degree..

The six port junction 10 of FIG. 1 comprises a reciprocal network in that power transfer will be the same for both transmission and reception and is further comprised of a pair of 3-dB couplers 14 and 16, a single 4.8-dB coupler 18 and three fixed phase shift elements 20, 22 and 24 which respectively provide fixed phase shifts of -90.degree., -240.degree., and -30.degree.. These phase shifters, moreover, employ sections of coupled strip transmission lines which operate in the TEM mode and typically consist of two parallel coupled lines 26 and 28 of equal length that are connected at one end. This type of device belongs to a class of broadband microwave phase shifters disclosed, for example, by B. M. Schiffman, in the IRE Transactions On Microwave Theory And Technicues, April, 1958, at pp. 230-237, inclusive.

The three couplers 14, 16 and 18 comprise four port quadrature couplers having two input ports and two output ports which supply signals at the respective output ports that are mutually 90.degree. out of phase. The 3-dB couplers 14 and 16 comprise quadrature couplers wherein power applied to either input port is split approximately equally between the two output ports, whereas the 4.8-dB coupler comprises a coupler that power applied to either of the input ports is split approximately 2 to 1 at the outputs with two thirds times the power input appearing at the output port directly opposite the input port receiving the power and one third times the input power appearing at the diagonally located output terminal.

The preferred embodiment of the invention as shown in FIG. 1 comprises the foregoing elements specifically connected in the following manner. One of the input ports 30 of the 4.8-dB coupler 18 and both input ports 32 and 34, while being shown separately and distinct, normally function as the three input ports A, B and C. One of the output ports 36 of the 3-dB coupler 14 is directly coupled to the other input port 38 of the 4.8-dB coupler 18. The -90.degree. fixed phase shifter 20 is coupled from the other output port 40 to one of the input ports 42 of the second 3-dB coupler 16. The other input port 44 of the 3-dB coupler 16 is directly connected to one output port 46 of the 4.8-dB coupler 18. The other output port 48 of the 4.8-dB coupler 18 is coupled to the output port c of the six port junction 10 by means of the fixed -240.degree. fixed phase shifter 22. The -30.degree. fixed phase shifter 24 couples one output 50 of the 3-dB coupler 16 to output port b of the six port junction 10 while the second output port 51 effectively forms and functions output port a of the six port junction.

The amplitudes and phases produced by the configuration shown in FIG. 1 are those required to form three orthogonal sin x/x beams with a three element array; however, this specific 3.times.3 non-binary matrix 10 can be combined with one or more binary Butler matrices, which in its simplest form comprises a single 3-dB 2.times.2 branch line coupler, to provide the basic building blocks to configure orthogonal beam formers operable with 2.sup.n .times.3.sup.m elements respectively forming 2.sup.n .times.3.sup.m orthogonal sin x/x beams. Examples of such networks are shown in FIGS. 2 through 4.

Referring now to FIG. 2, shown therein is a 6.times.6 beamformer matrix comprised of two 3.times.3 matrices 10.sub.1 and 10.sub.2 having their respective output ports a.sub.1, b.sub.1, c.sub.1 and a.sub.2, b.sub.2, c.sub.2 interlaced to a second level of 3-dB quadrature four port couplers 52, 54 and 56 through a set of four fixed phase shifters 58, 60, 62 and 64. Further as shown, the two input ports of the 3-dB coupler 52 are coupled to the output ports a.sub.1 and a.sub.2 of the 3.times.3 matrices 10.sub.1 and 10.sub.2 with the exception that the output port a.sub.2 is coupled to the input port of 3-dB coupler 52 by means of the fixed phase shifter 62 which provides a -60.degree. phase shift. The two input ports of the 3-dB coupler 54 are coupled to the intermediate output ports b.sub.1 and b.sub.2 through the fixed phase shifters 58 and 64 which provide a phase shift of -30.degree.. The two input ports of the 3-dB coupler 56 are coupled to the intermediate output ports c.sub.1 and c.sub.2 ; however, a -60.degree. fixed phase shifter 60 couples port c.sub.1 to one input of coupler 56 while the other input is directly coupled to intermediate output terminal c.sub.2. It can be seen that the arrangement is nevertheless symmetrical. With beam B6 being considered the left-most beam, six beams B1 through B6 are separately provided at the input ports A.sub.1, B.sub.1, C.sub.1, A.sub.2, B.sub.2, C.sub.2 as shown and the progressive phase fronts defined by angle .alpha. for the six beam ranges between -150.degree. and +150.degree. where the output port P1 is considered the left-most port coupled to respective antenna elements, not shown, of a multi-element array.

Referring now to FIG. 3, shown therein is a 9.times.9 beamformer matrix comprised of a first level of 3.times.3 matrices 10.sub.1, 10.sub.2 and 10.sub.3 interlaced with a second level of 3.times.3 matrices 10.sub.4, 10.sub.5 and 10.sub.6. As illustrated, the intermediate output ports a.sub.1, b.sub.1 and c.sub.1 of matrix 10.sub.1 are respectively coupled to the first input port A.sub.4, A.sub.5 and A.sub.6 of the second level matrices 10.sub.4, 10.sub.5 and 10.sub.6. The intermediate output ports a.sub.2, b.sub.2 and c.sub.2 of matrix 10.sub.2 are respectively coupled to the second input port B.sub.4, B.sub.5 and B.sub.6 with two fixed phase shifters 66 and 68 having phase shifts of -40.degree. and -80.degree. coupled into the lines between ports b.sub.2 and B.sub.5 and c.sub.2 and B.sub.6. The three intermediate output ports a.sub.3 , b.sub.3 and c.sub.3 of matrix 10.sub.3 respectively couple to the input ports C.sub.4, C.sub.5 and C.sub.6 but with a pair of fixed phase shifters 70 and 72 respectively being coupled in the lines between ports a.sub.3 and C.sub.4 and b.sub.3 and C.sub.5. Such an arrangement provides nine output ports P1, P2, P3 . . . P9 which respectively correspond to the ports a.sub.4, a.sub.5, a.sub.6 . . . c.sub.6 as shown in FIG. 3. Nine beams B1, B2, B3 . . . B9 are associated with the input ports C.sub.3, C.sub.1, C.sub.2 . . . B.sub.2 having a progressive phase front which varies between -160.degree. and +160.degree. where beam B.sub.1 corresponds to the left-most beam for a left to right phase front impinging on output ports P1 through P9.

When desirable, the 9.times.9 beam former configuration shown in FIG. 3 can be expanded to an 18.times.18 beam former by a parallel duplication of the embodiment shown in FIG. 3, but with suitable phase shifters inserted into the input lines connected to the ports A, B and C of the first level of 3.times.3 matrices. Although these components are not shown, it is well within the purview of one skilled in the art.

Referring now to FIG. 4, shown therein is a 12.times.12 beamformer matrix comprised of four 3.times.3 matrices 10.sub.1, 10.sub.2, 10.sub.3 and 10.sub.4 each having three input ports A, B and C to provide twelve beam input ports, A.sub.1, B.sub.1, A.sub.2, B.sub.2 . . . C.sub.4 and providing a respective number of intermediate output ports a.sub.1, b.sub.1 . . . c.sub.4. These intermediate output ports are interlaced to a second level of couplers comprising the 3-dB quadrature couplers 74, 76, 78, 80, 82 and 84 which in turn are coupled to a third level of 3-dB quadrature couplers 86, 88, 90, 92, 94 and 96 which provide twelve output ports P1, P2, P3 . . . P12 as shown. Moreover, the two levels of quadrature couplers 74 . . . 84 and 86 . . . 96 furthermore are coupled together in sets of four couplers along with fixed 45.degree. phase shifters in the outside lines which thereby implement three 4.times.4 binary Butler matrices 98.sub.1, 98.sub.2 and 98.sub.3 networks which are well known to those skilled in the state of the art. This configuration is similar to the 6.times.6 beam forming matrix shown in FIG. 2 with the exception that different values of fixed phase shifters are utilized in the connecting lines between the intermediate output ports of the 3.times.3 matrices 10.sub.1, 10.sub.2, 10.sub.3 and 10.sub.4 and the upper level 3-dB couplers 74, 76, 78, 80, 82 and 84. As illustrated in FIG. 4, a pair of fixed phase shifters 100 and 102 having fixed phase shifts of -15.degree. and 30.degree., respectively, are included in the lines coupled between intermediate output ports b.sub.1 and c.sub.1 to one of the input ports of the 3-dB couplers 78 and 82. The intermediate output ports b.sub.2 and c.sub.2 of the second 3.times.3 matrix 10.sub.2 are coupled to the other input ports of the 3-dB couplers 78 and 82 by means of the fixed phase shifters 104 and 106 which provide phase shifts of -45.degree. and -90.degree., respectively. As to the third 3.times.3 matrix 10.sub.3, the intermediate output ports a.sub.3 and b.sub.3 are respectively coupled to one of the input ports of the 3-dB couplers 76 and 80 by means of fixed phase shifters 108 and 110 which provide fixed phase shifts of -90.degree. and -45.degree.. And finally, the fourth 3.times.3 matrix 10.sub.4 has its intermediate output ports a.sub.4 and b.sub.4 coupled to the other input ports of the two 3-dB couplers 76 and 80 by means of fixed phase shifters 112 and 114 respectively providing phase shifts of -30.degree. and -15.degree.. It can be seen that the phase shifts coupled to the outputs of the intermediate output ports of the four matrices 10.sub.1, 10.sub.2, 10.sub.3 and 10.sub.4 are symmetrical on either side of the two intermediate matrices 12.sub.2 and 12.sub.3. Thus where the output port P1 is considered to be the left-most port a progressive phase front for the beams B1 through B12 ranging between -135.degree. and +135.degree. for the ports A.sub.1 through C.sub.4 is provided as shown.

Thus what has been set forth in this detailed description of the invention is an improved non-binary matrix in the form of a six port junction which can either be duplicated and/or combined with other binary matrices, the simplest of which comprises a four port Butler matrix in the form of a 3-dB quadrature coupler, for forming a predetermined number (2.sup.n .times.3.sup.m) of orthogonal sin x/x beams.

While there has been shown and described what is at present considered to be the preferred embodiments of the invention, it should be noted that the foregoing has been made by way of illustration and not limitation. Accordingly, all modifications, alterations and changes coming within the spirit and scope of the invention as defined in the appended claims are herein meant to be included.