EP0215971A1

EP0215971A1 - Antenna feed network

Info

Publication number: EP0215971A1
Application number: EP85112056A
Authority: EP
Inventors: Richard Paul Flam
Original assignee: Allied Corp
Current assignee: Allied Corp
Priority date: 1985-09-24
Filing date: 1985-09-24
Publication date: 1987-04-01

Abstract

An antenna feed network is described incorporating a plurality of transmission lines (47, 155, 66), a plurality of couplers, (108, 109) and a plurality of resistors (57, 150) for distributing microwave signals to overlapping subarrays of antenna elements (11 to 24) with minimum power loss.

Description

Background of the Invention

Field of the Invention:

This invention relates to antenna feed networks and more particularly to a microwave network for coupling a plurality of microwave signals to respective subarrays of antenna elements of an antenna.

Description of the Prior Art:

Phased array antenna typically have a plurality of radiating elements along a path. Each radiating element is fed with a microwave signal having a particular phase. In the general case, a phase shifter is provided between a microwave signal and each element so that the phase of the microwave signal at each element may be controlled. In order to reduce the number of phase shifters required to drive a phased array antenna, subarrays are fed with a microwave signal through a single phase shifter. The subarray which may comprise several antenna elements, such as two or greater, are fed with an antenna freed network where the microwave signal, after leaving the phase shifter, is divided and the signal power is prorated in a predetermined manner along the subarray elements. The amount of power distributed to each element is also known as the illumination function and by providing a predetermined illumination function such as a sin x/x pattern, a beam of a predetermined width may be generated in the far field. The power distributed to the radiating elements of the subarray may also be adjusted to provide a Taylor, uniform, Chebycheff, or binomial function which is well known in the art. The subarrays of a phased array antenna may be spaced apart by a predetermined distance or may be overlapped with other subarrays. With overlapped subarrays, common elements are used for each subarray and the antenna feed network must combine the microwave signals for each subarray together before feeding the common antenna element.
By overlapping subarrays, grating lobes may be suppressed while the antenna is scanned over a predetermined angular range.
In U.S. Patent 4,321,605 which issued on March 23, 1982 to Alfred R. Lopez, an array antenna is described. In Fig. 4, a plurality of 2N first transmission lines are shown for supplying wave energy to one of the element groups. Second transmission lines having a signal input end intersect a selected number of first transmission lines before being terminated at its other end. Directional couplers are provided for coupling the second transmission lines to the first transmission lines.
In U.S. Patent 4,143,379 which issued on March 6, 1979 to H. A. Wheeler, an antenna feed network is shown such as in Figs. 3 and 7 for feeding a phased array antenna having overlapped subarrays. In Fig. 3, an 8 element subarray is shown being fed at input port 31d from one phase shifter wherein elements 2 and 7 in the subarray are not fed to provide a resulting sin x/x illumination pattern. The adjacent subarray, being fed at input port 31c, overlaps the subarray fed by input port 31d by 6 elements.
Fig. 7 shows a modular coupling network 94d with input port 31d which, when combined with a number of similar modules, provides a coupling network to several overlapped subarrays. In Fig. 7, branch line directional couplers, shown in more detail in Fig. 5, are used to divide the power further from power divider 36d. Zero db couplers are shown such as 82_a through 82_e for providing crossover networks in a single wiring plane. A more detailed description of the zero db couplers is found in column 5 and Fig. 6. As shown in Figs. 3 and 7, the microwave signal from input port 31d is divided by power divider 36d and fed over two transmission lines to antenna element terminals 110d and 112d. Signals for other elements of the subarray are coupled from the two transmission lines feeding elements 110d and 112d.
As shown in Fig. 7, each coupler 60a-60h has one port terminated by a resistive load shown by the black circle. In couplers 60a-60d very little power is dissipated in the resistive load since it is the isolated port. In couplers 60e-60h, however, considerable microwave signal power is dissipated by the resistive load since the resistive load is attached to one of the output ports. Thus, each microwave signal coupled to an antenna element through a coupler results in microwave signal power being dissipated in the resistive load.
In U.S. Patent 4,041,501 which issued on August 9, 1977 to R. F. Frazita et al., a phased array antenna system is described using coupling circuits to reduce the number of phase shifters required. In Fig. 6 phase shifter 13a provides a microwave signal to power divider 48 which divides the signal and provides it on transmission lines 50 and 52 to antenna elements 12a through 12d. In addition, couplers 58 and 60 couple microwave energy from transmission lines 50 and 52, respectively, onto transmission lines 56 and 54, respectively. Transmission lines 56 and 54 have attenuators 66 and 64 in the line to couple a predetermined amount of microwave energy to other antenna elements by way of couplers 58 and 60, respectively. As may be seen in Fig. 6, each phase shifter 13a through 13f provides a microwave signal to a respective module which in turn directly drives its antenna elements and at the same time couples power off to other antenna elements in other modules so as to provide overlapping subarrays with each subarray having a predetermined illumination function. Frazita et al. also shows in Fig. 2 and discusses in column 4, at lines 17-36, the spacing of the subarrays so that the grating lobe does not enter the subarray pattern when the array is scanned.
U.S. Patent 3,803,625 which issued on April 9, 1974 to J. T. Nemit, a network approach is described for reducing the number of phase shifters in a limited scan phased array. Fig. 5 of Nemit shows a three element subarray being fed by a microwave signal from phase shifter 29. The subarray and an adjacent subarray are overlapped by one antenna element. For example, element 20 is fed with microwave signals from phase shifters 28 and 29 and combined together by coupler 25.
It is therefore desirable to provide an antenna feed network for coupling a plurality of microwave signals to respective subarrays of antenna elements wherein the feed network associated with each microwave signal is symmetric about a reference line dividing the subarray in half.
It is further desirable to provide an antenna feed network for coupling a plurality of microwave signals to respective subarrays of antenna elements utilizing topological symmetry to minimize the number of couplers having different coupling values.
It is further desirable to provide an antenna feed network for coupling a plurality of microwave signals to respective subarrays of antenna elements utilizing topological symmetry to minimize the range of coupler values required.
It is further desirable to provide an antenna feed network utilizing a distribution transmission line having short feed lengths from a respective microwave signal source to the antenna elements it is coupled to.
It is further desirable to provide an antenna feed network utilizing a distribution transmission line having a termination resistor at both ends.
It is further desirable to provide an antenna feed network for feeding a plurality of overlapping subarrays of antenna elements wherein each antenna element may receive a plurality of microwave signals using four port branch line couplers and wherein one resistor or less per antenna element absorbs microwave energy and wherein a second resistor per antenna element terminates the end of a transmission line opposite the end coupled to the antenna element wherein the antenna feed network is operable over a broad band and has low power loss.
It is further desirable to provide an antenna feed network for coupling a plurality of microwave signals to be respective subarrays of antenna elements wherein each microwave signal passes through the least number of couplers to minimize the build up of coupling errors from the couplers.
It is further desirable to provide an antenna feed network wherein microwave signal power is distributed from first transmission lines to second transmission lines using couplers of the branch line type and wherein the power in the network flows forward towards the antenna elements in all cases along all paths by action of the couplers.

Summary of the Invention

An antenna feed network for coupling a plurality of microwave signals to respective subarrays of antenna elements is described comprising a plurality of first transmission lines spaced apart and adjacent one another, a first end of each of the first transmission lines adapted for coupling to a respective antenna element, a second end of each of the first transmission lines coupled to an impedance for terminating the first transmission line, a plurality of couplers each having first through fourth ports and interconnected into each of the first transmission lines at predetermined locations, and a plurality of second transmission lines, each interconnecting a coupler on a plurality of selected first transmission lines, each second transmission line having a node interior of first and second ends adapted for coupling to a respective microwave signal, the first and second ends coupled to a respective impedance for terminatting the second transmission line.

Brief Description of the Drawing

Fig. 1 is one embodiment of the invention.
Figs. 2A, and 2C through 2D show schematic diagrams of a four port coupler.
Fig. 2B shows a plan view of a four port branch line coupler.
Fig. 3 is an alternate embodiment of the invention.
Fig. 4 is another alternate embodiment of the invention.
Fig. 5 is a plan view of one printed circuit board layout of module 159 shown in Fig. 1.

Description of the Preferred Embodiment

Referring to the drawing and more particularly to Fig. 1, antenna feed network 10 is shown for coupling plurality microwave signals 0̸₁-0̸₄ to respective subarrays of antenna elements 11 through 24. Antenna elements 11-24 form a phased array antenna 25 having an aperture determined by the spacing and number of antenna elements. For example, additional antenna elements may be spaced on either side of antenna elements 11 and 24 to provide a wider aperture than what is shown in Fig. 1. Antenna elements 11-24 may be spaced apart unevenly and follow a predetermined path which may be, for example, a straight line or they may be spaced apart evenly along a predetermined path. Microwave energy is coupled to antenna elements 11-24 which may from element to element be a signal of predetermined power and phase or a combination of signals each having a predetermined power and phase.
Oscillator 26 functions to provide a microwave signal at a predetermined frequency which is coupled over line 27 to phase shifters 28-31. Phase shifters 28-31 function in response to control signals on lines 32-35, respectively, to provide a predetermined phase shift. Phase shifters 28-31 provide an output signal on lines 36-39, respectively, of microwave signals 0̸₁-0̸₄, respectively, to antenna feed network 10.
A plurality of transmission lines 42 through 51 are spaced apart and adjacent one another and may be, for example, parallel to one another having a first end coupled to antenna elements 11, 12, 14, 15, 17, 18, 20, 21, 23 and 24, respectively. Transmission lines 42-51 have a second end coupled to impedances 52-61 respectively, which may be, for example, a resistor having a value equal to the impedance of the respective transmission line for terminating the transmission line. Transmission lines 42-51 may be, for example, 50 ohms or less and may be a conductor having a predetermined width on a printed circuit board with a ground plane on the other side. Transmission lines 64-67 have one end coupled to antenna elements 13, 16, 19 and 22, respectively, and the other end coupled to node 68-71, respectively, which receives a microwave signal over lines 36-39, respectively. Transmission line 64, for example, is spaced apart and adjacent transmission lines 43 and 44. Transmission line 65 is spaced apart and adjacent transmission lines 45 and 46. Transmission line 66 is spaced apart and adjacent transmission lines 47 and 48. Transmission line 67 is spaced apart and adjacent transmission lines 49 and 50.
Transmission line 42 has four couplers interconnected at predetermined locations along the line. Couplers 74-77 function to couple microwave energy from a transmission line traversing transmission line 42 onto transmission line 42 in the direction toward antenna element 11. Each coupler 74-77 has four ports as shown in Figs. 2C and 2E. Couplers 74-77 may, for example, be a branch line coupler having the characteristics shown in Fig. 2A, wherein an input signal on line 78 shown by arrow 79 to coupler 80 provides an output signal on line 81 having an amplitude K and an output signal on line 82 having an amplitude
with a 90° phase shift with respect to the output signal on line 81. A second port to coupler 80 on line 83 receives zero power from coupler 80 when an input signal is entered on line 78. If the value of K is equal to 1/√2 then the output signals on lines 81 and 82 are even and coupler 80 is described as a three db coupler well known in the art. Fig. 2B shows a plan view of one embodiment of Fig. 2A. In Fig. 2B a four port branch line coupler 80 is shown having conductors of predetermined width and length on a printed circuit board 84 having a ground plane 85 on the lower surface. A dielectric material 86 separates ground plane 85 from metallization 87 on the upper surface.
The coupler shown in Fig. 2A, which is well known in the prior art, may also be represented by the symbols or schematics shown in Fig. 2C and 2E. For example, couplers 74 and 76 shown in Fig. 1 correspond to the symbol for a coupler shown in Fig. 2C. Couplers 75 and 77 shown in Fig. 1 correspond to the symbol for a coupler shown in Fig. 2E. The four ports of coupler 74 are shown in Fig. 1 as 78' and 81'-83'. The four ports of coupler 75 are shown in Fig. 1 as 78" and 81"-83".
Transmission line 43, as well as transmission lines 44-51, each have four couplers interconnected into its transmission line at predetermined locations. Each coupler has four ports and have functions corresponding to Figs. 2C and 2E. Couplers 74-77 and 90-125 provide a means for coupling microwave signals to each antenna element via the respective transmission line the coupler is located in. As may be seen in Fig. 1, Figs. 2C and 2E port 83 shown in Fig. 1 as 83' and 83" of each coupler is always connected towards the termination resistor or away from the antenna element. Port 82 of each coupler is always interconnected into the transmission line on the side towards the antenna element.
A plurality of transmission lines 128-133 are positioned transverse to transmission lines 42-51 and are interconnected to couplers on selected transmission lines 42-51. Each transmission line 128-133 has a first end adapted for coupling to a respective microwave signal such as signals 0̸₁-0̸₆ and a second end coupled to an impedance 139 and 134-138, respectively, for terminating the transmission line. Impedances 134-139 may, for example, be a resistor having an ohmic value equal to the impedance of its respective transmission line. Transmission line 128 is terminated by impedance 139 or if transmission line 128 continues to other couplers (not shown) the impedance would be moved to the end of the line.
Each coupler interconnected to one of transmission lines 128-133 has its port 78 as shown in Fig. 2E or 78" as shown in Fig. 1, port 78 is coupled to the transmission line towards the first end where the microwave signal is connected. Port 81 as shown in Fig. 2E or 81" as shown in Fig. 1 is coupled to the transmission line on the side towards the second end or towards the termination impedance. In this manner, all microwave signals traveling down transmission lines 128-133 will either pass through coupler 80 and out port 82 towards an antenna element or out port 81 and continue along the transmission line. Substantially no microwave signal energy is coupled out port 83.
In Fig. 1, transmission line 142 has one end coupled to microwave signal 0̸₈. Transmission line 142 is interconnected to couplers 76 and 93 and terminated at its other end by impedance 148. Transmission line 143 has one end coupled to microwave signal 0̸₇ and passes through couplers 74, 91, 96 and 101. The second end of transmission line 143 is coupled to impedance 149. Transmission line 144 has one end coupled to node 68 which receives microwave signal 0̸₁. Transmission line 144 is interconnected to couplers 94, 99, 104 and 109 with its second end coupled to impedance 150. Transmission line 145 has one end coupled to node 69 which is coupled to microwave signal 0̸₂. Transmission line 145 is interconnected to couplers 102, 107, 112 and 117. The other end of transmission line 145 is coupled to impedance 151. Transmission line 146 has one end coupled to node 70 which is coupled to microwave signal 0̸₃. Transmission line 146 is coupled through couplers 110, 115, 120 and 125. The other end of transmission line 146 is coupled to impedance 152. Transmission line 147 has one end coupled to node 71 which is coupled to microwave signal 0̸₄. Transmission line 147 is interconnected to couplers 118 and 123. The other end of transmission line 147 is coupled to impedance 153 as shown in Fig. 1 or transmission line 147 may extend through additional couplers, not shown, with impedance 153 moved to the end of the line.
As shown in Fig. 1 microwave signal 0̸₂ is coupled by way of node 69 which divides the microwave signal along transmission lines 129 and 145 to antenna elements 11-15 and 17-21, respectively. Transmission lines 129 and 145 may also be one single transmission line 154 with an electrical tap or node 69 interior of the ends of transmission line 154 adapted for coupling to microwave signal 0̸₂. Transmission line 65 coupled to node 69 is coupled directly to antenna element 16. Microwave signal 0̸₂ is therefore coupled to antenna elements 11-21 to provide an 11 element subarray. As may be seen in Fig. 1, antenna elements 13 and 19 are not coupled to microwave signal 0̸₂ since the selected illumination function calls for the antenna element at this location to be driven with zero power. Antenna elements 13 and 19 are considered to be, however, part of the 11 element subarray since the illumination function of the 11 elements provides a predetermined pattern in the far field of antenna 25.
Microwave signal 0̸₃ is coupled by way of transmission line 130 to antenna elements 14-18 and by transmission line 146 to antenna elements 20-24. Transmission lines 130 and 146 may also be one single transmission line 155 with an electrical tap or node 70 interior of the ends of transmission line 155 for coupling to microwave signal 0̸₃. Transmission line 66 is coupled to microwave signal 0̸₃ at node 70 and directly to antenna element 19. Microwave signal 0̸₃ therefore is coupled to antenna elements 14-24 to provide an 11 element subarray. Again it is understood that antenna elements 16 and 22 receive no power from microwave signals 0̸₃ due to the selected illumination function but is still considered part of the 11 element subarray. As may be seen in Fig. 1, the subarray associated with microwave signal 0̸₂ and the subarray associated with microwave signal 0̸₃ have an 8 antenna element overlap, that is to say over a width of 8 antenna elements some individual elements receive both microwave signals 0̸₂ and 0̸₃. Microwave signal 0̸₁ is shown coupled to a subarray of 8 antenna elements, elements 11-18. Typically, microwave signal 0̸₁ would be coupled to 11 antenna elements by extending the left-hand portion of the drawing to provide a complete subarray similar to the subarray associated with microwave signal 0̸₂. Microwave signal 0̸₁ shows an 8 element overlap with microwave signal 0̸₂. Likewise, microwave signal 0̸₄ is shown coupled to 8 antenna elements, elements 17-24 to provide an 8 element subarray having all 8 elements overlapped with microwave signal 0̸₃. The subarray associated with microwave signal 0̸₄ may be extended on the right-hand portion of the drawing to provide a complete subarray of 11 elements similar to the subarray associated with microwave signal 0̸₃.
In Fig. 1 coupler 74 shows the interconnections of lines 78', and 81'-83'. Since no input signal on line 78' is coupled out on line 83' towards impedance 52, the microwave signal 0̸₇ is coupled towards antenna elements 11, 12, 14 and 15 by way of couplers 74, 91, 96 and 101. Substantially no microwave signal is coupled from couplers 74, 91, 96 and 101 towards impedances 52-55. Thus as may be seen in Fig. 1, from the arrangement of all of the couplers, the microwave signals or energy is always coupled forward towards antenna elements 11-24. In order to provide a predetermined phase front at the antenna elements of a subarray in a preferred embodiment, couplers are spaced a predetermined distance apart, such as by one-half wavelength along the distributing transmission line such as transmission line 129 or 145. Phase reversal at an antenna element may be provided by appropriate spacing.
Microwave signal 0̸₂ coupled through coupler 102, for example, from transmission line 145 to transmission line 46 will travel down transmission line 46 towards coupler 103. At coupler 103, some of the signal will continue along microwave transmission line 46 and some will be diverted along transmission line 130 towards coupler 100. Some of this microwave signal 0̸₂ traveling along transmission line 130 will be coupled at coupler 100 to antenna element 15 and some will continue to coupler 97, where it will be coupled either to antenna element 14 or to impedance 135. Therefore, in determining the parameter K for each coupler to provide a predetermined subarray illumination function, in the antenna feed network additional microwave energy from indirect or sneak paths must be factored in. As may be seen in Fig. 1, since each transmission line associated with the antenna elements such as 42-51 have four couplers associated therewith, equations may be written for the division of the microwave signals for each subarray of antenna elements and solved to provide the value K for each of the couplers as a function of its location in the antenna feed network. It is noted that the longest path of any microwave signal is through four couplers to an antenna element. Furthermore, it is noted that all the direct and indirect paths through the couplers result in microwave energy being coupled towards the antenna element with substantially no microwave energy being coupled towards the termination resistors 52-61. However, resistors 134-139 and 148-153 at the end of the distributing transmission line 128-133 and 142-147 dissipate residual microwave energy not coupled to one of antenna elements 11-14.
Antenna feed network 10 may be subdivided into a plurality of identical modules 158 through 161 each having a microwave signal input and 3 outputs coupled to respective antenna elements. By utilizing the circuitry of a module such as module 158, a plurality of subarrays of antenna elements may be interconnected to a respective microwave signals by using a number of modules. In this manner, any antenna aperture size may be accommodated or provided wherein each subarray has 11 antenna elements Each subarray has an 8 element overlap with the adjacent subarray and a 5 element overlap with the next adjacent subarray and a 2 element overlap with the third adjacent subarray on each side.
Modules 162 and 163 are end modules of the antenna feed network 10 which merely terminate the transmission lines not having additional antenna elements to feed. Microwave signals 0̸₈ and 0̸₇ may be removed, for example, and couplers 74, 76, 91, 93, 96 and 101 associated therewith may be removed. However, if removal will interface with the calculation of the additional microwave signal paths then these couplers and transmission lines may remain with an appropriate microwave signal 0̸₈ or 0̸₇ coupled thereto or with the input left open or terminated in the characteristic impedance of the transmission line.
Fig. 3 is an alternate embodiment of the invention. In Fig. 3 like references are used for functions corresponding to the apparatus of Fig. 1. In Fig. 3 antenna feed network 10' includes modules 158'-161'. Microwave signals 0̸₁-0̸₈ are coupled by way of antenna feed network 10' to a respective subarray of an even number of elements, such as 8 elements for microwave signal 0̸₂, with a 6 element overlap with the adjacent subarray. Nodes 68'-71' divide the microwave signal received on lines 36-39, respectively, onto two transmission lines for distribution such as transmission lines 128 and 144 for microwave signal 0̸₁. Antenna elements 11, 12, 13, 14, 15, 17, 18, 20, 21, 23 and 24 are shown in Fig. 3 as unevenly spaced apart. In the normal practice of the invention, the above antenna elements would be evenly spaced apart.
Fig. 4 shows an alternate embodiment of the invention. In Fig. 4 like references are used for functions corresponding to the appratus of Figs. 1, 2A, 2B, 2D and 2F. In Fig. 4 antenna feed network 10" provides a distribution network for forming a plurality of 17 element subarrays with a 14 element overlap with the adjacent subarray.
Fig. 5 is a plan view of a printed circuit board layout of module 159 shown in Fig. 1. In Fig. 5 like references are used for functions corresponding to the apparatus of Figs. 1, 2A, 2B, 2C and 2E. Fig. 5 shows module 159 utilizing branch line hybrid couplers 98-105. The metallization lines have a predetermined width on a printed circuit board 84 having a dielectric material 86 of a predetermined thickness with a ground plane 85 on the lower surface to provide a transmission line characteristic. The crossover of transmission lines 143 and 130 is provided by coupler 170. The crossover of transmission lines 129 and 144 is provided by coupler 171. The crossover of transmission lines 131 and 144 is provided by coupler 172. The crossover of transmission lines 165 and 144 is provided by coupler 173. The crossover of transmission lines 165 and 130 is provided by coupler 174.
As shown in Fig. 5, node 69 includes a coupler 175 having the isolated port terminated by impedance 176. An output of coupler 175 is coupled over line 177 to divider 178 which functions to divide the microwave signal received on line 177 into equal parts on to transmission lines 129 and 145. Divider 178 has a resistive impedance 179 across lines 129 and 145.
The antenna feed networks as shown in Figs. 1, 3 and 4 are suitable for collecting the microwave signals received by the antenna elements and coupling them to the phase shifters. If the phase shifters have the same delay in both directions, such as diode phase shifters, a receiver may be positioned after the phase shifter, such as on line 27 in Fig. 1, with the oscillator 26 disconnected. The antenna feed networks are reciprocal and may either transmit or receive microwave signals via antenna elements in the array antenna.
An antenna feed network for coupling a plurality of microwave signals to be transmitted to respective subarrays of antenna elements and for collecting microwave signals received by respective subarrays of antenna elements has been described incorporating a plurality of first transmission lines spaced apart and adjacent to one another, having one end of each transmission line adapted for coupling to a respective antenna element and a second end of each first transmission line coupled to an impedance for terminating the first transmission line, a plurality of couplers each having four ports interconnected into each transmission line at predetermined locations, and a plurality of second transmission lines, each interconnecting a coupler on a plurality of selected first transmission lines, each second transmission line having a node interior of first and second ends adapted for coupling to a respective microwave signal and said first and second ends coupled to a respective impedance for terminating the second transmission line.

Claims

1. An antenna feed network for coupling a plurality of microwave signals ( 0̸₃, 0̸₄) to respective subarrays of antenna elements comprising,
a plurality of first transmission lines (47, 48) spaced apart, a first end of each of said first transmission lines adapted for coupling to a respective antenna element (18, 20), a second end of each said first transmission lines coupled to an impedance (57, 58) for terminating said first transmission line (47, 48),
a plurality of couplers (106, 108, 110, 111) each having first through fourth ports, at least two of said couplers (106, 108, 110, 111) interconnected into each of said first transmission lines at predetermined locations, and
a plurality of second transmission lines (155, 131, 147), each interconnecting a coupler on a plurality of selected first transmission lines, characterized by each said second transmission line (155, 131, 147) having a node (70, 71) interior of first and second ends adapted for coupling to a respective microwave signal (0̸₃, 0̸₄), said first and second ends coupled to a respective impedance (135, 152, 136, 153) for terminating said second transmission line (155, 131, 147).

2. The antenna feed network of claim 1 wherein one of said plurality of couplers (106) is characterized by a branch line coupler having first through fourth ports wherein an input signal on said first port will provide a K output on said third port, a

output on said fourth port where K is a constant and substantially zero output on said second port, said first port coupled to said second transmission line (155) leading towards said node (70), said third port coupled to said second transmission line (155) leading towards one of said first and second ends.

3. The antenna feed network of claim 1 wherein one of said couplers (106) is characterized by a branch line coupler having first through fourth ports wherein an input signal on said first port will provide a K output on said third port, a

output on said fourth port where K is a constant and substantially no output on said second port, said second port coupled to said first transmission line (47) leading towards said second end and said fourth port coupled to said first transmission line (155) leading towards said first end.

4. The antenna feed network of claim 1 further characterized by
third transmission lines (66, 67), each positioned between two adjacent first transmission lines (47, 48, 49, 50), a first end of each of said third transmission lines adapted for coupling to a respective antenna element (19, 22), a second end of each of said third transmission lines (66, 67) coupled to one of said second transmission lines (155, 131, 147).

5. The antenna feed network of claim 4 characterized by said second end of each of said third transmission lines (66, 67) coupled to one of said second transmission lines (155, 131, 147) at said node (70, 71).

6. The antenna feed network of claim 5 characterized by said node (70) positioned interior of said ends and between an equal number of selected first transmission lines.

7. The antenna feed network of claim 1 characterized by said node (70) positioned interior of said ends and at least one selected first transmission line (47).

8. The antenna feed network of claim 1 characterized by said node (70) of at least one of said second transmission lines (155) positioned between two of said plurality of first transmission lines (47, 48).

9. The antenna feed network of claim 1 characterized by said node (70) on at least one of said second transmission lines (155) substantially equidistance from said first and second ends.