CA2030600C - Aircraft antenna with coning and banking correction - Google Patents

Abstract

Array antennas for aircraft use have a shiftable center of radiation. The antenna beam of a group of laterally spaced array antennas is steered and the beam shape is controlled by relative shifting of the centers of radiation of the arrays. Beam tilting in afuselage mounted system of array antennas uses controlled selection of active antennas.

Description

Docket R4454.U1 EAO:cjf 3 The Present invention relates to array 4 antennas and multiple array systems for radiating and receiving electromagnetic signals and, in particular, 6 to antennas adapted for use on aircraft which permit 7 the antenna beam to be steered in azimuth or tilted, 8 or both.

Identification Friend or Foe (IFF) 11 systems are used to enable aircraft to transmit and 12. receive signals fox identification of other aircraFt.
13 Airborne radar systems axe also used for target 14 location without identification capabilities. The higher frequencies typically used fox airborne radar 16 permit use of antennas providing reasonable beam 17 resolution both vertically and horizontally. Airborne 18 linear array antennas used fox IFF may, by contrast, 19 lact the capability of providing significant vertical resolution. Without vertical, or elevation, resolving ~~ 3~~~~
1 capability, no elevation inForrnation is provided by 2 the system. Furthermore, the straight vertical 'Fan 3 beam that the antenna provides in the on-boresight 4 direction perpendicular to the linear array becomes curved or conical in shape when the beam is scanned off 6 boresight. As a result, as illustrated in Fig. 1, if 7 a target exists at a location (a) (15° right and at 8 the same altitude as the reference aircraft) the IFF
9 display would accurately indicate a target at 15°
right. If however, a target were at location (b) (again 11 15° right, but at a higher altitude) the IFF display 12 would indicate a target at azimuth (c), displaced from 13 the actual 15° position of the target. The error is 14 introduced by a "coning" of the antenna beam as it is scanned to the right and effectively assumes a profile 16 of a form shown by curved line (d). The resulting 17 errors introduced by off-boresight coning of the IFF
18 beam, in addition to affecting the accuracy of the IFF
19 target display, can introduce a displacement between the IFF and radar returns displayed for 'the same 21 target.
22 Additional errors are introduced as a 23 result of aircraft banking. In the absence of 24 accurate elevation information, the azimuth of a ~~~~~o~~
1 target cannot be accurately determined ~as the banking 2 maneuver tilts the antenna beam away from the 3 horizontal reference.

In accordance with the present invention, 6 a switchable array antenna, with a plurality of 7 antenna elements arranged for excitation in subsets 8 of at least three elements, includes terminal means 9 for coupling signals and a plurality of antenna elements each comprising a linear array of at 11 least four elements arranged for use in subsets 12 each having first, second and third elements.
13 First excitation means, coupled to the terminal 14 means, includes signal transmission means for coupling forward and rear element signal cornponents of 16 predetermined relative phase and amplitude to first 17 and third elements by way of a point of common 18 voltage. Second excitation means, coupled to the 19 terminal means, includes means for coupling to a second element a middle elernent signal component of 21 predetermined phase and amplitude relative to signal 22 components coupled to the first and third elements.

1 The antenna also includes shifting rneans for 2 selertively coupling the first and second excitation 3 means to subsets of said antenna elements so that forward, middle and rear signal components can be respectively shifted to different first, second and 6 third element subsets of the plurality of antenna 7 elements.
8 Also in accordance with the invention, a 9 steerable antenna array system includes a plurality of switchable array antennas spaced laterally in 11 relation to a first radiation direction, each such 12 array antenna comprising a linear array of antenna 13 elements, excitation means for coupling signal 1~~ components of predetermined relative phase and amplitude to selected antenna elements, and 16 shifting means coupled to said excitation means 'For 17 altering the coupling of signal components to the 18 antenna elements so as to selectively shift the 19 effective radiation center of each array antenna along its length. The antenna system alsa includes azimuth 21 control means, coupled to said array antennas, for 22 selectively controlling the shifting means of 23 respective antennas.
24 Further in accordance with the invention, a beam tilt antenna array system includes terminal 26 means far coupling signals, a plurality of switchable 27 array antennas such as described above and beam tilt _ ~ _ ~~e'~~~~~
1 control means for coupling a selected plurality of the 2 array antennas to the terminal rneans. A bearn tilt 3 antenna array system rnay additionally include azirnuth 4 control means, coupled to the array antennas, for selectively controlling the shifting means of the 6 antennas, whereby the antenna beam of the antenna 7 system can be independently steered in azimuth and 8 tilted.
9 For a better understanding of the present 1D invention, together with other and further objects, 11 reference is made to the following description, 'taken 12 in conjunction with the accompanying drawings, and its 13 scope will be pointed out in the appended claims.
14 BRIEF DESCRIPTION OF THE DRA~~VINGS

Fig. 1 illustrates the effect of scanning 16 a linear arrayantenna off axis.

17 Fig. 2 shows orthogonal and simplified 18 exploded viewsof a low-profile array antenna 19 containing ee antenna elements.
thr Fig. 3 shows an array antenna system 21 including Fig. 2 array antennas.
five 22 Fig. 4 is a block diagram of an array 23 antenna containing three antenna elements.

24 Fig. 5 shows desirable current relationshipsfor an end-fire array.

2~~~~~r 1 Fig. 6 is a circuit diagrarn of a three 2 monopole array antenna.
3 Figs. 7 and 8 are circuit diagrams of 4 alternative forrns of the Fig. 6 antenna.
Fig. 9 is an antenna pattern far operation G of an array antenna of the type shown in fig. 6, 7 Fig. 10 illstrates component parts of an 8 array antenna of 'the type shown in Fig. 6.
9 Fig. 11 is a circuit diagram of a three slot array antenna.
11 Figs. 12 and 13 are circuit diagrams of 12 alternative forms of the Fig. 11 antenna.
13 Fig. 14 is a circuit diagram of a five 14 monopole array antenna.
Fig. 15 is a circuit diagram of a five 16 monopole switchable array antenna in accordance with 17 the invention.
18 Fig. 16 shows an alternative form of the 19 Fig. 15 antenna utilizing slots.
Fig. 17 shows a steerable antenna array 21 system in accordance with the invention.
22 Fig. 18 showy excitation alternatives 23 useful in describing operation of the Fig. 17 antenna 24 system.
Fig. 1~ shows the straight fan beams that are 26 provided by the Fig, 17 antenna system.
_ (, 1 Fig. 20 illustrates roll conditions in 2 aircraft banking maneuvers.
3 Fig. 21 shows a steerable beam tilt 4 antenna system in accordance with the inventon.
Fig. 22 shows an alternative form of 6 signal distribution network usable with the Fig. 21 7 antenna system.

DETAILED DESCRIPTION OF THE INVENTION

Refe>=ring now to Fig, 2, there is shown.
11 the physical configuration of an array antenna.

14 An understanding of antennas of this type is important to an understanding of the present invention, which 16 provides further improvements in such antennas and 17 systems utilizing them. The present invention is more 18 specifically described under the heading "Description 19 of Figs. 15-21."
Fig. 2a is an orthogonal view of the complete 21 antenna including protective cover 12, of a radiation 22 transmissive material such as fiberglass or a suitable 23 plastic, base member 14, of metal or suitable 24 conductive material to serve as a mounting flange and ground plane connection, and terminal means 16, shown _ 7 _ _ ~~~~f.~~u~
1 as a coaxial connector suitable for coupling FiF
2 signals.
3 Fig. 2b and c are exploded end and 4 side views, respectively, of the array antenna 10, showing cover 12 and base member 14 with connector 16 6 attached. Also shown are a 'First printed circuit 7 card 18 bearing a first planar conductor pattexn of 8 forward, middle and rear monopole antenna elements 20, 9 22 and 24, respectively, and a second printed circuit card 26 bearing a second planar conductor pattern on 11 surface 28. The conductor pattern on surface 28, 12 which is not visible in these views, will be described 13 below, 14 In a specific embodiment of the antenna 10, the assembled combination of the cover 12 and base 16 14 had a height of approximately one-tenth wavelength 17 and length of about three-quarter wavelength.
18 References to dimensions measured in wavelength refer 19 to approximately the average design frequency, so that for a design frequency range or bandwidth of 1,020 to 21 1,100 MHz, for example, the average design frequency 22 would be 1,060 MHz, corresponding to a wavelngth of 23 about 11.1 inches. Dimensions are stated in order to 24 characterize the invention and differentiate over prior art antennas, and are not intended to suggest that 26 the invention is limited to precise dimensions or to 27 exclude antennas representing appropriate applications _ g 1 of the invention. As shown in Fig. 2, the lower 2 surface of base member 14 is flat, but in other 3 embodiments it rnay be a curved surface corresponding 4 to the curved surface of an aircraft to which it is to be mounted. For mounting, screws are typically 6 fastened through the mounting holes shown in Fig. 2a 7 and a clearance hole through the outer surface of the 8 aircraft is provided for the connector 16, so that it 9 can be joined to a mating connector for caupling signals to cabling and signal processing equipment 11 carried within the aircraft.

12 Fig. 3 shows a typical antenna system 13 including 'Five array antennas 10a, through l0e 14 supported in a laterally spaced configuration on a curved metal surface 30 such as the fuselage of an 16 aircraft, forward of the pilots' windshield. It will 17 be apparent that in such an installation, use of array 18 antennas one inch in height provides a dramatic 19 improvement in the pilot's visibility, as compared to use of prior art antennas three inches in height. In 21 an installation of this type, the individual array 22 antennas can be excited in groupings selected to 23 provide desired antenna beam characteristics, in 24 accordance with known principles of array antenna excitation. An antenna system as shown in Fig. 3, 26 when installed an the upper forward surface of an 27 aircraft, can provide broad horizontal coverage 1 forward of the aircraft and good vertical coverage, 2 except below the aircraft. A similar antenna system 3 installed on the lower forward surface of the aircraft 4 would permit full vertical and horizontal coverage forward of the aircraft. Alternatively, antenna 6 systems mounted near the leading edge of a wing 7 could provide complete vertical coverage, but would 8 probably require similar systems on the other wing in 9 order to provide complete horizontal coverage free of blockage by the nose of the aircraft.
11 Fig. 4 is a simplified block diagram of an 12 array antenna in accordance with the invention, shown 13 in two sections 18a and 26a corresponding basically to 14 the printed circuit cards 18 and 26 in Fig. 2. The antenna is used to alternatively radiate and receive 16 signals, in the range of 1,020 MHz to 1,100 MHz, which 17 are coupled to and from the antenna by way of 'the 18 terminal means 16a corresponding to connector 16 in 19 Fig. 2. The cover and base components, 12 and l~G, are not represented in Fig. 4. As noted, the antenna is 21 used both to radiate and receive signals, and 22 description of how signals are processed by various 23 portions of the antenna when radiating, for example, 24 will be understood to be equally relevant in a reverse relationship during reception.

2~~~~~
1 The Fig. 4 antenna :inclurJes first, second 2 and third antenna elernents 20, 22 and 24, which in 3 accordance with the invention rnay be rnonopoles of the 4 order of one-tenth wavelength in height arranged in a spaced linear array. While the desirability of using 6 antenna elements one-tenth wavelength high as compared 7 to prior art elements one-quarter wavelength high may 8 be readily apparent, the severe operational bandwidth 9 degradation narmally associated with short antenna elements such as monopoles has been a limiting factor 11 contributing to the continuing reliance on quarter 12 wave elements in the prior art. In addition, attempts 13 to use elements shorter than a quarter wavelength in 14 an array configuration with prior art excitation arrangements have been subject to severe effects of 16 intercoupling between adjacent and other combinations 17 of the antenna elements and nearby surfaces, as a 18 result of effects of unequal and complex mutual 19 impedances between individual antenna elements in an array. These effects, which do not readily yield to 21 design cornpensation, largely determine the actual 22 currents in the antenna elements and the resulting 23 antenna pattern. It will be appreciated that if the 24 currents in the various elements cannot be acurately deterrnined and proportioned, neither can a desired 26 antenna pattern be provided. While the basic 27 description of the invention will be in the context of 1 arrays of three elernents, denoted as "first; -second 2 and third" elements, additional elements rnay be 3 included as will be described. Flowever, regardless of 4 the total number of antenna elements, each antenna will include three elements meeting the description 6 and function of the first, second and third elements 7 as set out and claimed.
8 Section 26a of the Fig. 4 antenna as shaven 9 comprises excitation and tuning means which are effective to cause signal currents in the antenna 11 elements 20, 22 and 24 to have a predetermined 12 relationship of phase and amplitude substantially 13 independent of impedance interaction, and are able 14 to accomplish this over a significant band or range of operating frequencies. As shown, antenna portion 26a 16 includes first excitation means shown as excitation 17 circuit 40, coupled between terminal 16a and the first 18 and third elements 20 and 24, comprising signal 19 transmission means (as will be discussed in more detail with reference to Fig. 6) for coupling signal 21 components to elements 20 and 24 by way of a point of 22 common voltage, shown as point 42 on the connection 23 between excitation circuit 40 and double tuning 24 circuit 44. Tuning circuit 44, provides double tuning of the impedance characteristics of the 26 antenna ciruits to optimize for operation in a desired 27 frequency range. While circuit 44 is shown as being 1 connected in series between terrninal 16a and poi~t~ ~~,~
2 its function .is to provide wideband impedance matching 3 and it may compr9.se discrete or distributed reactances 4 coupled to point 42 in series as shown, or in parallel to ground, ar rnay utilize appropriate lengths of 6 transmission line, as will be apparent to those 7 skilled in the art. Section 26a also includes means 8 ~c6 shown as including second excitation circuit 48, 9 coupled between terminal 16a and second element 22, comprising means for coupling a signal component to 11 the element 22 which has a predetermined phase and 12 amplitude relative to the components coupled to 13 elements 20 and 24 via first excitation means 40. As 14 shown in Fig. 4, excitation circuit 48 functions as a power divider coupling a portion of the input signal 16 From terminal 16a to element 22, while the remaining 17 portion of the input signal flows from the terminal 18 16a to the other elements. This power divider 19 function of circuit 48 may be provided by a directional coupler (as will be discussed with 21 .reference to Fig. 6) or other menus. In Fig. 4, means 22 46 also includes double tuning circuit 50 for 23 providing double tuning of the impedance 22 characteristics of 'the middle element 22 for operation in a desired frequency band or range. Where 26 distributed reactances or transmission lines in 27 excitation means 48 are used to provide the double J
1 tuning function, means 50 may not appear as a discrete 2 element.
3 Fig. 5 shows a three manopole array 4 arranged to provide an end-fire pattern and Fig. 6 shows such an array antenna with an excitation 6 system in accordance evith the invention. A good 7 end-fire pattern is obtainable from the Fig. 5 8 array if the elements have the spacings and the phase 9 and amplitude of currents shown. Fig. 6 shows an antenna with an excitation system effective to provide 11 "forced excitation" to cause signal component currents 12 in the antenna elements to have such a predetermined l3 relationship of phase and amplitude, substantially 14 independently of intercoupling affecting the antenna elements, with double tuning to provide for operation 16 over a significant range of frequencies. "Forced 17 excitation" is defined as an excitation arrangement 18 which forces or predetermines the currents in the 19 elements of an array antenna so as to result in currents of desired relative magnitude and phase, 21 substantially independently of mutual and other 22 coupling and impedance effects.
23 In Fig. 6 there are included first, second 24 and third antenna elements, shown as short monopoles 20, 22 and 24 rnounted through and above a conductive 26 ground plane 14a. The Fig. 6 array antenna includes 27 first excitation means comprising quarter wave 1 transforrner S6 coupled to third monopole 24, and 2 quarter wave transforrner 58 and half wave transmission 3 line 6U coupled to first monopole 20. Transformer 56 4 and line 60 are also shown coupled to common voltage point 42, as is tuning means 62 which is also coupled 6 to signal input and output terminal 16a. Tuning means 7 62 is a series resonant LC circuit arranged fnr double 8 tuning the impedance of rear and forward monopoles 24 9 and 20. Each of the monopoles is shown as having a series inductance at its base, such as indicator 64 at 11 element 24, for tuning out the capacitive impedances 12 of the short monopole element at one frequency near 13 midband. This narrow band tuning is augmented by the 14 double 'tuning means 62 to provide substantially increased bandwidth. The Fig. 6 antenna also includes 16 second excitation means comprising a directional 17 coupler 66, for coupling signals of predetermined 18 relative amplitude to the second monopole 22, and 19 second tuning rneans 68. As shown, coupler 66 is coupled to terminal 16a and is effective to transfer a 21 portion of a signal input to the antenna to rnonopole 22 22 by way of transmission line section 70. Second 23 tuning means 68 is a parallel resonant LC circuit 24 arranged for double tuning the impedance of second monopole 22, and the length of line 70 is chosen so 26 that signals reaching monopole 22 have the desired ~~3~~'~~
relative phase as compared to signals at monopo:Les 20 2 and 24.
3 In operatian of the Fig. 6 array antenna, 4 the twa quarter wave transformers 56 and 58 farce the S currents Ia and Ic in the third and first monapoles 24 6 and 20 to be dependent substantially wholly on the 7 voltage at the common voltage point 42. Thus, Ia and 8 Ic are forced to be in the ratio IatIc = Zoc/Zoa, where 9 the latter are the respective transmission line impedances of the transformers 58 and 56. The half 11 wave line 60 introduces a reversal in the polarity of 12 Ic at element 20, relative to Ia at element 24. The 13 ratio of Ib to the Ia and Ic currents is not forced 14 and cannot be forced because of the 90° phase difference needed to obtain the desired signal 16 component relationship of Ia=j, Ib=2 and is=-j, as 17 shown in Fig. S. However, if Ia=-Ic then the second 18 monopole 22 will effectively be at a null point midway 19 between the equal and opposite signals at elements 20 and 24 and no net signal from those monopoles will be 21 coupled to element 2.2. In this case 'there is no need 22 for Ib to element 22. to be forced.
23 As a specific example, computations of 24 impedance were made using a commercial computer program for three monopoles arranged as in Fig. S with 26 currents as in Fig. 5. The computations were made at 27 1,030 MHz, 1,060 MHz, and 1,090 MHz for an array of 1 three identical monopoles one inch high, ~~~ ~~~~
2 wide at the top and with center-to-.center spacing of 3 2.78 inches. Computed results were as Follows:

Za -0.89-j61.8 -0.6-j57.0 -0.31-j52.7 6 Zb 6.0 -j57.4 6.4-j52.6 6.8 -j48.1 7 Zc 14.7 -j47.5 15.7-j42.4 16.7-j37.8 8 Za -f Zc 13.8 - j109.3 15.1-j99.4 16.4-j90.5 9 With reference to Fig. 6:
Ys = Ya' + Yc' il For quarter wave transformerss 12 Ya' - Za/Zoa2 Yc' - Zc/Zoc2 13 Let Zoa = kZoc 14 Zs = Zoa2/(Za + k2Zc) _ Zo2/(Za -~ Zc), if k=1 16 where Zoa = Zoc = Zo 17 From the table above, with the reactance tuned 18 out at midband by the series inductances such as 64, 19 Za + Zc is approximately equal to 15 ohms.
From the last equation, and assuming we want Zs 21 to be 50 ohms:
22 Zo2 - Zs (Za -~ Zc) 23 - 50 (1.5) 2LV Zo _ 27.4 ohms Note that in Fig. 6, the quarterwave 26 transformers and transmission line sections are shown 27 as being sections of microstrip transmission line that 28 is dimensioned to provide 'the desired characteristic _ 17 -1 impedances, Thus, lines 60 and 70 in-this example 2 would be 50 ohrn line sections and transformers 56 and 3 58 wou:Ld be 27.4 ohrn sections pne quarter wavelength 4 long at a frequency of 1,060 M~iz. Reactive tuning S circuits 62 and 68 are used to optimize antenna 6 performance at 1,030 MHz and 1,090 MHzf i.e., are 7 adjusted to double tune the respective antenna 8 elements at those frequencies). Note also that, 9 because of mutual coupling, Za has negative resistance, making it very difficult to precisely and 11 efficiently provide the desired Ia over a frequency 12 band, in the absence of the invention. However, (Za +
13 Zc) has a substantial positive resistance which can be 14 efficiently double tuned while providing the desired Ia and Ic values, in accordance with the invention.
16 Achievement of an array antenna pattern with a high 17 front-to--back ratio and strong radiation over a wide 18 angle in the front sector requires precise control of 19 the relative currents in the array elements, as made possible by the present invention.
21 Referring now to Figs. 7 and 8, there are 22 shown alternative excitation circuits for array 23 antennas similar to the Fig. 6 antenna. For 'the 24 Figs. 7 and 8 antennas, the monopoles and the 2S excitation means between point 42 and the monopales 26 20 and 24 are the same as shown in Fig. 6. In 27 Fig. 7 the excitation means far the second element - 18 _ ~~ i~.~~'~
1 includes a quarte.c wave transformer 72 simi:la:r to 2 transformers 56 and 58 in Fig. 6. Zo of 72 should be 3 different than Zo of 56 and 58. In the Fig. 7 antenna 4 the tuning function can be provided by a series resonant LC circuit 68a and the length of line 70a 6 can be reduced, otherwise operation corresponds to 7 operation of the Fig. 6 antenna. In Fig. 8 the 8 excitation means for the forward and rear elernents 9 includes a quarter wave transformer 78 similar 'to transformer 72 included in the second element 11 excitation means in Fig. 7. Tn the Fig. 8 12 arrangement the parallel resonant LC circuit 62a 13 provides the tuning 'Function, and operation again 14 corresponds to opertion of the Fig. 6 antenna. The LC
circuits, such as 68a and 62a, may use discrete 16 reactance components or appropriate lengths of 17 transmission line, as will be apparent to those 18 skilled in 'the art.
19 Fig. 9 is an actual measured azimuth antenna pattern at 1,060 MHz for an array antenna with 21 three rnonopoles resembling those shown in Fig. 2c, 22 with a monopole width of 2 inches, spacing of 2.78 23 inches and height of .91 inches, after adjustments for 24 the excitation circuits intended to optimize the results achieved. Note that the front-to-bank ratio 26 is greater than 20dB, and the pattern remains strong 27 over a wide angle in the front sector. Similar 1 results were obtained at 1030 and 1090~MHz, It is 2 believed that the antenna performance reflected in 3 this data is clearly beyond the perFormance of any 4 known prior art monopole array antenna of comparable dimensions.
6 Fig. 10 shows printed circuit cards 18 and 7 26 designed for this antenna. On card 18, three 8 monopoles 20, 22 and 24 as shown were formed by 9 etching a copper layer on dielectric card 18 to leave conductive patterns in the form of 'the monopoles. The 11 pattern shown on surface 28 of the card 26 was 12 similarly formed. The actual pattern shown on card 26 13 represents microstip transmission line sections of 14 various lengths and characteristic impedances, together with interconnecting points and sections, 16 desiged to implement the antenna in a physically 17 simple form providing ease of production and assembly, 18 consistent electrical characteristics, inherently high 19 reliability and good durability under shock and vibration conditions common in high-performance 21 aircraft applications. While reference numerals 22 corresponding to the Fig. 6 antenna, with subs'titu'tion 23 of the alternative excitation circuit of Fig. 8, have 24 been included in Fig. 10, it will be understood that reducing the antenna to a microstip layout, and 26 reFining that configuration for maximum performance, 27 results in a final physical embodiment of the 2p~~~~;~~
1 invention in this example in which there is a degree 2 of inherent masking of the indentification of discrete 3 components. Thus, while portions of the conductive 4 pattern on card 26 in Fig. 10 have been given identifying numerals, it may be difficult or not 6 possible to specifically identify the metes and bounds 7 of a particular component so as to separate it from 8 the remainder of the circuit.
9 Fig. l1 shows an array antenna in accordance with the invention wherein the individual 11 radiating elements are slots. A three element slot 12 array, as shown, is subject to disruptive mutual 13 coupling efFects similar 'to those previously discussed 14 with reference to monopoles. Slots 80, 82 and 84 in Fig. 11 may simply be openings in a conductive 16 covering 86 on the forward side of a dielectric sheet 17 88. Conductive covering 86 and dielectric sheet 88 are 18 both shown as being transparent for ease of 19 illustration in order to make visible 'the other elements which may be deposed an the backside of the 21 dielectric sheet, as shown.

22 Each of the slots or windows 80, 82 and 84 23 in the conductive member 86 may typically be a half 24 wavelength long or, alternatively, may be shorter with shunt capaeitances inserted across the center of the 26 slot at one frequency near midband. The slots in the 27 array are spaced by a quarter wavelength, with a width 1 equal to a fraction of the spacing. Dirnens:ions can be 2 selected far particular applications using known 3 design techniques. As shown, each slot is excited by 4 a conductor passing across the slat on the back of the dielectric sheet, as shown at 90, and passing Forward 6 or upward through the dielectric 88 to terrninate at a 7 point 92 in electrical contact with the conductive 8 covering 86 at the side of slot 80. As shown, slot 80 9 has an excitation conductor termination point 92 at its right side and will be excited with a phase or a 11 polarity of excitation opposite to that of slot 84, 12 which has such termination point at 96 at its left 13 side. Although not shown, each slot is typically 14 backed by a metallic box or conductive cavity to allow .radiation only in the forward or outward 16 direction from each sot. It will be appreciated that 17 an antenna in the form of an array of slots is 18 particularly advantageous for implementation in a 19 configuration flush with the surface of an aircraft.
The present invention is readily adaptable to such 21 applications.
22 The Fig. 11 antenna includes first 23 excitation means shown as half-wave transmission lines 24 98 and 100 coupling the third and first elements 84 and 80 to the terminal means 16a via common voltage 26 point 102. Reactive means 62a is shown coupled between 27 point 102 and terminal 16a for providing double tuning 1 in a desired frequency range. Second excitation means, 2 shawn as directional coupler 66a, is coupled between 3 terrninal 16a and second element 82, via transmissian 4 line section 70a and reactive means shown as LC
circuit 68a. Operation of the Fig. 11 antenna is 6 similar to the Fig. 6 antenna. Characteristics of 7 slots permit use of transmission line sections 98 and 8 100 without provision for quarter wave transformers in 9 providing a common voltage point enabling forcing of the voltages across the slots to have the desired 11 magnitude and phase, substantially independently of 12 mutual and other coupling and impedance effects. With 13 slot radiators the significant signal component that 14 determines the radiat~.on pattern of an array is 'the slot voltage, in contrast to monopole or dipole 16 radiators which have their currents as the significant 17 signal components. Desired slot voltages for a good 18 end--fire pattern with the Fig. 11 array have phase and 19 amplitude values similar to 'the monopole currents shomvn in Fig. 5. The Fig, 11 system can provide this 21 forced excitation together with double tuning for 22 increased bandwidth.
23 Figs. 12 and 13 show alternative 24 embodiments regarding the means connecting points 96 and 92 to point 102 in antennas which otherwise 26 correspond to Fig. 11. In Fig. 12 'the half wave 27 transmission lines 98 and 100 have each been replaced _ 23 _ 1 be a series combination of two quarter wave 2 transformers, such as transformers 104 and 106 .show n 3 as replacing line 100 between points 92 and 102. This 4 arrangement provides wideband transformation of the S slot conductance to a convenient value such as 50 ohms 6 at point 102. In Fig. 13, half wave lines 98 and 100 7 have been replaced by a single full wavelength 8 transmission line segment 108 connecting points 9 96 and 92, and reactive tuning circuit 62a connects to a point 102a in the vicinity of point 96.
11 Variations such as shown in Fig. 13 can provide 12 flexibility in particular applications.
13 The preceding embodiments are particularly 14 shown and described in the context of an array of three radiating elements, however, it will be apparent 16 that in some applications it may be desirable to 17 provide one or more array antennas, each of which 18 includes four or more radiating elements with forced 19 excitation in accordance with the invention.
Referring now to Fig. 14, there is illustrated 21 an embodiment of the invention comprising a linear 22 array of f5.ve antenna elements shown as monopoles 20a 23 through 24a. As shown, the first, second and third 24 elements 20a, 22a and 24a (corresponding to the first, second and third elements of Fig. 6~ have been 26 supplemented by a leading element 21a, ahead of 27 element 20a, and a trailing elernent 23a, following 1 elernent 24a. In considering the Fig. 14 antenna, it is 2 important to note that the arrangement and functioning 3 of elements 20a, 22a and 24a are as described with 4 reference to a three element array, the three elernent array of first, second and third elements being a 6 basic subset used in antennas utilizing the invention.
7 In Fig. 14, elements 20a, 22a and 24a 8 correspond to elements 20, 22 and 24 of Fig. 6. The 9 Fig. 14 excitation system corresponds to the ZO alternative excitation system of Fig. 9, with 11 modification for excitation of the additional elements 12 21a and 23a. As shown in Fig. 14, a first group of 13 non-adjacent antenna elements 20a and 24a are coupled 14 to first excitation means shown as signal transmission means including halfwave transmission line 60 and 16 quarterwave transformers 56 and 58. The remaining 17 elements, middle element 22a, leading element 2la and 18 trailing element 23a, are coupled to second excitation 19 means shown as directional coupler 66, transmission line section 70a, quarterwave transforrners 72, 73 and 21 74, and half and full wavelength transmission lines 75 22 and 76, respectively. Signals are coupled by the 23 excitation means to elements 20a and 24a by way of 24 common voltage point 42 and to elements 21a, 22a and 23a by way of a second common voltage point 43, 26 permitting forced excitation.
_ 25 ~~e~7~i~~

1 If there were only four elernents, the 2 element 21a, transformer %3 line 76 could be and 3 eliminated. For any number elernents there are of 4 actually two voltage points accordance with the in invention, to which signals fed. For three are 6 elements, one of these voltagepoints is a common 7 voltage point for two elements,permitting 8 predetermined magnitudes and ases of current to ph be 9 provided. For more than threeelements the invention makes available two common age points, 42 and volt 43 11 for example, each connecting two or more elements.
to 12 DESCRIPTION OF FIGS. 15 - 22 13 Referring now to Fig. 15, there is shown a 14 switchable array antenna in accordance with the present invention. Fig. 15 includes five antenna 16 elernents, shown as monopoles 110, 112, 114, 116 and 17 118, supported above a ground plane 121 in a linear 18 array and arranged for excitation in subsets of three 19 elements. Shifting means shown, as switch 122, selectively connects the subsets of elements (i.e., 21 elements 110, 112, and 114, elements 112, 114 and 116, 22 or elements 114, 116 and 118) to excitation means for 23 coupling signal components from and to the selected 24 elements during reception and transmission of radiated signals. Thus, during transmission, shifting means 26 122, which may comprise mechanical or electronic i~
1 individual switching means such as switches 123 and 2 124, selectively shiFt the coupling of signal 3 components appearing at terrninals 126, 128 and 130 to 4 different first, second and third element subsets of the antenna elements 110, 112, 114, 116 and 118. For 6 example, with shifting means 122 in the position 7 illustrated in Fig. 15, forward, middle and rear 8 signal components for achieving an end-fire antenna 9 pattern directed toward the right are respectively coupled to a first element 114, a second element 112 11 and a third element 110. As will ba described 12 further, when the forward, middle and rear signal 13 components are shifted to a different three element 14 subset, such as a first element 118, a second element 116, and a third element 114, the effective element 16 radiation center of the array is shifted forward from 17 the vicinity of element 112 to the vicinity of element 18 116.
19 As shown in Fig. 15, the switchable array antenna also includes terminal means, first excitation 21 means, second excitation means and tuning means 22 substantially as described above with reference to 23 Figs. 6 and 7. The terminal means is illustrated as 24 terminal 16a for coupling signals to and from the antenna. First excitation means is shown as including 26 a half-.wavelength transmission line 60 for coupling a 27 signal component to 'terminal 130 with a phase ~~~~~0~
1 reversal, as compared to the sign al component coupled 2 to terminal 126, and a set of 'two guarter wave 3 transformers for coupling such signal components to 4 first and third elements, respectively, of a selected three element subset of the five antenna elements 6 illustrated. With switching means 122 in the position 7 shown, it will be seen that the first excitation means 8 utilizes a quarter wave transformer 144 coupling to 9 element 114 and transformer 132 coupling to element 110. Second excitation means is shown as including 11 directional coupler 66 for coupling a signal componen t 12 of predetermined amplitude to terminal 128 and a 13 quarter wave transformer 138 coupling to element 112, 14 which is the second element of the selected 114, 112, 110 element subset in this example. Tuning means, 16 shown as series LC circuit 68a coupled to the first 17 excitation means via common voltage point 42 and 18 series LC circuit 62 coupled to the second excitation 19 means, provide double tuning of the antenna elements.
The structure and operation generally and 21 as to individual elements of the excitation and 'tuning 22 means are covered more specifically in the description 23 of Figs. 6 and 7 wherein corresponding reference 24 numerals refer to similar components. It will be seen, however that the functions of three quarterwave 26 transformers (56, 58 and 72 in Fig. 6 with the Fig. 7 27 modification) are provided in Fig. 15 by quarterwave 1 transformers 132, 134, 136, 138, 140, 142, 144, 146 2 and 148, which are utilized in sets of three dependent 3 on the operation of switch 122. Alternatively, 'three 4 quarterwave transformers can be inserted at points 126, 128 and 130, respectively, and the nine 6 quarterwave transformers in Fig. 15 replaced by nine 7 halfwave transmission lines.
8 The operation of the Fig. 15 antenna is 9 basically as described with reference to Fig. 6. By enabling the phase and amplitude of the respective 11 currents in first, second and third elements, such as 12 elements 114, 112 and 110, to be forced to have 13 predetermined values, and end-fire or other desired 14 antenna pattern is achieved. The Fig. 15 antenna differs in permitting the radiation center of the 16 array to be shifted to be in the vicinity of element 17 112, 14 or 116, depending on whether excitation is 18 applied to a first, second and third element subset 19 114, 112, 110 or 116, 114, 112 or 118, 116, 114 by action of the shifting means shown as switch 122, 21 With reference to Fig. 2, it wi:l1 be 22 understood that the Fig. 15 antenna can be constructed 23 as in Fig. 2 with a protective cover and base member 24 similar to elements 12 and l4~in Fig. 2, Also, antennas in accordance with Fig. 15 can be arranged in 26 a laterally spaced array supported on a surface such 27 as the fuselage of an aircraft, as shown in Fig. 3.

1 In Fig. 15, the radiating element array 2 120 .is shown as cornprising five rnonopoles coupled to 3 points 111, 113, 115, 117 and 119. Fig. 16 shows an 4 alternative form of radiating element array 120a which can be substituted in a rnadified Fig. 15. As shown, 6 array 120a enmprises five slots 110a, 112a, 114a, 116a 7 and 118a illustrated as elongated openings in a 8 conductive layer or surface 86 coupled to an 9 insulative layer or rnember 88. As discussed with reference to Fig. 11, members 86 and 88 are shown as 11 being transparent to show the connection 'From point 12 111, in an insulated relationship across slot 110a 13 behind layer 88, and passing through layer 88 to 14 terminate in contact with layer 86 on the left side of the slot at point 150. Whereas in Fig. 11 a relative 16 phase reversal was introduced for the signal companent 17 fed to slot 80 by virtue of the feed conductor 18 crossing to a contact point 92 on the right side of 19 slot 80, in Fig. 7.6 all contact points are to the left side of the respective slats. For the Fig. 16 21 antenna, a phase reversal is introduced by the half 22 wave line section 60 shown in fig. 15, so that the 23 Fig. 15 and 16 antennas can both use a similar 24 excitation system such as shown in Fig. 15. However, with the Fig. 16 antennas, the quarterwave 26 transformers in Fig. 15 must be replaced by halfwave 27 transmission lines.

~~~~~~i~
1 Operation of the Fig. 16 alte:cnative forrn 2 of antenna is basically as described with reference to 3 Fig. 11, with the additional capability of shifting of 4 the effective radiation center dependent upon the selected excitation of first, second and third element 6 subset 114a, 112a, 110a, or subset 116a, 114a, 112a, 7 or subset 118a, 116a or 114a.
8 Referring now to Fig. 17, these is shown a 9 simplified schematic of a steerable antenna array system in accordance with the invention. As 11 illustrated, the array system includes a plurality of 12 switchable array antennas shown as three identical 13 antennas 152a, 152b and 152c, which may be of the type 14 shown in Fig. 15 or 16. The three switchable axray antennas are spaced laterally relative to an end-fire 16 radiation direction to the right in the drawing. In 17 Fig. 17, signals supplied to terminal 154 axe coupled 18 to the three antenna terminals 16a, 16b, and 16c, 19 corresponding to terminal 16a in Fig. 15. As in fig.
15, in each of antennas 152a, 152b, 152c, signal 21 components will be coupled by 'the respective 22 excitation means to a selected first, second and third 23 element subset of the five elements 110, 112, 114, 116 24 and 118, which are represented by dots, such as dot 118. Assuming shifting means 122 of antenna 152a to 26 be in the position shown in Fig. 15, the active 27 element subset in antenna 152a will be elements 114, 1 112, and 110, which are circled to indicate the active 2 subset.
3 Tn Fig. 17 there is also included azimuth 4 control means shown as switch controller 156 coupled to the respective shifting means (122 in Fig. 15) of 6 each of the antennas via terminals 122a, 122b and 7 122c. The shifting means 122 of each antenna may be 8 activated to select one of the three different element 9 subsets and controller l52 comprises a control circuit or mechanism for adjusting the shifting means so that 11 the effective radiation center of each antenna is in a 12 selected position.
13 As indicated by the cirled dots in Fig. 17 14 representing the activated antenna elements, the shifting means are adjusted in this example so that 16 the effective radiation center is in the vicinity of 17 element il2 for antenna 152a, in the vicinity of 18 element 114 for antenna 152b, and in the vicinity of 19 elernent 116 for antenna 152c. Based on well known concepts of theory and operation of phased array 21 antennas, three Fig. 15 antennas laterally spaced as 22 in Fig. 17 and identically excited in an end-fire mode 23 would produce a beam directed to 'the right in Fig.
24 17. However, excitation of the antennas with different centers of radiation as indicated in Fig. 17 26 would steer the beam to an angle while preserving 'the 27 straight shape of the fan beam.

1 This is better illustrated in Fig. 18, 2 which is a simplified representation of three spaced 3 array antennas excited in five difFerent modes. In 4 Fig. 18a the circles identifying the active elements indicate that the three antennas, such as 152a, b and 6 c in Fig. 17, are identically excited with their 7 centers of radiation along the line 158. Line 158 8 effectively represents the wavefront for this 9 excitation and would result in a beam direction normal to line 158. In Fig. 18b excitation is as indicated 11 in Fig. 17, resulting in a rotated wavefront line 12 producing a normal beam direction angled to the left 13 of the original beam direction by an angle of 30°, 14 for example, dependent on the actual dimensioning of the antennas. Fig. 18c indicates a wavefront for a 16 beam angled to the right and Fig. 18d shows a 17 segmented wavefront resulting in a beam direction 18 angled to 'the right less than the Fig. 18c beam 19 direction. On a simplified basis, the beam direction in Fig. 18d can be considered 'to be the mean of 21 partial beams normal to the two wave Fronts 22 represented. The actual beam direction for excitation 23 as in Figs. 18d and a can be calculated or rneasured 24 based on actual dimensions and characteristics of the antennas to be used, the important point being that 26 relative positioning of the effective radiation 27 centers determines the wavefront and beam position.

1 X111 of the beams resulting frorn the ex.c:itations shown 2 in Fig. 18 preserve the straight shape of the fan 3 beam. This is shown in Fig. 19.
4 Fig. 17 also shows phase shifters 127a and 127b in channels 16a and 16c, respectively. These 6 phase shifters can provide two benefits. First, they 7 may be used to reduce the bend in 'the wavefront of 8 Fig. 18d or 18e while still preserving the beam 9 direction and the straight shape of the fan beam.
Second, they may be used to steer the beam to any 11 azimuth angle between or beyond the five angles shown 12 in Fig. 19. In this case the fan beams become curved, 13 but typically much less curved than the prior art case 14 of Fig. 1.
It should be understood that although 16 three array antennas each containing five antenna 17 elements have been shown in Figs. 15, 16, 17 and 18, 18 the number of array antennas and antenna elements in 19 each array antenna can be greater. Also the nurnber of active antenna elements in each array antenna can be 21 greater than 'three.
22 In operation, a laterally spaced 23 combination of array antennas with effective radiation 24 centers controlled by azimuth control means can have its antenna beam selectively steered. In this way, a 26 target which is off-boresight relative to the antenna 27 system, need not be off--boresight relative to the 1 active elements in the antenna system. With the beam 2 steered toward the l:arget, of F-boresight errors 3 associated with coning of the beam are reduced, 4 thereby improving the accuracy of the indicated azimuth bearings of targets at varying altitudes 6 relative to the base aircraft.
7 Fig. 20a is a simplified representation of 8 a section of aircraft fuselage with seven array 9 antennas 152a-g mounted on it (for example, seven Fig.
15 antennas seen in end view). The vertical line 159 11 in Fig. 20a indicates that the vertical axis of the 12 aircraft is not tilted (i.e. the aircraft is not 13 banking). Assurne that the three central antennas 14 152c, d and a in Fig. 20a represent an antenna system with adequate performance in the absence of banking, 16 but that during banking the antenna system is tilted, 17 compromising the performance. As shown in Figs. 20b 18 and c, during banking conditions as indicated the 19 invention permits compensation by selection of an operative group of antennas (identified by the 21 bracket) eFfectively representing a three antenna 22 system (152d, a and f or 152e, f and g) w hick is level 23 at a particular degree of roll caused by banking. In 24 Figs. 20b and 20c selection of the three indicated antennas results in a fan beam which remains 26 vertically oriented relative to the horizon. However, 27 since the vertical axis of the aircraft represented by ~~ )~~~
1 line 155 has ro:l).ed left, the desired beam 2 compensation has actually been accomplished by tilting 3 the fan beam of the antenna system relative to the 4 aircraft on which it is mounted.
Referring now to Fig. 21, there is shown a 6 beam tilt antenna array system able to compensate for 7 aircraft roll and also permitting antenna beam 8 steering while preserving straight fan beams. The 9 antenna will first be described independently of the beam steering capability. As illustrated, the antenna 11 array system includes a plurality of array antennas 12 shown as seven antennas 152a-g, which may be the type 13 shown in Fig. 15 or 16. 'The antennas are arranged to 14 radiate principally in a forward direction (upward, in the drawing) and are spaced laterally, the spacing 16 being such that in the context of an aircraft fuselage 17 the awtennas have displacements in a 'third direction 18 (which is the vertical direction in the Fig. 20a 19 view), due to the curvature of 'the fuselage. Thus, the antennas are basically shown in 'top view in Fig.
21 21 and end view in Fig. 20, so that the relative 22 vertical displacements in Fig. 20 are essentially 23 normal to Fig. 21.
24 The Fig. 21 antenna system also includes beam tilt control means, shown as beam tilt control 26 means 160, for selectively activating signal 27 distribution means 162 for deterrnining which group of 1 antennas is active during particular roll conditions.
2 Information representing the degree of roll may be 3 supplied to means 160 or sensed by any appropriate 4 means therein. In either case, tilt control means 16U
controls electronic or other switching means 162, 6 shown as including a series of switches such as 162a 7 and 162b, to couple signals between terminal 154a and 8 selected group of antennas, such as antennas 152d, 9 152e and 152f as shown in Fig. 21, corresponding to compensation for the roll condition shown in Fig. 20b.
11 In accordance with the invention, a beam 12 tilt antenna may also include beam steering as 13 discussed with reference to Fig. 17. Thus, in Fig.
14 2l,switch control 156a functions in the same manner as switch controller 156 in Fig. 17 to selectively 16 control the shifting means of each active antenna. In 17 the case of Fig. 21, information from tilt control 18 means 160, indicative of which three of antennas 19 152a-g are activated at a particular time, is used in means 156a to direct shifting means control 21 infarrnation to the currently active antennas. In 22 Figs. 17 and 21 individual radiating elements in an 23 array antenna such as 152x, are indicated by dots and 24 active elements by circles for ease of illustration and explanation. The actual elements may be 26 monopoles, slots, etc. as shown and described in 27 greater detail with reference to the other drawings, 1 such as Figs. 6, 11, 15 and 16, and the various 2 alternatives already covered, Phase shifters 127a and 3 127b for additional azimuth beam control are now 4 located below switching raeans 162.
In operation of the Fig. 21 antenna, the 6 antenna fan beam is tilted to the right as the 7 aircraft rolls left, and vice versa, to provide a 8 range of compensation as the 'Fan beam would otherwise 9 deviate from its normal .reference or vertical orientation. At the same time, the antenna beam may 11 be steered as described with reference to Figs. 17 and 12 18, and the beam steering and tilting can be 13 accomplished independently of each other. Fig. 22 is 14 included to indicate that, where desired, alternative forms of signal feed arrangements known in the prior 16 art may be substituted for the switching approach 17 utilized, in place of signal distribution means 162 as 18 shown in Fig. 21. The phase shifters 164a - 164g in 19 combination with the Butler Matrix and 'Feed network smoothly shift the active portions of the array to 21 compensate for aircraft roll. The phase shifters 166a 22 - 166g provide 'the additional azimuth beam control.

Claims (28)

1. Claim 1. A switchable array antenna, having a plurality of antenna elements arranged for excitation in subsets of at least three elements, comprising:
   terminal means for coupling signals;
   a plurality of antenna elements comprising a linear array of at least four elements arranged for use in subsets, each subset having first, second and third antenna elements;
   first excitation means, coupled to said terminal means, comprising signal transmission means for coupling forward and rear element signal components of predetermined relative phase and amplitude to first and third elements of a subset by way of a point of common voltage;
   second excitation means, coupled to said terminal means, comprising rneans for coupling to a second element of said subset a middle element signal component of predetermined phase and amplitude relative to said signal components coupled to said first and third elements;
   shifting means for selectively coupling said first and second excitation means to different subsets of said elements so that said forward, middle and rear signal components can be respectively shifted to different element subsets of said plurality of elements;
   whereby the effective radiation center of said linear array is selectively shifted along the linear array by activation of said shifting means.
2. Claim 2. A switchable array antenna as in claim 1, in which there are five antenna elements arranged for selective excitation so that the forward, middle and rear signal components may be selectively coupled so that the first, second and third elements of a subset may be any three adjacent elements of the linear array of five elements.
3. Claim 3. A switchable array antenna as in claim 1 or 2, in which said antenna elements are monopoles.
4. Claim 4. A switchable array antenna as in claim 1, in which said antenna elements are monopoles and said first excitation means comprises two quarter wavelength transformers for coupling between said common voltage point and said first and third elements, respectively, said wavelength corresponding to approximately the average design frequency of said antenna.
5. Claim 5. A switchable array antenna as in claim 4, in which said second excitation means comprises a directional coupler.
6. Claim 6. A switchable array antenna as in claim 5, in which said first excitation means additionally comprises half wavelength transmission line means, for coupling signals between said first element and said common voltage point with a reversal in phase, said wavelength corresponding to approximately the average design frequency of said antenna.
7. Claim 7. A switchable array antenna as in claim 1, 2, 4, 5 or 6, in which said antenna additionally comprises a protective cover of radiation transmissive material and a base member having a reflective surface serving as a ground plane for said elements.
8. Claim 8. A switchable array antenna as in claim 1 or 2, in which said elements are slots in the form of elongated windows in a conductive surface.
9. Claim 9. A switchable array antenna as in claim 1, in which said elements are slots and said first excitation means comprises two half wavelength transmission lines for coupling between said common voltage point and said first and third elements, respectively, said wavelength corresponding to approximately the average design frequency of said antenna.
10. Claim 10. A switchable array antenna as in claim 1, in which said elements are slots and said first excitation means comprises a full wavelength transmission line for coupling between said common voltage point and first and third elements, respectively, said wavelength corresponding to approximately the average design frequency of said antenna.
11. Claim 11. A switchable array antenna as in claim l, in which said elements are slots and said first excitation means comprises two series combinations of two quarter wavelength transformers of different impedances, one such combination far coupling between said common voltage point and each of first and third elements, respectively, said wavelength corresponding to approximately the average design frequency of said antenna.
12. Claim 12. A switchable array antenna as in claim 9, 10 or 11, in which said second excitation means comprises directional coupler means for coupling said middle element signal component to said second element and tuning means for providing tuning in a desired frequency range.
13. Claim 13. A switchable end-fire array antenna, comprising:
    terminal means for coupling signals;
    a plurality of antenna elements, comprising a linear array of five monopoles arranged for use in subsets of three, each subset having first, second and third monopole elements;
    first excitation means for coupling signal components from said terminal means to first and third elements of a subset, for providing radiated signals of opposite phase at one element relative to the other;
    second excitation means for coupling a signal component from said terminal means to a second element of said subset with a predetermined phase and amplitude different from said signals coupled to said first and third elements; and shifting means for selectively coupling said first and second excitation means to different subsets of said antenna elements so that said signal components can be respectively shifted to different first, second and third element subsets of said plurality of elements;
    whereby the effective radiation center of the antenna is selectively shifted along the linear array by activation of said .shifting means.
14. Claim 14. A switchable end-fire slot array antenna, comprising:
    terminal means for coupling signals;
    a plurality of slot antenna elements, comprising a linear array of five slots arranged for use in subsets of three, each subset having first, second and third slot elements;
    first excitation means for coupling signal components from said terminal means to first and third elements of a subset, for providing radiated signals of opposite phase at one element relative to the other;
    second excitation means for coupling a signal component from said terminal means to a second element of said subset with a predetermined phase and amplitude different from said signals coupled to said first and third elements; and shifting means for selectively coupling said first and second excitation means to different subsets of said antenna elements so that said signal components can be respectively shifted to different first, second and third element subsets of said plurality of elements;
    whereby the effective radiation center of the antenna is selectively shifted along the linear array by activation of said shifting means.
15. Claim 15. A switchable end-fire array antenna as in claim 13 or 14, which additionally comprises tuning means coupled to said first excitation means for providing tuning in a desired frequency range.
16. Claim 16. A switchable end-fire array antenna as in claim 13 or 14, in which said first excitation means comprises two quarter wavelength transformers for coupling to said first and third elements, said wavelength corresponding to approximately the average design frequency.
17. Claim 17. A switchable end-fire array antenna as in claim 13 or 14, in which said first excitation means comprises two quarter wavelength transformers and a halfwave transmission line, said second excitation means comprises a directional coupler, and which additionally comprises tuning means coupled to said first excitation means for providing tuning in a desired frequency range, said wavelength corresponding to approximately the average design frequency.
18. Claim 18. A steerable antenna array system, comprising:
    a plurality of switchable array antennas spaced laterally in relation to a first radiation direction, each such array antenna comprising a linear array of antenna elements, excitation means for coupling signal components of predetermined relative phase and amplitude to selected elements of each array antenna, and shifting means coupled to said excitation means for altering the coupling of signal components to said elements so as to selectively shift the effective radiation center of each linear array antenna along its length; and azimuth control means, coupled to said array antennas, for selectively controlling the shifting means of respective antennas;
    whereby the radiation direction and beam shape of said antenna array system is controlled by relative adjustment of the effective radiation centers of said array antennas.
19. Claim 19. A steerable antenna array system as in claim 18, in which the plurality of array antennas is three antennas and each linear array of antenna elements includes five antenna elements.
20. Claim 20. A steerable antenna array system, comprising:
    a plurality of switchable array antennas as in claim l, 2, 4, 5, 6, 9, 10, 11, 13 or 14, said antennas spaced laterally in relation to a first radiation direction; and azimuth control means, coupled to said array antennas, for selectively controlling the shifting means of respective antennas;
    whereby, the radiation direction and beam shape of said antenna array system, is controlled by relative adjustment of the effective radiation centers of said array antennas.
21. Claim 21. A tilting beam antenna array system, comprising:
    terminal means for coupling signals;
    a plurality of array antennas, each comprising a linear array of elements arranged to radiate principally in a forward direction, said antennas being spaced in a lateral direction normal to said forward direction and one or more of said antennas having different displacements in a third direction substantially normal to said forward and lateral directions; and beam tilt control means for coupling a selected plurality of said array antennas to said terminal means;
    whereby the relative displacement of the selected antennas in said third direction determines the tilt of the composite antenna beam pattern.
22. Claim 22. A tilting beam antenna array system as in claim 21, in which said plurality of array antennas are supported on the curved fuselage of an aircraft, the longitudinal axis of each linear array of elements corresponding substantially to the longitudingal axis of the aircraft, and the antennas are spaced in a lateral direction so that the surface curvature of the fuselage results in one or more of the antennas having different vertical displacements substantially normal to said axis and said lateral direction,
23. Claim 23. A tilting beam antenna array system, comprising:
    terminal means for coupling signals;
    a plurality of switchable array antennas, each comprising an array of antenna elements with a linear axis, said antennas being spaced laterally in a first direction normal to said axis and one or more of said antennas having different displacements in a second direction substantially normal to said axis and said normal direction; each such array antenna additionally comprising excitation means for coupling signal components of predetermined relative phase and amplitude to selected elements, and shifting means coupled to said excitation means for altering the coupling of signal components to said elements so as to selectively shift the effective radiation center of each array antenna; and beam tilt control means for coupling a selected plurality of said array antennas to said terminal means;
    whereby the relative displacement of the selected array antennas in said second direction determines the tilt of the composite antenna beam pattern.
24. claim 24. A tilting beam antenna array system as in claim 23, additionally including azimuth control means, coupled to said array antennas, for selectively controlling the shifting means of said antennas, whereby the antenna beam of said antenna system can be independently steered in azimuth and tilted.
25. Claim 25. A beam tilt antenna system as in claim 21, 22, 23 or 24, in which said plurality of array antennas is seven antennas and said tilt control means couples a selected plurality of three adjacent array antennas to the terminal means.
26. Claim 26. A tilting beam antenna system as in claim 23 or 24, in which said different displacements of array antennas are the result of the array antennas being mounted on a substantially cylindrical fuselage of an aircraft.
27. Claim 27. A tilting beam antenna array system, comprising:
    terminal means for coupling signals;
    a plurality of array antennas as in claim l, 2, 4, 5, 6, 9, 10, 11, 13 or 14, said antennas being spaced laterally in a first direction and one or more of said antennas having different displacements in a second direction substantially normal to said first direction;
    azimuth control means, coupled to said array antennas, for selectively controlling the shifting means of said antennas; and beam tilt control means for coupling a selected plurality of said array antennas to said terminal means;
    
    whereby the antenna beam of said antenna system can be independently steered in azimuth and tilted.
28. Claim 28. A tilting beam antenna array system as in claim 27, in which said antennas are on the surface of a vehicle, in the vehicle's normal attitude said first and second directions are substantially horizontal and vertical, respectively, and the beam steering is adapted for effective steering in azimuth with respective to an axis of said vehicle, and the beam tilting is adapted for compensation for vehicle banking.