Note: Descriptions are shown in the official language in which they were submitted.
Docket R4454.U1
EAO:cjf
1 AIRCRAFT ANTENNA WITH CONING AND BANKING CORRECTION
2 BACKGROUND OF THE INVENTION
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.
9 DESCRIPTION OF RELATED ART
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
- 1 --
~~ 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
- 2 -
~~~~~o~~
1 target cannot be accurately determined ~as the banking
2 maneuver tilts the antenna beam away from the
3 horizontal reference.
4 SUMMARY OF THE INVENTION
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.
- 3 -
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.
- 5 -
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.
_ (,
CA 02030600 1999-06-08
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.
8
DETAILED DESCRIPTION OF THE INVENTION
9
Refe>=ring now to Fig, 2, there is shown.
11 the physical configuration of an array antenna.
12
13
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
- 9 --
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.
- 10 -
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
- 11 -
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
- 12 -
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
- 13 -
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
- 14 -
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
- 15 -
~~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
- 16 -
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:
4 1030 1060 1090
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
- 19 -
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
- 20 -
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
- 21 -
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
- 22 -
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
- 24 -
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
- 26 -
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
- 27 -
~~~~~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
- 28 -
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.
- 29 -
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.
- 30 -
~~~~~~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,
- 31 -
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.
- 32 -
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.
- 33
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
- 34 -
1
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
- 35
~~ )~~~
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
- 36 -
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,
- 37 -
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.
- 38 -
