BACKGROUND OF THE INVENTION
[0001] The Present invention relates to array antennas and multiple array systems for radiating
and receiving electromagnetic signals and, in particular, to antennas adapted for
use on aircraft which permit the antenna beam to be steered in azimuth or tilted,
or both.
DESCRIPTION OF RELATED ART
[0002] Identification Friend or Foe (IFF) systems are used to enable aircraft to transmit
and receive signals for identification of other aircraft. Airborne radar systems are
also used for target location without identification capabilities. The higher frequencies
typically used for airborne radar permit use of antennas providing reasonable beam
resolution both vertically and horizontally. Airborne linear array antennas used for
IFF may, by contrast, lack the capability of providing significant vertical resolution.
Without vertical, or elevation, resolving capability, no elevation information is
provided by the system. Furthermore, the straight vertical fan beam that the antenna
provides in the on-boresight direction perpendicular to the linear array becomes curved
or conical in shape when the beam is scanned off boresight. As a result, as illustrated
in Fig. 1, if a target exists at a location (a) (15° right and at the same altitude
as the reference aircraft) the IFF display would accurately indicate a target at 15°
right. If however, a target were at location (b) (again 15° right, but at a higher
altitude) the IFF display would indicate a target at azimuth (c), displaced from the
actual 15° position of the target. The error is introduced by a "coning" of the antenna
beam as it is scanned to the right and effectively assumes a profile of a form shown
by curved line (d). The resulting errors introduced by off-boresight coning of the
IFF beam, in addition to affecting the accuracy of the IFF target display, can introduce
a displacement between the IFF and radar returns displayed for the same target.
[0003] Additional errors are introduced as a result of aircraft banking. In the absence
of accurate elevation information, the azimuth of a target cannot be accurately determined
as the banking maneuver tilts the antenna beam away from the horizontal reference.
SUMMARY OF THE INVENTION
[0004] In accordance with the present invention, a switchable array antenna, with a plurality
of antenna elements arranged for excitation in subsets of at least three elements,
includes terminal means for coupling signals and a plurality of antenna elements each
comprising a linear array of at least four elements arranged for use in subsets each
having first, second and third elements. First excitation means, coupled to the terminal
means, includes signal transmission means for coupling forward and rear element signal
components of predetermined relative phase and amplitude to first and third elements
by way of a point of common voltage. Second excitation means, coupled to the terminal
means, includes means for coupling to a second element a middle element signal component
of predetermined phase and amplitude relative to signal components coupled to the
first and third elements. The antenna also includes shifting means for selectively
coupling the first and second excitation means to subsets of said antenna elements
so that forward, middle and rear signal components can be respectively shifted to
different first, second and third element subsets of the plurality of antenna elements.
[0005] Also in accordance with the invention, a steerable antenna array system includes
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 antenna elements, and shifting means coupled to said excitation
means for altering the coupling of signal components to the antenna elements so as
to selectively shift the effective radiation center of each array antenna along its
length. The antenna system also includes azimuth control means, coupled to said array
antennas, for selectively controlling the shifting means of respective antennas.
[0006] Further in accordance with the invention, a beam tilt antenna array system includes
terminal means for coupling signals, a plurality of switchable array antennas such
as described above and beam tilt control means for coupling a selected plurality of
the array antennas to the terminal means. A beam tilt antenna array system may additionally
include azimuth control means, coupled to the array antennas, for selectively controlling
the shifting means of the antennas, whereby the antenna beam of the antenna system
can be independently steered in azimuth and tilted.
[0007] An embodiment of the present invention will now be described, by way of example,
with reference to the accompanying drawings in which:
[0008] Fig. 1 illustrates the effect of scanning a linear array antenna off axis.
[0009] Figs. 2a, 2b, and 2c show orthogonal and simplified exploded views of a low-profile
array antenna containing three antenna elements.
[0010] Fig. 3 shows an array antenna system including five Fig. 2 array antennas.
[0011] Fig. 4 is a block diagram of an array antenna containing three antenna elements.
[0012] Fig. 5 shows desirable current relationships for an end-fire array.
[0013] Fig. 6 is a circuit diagram of a three monopole array antenna.
[0014] Figs. 7 and 8 are circuit diagrams of alternative forms of the Fig. 6 antenna.
[0015] Fig. 9 is an antenna pattern for operation of an array antenna of the type shown
in fig. 6.
[0016] Figs. 10a and 10b illustrate component parts of an array antenna of the type shown
in Fig. 6.
[0017] Fig. 11 is a circuit diagram of a three slot array antenna.
[0018] Figs. 12 and 13 are circuit diagrams of alternative forms of the Fig. 11 antenna.
[0019] Fig. 14 is a circuit diagram of a five monopole array antenna.
[0020] Fig. 15 is a circuit diagram of a five monopole switchable array antenna in accordance
with the invention.
[0021] Fig. 16 shows an alternative form of the Fig. 15 antenna utilizing slots.
[0022] Fig. 17 shows a steerable antenna array system in accordance with the invention.
[0023] Fig. 18 shows excitation alternatives useful in describing operation of the Fig.
17 antenna system.
[0024] Fig. 19 shows the straight fan beams that are provided by the Fig. 17 antenna system.
[0025] Fig. 20 illustrates roll conditions in aircraft banking maneuvers.
[0026] Fig. 21 shows a steerable beam tilt antenna system in accordance with the inventon.
[0027] Fig. 22 shows an alternative form of signal distribution network usable with the
Fig. 21 antenna system.
DETAILED DESCRIPTION OF THE INVENTION
[0028] Referring now to Fig. 2, there is shown the physical configuration of an array antenna
10 as disclosed in the inventor's copending application entitled "Array Antenna With
Forced Excitation," filed December 19, 1990, Application Number 90313961.6. An understanding
of antennas of this type is important to an understanding of the present invention,
which provides further improvements in such antennas and systems utilizing them. The
present invention is more specifically described under the heading "Description of
Figs. 15-22."
[0029] Fig. 2a is an orthogonal view of the complete antenna including protective cover
12, of a radiation transmissive material such as fiberglass or a suitable plastic,
base member 14, of metal or suitable conductive material to serve as a mounting flange
and ground plane connection, and terminal means 16, shown as a coaxial connector suitable
for coupling RF signals.
[0030] Figs. 2b and c are exploded end and side views, respectively, of the array antenna
10, showing cover 12 and base member 14 with connector 16 attached. Also shown are
a first printed circuit card 18 bearing a first planar conductor pattern of forward,
middle and rear monopole antenna elements 20, 22 and 24, respectively, and a second
printed circuit card 26 bearing a second planar conductor pattern on surface 28. The
conductor pattern on surface 28, which is not visible in these views, will be described
below.
[0031] In a specific embodiment of the antenna 10, the assembled combination of the cover
12 and base 14 have a height of approximately one-tenth wavelength and length of about
three-quarter wavelength. References to dimensions measured in wavelength refer to
approximately the average design frequency, so that for a design frequency range or
bandwidth of 1,020 to 1,100 MHz, for example, the average design frequency would be
1,060 MHz, corresponding to a wavelength of about 11.1 inches. Dimensions are stated
in order to characterize the invention and differentiate over prior art antennas,
and are not intended to suggest that the invention is limited to precise dimensions
or to exclude antennas representing appropriate applications of the invention. As
shown in Fig. 2, the lower surface of base member 14 is flat, but in other embodiments
it may be a curved surface corresponding to the curved surface of an aircraft to which
it is to be mounted. For mounting, screws are typically fastened through the mounting
holes shown in Fig. 2a and a clearance hole through the outer surface of the aircraft
is provided for the connector 16, so that it can be joined to a mating connector for
coupling signals to cabling and signal processing equipment carried within the aircraft.
[0032] Fig. 3 shows a typical antenna system including five array antennas 10a through 10e,
supported in a laterally spaced configuration on a curved metal surface 30 such as
the fuselage of an aircraft, forward of the pilots' windshield. It will be apparent
that in such an installation, use of array antennas one inch in height provides a
dramatic improvement in the pilot's visibility, as compared to use of prior art antennas
three inches in height. In an installation of this type, the individual array antennas
can be excited in groupings selected to provide desired antenna beam characteristics,
in accordance with known principles of array antenna excitation. An antenna system
as shown in Fig. 3, when installed on the upper forward surface of an aircraft, can
provide broad horizontal coverage forward of the aircraft and good vertical coverage,
except below the aircraft. A similar antenna system installed on the lower forward
surface of the aircraft would permit full vertical and horizontal coverage forward
of the aircraft. Alternatively, antenna systems mounted near the leading edge of a
wing could provide complete vertical coverage, but would probably require similar
systems on the other wing in order to provide complete horizontal coverage free of
blockage by the nose of the aircraft.
[0033] Fig. 4 is a simplified block diagram of an array antenna in accordance with the invention,
shown in two sections 18a and 26a corresponding basically to the printed circuit cards
18 and 26 in Fig. 2. The antenna is used to alternatively radiate and receive signals,
in the range of 1,020 MHz to 1,100 MHz, which are coupled to and from the antenna
by way of the terminal means 16a corresponding to connector 16 in Fig. 2. The cover
and base components, 12 and 14, are not represented in Fig. 4. As noted, the antenna
is used both to radiate and receive signals, and description of how signals are processed
by various portions of the antenna when radiating, for example, will be understood
to be equally relevant in a reverse relationship during reception.
[0034] The Fig. 4 antenna includes first, second and third antenna elements 20, 22 and 24,
which in accordance with the invention may be monopoles of the order of one-tenth
wavelength in height arranged in a spaced linear array. While the desirability of
using antenna elements one-tenth wavelength high as compared to prior art elements
one-quarter wavelength high may be readily apparent, the severe operational bandwidth
degradation normally associated with short antenna elements such as monopoles has
been a limiting factor contributing to the continuing reliance on quarter wave elements
in the prior art. In addition, attempts to use elements shorter than a quarter wavelength
in an array configuration with prior art excitation arrangements have been subject
to severe effects of intercoupling between adjacent and other combinations of the
antenna elements and nearby surfaces, as a result of effects of unequal and complex
mutual impedances between individual antenna elements in an array. These effects,
which do not readily yield to design compensation, largely determine the actual currents
in the antenna elements and the resulting antenna pattern. It will be appreciated
that if the currents in the various elements cannot be acurately determined and proportioned,
neither can a desired antenna pattern be provided. While the basic description of
the invention will be in the context of arrays of three elements, denoted as "first,
second and third" elements, additional elements may be included as will be described.
However, regardless of the total number of antenna elements, each antenna will include
three elements meeting the description and function of the first, second and third
elements as set out and claimed.
[0035] Section 26a of the Fig. 4 antenna as shown comprises excitation and tuning means
which are effective to cause signal currents in the antenna elements 20, 22 and 24
to have a predetermined relationship of phase and amplitude substantially independent
of impedance interaction, and are able to accomplish this over a significant band
or range of operating frequencies. As shown, antenna portion 26a includes first excitation
means shown as excitation circuit 40, coupled between terminal 16a and the first and
third elements 20 and 24, comprising signal transmission means (as will be discussed
in more detail with reference to Fig. 6) for coupling signal components to elements
20 and 24 by way of a point of common voltage, shown as point 42 on the connection
between excitation circuit 40 and double tuning circuit 44. Tuning circuit 44, provides
double tuning of the impedance characteristics of the antenna ciruits to optimize
for operation in a desired frequency range. While circuit 44 is shown as being connected
in series between terminal 16a and point 42, its function is to provide wideband impedance
matching and it may comprise discrete or distributed reactances coupled to point 42
in series as shown, or in parallel to ground, or may utilize appropriate lengths of
transmission line, as will be apparent to those skilled in the art. Section 26a also
includes means 46 shown as including second excitation circuit 48, coupled between
terminal 16a and second element 22, comprising means for coupling a signal component
to the element 22 which has a predetermined phase and amplitude relative to the components
coupled to elements 20 and 24 via first excitation means 40. As shown in Fig. 4, excitation
circuit 48 functions as a power divider coupling a portion of the input signal from
terminal 16a to element 22, while the remaining portion of the input signal flows
from the terminal 16a to the other elements. This power divider function of circuit
48 may be provided by a directional coupler (as will be discussed with reference to
Fig. 6) or other means. In Fig. 4, means 46 also includes double tuning circuit 50
for providing double tuning of the impedance characteristics of the middle element
22 for operation in a desired frequency band or range. Where distributed reactances
or transmission lines in excitation means 48 are used to provide the double tuning
function, means 50 may not appear as a discrete element.
[0036] Fig. 5 shows a three monopole array arranged to provide an end-fire pattern and Fig,
6 shows such an array antenna with an excitation system in accordance with the invention.
A good end-fire pattern is obtainable from the Fig. 5 array if the elements have the
spacings and the phase and amplitude of currents shown, Fig. 6 shows an antenna with
an excitation system effective to provide "forced excitation" to cause signal component
currents in the antenna elements to have such a predetermined relationship of phase
and amplitude, substantially independently of intercoupling affecting the antenna
elements, with double tuning to provide for operation over a significant range of
frequencies, "Forced excitation" is defined as an excitation arrangement which forces
or predetermines the currents in the elements of an array antenna so as to result
in currents of desired relative magnitude and phase, substantially independently of
mutual and other coupling and impedance effects.
[0037] In Fig. 6 there are included first, second and third antenna elements, shown as short
monopoles 20, 22 and 24 mounted through and above a conductive ground plane 14a. The
Fig. 6 array antenna includes first excitation means comprising quarter wave transformer
56 coupled to third monopole 24, and quarter wave transformer 58 arid half wave transmission
line 60 coupled to first monopole 20. Transformer 56 and line 60 are also shown coupled
to common voltage point 42, as is tuning means 62 which is also coupled to signal
input and output terminal 16a. Tuning means 62 is a series resonant LC circuit arranged
for double tuning the impedance of rear and forward monopoles 24 and 20. Each of the
monopoles is shown as having a series inductance at its base, such as indicator 64
at element 24, for tuning out the capacitive impedances of the short monopole element
at one frequency near midband. This narrow band tuning is augmented by the double
tuning means 62 to provide substantially increased bandwidth. The Fig. 6 antenna also
includes second excitation means comprising a directional coupler 66, for coupling
signals of predetermined relative amplitude to the second monopole 22, and second
tuning means 68. As shown, coupler 66 is coupled to terminal 16a and is effective
to transfer a portion of a signal input to the antenna to monopole 22 by way of transmission
line section 70. .Second tuning means 68 is a parallel resonant LC circuit arranged
for double tuning the impedance of second monopole 22, and the length of line 70 is
chosen so that signals reaching monopole 22 have the desired relative phase as compared
to signals at monopoles 20 and 24.
[0038] In operation of the Fig. 6 array antenna, the two quarter wave transformers 56 and
58 force the currents Ia and Ic in the third and first monopoles 24 and 20 to be dependent
substantially wholly on the voltage at the common voltage point 42. Thus, Ia and Ic
are forced to be in the ratio Ia/Ic = Zoc/Zoa, where the latter are the respective
transmission line impedances of the transformers 58 and 56. The half wave line 60
introduces a reversal in the polarity of Ic at element 20, relative to Ia at element
24. The ratio of Ib to the Ia and Ic currents is not forced and cannot be forced because
of the 90° phase difference needed to obtain the desired signal component relationship
of Ia=j, Ib=2 and Ic=-j, as shown in Fig. 5. However, if Ia=-Ic then the second monopole
22 will effectively be at a null point midway between the equal and opposite signals
at elements 20 and 24 and no net signal from those monopoles will he coupled to element
22. In this case there is no need for Ib to element 22 to be forced.
[0039] As a specific example, computations of impedance were made using a commercial computer
program for three monopoles arranged as in Fig. 5 with currents as in Fig. 5. The
computations were made at 1,030 MHz, 1,060 MHz, and 1,090 MHz for an array of three
identical monopoles one inch high, 1.6 inches wide at the top and with center-to-center
spacing of 2.78 inches. Computed results were as follows:

With reference to Fig. 6:

For quarter wave transformers:

Let Zoa = kZoc
- Zs
- = Zoa²/(Za + k²Zc)
-
- = Zo²/(Za + Zc), if k=1
where Zoa = Zoc = Zo
[0040] From the table above, with the reactance tuned out at midband by the series inductances
such as 64, Za + Zc is approximately equal to 15 ohms.
[0041] From the last equation, and assuming we want Zs to be 50 ohms:
- Zo²
- = Zs (Za + Zc)
-
- = 50 (15)
- Zo
- = 27.4 ohms
[0042] Note that in Fig. 6, the quarterwave transformers and transmission line sections
are shown as being sections of microstrip transmission line that are dimensioned to
provide the desired characteristic impedances. Thus, lines 60 and 70 in this example
would be 50 ohm line sections and transformers 56 and 58 would be 27.4 ohm sections
one quarter wavelength long at a frequency of 1,060 MHz. Reactive tuning circuits
62 and 68 are used to optimize antenna performance at 1,030 MHz and 1,090 MHz( i.e.,
are adjusted to double tune the respective antenna elements at those frequencies).
Note also that, because of mutual coupling, Za has negative resistance, making it
very difficult to precisely and efficiently provide the desired Ia over a frequency
band, in the absence of the invention. However, (Za + Zc) has a substantial positive
resistance which can be efficiently double tuned while providing the desired Ia and
Ic values, in accordance with the invention. Achievement of an array antenna pattern
with a high front-to-back ratio and strong radiation over a wide angle in the front
sector requires precise control of the relative currents in the array elements, as
made possible by the present invention.
[0043] Referring now to Figs. 7 and 8, there are shown alternative excitation circuits for
array antennas similar to the Fig. 6 antenna. For the Figs. 7 and 8 antennas, the
monopoles and the excitation means between point 42 and the monopoles 20 and 24 are
the same as shown in Fig. 6. In Fig. 7 the excitation means for the second element
includes a quarter wave transformer 72 similar to transformers 56 and 58 in Fig. 6.
Zo of 72 should be different than Zo of 56 and 58. In the Fig. 7 antenna, the tuning
function can he provided by a series resonant LC circuit 68a and the length of line
70a can be reduced, otherwise operation corresponds to operation of the Fig. 6 antenna.
In Fig. 8, the excitation means for the forward and rear elements includes a quarter
wave transformer 78 similar to transformer 72 included in the second element excitation
means in Fig. 7. In the Fig. 8 arrangement, the parallel resonant LC circuit 62a provides
the tuning function, and operation again corresponds to operation of the Fig. 6 antenna.
The LC circuits, such as 68a and 62a, may use discrete reactance components or appropriate
lengths of transmission line, as will be apparent to those skilled in the art.
[0044] Fig. 9 is an actual measured azimuth antenna pattern at 1,060 MHz for an array antenna
with three monopoles resembling those shown in Fig. 2c, with a monopole width of 2
inches, spacing of 2.78 inches and height of .91 inches, after adjustments for the
excitation circuits intended to optimize the results achieved. Note that the front-to-back
ratio is greater than 20dB, and the pattern remains strong over a wide angle in the
front sector. .Similar results were obtained at 1030 and 1090 MHz. It is believed
that the antenna performance reflected in this data is clearly beyond the performance
of any known prior art monopole array antenna of comparable dimensions.
[0045] Figs. 10a and 10b show printed circuit cards 18 and 26 designed for this antenna.
On card 18, three monopoles 20, 22 and 24 as shown were formed by etching a copper
layer on dielectric card 18 to leave conductive patterns in the form of the monopoles.
The pattern shown on surface 28 of the card 26 was similarly formed. The actual pattern
shown on card 26 represents microstip transmission line sections of various lengths
and characteristic impedances, together with interconnecting points and sections,
desiged to implement the antenna in a physically simple form providing ease of production
and assembly, consistent electrical characteristics, inherently high reliability and
good durability under shock and vibration conditions common in high-performance aircraft
applications. While reference numerals corresponding to the Fig. 6 antenna, with substitution
of the alternative excitation circuit of Fig. 8, have been included in Fig. 10, it
will be understood that reducing the antenna to a microstip layout, and refining that
configuration for maximum performance, results in a final physical embodiment of the
invention in this example in which there is a degree of inherent masking of the indentification
of discrete components. Thus, while portions of the conductive pattern on card 26
in Fig. 10 have been given identifying numerals, it may be difficult or not possible
to specifically identify the metes and bounds of a particular component so as to separate
it from the remainder of the circuit.
[0046] Fig. 11 shows an array antenna in accordance with the invention wherein the individual
radiating elements are slots. A three element slot array, as shown, is subject to
disruptive mutual coupling effects similar to those previously discussed with reference
to monopoles. Slots 80, 82 and 84 in Fig. 11 may simply be openings in a conductive
covering 86 on the forward side of a dielectric sheet 88. Conductive covering 86 and
dielectric sheet 88 are both shown as being transparent for ease of illustration in
order to make visible the other elements which may be disposed on the backside of
the dielectric sheet, as shown.
[0047] Each of the slots or windows 80, 82 and 84 in the conductive member 86 may typically
be a half wavelength long or, alternatively, may be shorter with shunt capacitances
inserted across the center of the slot at one frequency near midband. The slots in
the array are spaced by a quarter wavelength, with a width equal to a fraction of
the spacing. Dimensions can be selected for particular applications using known design
techniques. As shown, each slot is excited by a conductor passing across the slot
on the back of the dielectric sheet, as shown at 90, and passing forward or upward
through the dielectric 88 to terminate at a point 92 in electrical contact with the
conductive covering 86 at the side of slot 80. As shown, slot 80 has an excitation
conductor termination point 92 at its right side and will be excited with a phase
or a polarity of excitation opposite to that of slot 84, which has such termination
point at 96 at its left side. Although not shown, each slot is typically backed by
a metallic box or conductive cavity to allow radiation only in the forward or outward
direction from each slot. It will be appreciated that an antenna in the form of an
array of slots is particularly advantageous for implementation in a configuration
flush with the surface of an aircraft. The present invention is readily adaptable
to such applications.
[0048] The Fig. 11 antenna includes first excitation means shown as half-wave transmission
lines 98 and 100 coupling the third and first elements 84 and 80 to the terminal means
16a via common voltage point 102. Reactive means 62a is shown coupled between point
102 and terminal 16a for providing double tuning in a desired frequency range. Second
excitation means, shown as directional coupler 66a, is coupled between terminal 16a
and second element 82, via transmission line section 70a and reactive means shown
as LC circuit 68a. Operation of the Fig. 11 antenna is similar to the Fig. 6 antenna.
Characteristics of slots permit use of transmission line sections 98 and 100 without
provision for quarter wave transformers in providing a common voltage point enabling
forcing of the voltages across the slots to have the desired magnitude and phase,
substantially independently of mutual and other coupling and impedance effects. With
slot radiators, the significant signal component that determines the radiation pattern
of an array is the slot voltage, in contrast to monopole or dipole radiators which
have their currents as the significant signal components. Desired slot voltages for
a good end-fire pattern with the Fig. 11 array have phase and amplitude values similar
to the monopole currents shown in Fig. 5. The Fig. 11 system can provide this forced
excitation together with double tuning for increased bandwidth.
[0049] Figs. 12 and 13 show alternative embodiments regarding the means connecting points
96 and 92 to point 102 in antennas which otherwise correspond to Fig. 11. In Fig.
12 the half wave transmission lines 98 and 100 have each been replaced be a series
combination of two quarter wave transformers, such as transformers 104 and 106 shown
as replacing line 100 between points 92 and 102. This arrangement provides wideband
transformation of the slot conductance to a convenient value such as 50 ohms at point
102. In Fig. 13, half wave lines 98 and 100 have been replaced by a single full wavelength
transmission line segment 108 connecting points 96 and 92, and reactive tuning circuit
62a connects to a point 102a in the vicinity of point 96. Variations such as shown
in Fig. 13 can provide flexibility in particular applications.
[0050] The preceding embodiments are particularly shown and described in the context of
an array of three radiating elements, however, it will be apparent that in some applications
it may be desirable to provide one or more array antennas, each of which includes
four or more radiating elements with forced excitation in accordance with the invention.
[0051] Referring now to Fig. 14, there is illustrated an embodiment of the invention comprising
a linear array of five antenna elements shown as monopoles 20a through 24a. As shown,
the first, second and third elements 20a, 22a and 24a (corresponding to the first,
second and third elements of Fig. 6) have been supplemented by a leading element 21a,
ahead of element 20a, and a trailing element 23a, following element 24a. In considering
the Fig. 14 antenna, it is important to note that the arrangement and functioning
of elements 20a, 22a and 24a are as described with reference to a three element array,
the three element array of first, second and third elements being a basic subset used
in antennas utilizing the invention.
[0052] In Fig. 14, elements 20a, 22a and 24a correspond to elements 20, 22 and 24 of Fig.
6. The Fig. 14 excitation system corresponds to the alternative excitation system
of Fig. 9, with modification for excitation of the additional elements 21a and 23a.
As shown in Fig. 14, a first group of non-adjacent antenna elements 20a and 24a are
coupled to first excitation means shown as signal transmission means including halfwave
transmission line 60 and quarterwave transformers 56 and 58. The remaining elements,
middle element 22a, leading element 21a and trailing element 23a, are coupled to second
excitation means shown as directional coupler 66, transmission line section 70a, quarterwave
transformers 72, 73 and 74, and half and full wavelength transmission lines 75 and
76, respectively. .Signals are coupled by the excitation means to elements 20a and
24a by way of common voltage point 42 and to elements 21a, 22a and 23a by way of a
second common voltage point 43, permitting forced excitation.
[0053] If there were only four elements, the element 21a, transformer 73 and line 76 could
be eliminated. For any number of elements there are actually two voltage points in
accordance with the invention, to which signals are fed. For three elements, one of
these voltage points is a common voltage point for two elements, permitting predetermined
magnitudes and phases of current to be provided. For more than three elements the
invention makes available two common voltage points, 42 and 43 for example, each connecting
to two or more elements.
DESCRIPTION OF FIGS. 15 - 22
[0054] Referring now to Fig. 15, there is shown a switchable array antenna in accordance
with the present invention. Fig. 15 includes five antenna elements, shown as monopoles
110, 112, 114, 116 and 118, supported above a ground plane 121 in a linear array and
arranged for excitation in subsets of three elements. Shifting means, shown as switch
122, selectively connects the subsets of elements (i.e., elements 110, 112, and 114,
elements 112, 114 and 116, or elements 114, 116 and 118) to excitation means for coupling
signal components from and to the selected elements during reception and transmission
of radiated signals. Thus, during transmission, shifting means 122, which may comprise
mechanical or electronic individual switching means such as switches 123 and 124,
selectively shift the coupling of signal components appearing at terminals 126, 128
and 130 to different first, second and third element subsets of the antenna elements
110, 112, 114, 116 and 118. For example, with shifting means 122 in the position illustrated
in Fig. 15, forward, middle and rear signal components for achieving an end-fire antenna
pattern directed toward the right are respectively coupled to a first element 114,
a second element 112 and a third element 110. As will be described further, when the
forward, middle and rear signal components are shifted to a different three element
subset, such as a first element 118, a second element 116, and a third element 114,
the effective element radiation center of the array is shifted forward from the vicinity
of element 112 to the vicinity of element 116.
[0055] As shown in Fig. 15, the switchable array antenna also includes terminal means, first
excitation means, second excitation means and tuning means substantially as described
above with reference to Figs. 6 and 7. The terminal means is illustrated as terminal
16a for coupling signals to and from the antenna. First excitation means is shown
as including a half-wavelength transmission line 60 for coupling a signal component
to terminal 130 with a phase reversal, as compared to the signal component coupled
to terminal 126, and a set of two quarter wave transformers for coupling such signal
components to first and third elements, respectively, of a selected three element
subset of the five antenna elements illustrated. With switching means 122 in the position
shown, it will be seen that the first excitation means utilizes a quarter wave transformer
144 coupling to element 114 and transformer 132 coupling to element 110. Second excitation
means is shown as including directional coupler 66 for coupling a signal component
of predetermined amplitude to terminal 128 and a quarter wave transformer 138 coupling
to element 112, which is the second element of the selected 114, 112, 110 element
subset in this example. Tuning means, shown as series LC circuit 68a coupled to the
first excitation means via common voltage point 42 and series LC circuit 62 coupled
to the second excitation means, provide double tuning of the antenna elements.
[0056] The structure and operation generally and as to individual elements of the excitation
and tuning means are covered more specifically in the description of Figs. 6 and 7
wherein corresponding reference numerals refer to similar components. It will be seen,
however that the functions of three quarterwave transformers (56, 58 and 72 in Fig.
6 with the Fig. 7 modification) are provided in Fig. 15 by quarterwave transformers
132, 134, 136, 138, 140, 142, 144, 146 and 148, which are utilized in sets of three
dependent on the operation of switch 122. Alternatively, three quarterwave transformers
can be inserted at points 126, 128 and 130, respectively, and the nine quarterwave
transformers in Fig. 15 replaced by nine halfwave transmission lines.
[0057] The operation of the Fig. 15 antenna is basically as described with reference to
Fig. 6. By enabling the phase and amplitude of the respective currents in first, second
and third elements, such as elements 114, 112 and 110, to be forced to have predetermined
values, an end-fire or other desired antenna pattern is achieved. The Fig. 15 antenna
differs in permitting the radiation center of the array to be shifted to be in the
vicinity of element 112, 114 or 116, depending on whether excitation is applied to
a first, second and third element subset 114, 112, 110 or 116, 114, 112 or 118, 116,
114 by action of the shifting means shown as switch 122.
[0058] With reference to Fig. 2, it will be understood that the Fig. 15 antenna can be constructed
as in Fig. 2 with a protective cover and base member similar to elements 12 and 14
in Fig. 2. Also, antennas in accordance with Fig. 15 can be arranged in a laterally
spaced array supported on a surface such as the fuselage of an aircraft, as shown
in Fig. 3.
[0059] In Fig. 15, the radiating element array 120 is shown as comprising five monopoles
coupled to points 111, 113, 115, 117 and 119. Fig. 16 shows an alternative form of
radiating element array 120a which can be substituted in a modified Fig. 15. As shown,
array 120a comprises five slots 110a, 112a, 114a, 116a and 118a illustrated as elongated
openings in a conductive layer or surface 86 coupled to an insulative layer or member
88. As discussed with reference to Fig. 11, members 86 and 88 are shown as being transparent
to show the connection from point 111, in an insulated relationship across slot 110a
behind layer 88, and passing through layer 88 to terminate in contact with layer 86
on the left side of the slot at point 150. Whereas in Fig. 11 a relative phase reversal
was introduced for the signal component fed to slot 80 by virtue of the feed conductor
crossing to a contact point 92 on the right side of slot 80, in Fig. 16 all contact
points are to the left side of the respective slots. For the Fig. 16 antenna, a phase
reversal is introduced by the half wave line section 60 shown in fig. 15, so that
the Fig. 15 and 16 antennas can both use a similar excitation system such as shown
in Fig. 15. However, with the Fig. 16 antennas, the quarterwave transformers in Fig.
15 must be replaced by halfwave transmission lines.
[0060] Operation of the Fig. 16 alternative form of antenna is basically as described with
reference to Fig. 11, with the additional capability of shifting of the effective
radiation center dependent upon the selected excitation of first, second and third
element subset 114a, 112a, 110a, or subset 116a, 114a, 112a, or subset 118a, 116a
or 114a.
[0061] Referring now to Fig. 17, there is shown a simplified schematic of a steerable antenna
array system in accordance with the invention. As illustrated, the array system includes
a plurality of switchable array antennas shown as three identical antennas 152a, 152b
and 152c, which may be of the type shown in Fig. 15 or 16. The three switchable array
antennas are spaced laterally relative to an end-fire radiation direction to the right
in the drawing. In Fig. 17, signals supplied to terminal 154 are coupled to the three
antenna terminals 16a, 16b, and 16c, corresponding to terminal 16a in Fig. 15. As
in Fig. 15, in each of antennas 152a, 152b, 152c, signal components will be coupled
by the respective excitation means to a selected first, second and third element subset
of the five elements 110, 112, 114, 116 and 118, which are represented by dots, such
as dot 118. Assuming shifting means 122 of antenna 152a to be in the position shown
in Fig. 15, the active element subset in antenna 152a will be elements 114, 112, and
110, which are circled to indicate the active subset.
[0062] In Fig. 17 there is also included azimuth control means shown as switch controller
156 coupled to the respective shifting means (122 in Fig. 15) of each of the antennas
via terminals 122a, 122b and 122c. The shifting means 122 of each antenna may be activated
to select one of the three different element subsets and controller 152 comprises
a control circuit or mechanism for adjusting the shifting means so that the effective
radiation center of each antenna is in a selected position.
[0063] As indicated by the circled dots in Fig. 17 representing the activated antenna elements,
the shifting means are adjusted in this example so that the effective radiation center
is in the vicinity of element 112 for antenna 152a, in the vicinity of element 114
for antenna 152b, and in the vicinity of element 116 for antenna 152c. Based on well
known concepts of theory and operation of phased array antennas, three Fig. 15 antennas
laterally spaced as in Fig. 17 and identically excited in an end-fire mode would produce
a beam directed to the right in Fig. 17. However, excitation of the antennas with
different centers of radiation as indicated in Fig. 17 would steer the beam to an
angle while preserving the straight shape of the fan beam.
[0064] This is better illustrated in Fig. 18, which is a simplified representation of three
spaced array antennas excited in five different modes. In Fig. 18a the circles identifying
the active elements indicate that the three antennas, such as 152a, b and c in Fig.
17, are identically excited with their centers of radiation along the line 158. Line
158 effectively represents the wavefront for this excitation and would result in a
beam direction normal to line 158. In Fig. 18b excitation is as indicated in Fig.
17, resulting in a rotated wavefront line producing a normal beam direction angled
to the left of the original beam direction by an angle of 30°, for example, dependent
on the actual dimensioning of the antennas. Fig. 18c indicates a wavefront for a beam
angled to the right and Fig. 18d shows a segmented wavefront resulting in a beam direction
angled to the right less than the Fig. 18c beam direction. On a simplified basis,
the beam direction in Fig. 18d can be considered to be the mean of partial beams normal
to the two wavefronts represented. The actual beam direction for excitation as in
Figs. 18d and e can be calculated or measured based on actual dimensions and characteristics
of the antennas to be used, the important point being that relative positioning of
the effective radiation centers determines the wavefront and beam position. All of
the beams resulting from the excitations shown in Fig. 18 preserve the straight shape
of the fan beam. This is shown in Fig. 19.
[0065] Fig. 17 also shows phase shifters 127a and 127b in channels 16a and 16c, respectively.
These phase shifters can provide two benefits. First, they may be used to reduce the
bend in the wavefront of Fig. 18d or 18e while still preserving the beam direction
and the straight shape of the fan beam. Second, they may be used to steer the beam
to any azimuth angle between or beyond the five angles shown in Fig. 19. In this case
the fan beams become curved, but typically much less curved than the prior art case
of Fig. 1.
[0066] It should be understood that although three array antennas each containing five antenna
elements have been shown in Figs. 15, 16, 17 and 18, the number of array antennas
and antenna elements in each array antenna can be greater. Also the number of active
antenna elements in each array antenna can be greater than three.
[0067] In operation, a laterally spaced combination of array antennas with effective radiation
centers controlled by azimuth control means can have its antenna beam selectively
steered. In this way, a target which is off-boresight relative to the antenna system,
need not be off-boresight relative to the active elements in the antenna system. With
the beam steered toward the target, off-boresight errors associated with coning of
the beam are reduced, thereby improving the accuracy of the indicated azimuth bearings
of targets at varying altitudes relative to the base aircraft.
[0068] Fig. 20a is a simplified representation of a section of aircraft fuselage with seven
array antennas 152a-g mounted on it (for example, seven Fig. 15 antennas seen in end
view). The vertical line 159 in Fig. 20a indicates that the vertical axis of the aircraft
is not tilted (i.e. the aircraft is not banking). Assume that the three central antennas
152c, d and e in Fig. 20a represent an antenna system with adequate performance in
the absence of banking, but that during banking the antenna system is tilted, compromising
the performance. As shown in Figs. 20b and c, during banking conditions as indicated
the invention permits compensation by selection of an operative group of antennas
(identified by the bracket) effectively representing a three antenna system (152d,
e and f or 152e, f and g) which is level at a particular degree of roll caused by
banking. In Figs. 20b and 20c selection of the three indicated antennas results in
a fan beam which remains vertically oriented relative to the horizon. However, since
the vertical axis of the aircraft represented by line 159 has rolled left, the desired
beam compensation has actually been accomplished by tilting the fan beam of the antenna
system relative to the aircraft on which it is mounted.
[0069] Referring now to Fig. 21, there is shown a beam tilt antenna array system able to
compensate for aircraft roll and also permitting antenna beam steering while preserving
straight fan beams. The antenna will first be described independently of the beam
steering capability. As illustrated, the antenna array system includes a plurality
of array antennas shown as seven antennas 152a-g, which may be the type shown in Fig.
15 or 16. The antennas are arranged to radiate principally in a forward direction
(upward, in the drawing) and are spaced laterally, the spacing being such that in
the context of an aircraft fuselage the antennas have displacements in a third direction
(which is the vertical direction in the Fig. 20a view), due to the curvature of the
fuselage. Thus, the antennas are basically shown in top view in Fig. 21 and end view
in Fig. 20, so that the relative vertical displacements in Fig. 20 are essentially
normal to Fig. 2.1.
[0070] The Fig. 21 antenna system also includes beam tilt control means, shown as beam tilt
control means 160, for selectively activating signal distribution means 162 for determining
which group of antennas is active during particular roll conditions. Information representing
the degree of roll may be supplied to means 160 or sensed by any appropriate means
therein. In either case, tilt control means 160 controls electronic or other switching
means 162, shown as including a series of switches such as 162a and 162b, to couple
signals between terminal 154a and selected group of antennas, such as antennas 152d,
152e and 152f as shown in Fig. 21, corresponding to compensation for the roll condition
shown in Fig. 20b.
[0071] In accordance with the invention, a beam tilt antenna may also include beam steering
as discussed with reference to Fig. 17. Thus, in Fig. 21,switch control 156a functions
in the same manner as switch controller 156 in Fig. 17 to selectively control the
shifting means of each active antenna. In the case of Fig. 21, information from tilt
control means 160, indicative of which three of antennas 152a-g are activated at a
particular time, is used in means 156a to direct shifting means control information
to the currently active antennas. In Figs. .17 and 21 individual radiating elements
in an array antenna such as 152a, are indicated by dots and active elements by circles
for ease of illustration and explanation. The actual elements may be monopoles, slots,
etc. as shown and described in greater detail with reference to the other drawings,
such as Figs. 6, 11, 15 and 16, and the various alternatives already covered. Phase
shifters 127a and 127b for additional azimuth beam control are now located below switching
means 162.
[0072] In operation of the Fig. 21 antenna, the antenna fan beam is tilted to the right
as the aircraft rolls left, and vice versa, to provide a range of compensation as
the fan beam would otherwise deviate from its normal reference or vertical orientation.
At the same time, the antenna beam may be steered as described with reference to Figs.
17 and 18, and the beam steering and tilting can be accomplished independently of
each other. Fig. 22 is included to indicate that, where desired, alternative forms
of signal feed arrangements known in the prior art may be substituted for the switching
approach utilized, in place of signal distribution means 162 as shown in Fig. 21.
The phase shifters 164a - 164g in combination with the Butler Matrix and feed network
smoothly shift the active portions of the array to compensate for aircraft roll. The
phase shifters 166a - 166g provide the additional azimuth beam control.
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 means 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 switching means
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.
Claim 3. A switchable array antenna as in claim 1 or 2, in which said antenna elements
are monopoles.
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.
Claim 5. A switchable array antenna as in claim 4, in which said second excitation
means comprises a directional coupler.
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.
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.
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.
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.
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.
Claim 11. A switchable array antenna as in claim 1, 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 for 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.
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 second reactive means for providing tuning
in a desired frequency range.
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 switching means.
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 switching means
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.
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.
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.
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.
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.
Claim 20. A steerable antanna array system, comprising:
a plurality of switchable array antennas as in claim 1, 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.
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.
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 longitudingal
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 directions.
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.
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.
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.
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.
Claim 27. A tilting beam antenna array system, comprising:
terminal means for coupling signals;
a plurality of array antennas as in claim 1, 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.
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 azimuths with respective to an axis of said vehicle,
and the beam tilting is adapted for compensation for vehicle banking.