Communications: directive radio wave systems and devices (e.g. – Directive – Including polarized signal communication transmitter or...
Reexamination Certificate
2002-03-20
2004-03-09
Tarcza, Thomas H. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Including polarized signal communication transmitter or...
C342S368000
Reexamination Certificate
active
06703974
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to antennas and associated methods for communication and, more particularly, to antennas systems having active polarization correction and associated communication methods.
BACKGROUND OF THE INVENTION
Antennas are widely utilized in order to transmit and receive a variety of signals. For example, antennas are widely utilized in radio frequency communication systems. Radio frequency antennas are commonly capable of simultaneously transmitting and/or receiving signals having different polarizations, such as orthogonally polarized signals, in order to increase the transmission and/or reception capacity of the antenna. In order to effectively transmit and/or receive signals that are orthogonally polarized, an antenna must have relatively high polarization purity such that there is minimal interference between the orthogonally polarized signals. In some applications, for example, the required cross-polarization isolation may be 30 dB or more.
One common type of antenna utilized for high-data rate Communications with moving platforms is a phased array antenna. Among other advantages, phased array antennas are capable of communicating simultaneously with two or more spatially separate sources. In addition, phased array antennas are relatively easy to install, operate and maintain on moving platforms such as aircraft, ships and motor vehicles since they generally have a relatively low profile, are capable of rapidly tracking and have no moving parts.
Phased array antennas generally include a number of identical radiating elements and a beam former connected to the radiating elements. Each element may include a phase shifter and/or a time delay circuit. In addition, each element may include an amplifier, if desired. In one phased array antenna, each element includes a phase shifter and groups of elements are interconnected by a time delay circuit. By adjusting the phase shift of each element and the time delay of each group of elements, the beam transmitted and/or received by the phased array antenna may be formed electronically and steered without physical movement of the antenna aperture over a wide instantaneous bandwidth. Moreover, by incorporating multiple beam formers and multiple phase shifters and time delay circuits associated with each radiating element, a phased array antenna that is capable of forming multiple simultaneous independent beams may be constructed.
Phase array antennas are capable of transmitting and/or receiving signals having any desired polarization. In this regard, a schematic representation of the architecture of a phased array antenna capable of sensing signals having either circular polarization or arbitrarily oriented linear polarization is shown in
FIG. 1. A
phased array antenna having the architecture depicted in
FIG. 1
includes a plurality of modules
1
, each of which includes two amplifiers
2
connected to the orthogonal radiating elements
3
. The output of each amplifier connected to a 90° hybrid
4
which forms left circular polarization out of one port and right circular polarization out of the other port. Each port is connected to a phase shifter
5
which, in turn, is connected to an independent beam forming network
6
. The output of each beam former is therefore a left or right circularly polarized (CP) beam that is redirected independently of the other beam. A phased array antenna having the construction depicted in
FIG. 1
may also operate in a linearly polarized (LP) mode. In this mode, the two beams are co-pointed and the beam former outputs are recombined in a quadrature hybrid
7
to recover two orthogonal linear polarizations from a single source. By controlling the two phase shifts
5
to have a constant offset therebetween, the two orthogonal linear polarization axes may be spatially rotated so as to be aligned with the polarization axes of a source, such as a satellite that radiates orthogonal linearly polarized signals Although phased array antennas offer a number of advantages, phased array antennas, in particular, and electronically scanned antennas, in general, are typically unable to provide the degree of polarization purity over the entire range of scan angles as that provided by at least some mechanically scanned antennas. The limitations with respect to the polarization purity of electronically scanned antennas are created by construction constraints within the modules, inherent radiating element cross-polarization characteristics and the active impedance to which a module is subjected once a module is placed in an array. Notably, this disadvantageous cross-polarization coupling between signals having orthogonal polarizations is within the antenna itself and is independent of any cross-coupling between signals having orthogonal polarizations that may occur in the propagation medium.
For signals transmitted and/or received in a near broadside direction, the cross-polarization isolation is determined largely by the degree of cross-coupling between orthogonal radiating elements. While phased array antennas can be constructed with near broadside polarization isolation approaching that of mechanically scanned antennas, the cost of the modules that must be constructed generally increases substantially. Unfortunately, as the scan angle increases away from broadside, the cross-polarization isolation degrades due to divergence between the E and H-plane active impedances seen by each module in the array. The degree of divergence typically increases monotonically with elevation scan and varies smoothly and periodically with azimuth scan. At an elevation scan of 60°, for example, the degree of degradation of the cross-polarization isolation relative to that provided near broadside will vary as the antenna is scanned in an azimuthal direction by as much as 10 dB. The internal coupling between the orthogonally polarized signals within an antenna
20
may be graphically depicted as shown in FIG.
2
. In this regard, the antenna is represented by the combination of two blocks, one block
22
depicting an ideal antenna having no internal coupling between the orthogonally polarized signals and another block
24
depicting the internal coupling between the orthogonally polarized signals. As will be apparent, although the antenna is depicted for purposes of discussion as being separated into two boxes, the antenna cannot physically be separated in the same manner as the internal coupling between the orthogonally polarized signals is inherent within the antenna as a result of its construction and design.
Referring to
FIG. 2
, the antenna
20
includes a pair of terminals
26
and at least one pair of orthogonally polarized radiating elements
28
. In the transmission mode, two orthogonally polarized signals T
1
and T
2
are presented at the antenna terminals and are amplified by the ideal antenna
22
by a gain designated A. These amplified signals are then subjected to undesirable cross-polarization coupling as represented by block
24
. As indicated within block
24
, the ratio of the cross-coupled voltage to the signal voltage is designated &dgr;. As such, the signals transmitted by the dual orthogonally polarized radiating element are not merely the amplified inputs designated AT
1
and AT
2
, but are instead more complex signals in which each signal includes components having both polarizations. In the illustrated example, the radiating element designed to radiate signals having the first polarization p
1
actually radiates a signal defined as A[T
1
(1−&dgr;
2
)
1/2
{circumflex over (p)}
1
+&dgr;T
2
{circumflex over (p)}
2
], while the radiating element designed to radiate signals having the second polarization p
2
actually radiates a signal represented as A[&dgr;T
1
{circumflex over (p)}
1
+(1−&dgr;
2
)
1/2
T
2
{circumflex over (p)}
2
].
Similarly, in the reception mode, dual orthogonally polarized signals are presented to the dual orthogonally polarized radiating elements
28
as designated R
1
1
and R
2
2
. Instead of bein
Sanzgiri Shashi M.
White Geoffrey O.
Alston & Bird LLP
Mull F H
Tarcza Thomas H.
The Boeing Company
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