Communications: radio wave antennas – Antennas – Microstrip
Reexamination Certificate
2001-09-17
2003-07-22
Le, Hoanganh (Department: 2821)
Communications: radio wave antennas
Antennas
Microstrip
C343S846000
Reexamination Certificate
active
06597316
ABSTRACT:
The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment of any royalty thereon.
FIELDS OF THE INVENTION
The present invention relates generally to radio-frequency antenna structures. More specifically, the present invention relates to microstrip antenna arrays for use in navigation systems, such as the Global Positioning System (GPS), and in wireless and satellite communications systems. The present invention further relates to generating spatial nulls with pairs of microstrip antenna elements excited in fundamental and higher order modes. The present invention also relates to multiple frequency band applications in the aforementioned fields.
BACKGROUND OF THE INVENTION
Any communications or navigation system is susceptible to degradation due to interfering conditions. The carrier signal is vulnerable to interruption by natural phenomena, interference from other signals or countermeasures. Countermeasures may take the form of a variety of jamming schemes whose sole purpose is to disrupt the operation of a receiver.
A variety of techniques are currently used to decrease the effects of interference in receivers. Adaptive nulling involves the cancellation of a signal received by one antenna element relative to another. A conventional, multi-element adaptive array requires “N” number of elements to null out “N−1” interference sources. For example, a seven-element array can, at the most, suppress six broadband interference sources. Since each antenna element needs its own receiver and also a complex weighting network to adapt the antenna pattern, the high cost and technical complexity of such a multi-element antenna array may make it unattractive for many commercial and military systems in which cost and simplicity are important considerations. Thus, a need exists for a simple adaptive array as an alternative to more complex and expensive multi-element adaptive arrays.
Due to limited space availability in airborne platforms, antennas used by various avionics systems are placed very close together resulting in significant co-site interference from harmonics of the signals radiated by the neighboring antennas, or from “splatter” of the transmitted energy outside their specified frequency band. A low profile means for suppressing co-site interference in antennas used for satellite navigation and communications without affecting the ability of the antenna array to receive desired signals would clearly be beneficial.
Multipath is a significant problem in both navigational and communications systems. It degrades navigational accuracy in GPS systems and can be a source of interference in communications systems. Multipath can be caused by “structural” reflections (such as shown in
FIGS. 1
a
and
1
b
) from specular reflecting surfaces of numerous scattering sources common to an urban environment such as buildings, large vehicles, aircraft or ships. Alternatively, multipath can be caused by ground reflections at low grazing angles off the moist ground, rooftops, sea surface or a large body of water close to the antenna. Since the GPS satellites transmit right-handed circularly polarized (RHCP) signals, and the polarization of the multipath signal after reflection is normally reversed, the rejection of the cross-polarized (left-handed circularly polarized, LHCP) signals is important to avoid multipath problems.
Various types of antennas have been proposed for GPS multipath mitigation. Choke ring ground planes are circular ground planes with quarter wavelength slots to present a high impedance to currents flowing on the ground plane to prevent their interference with the antenna radiation. A typical choke ring ground plane has a diameter of about 14 to 16 inches, a height of about 3 inches or higher, and a weight of approximately 10 to 20 pounds. Such antennas are not suitable for airborne applications because of their construction and weight. Additionally, it is difficult to design choke ring ground plane antennas that operate simultaneously in the two GPS frequency bands (L
1
and L
2
). Other types of GPS multipath limiting antennas also exist, but have even larger physical sizes or profiles.
Microstrip patch antennas are attractive due to their compact structure, light weight due to the absence of heavy metal stamped or machined parts, and low manufacturing cost using printed circuit technology. They also provide low profiles, conformity to surfaces and direct integration with microwave circuitry. Consequently, microstrip patch antennas are used widely in antenna arrays.
Nurie and Langley have studied the use of concentric annular patches with circumferential slots as a dual frequency band microstrip antenna array.
Performance of Concentric Annular Patches as a Dual Frequency Band Microstrip Array Element
, Sixth International Conference on Antennas and Propagation, 1989. They experimented with exciting the annular ring patches in two different modes, a lower order TM
11
mode and a higher order TM
12
mode. However, they encountered difficulties in exciting the TM
12
mode due to the presence of other even higher order modes that were either close to or overlapping the frequency band of interest. They have attempted to suppress these higher order modes by cutting slots in the outer annular ring. They also operated the two antennas as separate entities to service two completely different communications or radar systems, but no attempt was made to adaptively combine the signals from these two antennas so as to generate a combined antenna pattern with a spatial null for mitigation of interference or to suppresses the cross polarized radiated signals to suppress multipath.
U.S. Pat. No. 5,099,249 to Seavey discloses two element antenna arrays, including at least one annular ring antenna excited in a higher order mode, exclusively for providing simultaneous satellite and terrestrial communications. However, the disclosed arrays again do not attempt to adaptively combine the signals from the at least one annular ring antenna and other antennas in the disclosed arrays to generate nulls for reducing multiple interference signals, co-site interference signals, or GPS multipath. In addition the radiation mode that was used for terrestrial communication was a higher order mode with a radiation pattern that has multiple lobes that is not optimum for terrestrial communications in all azimuthal directions.
SUMMARY OF THE INVENTION
Accordingly, it is an objective of the invention to address the needs described above by providing an antenna array capable of steering a wide spatial null for limiting multiple interference sources, such as natural multipath or electronic countermeasures at a desired elevation angle, preferably on or close to the horizon.
It is a further objective of the present invention to provide a lightweight, low cost alternative to more complex and expensive multi-element adaptive arrays by the use of microstrip patch elements. Advantages offered by this antenna array include its low profile making it attractive for airborne systems because of reduced aerodynamic drag, its low manufacturing cost using printed circuit technology, and its light weight due to the absence of heavy metal stamped or machined parts in its construction.
It is a further objective of the present invention to provide a low-profile means for suppressing co-site interference in antennas used for satellite navigation and communications without affecting the ability of the antenna array to receive desirable signals.
And it is yet another objective of the present invention to provide an antenna array capable of simultaneous satellite and terrestrial communication in a plurality of frequency bands, by generating two different, orthogonal types of antenna patterns—one directed towards zenith for communicating with satellites, and the other towards horizon to facilitate terrestrial communications.
In one embodiment, the present invention is a two element microstrip antenna array designed to place a deep spatial “ring” null in the radiat
Rao Barsur Rama
Rosario Edward N.
Choate Hall & Stewart
Le Hoang-anh
The Mitre Corporation
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