Communications: radio wave antennas – Antennas – Antenna components
Reexamination Certificate
2002-07-29
2004-11-23
Nguyen, Hoang V. (Department: 2821)
Communications: radio wave antennas
Antennas
Antenna components
C343S754000, C343S756000
Reexamination Certificate
active
06822622
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to reconfigurable microwave lenses and shutters. In particular, the present invention relates to reconfigurable microwave lenses and shutters using cascaded frequency selective surfaces and polyimide-macro-electro-mechanical systems.
BACKGROUND OF THE INVENTION
Antennas are used to radiate and receive radio frequency signals. The transmission and reception of radio frequency signals is useful in a broad range of activities. For instance, radio wave communication systems are desirable where communications are transmitted over large distances. In addition, the transmission and reception of radio wave signals is useful in connection with obtaining position information regarding distant objects.
Antennas are generally formed to receive and transmit signals having frequencies within defined ranges. In addition to such frequency selectivity, antennas having a beam that can be pointed or steered in space can be provided. The pointing of an antenna beam can be accomplished by physically moving the radiator element or elements of the antenna. The beam of an antenna can also be steered electronically. The steering of an antenna beam is useful because it allows an antenna to focus on a distant receiver or transmitter, maximizing the gain of the antenna with respect to the distant transmitter or receiver. In addition, the pointing of an antenna beam allows the location of distant objects to be determined with respect to the antenna. Furthermore, by moving (or scanning) a beam of radio frequency radiation, a wide area can be surveyed by a single antenna.
In order to control the frequencies received by or emitted from an antenna, frequency selective surfaces (FSS) are known. With reference now to
FIG. 1A
, a band pass FSS
100
in accordance with the prior art is illustrated. In the band pass FSS of
FIG. 1A
, resonant slots
104
are formed in a layer of metal
108
overlaying a substrate
112
. The slots behave in the same fashion as a resonant L-C shunt admittance pair, as illustrated in
FIG. 1B
, for which the resonant frequency occurs when
ω
2
=
1
L
C
.
The admittance, Y
p
of the L-C shunt admittance pair may be defined as a function of frequency as Y
p
=jB=j
(
ω
C
-
1
ω
L
)
.
By altering the width and length of the slots, and/or their relationship to one another, the effective values of L and C may be changed, thereby changing the resonant frequency response of the band pass FSS. Such band pass FSS structures can be designed to have very low transmission losses within the pass band. However, a conventional band pass FSS
100
such as the one illustrated in
FIG. 1
cannot be controlled to selectively alter its transmission pass band, and associated transmission phase, while the FSS
100
is operatively connected to an antenna. Therefore, a conventional band pass FSS
100
is not able to selectively modify an antenna beam, or, specifically, to scan the beam towards a target.
Microwave lenses that allow an antenna beam to be scanned by modifying the refractive index of a panel made from an artificial dielectric are known. For example, in a RADANT® lens an artificial dielectric is formed from grids of cut wires and continuous wires, with diodes bridging the gap between cut wire segments. By biasing the diodes either on or off the index of refraction can be changed, thereby altering the phase of transmitted radio frequency radiation. However, such devices require the integration of thousands of discrete, lossy components (e.g., diodes). In addition, RADANT® lenses are heavy, and therefore are difficult to deploy, particularly in mobile or in space-based applications.
Phased array antennas that provide scanning beams are also known. In a phased array antenna, the phase of the radio frequency signals provided to individual antenna radiator elements is altered across the surface of the antenna. Conventional phased array antennas typically require the use of a large number of semiconductor switches or micro-electro mechanical (MEMs) devices to control the phase of the individual radiator elements. Accordingly, conventional phased array antennas are complicated and expensive to implement. In addition, the use of lossy components such as semiconductor switches and traditional micro-electro-mechanical devices results in large insertion losses.
Radio frequency shutters that can be selectively opened or closed to transmit or reflect radio frequency signals are also known. For example, an electronic diode shutter may be constructed by connecting diodes across the midpoint of slot elements in a conducting FSS sheet. By biasing the diodes either on or off, the resonant characteristics of the slots can be changed, thereby detuning the slots and altering the transmission and reflection properties of the FSS. Such shutters may be used to control the radar cross section of antennas or to protect antenna receiver circuitry from being damaged by high-power incident radio frequency signals while in the off state. However, shutter implementations employing thousands of discrete components entail the same types of liabilities as do diode lenses. Namely, complexity, loss, operating power, and weight.
For the above stated reasons, it would be desirable to provide a lens for use in connection with radio frequency antennas that allowed the phase of a transmitted radio frequency wave to be controlled, while exhibiting low insertion losses. Furthermore, it would be advantageous to provide such a device to permit the scanning or pointing of radio frequency radiation that required low power to operate and was relatively simple to construct and implement. In addition, it would be desirable to provide such a lens that was reliable in operation and that was suitable for use in connection with a wide variety of applications. It would also be desirable to provide shutter capability to the aforementioned lens, or to any antenna, for use in control of antenna radar cross section and/or protection from antenna damage caused by incident high-power radio frequency signals.
SUMMARY OF THE INVENTION
In accordance with the present invention, a frequency selective surface (FSS) that can be electrically detuned to provide insertion phase and amplitude control of radio frequency radiation propagating through the structure is provided. In general, the present invention uses frequency selective surfaces that are locally detuned in order to control the localized admittance, and hence localized insertion phase, of each surface. Further, a method for implementing such localized de-tuning, and hence localized insertion phase control, is described wherein two or three tightly coupled frequency selective surfaces are separated from one another by a small distance that can be electro-mechanically altered. By cascading a sufficient number of individually controllable tightly coupled groups of such surfaces, a full 360 degree change in insertion phase can be produced through the aggregate of surfaces, which is sufficient to scan the beam of a fixed beam antenna that transmits or receives through them. The same detuning technique when applied globally to an FSS can be used to increase or decrease the transmission amplitude of the FSS, thereby producing the effect of a shutter within a fixed frequency band.
In accordance with an embodiment of the present invention, an electromechanically reconfigurable microwave lens is provided that uses frequency selective surfaces in conjunction with polyimide macro-electromechanical systems (PMEMS). The following embodiment describes a two-layer implementation. According to such an embodiment, a first FSS sheet comprising a first array of unit cells formed on a first surface is provided. A second FSS sheet comprising a second array of unit cells is formed on a second surface, positioned so that the first and second arrays occupy parallel planes and at least partially overlap. In accordance with an embodiment of the present invention, the unit cells consist of slots con
Boone Theresa C.
Crawford Thomas M.
Kelly P. Keith
Lalezari Farzin
Ball Aerospace & Technologies Corp
Nguyen Hoang V.
Sheridan & Ross P.C.
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