Electronically tunable reflector

Details Electronically tunable reflector Electronically tunable reflector

2000-03-29

2003-04-22

Phan, Tho (Department: 2821)

Communications: radio wave antennas

Antennas

Antenna components

C343S7000MS, C343S754000

Reexamination Certificate

active

06552696

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a surface which reflects radio-frequency, including microwave radiation, and which imparts a phase shift to the reflected wave which is electrically tunable, using liquid crystals or other electrically tunable medium.
BACKGROUND OF INVENTION
There is an existing need for materials and/or surfaces which can steer (or focus) a radio frequency electromagnetic beam. Such materials and/or surfaces can be very useful in various applications such as radio frequency communication systems, including satellite communication system.
The present application is related to (i) U.S. patent application Ser. No. 09/537,923 entitled “A Tunable Impedance Surface” filed Mar. 29, 2000 (ii) U.S. patent application Ser. No. 09/537,921 entitled “An End-Fire Antenna or Array on Surface with Tunable Impedance” filed Mar. 29, 2000 and to (iii) U.S. patent application Ser. No. 09/520,503 entitled “A Polarization Converting Radio Frequency Reflecting Surface” filed Mar. 8, 2000 the disclosures of which are all hereby incorporated herein by this reference. U.S. patent application Ser. No. 09/537,923 for a “Tunable Impedance Surface” describes a method and apparatus for mechanically tuning the surface impedance of a Hi-Z surface and thus its reflection phase using various mechanical methods. By programming the reflection phase as a function of position on this surface, the reflected beam can be steered or focused
Prior art approaches for radio frequency beam steering generally involve using phase shifters or mechanical gimbals. With the present invention, beam steering is accomplished electronically using variable capacitors, thus eliminating expensive phase shifters and unreliable mechanical gimbals. Furthermore, the reflective scanning approach disclosed herein eliminates the need for a conventional phased array, with separate phase shifters on each radiating element. The tunable surface disclosed herein surface can serve as a reflector for any static, highly directed feed antenna, thus removing much of the complexity and cost of conventional, steerable antenna systems.
It is known in the prior art that an ordinary metal surface reflects electromagnetic radiation with a &pgr; phase shift. However, a Hi-Z surface of the type disclosed in U.S. provisional patent application Ser. No. 60/079,953 is capable of reflecting radio frequency radiation with a zero phase shift.
A Hi-Z surface, shown in
FIG. 1
, consists of an array of metal protrusions or elements
12
disposed above a flat metal sheet or ground plane
14
. It can be fabricated using printed circuit board technology, in which case the vertical connections are formed by metal vias
16
, which connect the metal elements
12
formed on a top surface of a printed circuit board
18
(see
FIG. 2
) to a conducting ground plane
14
on the bottom surface of the printed circuit board
18
. The metal elements
12
are arranged in a two-dimensional lattice, and can be visualized as mushrooms or thumbtacks protruding from the flat metal ground plane surface
14
. The maximum dimension of the metal elements
12
on the flat upper surface is much less than one wavelength (&lgr;) of the frequency of interest. Similarly, the thickness of the structure measures also much less than one wavelength of the frequency of interest.
The properties of the Hi-Z surface can be explained using an effective media model, in which it is assigned a surface impedance equal to that of a parallel resonant LC circuit. The use of lumped parameters to describe this electromagnetic structure is valid when the wavelength of interest is much longer than the size of the individual features, such as is the case here. When an electromagnetic wave interacts with the Hi-Z surface, it causes charges to build up on the ends of the top metal elements
12
. This process can be described as governed by an effective capacitance C. As the charges travel back and forth, in response to the radio-frequency field, they flow around a long path through the vias
16
and the bottom ground plane
14
. Associated with these currents is a magnetic field, and thus an inductance L. The effective circuit elements are illustrated in FIG.
2
. The capacitance is controlled by the proximity of the adjacent metal elements
12
, while the inductance is controlled by the thickness of the structure (i.e. the distance between the metal elements
12
and the ground plane
14
).
The presence of an array or lattice of resonant LC circuits affects the reflection phase of the Hi-Z surface. For frequencies far from resonance, the surface reflects radio frequency waves with a &pgr; phase shift, just as an ordinary conductor does. However, at the resonant frequency, the surface reflects with a zero phase shift. As a frequency of the incident wave is tuned through the resonant frequency of the surface, the reflection phase changes by one complete cycle, or 2&pgr;. This is seen in both the calculated and measured reflection phases, as shown in
FIGS. 3 and 4
, respectively.
FIG. 3
shows the calculated reflection phase of the high-impedance surface, obtained from the effective medium model. The phase crosses through zero at the resonance frequency of the structure.
FIG. 4
shows that the measured reflection phase agrees well with the calculated reflection phase reinforcing the validity of the effective medium model.
When the reflection phase is near zero, the structure also effectively suppresses surface waves, which has been shown to be significant in antenna applications.
Structures of this type have been constructed in a variety of forms, including multi-layer versions with overlapping capacitor plates. Examples have been demonstrated with resonant frequencies ranging from hundreds of megahertz to tens of gigahertz, and the effective media model presented herein has proven to be an effective tool for analyzing and designing these materials, now known as Hi-Z surfaces.
BRIEF DESCRIPTION OF THE PRESENT INVENTION
The present invention involves a method and apparatus for tuning the reflection phase of the Hi-Z surface using a material which locally changes its dielectric constant in response to external stimuli. Liquid crystal materials can be used as the material which locally changes its dielectric constant. Alternatively, instead of liquid crystal materials, one can use suspended microtubules, suspended metal particles, ferroelectrics, or any other media which has an electrically, for example, tunable dielectric constant. Since this device is electronically reconfigurable, it requires no macroscopic mechanical motion. Instead, it uses electric field-induced molecular reorientation within a layer of liquid crystal material or other appropriate material to produce an electrically tunable capacitance. Tunable capacitors make up resonant elements which are distributed across the Hi-Z surface, and determine the reflection phase at each point on the surface. By varying the reflection phase as a function of position, a reflected wave can be steered electronically. In addition, this method and apparatus can be combined with mechanical techniques to create a hybrid structure which can allow for even more tunability.
Important features of the present invention include:
1. A structure which incorporates a liquid crystal material or other tunable material into the capacitive region of a Hi-Z surface to produce a surface with tunable reflection phase.
2. The disclosed structure and methods can be used to extend the useful bandwidth of a Hi-Z surface.
3. A method of steering or focusing a microwave or radio-frequency beam using a structure having a Hi-Z surface and a media which has an electrically tunable dielectric constant, such as a liquid crystal.
The present invention can be applied to a wide range of microwave and millimeter-wave antennas were quasi-optical elements can improve performance. The present invention has application in space-based radar and airborne communications node (ACN) systems whereby an aperture must be continually reconfigured for various functions. The prese

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