Tunable-impedance spiral

Details Tunable-impedance spiral Tunable-impedance spiral

2000-03-28

2001-11-27

Wong, Don (Department: 2821)

Communications: radio wave antennas

Antennas

Antenna components

C343S754000

Reexamination Certificate

active

06323826

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to the field of tunable electromagnetic devices and, in particular, to tunable polarizers and beam steering devices, such being particularly useful in modern antenna and communications systems.
BACKGROUND
In modern antenna and communication systems, particularly those involving microwave and millimeter waves, the steering of electromagnetic signals plays an important role in the transmission to, or interconnection of, various system elements, such as in satellite tracking systems. The properties and characteristics of physical surfaces associated with such signals, in turn, can affect the desired transmission or reflection of the signals.
For example, it has been known for decades that the electromagnetic properties of a metal surface can be changed by applying periodic corrugations to that surface, such as corrugated surfaces used in horn antennas to improve the radiation pattern. The corrugations are typically one-quarter wavelength thick, and serve as a resonant structure to transform a low-impedance metal surface into a high-impedance surface. This affects the reflection phase of the surface, and also the propagation of surface waves along it.
The same technique can also be applied to cylindrical structures such as wires. An example is shown in
FIG. 1
, which is adapted from FIG. 9.9
a
in Ramo et al.'s “Fields and Waves in Communication Electronics”, published by John Wiley & Sons, Third Edition, 1994. The structure succeeds in suppressing the propagation of AC currents along the wire at the resonance frequency. However, the entire structure is greater than one-half wavelength thick, which can be problematic in size/weight constrained areas, such as for use in orbiting satellites.
Therefore, there exists a need for an effective device which can improve performance of wide range of microwave and millimeter wave antennas and structures useful in satellite tracking systems, while being small in size and manufacturable at relatively low cost. The present invention provides a unique solution to meet such need.
SUMMARY OF THE INVENTION
A spiral resonant structure is used to make a wire with tunable reflection properties. The structure can be tuned by stretching the spiral, allowing one to vary the reflection properties as a function of frequency. The diameter of the spiral is small compared to the operating wavelength, and the structure can be easily fabricated as a spring. Near resonance, it is electrically isolated in that it provides as a highly reactive current path, instead of a low-impedance short. Such a structure can be applied to dispersive polarizing beam splitters, and a new class of wire grid reflectors for focusing radiative power, and, as such, can be a useful performance enhancement for antennas and other types of electromagnetic devices.
Accordingly, in accordance with the present invention, there is provided a resonant spiral wire structure, that:
(1) can be used in such a way that it appears transparent to electromagnetic radiation within a particular frequency band, while reflecting out-of-band radiation, or
(2) can be used to impart a frequency-dependent phase to the reflected wave that differs from that of an ordinary straight wire, and
(3) is tunable by merely stretching or compressing the spiral.
Utilizing the teachings of the present invention, a microwave polarizer can be formed wherein a layer of thin parallel wires is spaced less than a wavelength apart. The electric field component polarized along the wire is reflected, while the orthogonal component passes unreflected. With the resonant spiral, the polarization effect is frequency dependent, making the polarizer band selective. The resonant spiral approach also enables phase control of the reflected wave.
In accordance with one aspect of the present invention, a tuneable impedance structure is provided which includes at least one electrically conductive wire forming at least one elongate wire spiral, the at least one elongate wire spiral being defined by a plurality of spirals of said at least one wire, the spirals having a pitch and being spaced apart along a major axis of said elongate wire spiral; and an arrangement for varying the pitch of the spirals of said at least one wire to thereby tune the impedance of said tuneable impedance structure.
In another aspect the present invention provides a method of tuning a high impedance surface comprising: arranging a plurality of elongated wire spirals in a generally planar and parallel relationship, each spiral having a pitch associated therewith; and varying the pitch of each of the wire spirals to thereby tune the impedance of said high impedance surface.
In yet another aspect the present invention provides an antenna aperture for steering a radio frequency beam having two different polarizations, comprising two high impedance surfaces, the two high impedance surfaces each comprising an array of wire spirals arranged in a parallel relationship, the two high impedance surfaces being disposed proximate each other with the plurality of parallel wire spirals of one high impedance structure being arranged orthogonally relative to the plurality of parallel elongate wire spirals of the other high impedance structure, the two high impedance surfaces having different impedance characteristics.
The present invention, in another aspect thereof, provides an antenna aperture for steering a radio frequency beam using a high impedance surface, the high impedance surface comprising a plurality of wire spirals arranged in a generally parallel relationship to one another, neighboring wire spirals in said plurality having different impedance characteristics. A second high impedance surface may be provided comprising a second plurality of wire spirals arranged in a generally parallel relationship to one another, neighboring wire spirals in said second plurality having different impedance characteristics, the second plurality of wire spirals being disposed essentially orthogonally to the first mentioned plurality of wire spirals.


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