Tunable impedance surface

Details Tunable impedance surface Tunable impedance surface

2000-06-08

2002-11-19

Phan, Tho G. (Department: 2821)

Communications: radio wave antennas

Antennas

Antenna components

C343S7000MS, C343S754000, C343S853000

Reexamination Certificate

active

06483480

ABSTRACT:

TECHNICAL FIELD
This invention relates to a surface having a tunable electromagnetic impedance which acts as a reconfigurable beam steering reflector.
BACKGROUND OF THE INVENTION
Steerable antennas today are found in two common configurations: those with a single feed or reflector that is mechanically steered using a gimbal, and those with a stationary array of electronically phased radiating elements. Both have shortcomings, and the choice of system used is often a tradeoff between cost, speed, reliability, and RF (radio frequency) performance. Mechanically steered antennas are inexpensive, but moving parts can be slow and unreliable, and they can require an unnecessarily large volume of unobstructed free space for movement. Active phased arrays are faster and more reliable, but they are much more expensive, and can suffer from significant losses due to the complex feed structure required to supply the RF signal to and/or receive the RF signal from each active element of the phased array. Losses can be mitigated if an amplifier is included in each element or subarray, but this solution contributes to noise and power consumption and further increases the cost of the antenna.
One alternative is to use a reflectarray geometry, and replace the lossy corporate feed network with a free space feed. The actively phased elements operate in reflection mode, and are illuminated by a single feed antenna. The array steers the RF beam by forming an effective reflection surface defined by the gradient of the reflection phase across the array. Using current techniques, such a system still requires a large number of expensive phase shifters.
There is a need for a reflective surface, in which the reflection phase could be arbitrarily defined, and easily varied as a function of position. The surface should be less expensive than a comparably sized array of conventional phase shifters, yet hopefully offer similar RF performance. Such a surface could behave as a generic reconfigurable reflector, with the ability to perform a variety of important functions including steering or focusing of one or more RF beams. It is the object of this invention to fulfill this need.
The reconfigurable reflector disclosed herein is based a resonant textured ground plane, often known as the high-impedance surface or simply the Hi-Z surface. This electromagnetic structure has two important RF properties that are applicable to low profile antennas. It suppresses propagating surface currents, which improves the radiation pattern of antennas on finite ground planes and it provides a high-impedance boundary condition, acting as an artificial magnetic conductor, which allows radiating elements to lie in close proximity to the ground plane without being shorted out. It has origins in other well-known electromagnetic structures such as the corrugated surface and the photonic band gap surface. A prior art high-impedance surface is disclosed in a pending US patent application of D. Sievenpiper, E. Yablonovitch, “Circuit and Method for Eliminating Surface Currents on Metals”, U.S. provisional patent application Ser. No. 60/079,953, filed on Mar. 30, 1998.
A prior art high-impedance surface is shown in FIG.
1
. It consists of an array of metal top plates or elements
10
on a flat metal sheet
12
. It can be fabricated using printed circuit board technology with the metal plates or elements
10
formed on a top or first surface of a printed circuit board and a solid conducting ground or back plane
12
formed on a bottom or second surface of the printed circuit board. Vertical connections are formed as metal plated vias
14
in the printed circuit board, which connect the elements
10
with the underlying ground plane
12
. The metal members, comprising the top plates
10
and the vias
14
, are arranged in a two-dimensional lattice of cells or cavities, and can be visualized as mushroom-shaped or thumbtack-shaped members protruding from the flat metal surface
12
. The thickness of the structure, which is controlled by the thickness of the printed circuit board, is much less than one wavelength for the frequencies of interest. The sizes of the elements
10
are also kept less than one wavelength for the frequencies of interest. The printed circuit board is not shown for ease of illustration.
Turning to
FIG. 2
, the properties of this surface can be explained using an effective circuit model or cavity which is assigned a surface impedance equal to that of a parallel resonant LC circuit. The use of lumped cavities to describe electromagnetic structures is valid when the wavelength is much longer than the size of the individual features, as is the case here. When an electromagnetic wave interacts with the surface of
FIG. 1
, it causes charges to build up on the ends of the top metal plates
10
. This process can be described as governed by an effective capacitance C. As the charges slosh back and forth, in response to a radio-frequency field, they flow around a long path P through the vias
14
and the bottom metal surface
12
. Associated with these currents is a magnetic field, and thus an inductance L. The capacitance C is controlled by the proximity of the adjacent metal plates
10
while the inductance L is controlled by the thickness of the structure.
The structure is inductive below the resonance and capacitive above resonance. Near its resonance frequency,
ω
=
1
LC
,
the structure exhibits high electromagnetic surface impedance. The tangential electric field at the surface is finite, while the tangential magnetic field is zero. Thus, electromagnetic waves are reflected without the phase reversal that occurs on a flat metal sheet. In general, the reflection phase can be 0, &pgr;, or anything in between, depending on the relationship between the test frequency and the resonance frequency of the structure. The reflection phase as a function of frequency, calculated using the effective medium model, is shown in FIG.
3
. Far below resonance, it behaves like an ordinary metal surface, and reflects with a &pgr; phase shift. Near resonance, where the surface impedance is high, the reflection phase crosses through zero. At higher frequencies, the phase approaches −&pgr;. The calculated model of
FIG. 3
is supported by the measured reflection phase, shown for an example structure in FIG.
4
.
A large number of structures of the type shown in
FIG. 1
have been fabricated with a wide range of resonance frequencies, including various geometries and substrate materials. Some of the structures were designed with overlapping capacitor plates, to increase the capacitance and lower the frequency. The measured and calculated resonance frequencies for twenty three structures with various capacitance values are compared in FIG.
5
. Clearly, the resonance frequency is a predictable function of the capacitance. The dotted line in
FIG. 5
has a slope of unity, and indicates perfect agreement. The bars indicate the instantaneous bandwidth of the surface, defined by the frequencies where the phase is between &pgr;/2 and −&pgr;2.
For a more detailed description and analysis of the high-impedance surface, see D. Sievenpiper, L. Zhang, R. Broas, N. Alexopolous, E. Yablonovitch, “High-Impedance Electromagnetic Surfaces with a Forbidden Frequency Band”, IEEE Transactions on Microwave Theory and Techniques, vol. 47, pp. 2059-2074, 1999 and D. Sievenpiper, “High-Impedance Electromagnetic Surfaces”, Ph.D. dissertation, Department of Electrical Engineering, University of California, Los Angeles, Calif., 1999.
When the resonant cavities are much smaller than the wavelength of interest, the electromagnetic analysis can be simplified by considering them as lumped LC circuits. The proximity of the neighboring metal plates provides capacitance, while the conductive path that connects them provides inductance. The textured ground plane supports an electromagnetic boundary condition that can be characterized by the impedance of an effective parallel LC circuit, given by
Z
s
=
j
⁢
 
⁢
ω
⁢
 
⁢
L
1
-
ω

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