Communications: radio wave antennas – Antennas – With polarization filter or converter
Reexamination Certificate
2001-04-30
2003-02-25
Wong, Don (Department: 2821)
Communications: radio wave antennas
Antennas
With polarization filter or converter
C343S795000
Reexamination Certificate
active
06525695
ABSTRACT:
BACKGROUND
The present invention relates to the development of reconfigurable artificial magnetic conductor (RAMC) surfaces for low profile antennas. This device operates as a high-impedance surface over a tunable frequency range, and is electrically thin relative to the wavelength of interest, &lgr;.
A high impedance surface is a lossless, reactive surface, realized as a printed circuit board, whose equivalent surface impedance is an open circuit which inhibits the flow of equivalent tangential electric surface currents, thereby approximating a zero tangential magnetic field. A high-impedance surface is important because it offers a boundary condition which permits wire antennas (electric currents) to be well matched and to radiate efficiently when the wires are placed in very close proximity to this surface (<&lgr;/100 away). The opposite is true if the same wire antenna is placed very close to a metal or perfect electric conductor (PEC) surface. It will not radiate efficiently. The radiation pattern from the antenna on a high-impedance surface is confined to the upper half space above the high impedance surface. The performance is unaffected even if the high-impedance surface is placed on top of another metal surface. The promise of an electrically thin, efficient antenna is very appealing for countless wireless device and skin-embedded antenna applications.
One embodiment of a thin, high-impedance surface
100
is shown in FIG.
1
. It is a printed circuit structure forming an electrically thin, planar, periodic structure, having vertical and horizontal conductors, which can be fabricated using low cost printed circuit technologies. The high-impedance surface
100
includes a lower permittivity spacer layer
104
and a capacitive frequency selective surface (FSS)
102
formed on a metal backplane
106
. Metal vias
108
extend through the spacer layer
104
, and connect the metal backplane to the metal patches of the FSS layer. The thickness of the high impedance surface
100
is much less than &lgr;/4 at resonance, and typically on the order of &lgr;/50, as is indicated in FIG.
1
.
The FSS
102
of the prior art high impedance surface
100
is a periodic array of metal patches
110
which are edge coupled to form an effective sheet capacitance. This is referred to as a capacitive frequency selective surface (FSS). Each metal patch
110
defines a unit cell which extends through the thickness of the high impedance surface
100
. Each patch
110
is connected to the metal backplane
106
, which forms a ground plane, by means of a metal via
108
, which can be plated through holes. The spacer layer
104
through which the vias
108
pass is a relatively low permittivity dielectric typical of many printed circuit board substrates. The spacer layer
104
is the region occupied by the vias
108
and the low permittivity dielectric. The spacer layer is typically 10 to 100 times thicker than the FSS layer
102
. Also, the dimensions of a unit cell in the prior art high-impedance surface are much smaller than &lgr; at the fundamental resonance. The period is typically between &lgr;/40 and &lgr;/12.
Another embodiment of a thin, high-impedance surface is disclosed in U.S. patent application Ser. No. 09/678,128, entitled “Multi-Resonant, High-Impedance Electromagnetic Surfaces,” filed on Oct. 4, 2000, commonly assigned with the present application and incorporated herein by reference in its entirety. In that embodiment, an artificial magnetic conductor is resonant at multiple resonance frequencies. That embodiment has properties of an artificial magnetic conductor over a limited frequency band or bands, whereby, near its resonant frequency, the reflection amplitude is near unity and the reflection phase at the surface lies between +/−90 degrees. That embodiment also offers suppression of transverse electric (TE) and transverse magnetic (TM) mode surface waves over a band of frequencies near where it operates as a high impedance surface.
Another implementation of a high-impedance surface, or an artificial magnetic conductor (AMC), which has nearly an octave of +/−90° reflection phase, was developed under DARPA Contract Number F19628-99-C-0080. The size of this exemplary AMC is 10 in. by 16 in by 1.26 in thick (25.4 cm×40.64 cm×3.20 cm). The weight of the AMC is 3 lbs., 2oz. The 1.20 inch (3.05 cm) thick, low permittivity spacer layer is realized using foam. The FSS has a period of 298 mils (0.757 cm), and a sheet capacitance of 0.53 pF/sq.
The measured reflection coefficient phase of this broadband AMC, referenced to the top surface of the structure is shown in
FIG. 2
as a function of frequency. A ±90° phase bandwidth of 900 MHz to 1550 MHz is observed. Three curves are traced on the graph, each representing a different density of vias within the spacer layer. For curve AMC
1
-
2
, one out of every two possible vias is installed. For curve AMC
1
-
4
, one out of every four vias is installed. For curve AMC
1
-
18
, one out of every 18 vias is installed. As expected from the effective media model described in application Ser. No. 09/678,128, the density of vias does not have a strong effect on the reflection coefficient phase.
Transmission test set-ups are used to experimentally verify the existence of a surface wave bandgap for this broadband AMC. In each case, the transmission response (S
21
) is measured between two Vivaldi-notch radiators that are mounted so as to excite the dominant electric field polarization for transverse electric (TE) and transverse magnetic (TM) modes on the AMC surface. For the TE set-up, the antennas are oriented horizontally. For the TM set-up, the antennas are oriented vertically. Absorber is placed around the surface-under-test to minimize the space wave coupling between the antennas. The optimal configuration—defined empirically as “that which gives the smoothest, least-noisy response and cleanest surface wave cutoff”—is obtained by trial and error. The optimal configuration is obtained by varying the location of the antennas, the placement of the absorber, the height of absorber above the surface-under-test, the thickness of absorber, and by placing a conducting foil “wall” between layers of absorber. The measured S
21
for both configurations is shown in FIG.
3
. As can be seen, a sharp TM mode cutoff occurs near 950 MHz, and a gradual TE mode onset occurs near 1550 MHz. The difference between these two cutoff frequencies is referred to as a surface wave bandgap. This measured bandgap is correlated closely to the +/−90-degree reflection phase bandwidth of the AMC.
The resonant frequency of the prior art AMC, shown in
FIG. 1
, is given by Sievenpiper et. al. (
IEEE Trans. Microwave Theory and Techniques,
Vol. 47, No. 11, November 1999, pp. 2059-2074), (Also see Dan Sievenpiper's dissertation, High Impedance Electromagnetic Surfaces, UCLA, 1999) as ƒ
o
=1/(2&pgr;{square root over (LC)}) where C is the equivalent sheet capacitance of the FSS layer in Farads per square, and L=&mgr;
o
h is the permeance of the spacer layer, with h denoting the height or thickness of this layer.
In most wireless communications applications, it is desirable to make the antenna ground plane as small and light weight as possible so that it may be readily integrated into physically small, light weight platforms such as radiotelephones, personal digital assistants and other mobile or portable wireless devices. The relationship between the instantaneous bandwidth of an AMC with a non-magnetic spacer layer and its thickness is given by
BW
f
0
=
2
π
h
λ
0
where &lgr;
0
is the free space wavelength at resonance where a zero degree reflection phase is observed. Thus, to support a wide instantaneous bandwidth, the AMC thickness must be relatively large. For example, to accommodate an octave frequency range (BW/ƒ
0
=0.667), the AMC thickness must be at least 0.106&lgr;
0
, corresponding to a physical thickness of 1.4 inches at a center frequency of 900 MHz
Brinks Hofer Gilson & Lione
Clinger James
e-tenna Corporation
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