Multi-resonant, high-impedance electromagnetic surfaces

Details Multi-resonant, high-impedance electromagnetic surfaces Multi-resonant, high-impedance electromagnetic surfaces

2000-10-04

2003-01-28

Wong, Don (Department: 2821)

Communications: radio wave antennas

Antennas

Antenna components

C343S91100R, C343S842000

Reexamination Certificate

active

06512494

ABSTRACT:

BACKGROUND
The present invention relates generally to high-impedance surfaces. More particularly, the present invention relates to a multi-resonant, high-impedance electromagnetic surface.
A high impedance surface is a lossless, reactive surface whose equivalent surface impedance,
Z
s
=
E
tan
H
tan
,
approximates an open circuit and which inhibits the flow of equivalent tangential electric surface current, thereby approximating a zero tangential magnetic field, H
tan
≈0. E
tan
and H
tan
are the electric and magnetic fields, respectively, tangential to the surface. High impedance surfaces have been used in various antenna applications. These applications range from corrugated horns which are specially designed to offer equal E and H plane half power beamwidths to traveling wave antennas in planar or cylindrical form. However, in these applications, the corrugations or troughs are made of metal where the depth of the corrugations is one quarter of a free space wavelength, &lgr;/4, where &lgr; is the wavelength at the frequency of interest. At high microwave frequencies, &lgr;/4 is a small dimension, but at ultra-high frequencies (UHF, 300 MHz to 1 GHz), or even at low microwave frequencies (1-3 GHz), &lgr;/4 can be quite large. For antenna applications in these frequency ranges, an electrically-thin (&lgr;/100 to &lgr;/50 thick) and physically thin high impedance surface is desired.
One example of a thin high-impedance surface is disclosed in D. Sievenpiper, “High-impedance electromagnetic surfaces,” Ph.D. dissertation, UCLA electrical engineering department, filed January 1999, and in PCT Patent Application number PCT/US99/06884. This high impedance surface
100
is shown in FIG.
1
. 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 h of the high impedance surface
100
is much less than &lgr;/4 at resonance, and typically on the order of &lgr;/50, as 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 periodic array of metal vias
108
has been known in the prior art as a rodded media, so these vias are sometimes referred to as rods or posts. 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.
A frequency selective surface is a two-dimensional array of periodically arranged elements which may be etched on, or embedded within, one or multiple layers of dielectric laminates. Such elements may be either conductive dipoles, patches, loops, or even slots. As a thin periodic structure, it is often referred to as a periodic surface.
Frequency selective surfaces have historically found applications in out-of-band radar cross section reduction for antennas on military airborne and naval platforms. Frequency selective surfaces are also used as dichroic subreflectors in dual-band Cassegrain reflector antenna systems. In this application, the subreflector is transparent at frequency band f
1
and opaque or reflective at frequency band f
2
. This allows one to place the feed horn for band f
1
at the focal point for the main reflector, and another feed horn operating at f
2
at the Cassegrain focal point. One can achieve a significant weight and volume savings over using two conventional reflector antennas, which is critical for space-based platforms.
The prior art high-impedance surface
100
provides many advantages. The surface is constructed with relatively inexpensive printed circuit technology and can be made much lighter than a corrugated metal waveguide, which is typically machined from a block of aluminum. In printed circuit form, the prior art high-impedance surface can be 10 to 100 times less expensive for the same frequency of operation. Furthermore, the prior art surface offers a high surface impedance for both x and y components of tangential electric field, which is not possible with a corrugated waveguide. Corrugated waveguides offer a high surface impedance for one polarization of electric field only. According to the coordinate convention used herein, a surface lies in the xy plane and the z-axis is normal or perpendicular to the surface. Further, the prior art high-impedance surface provides a substantial advantage in its height reduction over a corrugated metal waveguide, and may be less than one-tenth the thickness of an air-filled corrugated metal waveguide.
A high-impedance surface is important because it offers a boundary condition which permits wire antennas conducting electric currents to be well matched and to radiate efficiently when the wires are placed in very close proximity to this surface (e.g., less than &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. The wire antenna/PEC surface combination will not radiate efficiently due to a very severe impedance mismatch. The radiation pattern from the antenna on a high-impedance surface is confined to the upper half space, and the performance is unaffected even if the high-impedance surface is placed on top of another metal surface. Accordingly, an electrically-thin, efficient antenna is very appealing for countless wireless devices and skin-embedded antenna applications.
FIG. 2
illustrates electrical properties of the prior art high-impedance surface. FIG.
2
(
a
) illustrates a plane wave normally incident upon the prior art high-impedance surface
100
. Let the reflection coefficient referenced to the surface be denoted by &Ggr;. The physical structure shown in FIG.
2
(
a
) has an equivalent transverse electromagnetic mode transmission line shown in FIG.
2
(
b
). The capacitive FSS
102
(
FIG. 1
) is modeled as a shunt capacitance C and the spacer layer
104
is modeled as a transmission line of length h which is terminated in a short circuit corresponding to the backplane
106
. FIG.
2
(
c
) shows a Smith chart in which the short is transformed into the stub impedance Z
stub
just below the FSS layer
102
. The admittance of this stub line is added to the capacitive susceptance to create a high impedance Z
in
at the outer surface. Note that the Z
in
locus on the Smith Chart in FIG.
2
(
c
) will always be found on the unit circle since our model is ideal and lossless. So &Ggr; has an amplitude of unity.
The reflection coefficient &Ggr; has a phase angle &thgr; which sweeps from 180° at DC, through 0° at the center of the high impedance band, and rotates into negative angles at higher frequencies where it becomes asymptotic to −180°. This is illustrated in FIG.
2
(
d
). Resonance is defined as that frequency corresponding to 0° reflection phase. Herein, the reflection phase bandwidth is defined as that bandwidth between the frequencies corresponding to the +90° and −90° phases. This reflection phase bandwidth also corresponds to the range of frequencies where the magnitude of the surface reactance exceeds the impedance of free space: |X|≧&eegr;
o
=377 ohms.
A perfect magnetic

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