Communications: radio wave antennas – Antennas – Microstrip
Reexamination Certificate
2001-10-12
2003-11-11
Clinger, James (Department: 2821)
Communications: radio wave antennas
Antennas
Microstrip
C333S238000
Reexamination Certificate
active
06646605
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to antennas and dielectric substrate materials therefor, and in particular, to a tunable microstrip antenna dielectric material that is capable of use in portable or mobile applications where minimal aperture size and weight are desired, and where high bandwidth is preferred.
2. Description of the Related Art
U.S. Pat. No. 6,075,485 to Lilly et al. entitled “Reduced Weight Artificial Dielectric Antennas and Method for Providing the Same” dramatically advanced the state of the art.
An artificial dielectric structure
10
according to U.S. Pat. No. 6,075,485 is shown in FIG.
1
. It comprises a periodic structure or stack of alternating layers of high and low permittivity isotropic dielectric materials
12
and
14
, having respective relative permittivities of ∈
r1
and ∈
r2
. As shown in the drawing, layers
12
and
14
have respective thicknesses of t
1
and t
2
, and the direction normal to the surface of the layers (i.e. the direction of stacking of the layers) is parallel with the Y axis. The number of alternating layers
12
and
14
used in the stack depends on their respective thicknesses and the overall size of the structure desired.
One of the merits of the structure of
FIG. 1
is that tensor permittivities in the dielectric structure can be engineered to be any value between ∈
r1
and ∈
r2
by appropriate selection of the respective thicknesses for given respective permittivities of layers
12
and
14
. The weight of the resulting structure
10
can be easily designed as well. In particular, a significant weight savings can be achieved by selecting a thin high permittivity dielectric material for layer
12
and a much thicker but very low weight dielectric material such as foam for layer
14
.
Even greater weight savings can be achieved when the high permittivity dielectric material layer
12
is itself an artificial dielectric material, such as a frequency selective surface (FSS). For example, a 0.020″ thick FSS can be designed to represent an equivalent capacitance of up to ∈
r
=800, while exhibiting a specific gravity of only about ~2.5 grams/cm
3
, further improving the results obtained in the above example.
As shown in
FIGS. 2 and 3
, a frequency selective surface (FSS)
20
for possible use as a high permittivity dielectric material
12
in structure
10
is an electrically thin layer of engineered material (typically planar in shape) which is typically comprised of periodic metallic patches or traces
22
laminated within a dielectric material
24
for environmental protection.
The electromagnetic interaction of an FSS with plane waves may be understood using circuit analog models in which lumped circuit elements are placed in series or parallel arrangements on an infinite transmission line which models the plane wave propagation. FSS structures are said to be capacitive when their circuit analog is a single shunt capacitance. This shunt capacitance, C (or equivalent sheet capacitance), is measured in units of Farads per square area. Equivalently, the reactance presented by the capacitive FSS can be expressed in units of ohms per square area. This shunt capacitance is a valid model at low frequencies where (&bgr;
1
t
1
) <<1, and t
1
is the FSS thickness. As a shunt capacitance, electromagnetic energy is stored by the electric fields between metal patches. Physical implementations of capacitive FSS structures usually contain periodic lattices of isolated metallic “islands” such as traces
22
upon which bound charges become separated with the application of an applied or incident electric field (an incident plane wave). The periods of this lattice are much less than a free space wavelength at frequencies where the capacitive model is valid. The equivalent relative dielectric constant of a capacitive FSS is given as ∈
r
=C/(∈
0
t
1
) where ∈
0
is the permittivity of free space. FSS structures can be made with ∈
r
values extending up to several hundred.
FIG. 2
is a top view of a conventional anisotropic FSS
20
comprised of square metal patches
22
where each patch is identical in size, and buried inside a dielectric layer
24
(such as FR-4).
FIG. 3
is a cross-sectional side view of
FIG. 2
taken along sectional line
3
—
3
of FIG.
2
. As shown, the gaps between patches
22
are denoted as g
x
in the x direction and g
z
in the z direction. If these variables are different dimensions, as shown in this figure, then the equivalent capacitance provided by the FSS is different for electric fields polarized in the x and z directions. Since g
x
is smaller than g
z
, the equivalent sheet capacitance for x-polarized E fields will be larger than for z-polarized E fields. For a given value of incident E field, more energy will be stored for the x polarized waves than for the z polarized waves. This leads to ∈
rx
>∈
rz
in the FSS, and ∈
x
>∈
z
in the equivalent bulk permittivity for a layered substrate when it is included in a non-homogeneous stacked dielectric substrate such as substrate
10
(assuming that the second layer is isotropic, such as foam).
FIGS. 4 through 6
illustrate a linearly-polarized patch antenna
40
according to U.S. Pat. No. 6,075,485.
FIG. 4
is a top view, and
FIGS. 5 and 6
are cross-sectional views taken along lines
5
—
5
and
6
—
6
, respectively. As shown, antenna
40
includes a substrate comprised of artificial dielectric material
10
, having alternating layers
12
and
14
of high and low permittivity dielectric materials, respectively, a microstrip patch
42
, a coaxial feed
44
and a metalized ground plane
46
.
To achieve the same resonant frequency in patch antenna
40
, having an artificial dielectric material substrate, as in a conventional patch antenna with a homogeneous substrate, the artificial dielectric substrate is oriented so that the uniaxial axis, that is, the axis of anisotropy (where ∈
x
=∈
z
>>∈
y
, for example) is perpendicular to the surfaces of the high dielectric layers (the y axis in
FIGS. 4 and 5
, i.e. the direction in which the layers are stacked), and is parallel to the surface of the microstrip patch
42
.
Antenna
40
can be, for example, a low weight UHF (240-320 MHz) patch antenna. For purposes of comparison, a conventional patch antenna for this application would include, for example, a homogeneous ceramic slab (8″×8″×1.6″) of material PD-13 from Pacific Ceramics of Sunnyvale, Calif. where ∈
r
=13 and the specific gravity is 3.45 grams/cm
3
. The weight of the homogeneous substrate having the required dimensions would thus be about 12.75 lbs. In the lightweight substrate design of U.S. Pat. No. 6,075,485, layer
12
of substrate
10
can be, for example, a 0.020″ thick FSS (such as part no. CD-800 of Atlantic Aerospace Electronics Corp., Greenbelt, Md. for example) designed to represent an equivalent capacitance of at least 300 for the x and z directions of FIG.
1
. This FSS is made from one 0.020″ thick layer of FR4 fiberglass whose specific gravity is approximately 2.5 grams/cm
3
. To achieve an effective relative permittivity of ∈
x
=∈
z
=13∈
0
, layer
14
can be, for example, a 0.500″ thick Rohacell foam of the same type used in the example above. Substrate
10
having these design parameters weighs approximately 6.5 oz., which represents a 97% weight reduction from the conventional homogeneous substrate for this antenna application.
For fixed-frequency UHF applications as described above, patch
42
of
FIG. 4
can be a six inch square patch (L=6″) printed on a 8″×8″×0.060″ thick Rogers R04003 printed circuit board (not shown). The circuit board is mounted face down so that patch
42
touches the ceramic slabs of the artificial dielectric substrate
10
. The fixed frequency patch antenna
40
built according to these specifications resona
Garrett Steven L.
Lilly James D.
McKinzie, III William E.
Clinger James
e-Tenna Corporation
Pillsbury & Winthrop LLP
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