Molded high impedance surface and a method of making same

Details Molded high impedance surface and a method of making same Molded high impedance surface and a method of making same

2001-07-13

2004-05-25

Tugbang, A. Dexter (Department: 3729)

Metal working

Electric condenser making

Solid dielectric type

C029S025410, C029S831000, C029S852000, C029S841000, C029S848000, C029S857000, C205S118000, C205S122000

Reexamination Certificate

active

06739028

ABSTRACT:

TECHNICAL FIELD
This invention improves upon current techniques for manufacturing high impedance surfaces which surfaces are also known as resonant textured ground planes or a “Hi-Z” surfaces and which surfaces are presently made using printed circuit board techniques. The present invention provides new methods of manufacturing such surfaces based on molding and/or related techniques, and also provides several structures that are manufacturable using these techniques. The invention allows Hi-Z surfaces to be mass-produced more rapidly and at a lower cost than the prior art techniques, which primarily involve printed circuit board technology. This invention also provides a Hi-Z structure in which the capacitors are vertical, instead of horizontal, so that they may be trimmed after manufacturing, for tuning purposes.
BACKGROUND OF THE INVENTION
Recently, a new kind of electromagnetic ground plane has been developed which is known as a high-impedance or Hi-Z surface. See D. Sievenpiper and E. Yablonovitch, “Circuit and Method for Eliminating Surface Currents on Metals” U.S. provisional patent application, Serial No. 60/079,953, filed on Mar. 30, 1998 by UCLA and a related PCT application published as WO 99/50929 on Oct. 7, 1999. This prior art structure consists of a metal ground plane covered with an array of tiny resonant cavities. These resonant cavities alter the effective electromagnetic impedance of the surface, so that it appears to have a high impedance (>>377 ohms), instead of a low impedance (≈0 ohm) like an ordinary metal surface. Because of its high impedance, the Hi-Z structure can support a finite tangential electric field at its surface, which is not possible with a smooth metal ground plane. This textured surface is important for various applications in the field of antennas. In particular, it is useful for low-profile antennas because radiating elements can be placed directly adjacent to the Hi-Z surface (i.e. spaced less than <<0.01 wavelength therefrom) without being shorted out. This provides an advantage compared to an ordinary metal ground plane, which normally requires a separation of roughly ¼ wavelength between the ground plane and the antenna, resulting in antennas that are at least ¼ wavelength thick. In addition to providing a way to produce very thin antennas, the Hi-Z surface also suppresses surface currents, which tend to interfere with the performance of the antenna by propagating across the ground plane and radiating from edges, corners, or other discontinuities. The radiation produced by these surface currents combines with the direct radiation from the antenna, and produces ripples in the radiation pattern, as well as significant radiation into the backward direction behind the ground plane. By suppressing these surface currents, one can produce antennas with much smoother radiation patterns, and with less backward radiation. In short, the antennas are both more compact and more efficient when made with a Hi-Z surface.
The Hi-Z structure can be most easily understood by considering the effective circuit that describes the resonant cavities. In the structure shown in
FIG. 1
, the Hi-Z surface is constructed as a lattice of overlapping “thumbtack”-like protrusions on a flat metal ground plane
22
. The protrusion consist of flat metal plates
10
connected to the ground plane by metal plated vias
13
. This prior art structure shown here is built using printed circuit board techniques. The printed circuit board is not shown for ease of illustration, but the flat metal plates
10
would appear on the printed circuit board's top surface while the ground plan
22
is disposed on its bottom surface. The capacitance of the structure is determined by the proximity and overlap area of the metal plates
10
. The inductance is controlled by the area of the current loop that connects adjacent plates, which is primarily determined by the thickness of the structure. The resonance frequency of the surface is then given by
ω
=
1
LC
.
Near the resonance frequency, the surface has high impedance, and can suppress the propagation of surface currents. The bandwidth of the surface, or the frequency band where the impedance is greater than 377 ohms, is given by
BW
=
L
/
C
μ
o
/
ϵ
o
.
This roughly determines the bandwidth of antennas that can be built on these surfaces.
Typically, in the prior art, Hi-Z surfaces are produced by printed circuit board techniques. In order to achieve a low resonant frequency (<10 GHz or so) in a thin structure (a few mm thick), a large amount of built-in capacitance is required. This is accomplished using a multi-layer structure, in which the capacitors are of a parallel-plate geometry. The vias
12
are made by drilling through both boards, and then plating the inside of the holes with metal
13
. The steps taken in fabrication are shown in side elevation in FIGS.
2
(
a
)-
2
(
f
). First, two printed circuit boards, one relatively thick and one relatively thin form the starting materials (see FIG.
2
(
a
)). The inner layers are patterned (see FIG.
2
(
b
)), and the boards are bonded together (see FIG.
2
(
c
)). Then holes
12
are drilled through the structure to define the positions of the vias (see FIG.
2
(
d
) and the plan view of FIG.
2
(
g
)). These are then plated with metal
13
(see FIG.
2
(
e
)). Finally, the outer layers are patterned (see FIG.
2
(
f
) and the plan view of FIG.
2
(
h
)). The most time-consuming and expensive task is drilling the vias
12
. A fast computer-controlled drill can drill on the order of one hole per second. Typical lattice periods for these structures are on the order of ¼ inch, which means that the total drilling time can approach one hour per square foot.
What is needed is a method of producing a similar structure by faster and more economic techniques, in which the holes do not need to be drilled individually, but instead can be produced en masse by some other technique. This invention provides techniques for producing such a structure by molding, as well as new geometries that are amenable to such manufacturing techniques. The resulting structure is less expensive and less time-consuming to fabricate. Furthermore, it has the additional benefit that certain embodiments thereof can be tuned after fabrication to adjust for variations in the manufacturing process. This feature also allows a single mold to be used to build structures with slightly different resonant frequencies.
BRIEF DESCRIPTION OF THE INVENTION
The present invention provides a Hi-Z surface that can be produced by injection molding, which permits large areas to be produced rapidly and at a low cost. Additionally, certain embodiments of the structure are also technically superior in that they can be tuned after manufacturing, to adjust for variations in the manufacturing process, thus allowing a single mold to be used for structures with slightly different resonance frequencies, and/or allowing different areas of a single Hi-Z surface to be tuned to different resonance frequencies.
In one aspect the present invention provides a method of making a high impedance surface comprising the steps of: molding a structure from a dielectric material to form the structure, the structure having a plurality of holes therein and a plurality of ridges on at least one major surface of the structure, the ridges having sidewalls; plating the structure, including the interiors of the holes therein and the sidewalls, with a layer of metal; removing at least a portion of the layer of metal which bridges across the ridges to thereby define capacitor plates on the sidewalls.
In another aspect the present invention provides a method of making a high impedance surface comprising the steps of: molding a structure from a dielectric material to form the structure, the structure having a plurality of holes therein and a plurality of trenches on at least one major surface of the structure, the trenches having sidewalls and bottom walls; and plating the structure, including the interiors

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