Communications: radio wave antennas – Antennas – Microstrip
Reexamination Certificate
1999-05-17
2001-10-23
Wimer, Michael C. (Department: 2821)
Communications: radio wave antennas
Antennas
Microstrip
Reexamination Certificate
active
06307509
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to antennas and, more particularly, to a novel and highly effective antenna that strikes an effective compromise between the dielectric constant and loss tangent of a separator provided between a signal radiator and a ground plane, and does so at an exceptionally low coast.
2. Description of the Prior Art
Those skilled in the art of antenna design are aware what there is a tradeoff between the dielectric constant and the loss tangent of a separator provided between the signal radiator and the ground plane of, for example, a patch antenna. A high dielectric constant is desirable because it enables reduction of the physical dimensions of the antenna. A low loss tangent is desirable because it enables an increase in the gain of the antenna. Unfortunately, measures taken to increase the dielectric constant tend to increase the loss tangent, and measures taken to reduce the loss tangent tend to reduce the dielectric constant.
Consider a plane wave propagating in a lossy dielectric. Maxwell's equations for a lossy region are
V×E=−j&ohgr;&mgr;H
V×H=j&ohgr;&egr;E+&sgr;E
where E and H are the electric and magnetic fields, respectively, expressed as vectors; &ohgr; is the angular frequency; &mgr; is the permeability; &egr; is the permitivity; and &sgr; is the conductivity. The second equation may be written in the form
∇
xH
=
jω
(
ε
+
σ
j
ω
)
E
=
jωε
c
E
where
ε
c
=
ε
+
σ
j
ω
=
ε
′
-
jε
″
The quantity &egr;′ is called the relative dielectric constant and the ratio &egr;″/&egr;′ is called the loss tangent, denoted tan &dgr;. It is called a loss tangent because it is a measure of the ohmic loss in the medium and thus is a measure of the quality of the dielectric.
The dielectric constant affects the dimensions of the distributed circuit components, and the loss tangent affects the loss in the circuit. In the case of the a microstrip patch antenna, a higher dielectric constant allows the patch to be smaller; however, a higher loss tangent reduces the gain of the antenna. While the gain of an antenna is often more important than its size, one would like to obtain a dielectric material that had both a high dielectric constant (for small size) and a low loss tangent (for high gain). In conventional practice, less-than-ideal choices must often be made.
The dielectric constant and loss tangent of some commercial materials employed in a conventional manner are shown in Table 1.
TABLE 1
DIELECTRIC
MATERIAL
CONSTANT
LOSS TANGENT
Air
1.00
0.0001
RT/Duroid ® 5880
2.20
0.0009
FR-4
4.20
0.0300
As Table 1 shows, air has a dielectric constant of 1.00 and a loss tangent of 0.0001. A patch antenna employing air as a separator may be taken as a reference to which other designs may be compared.
RT/Duroid® 5880, which is a registered trademark of Rogers Corporation for a material generically described as PTFE and reinforcing glass fibers, can also be employed as a separator between the signal radiator and the ground plane of a patch antenna. As Table 1 shows, RT/Duroid® 5880 has a dielectric constant of 2.20 and a loss tangent of 0.0009. While the dielectric constant is good, enabling a reduction in the size of the antenna as compared to an antenna employing air as the dielectric, the loss tangent is undesirably high and compromises the antenna gain. Moreover, RT/Duroid® 5880 is quite expensive and in many instances not economical for commercial use as a separator in a patch antenna assembly.
FR-4, which is a generic name for an inexpensive glass/epoxy laminate, described as a highly cross linked, brominated epoxy resin reinforced with woven glass cloth, can also be employed as a separator between the signal radiator and the ground plane of a patch antenna. As Table 1 shows, FR-4 has a dielectric constant of 4.20 and a loss tangent of 0.0300. While the dielectric constant is excellent, the loss tangent is high. Despite the low cost of FR-4, its high loss tangent renders it undesirable in conventional use as a separator between the radiating element and the ground plane of a patch antenna.
Many other materials have been tried as dielectric separators, but all have left something to be desired from the standpoint of dielectric constant, loss tangent, cost, weight, physical dimensions, or all of the above.
OBJECTS AND SUMMARY OF THE INVENTION
An object of the invention is to provide a separator for use between the radiating element and the ground plane of a patch antenna and that is constructed in such a manner as to have, separately and in combination:
a high dielectric constant;
a low loss tangent;
low weight;
small physical dimensions;
low cost.
Other objects of the invention are to provide an antenna employing such a separator and a method of employing the antenna especially to transmit or receive a GPS signal.
The foregoing and other objects are attained in accordance with one aspect of the invention by providing a dielectric separator for use in a patch antenna, wherein the separator comprises at least a first portion and a second portion, the first and second portions having dielectric constants that are different from each other.
In accordance with an independent aspect of the invention, there is provided a dielectric separator for use in a patch antenna, wherein the separator comprises a material that has a dielectric constant and loss tangent substantially higher than those of air and is formed with at least one void reducing the loss tangent of the separator. The material is preferably FR-4.
In accordance with another independent aspect of the invention, there is provided an antenna comprising a dielectric separator as described above.
In accordance with another aspect of the invention, there is provided a method comprising, as a step thereof, employing an antenna as described above to transmit or receive a signal, preferably a GPS signal.
The following features of the invention are also noteworthy:
The antenna is constructed as a patch antenna.
The material of which the separator is made has a dielectric constant higher than 2.00 and even as high as substantially 4.20 and a loss tangent of substantially 0.0300, and the void reduces the loss tangent of the separator to substantially 0.0004, while preserving a dielectric constant at least as high as 1.26.
The ratio of the area of the void to the area of the separator exceeds 0.1 and can be more than 0.9, so long as the signal radiator and ground plane are adequately supported.
The void is formed by a wall boundary to support dielectric material optimally provided above and below the void, or to support the radiating element and the ground plane directly. In accordance with the invention, there can be a single void having, for example, a star, hub-and-spoke, or serpentine shape, or a number of voids of hexagonal (honeycomb), rectangular (including square), triangular, elliptical (including circular), or other shape, or any combination of the above.
While the number and the shape of the void or voids have some importance from a structural and manufacturing standpoint, neither is critical to the invention as broadly conceived. The ratio of the area of the void(s) to the area of the separator is a more significant figure, since the void(s) in effect substitute the low loss tangent of air for the high loss tangent of the structural dielectric. The exact relationship between the effective dielectric constant and the amount of material remaining after the void(s) are formed is not precisely known but seems to be nonlinear.
REFERENCES:
patent: 4366484 (1982-12-01), Weiss et al.
patent: 4977406 (1990-12-01), Tsukamoto et al.
patent: 5319378 (1994-06-01), Nalbandian et al.
patent: 5635942 (1997-06-01), Kushihi et al.
patent: 5977915 (1999-11-01), Bergstedt et al.
patent: 6034637 (2000-03-01), McCoy et al.
Cooper & Dunham LLP
Dowden Donald S.
Pelton William E.
Trimble Navigation Limited
Wimer Michael C.
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