Dielectric loaded microstrip patch antenna

Details Dielectric loaded microstrip patch antenna Dielectric loaded microstrip patch antenna

1999-12-06

2001-08-28

Ho, Tan (Department: 2821)

Communications: radio wave antennas

Antennas

Microstrip

C343S846000, C343S848000

Reexamination Certificate

active

06281845

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to microstrip patch antennas and more particularly to a microstrip patch antenna spaced from a ground plane by a substance having a very low dielectric constant such as air.
BACKGROUND OF THE INVENTION
The performance of an antenna is determined by several parameters, one of which is efficiency. For a microstrip antenna, “efficiency” is defined as the power radiated divided by the power received by the input to the antenna. A one-hundred percent efficient antenna has zero power loss between the received power input and the radiated power output. In the design and construction of microstrip antennas it is desirable to produce antennas having a relatively high efficiency rating, preferably in the range of 95 to 99 percent.
One factor in constructing a high efficiency microstrip antenna is minimizing power loss, which may be caused by several factors including dielectric loss. Dielectric loss is due to the imperfect behavior of bound charges, and exists whenever a dielectric material is located in a time varying electrical field. Moreover, because dielectric loss increases with operating frequency, the problem of dielectric loss is aggravated when operating at higher frequencies.
The extent of dielectric loss for a particular microstrip antenna is determined by, inter alia, the permittivity, ∈, expressed in units of farads/meter (F/m), of the dielectric space between the radiator and the ground plane which varies somewhat with the operating frequency of the antenna system. As a more convenient alternative to permittivity, the relative dielectric constant, ∈
r
, of the dielectric space may be used. The relative dielectric constant is defined by the equation:
 ∈
r
=∈/∈
o
where ∈is the permittivity of the dielectric space and ∈
o
is the permittivity of free space (8.854.times.10.sup.−12 F/m). It is apparent from this equation that free space, or air for most purposes, has a relative dielectric constant approximately equal to unity.
A dielectric material having a relative dielectric constant close to one is considered a “good” dielectric material—that is, the dielectric material exhibits low dielectric loss at the operating frequency of interest. When a dielectric material having a relative dielectric constant equal to unity is used, dielectric loss is effectively eliminated. Therefore, one method for maintaining high efficiency in a microstrip antenna system involves the use of a material having a low relative dielectric constant in the dielectric space between the radiator patch and the ground plane.
Furthermore, the use of a material with a lower relative dielectric constant permits the use of wider transmission lines that, in turn, reduce conductor losses and further improve the efficiency of the microstrip antenna.
The use of a material with a low dielectric constant, however, is not without drawbacks. One typical drawback is that it is difficult to produce high-speed compact patch antennas spaced from a ground plane by a “good” dielectric. When a dielectric material disposed between a patch and a ground plane has a low dielectric constant (about 1), the resulting patch size is large (for example at 3.6 GHz patches of about 1550 mm
2
result). For mobile applications and for use in arrays, such a patch size is often problematic.
Another problem with antennas as described above is that the feed efficiency often degrades substantially as the patch is spaced further away from the ground plane. That said, more spacing of the patch from the ground plane is often advantageous and, as such, is usually accommodated using dielectric material with a higher dielectric constant to fill the space between the patch and the ground plane. Unfortunately, efficiency is substantially compromised in order to meet other design parameters.
It would be advantageous to provide a patch antenna that is spaced a distance from a ground plane and efficiently coupled to a feed absent a substrate having a high dielectric constant filling the space therebetween.
SUMMARY OF THE INVENTION
In accordance with the invention there is provided a patch antenna comprising a ground plane, a feed and a patch spaced from the ground plane by a predetermined distance. A dielectric material having a low dielectric constant is disposed therebetween. This dielectric material could be air, foam, or the like. In order to improve coupling efficiency between the patch and the feed, a piece of second dielectric material having a higher dielectric constant than the dielectric material is inserted between the patch and the ground plane in order to load the feed and thereby improve coupling efficiency between the feed and the patch. Exact placement of the piece of the second dielectric material is important for optimising antenna performance.
Further pieces of another dielectric material are optionally disposed between the ground plane and the patch to shape the radiation fed to the patch. This permits a smaller size patch than would be possible in a conventional air spaced patch antenna.
In accordance with the invention there is provided a microstrip patch antenna comprising:
a patch radiator;
a conducting ground plane spaced from the patch radiator by a first predetermined distance along a z-axis;
a first dielectric material having a low dielectric constant and disposed between the ground plane and the patch radiator;
a feed for providing the patch radiator with radio signal energy; and,
a second dielectric material having a relative dielectric constant greater than that of the first dielectric material for loading the feed and having a dimension along an axis orthogonal to the z-axis smaller than a dimension along a same axis of the patch and disposed between said patch radiator and said ground plane for determining operational characteristics of said microstrip patch antenna.
In accordance with another embodiment of the invention there is provided a microstrip patch antenna comprising:
a conducting ground plane having a thickness along a z axis and dimensions along an x and y axis orthogonal to the z axis;
a patch radiator spaced by a first dielectric material having a low dielectric constant from the ground plane along the z-axis orthogonal;
a slot feed for providing the patch radiator with radio signal energy across the space containing the first dielectric material; and,
a piece of second dielectric material adjacent the slot feed between the patch radiator and the ground plane for loading the feed and having a dimension along one of the x and y axes smaller than a dimension of the patch along a same axis, wherein the piece of second dielectric material determines operational characteristics of the microstrip patch antenna, the second dielectric material having a dielectric constant that is higher than the dielectric constant of the first dielectric material.
In accordance with another aspect of the present invention there is provided a method of designing a microstrip patch antenna comprising the steps of:
providing a design of a patch radiator;
providing a design of a conducting ground plane spaced from the patch radiator by a first predetermined distance along a z-axis;
providing a design for a feed for providing the patch radiator with radio signal energy; and,
providing a design for a second dielectric for loading the feed and having a dimension along an axis orthogonal to the z-axis smaller than a dimension along a same axis of the patch and disposed between said patch radiator and said ground plane for determining operational characteristics of said microstrip patch antenna simulating the provided designs; and,
adjusting the design of the second dielectric until a desired radiation pattern from the microstrip patch results.
Advantageously, an antenna according to the invention provides high speed, high efficiency, and reasonable bandwidth with reduced size over prior art air gap patch antennas.


REFERENCES:
patent: 4843400 (1989-06-01), Tsao et al.
patent: 4958162 (1990-09-01), Roberts et al.
patent: 5355143 (1994-10-01),

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