Communications: radio wave antennas – Antennas – Microstrip
Reexamination Certificate
2001-07-06
2002-06-11
Phan, Tho G. (Department: 2821)
Communications: radio wave antennas
Antennas
Microstrip
C343S741000, C343S744000, C343S745000
Reexamination Certificate
active
06404391
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains to the field of antennas and more particularly, to a meander line loaded antenna configured as a small, tunable patch antenna.
BACKGROUND OF THE INVENTION
Microstrip patch antennas are known in the art, and generally have a dielectric substrate with a ground plane on one surface and a strip conductor feeding a large patch. The patch is generally large, with the length being a little less than half a wavelength at the operating frequency and the width selected for the appropriate radiating resistance.
Suspended patch antennas are also known in the art, wherein the patch is suspended substantially parallel and above the ground plane. The suspended design has an increased efficiency, but is otherwise restrained with the limitations of the standard patch antenna, namely being relatively large.
In the prior art, efficient antennas have typically required structures with minimum dimensions on the order of a quarter wavelength of their intended radiating frequency. These dimensions allowed the antennas to be easily excited and to be operated at or near their resonance, limiting the energy dissipated in resistive losses and maximizing the transmitted energy. These antennas tended to be large in size at their resonant wavelengths. Further, as the operating frequency decreased, the antenna's dimensions were increased proportionally.
In order to address the shortcomings of traditional antenna design and functionality, the meander line loaded antenna (MLA) was developed. The basic theory and design of the MLA is presented in U.S. Pat. No. 5,790,080. An example of a basic MLA, also known as a variable impedance transmission line (VITL) antenna, is shown in
FIG. 1
, generally at reference number
100
. The antenna
100
consists of two vertical sections (i.e., plates)
102
and a horizontal section
104
. The vertical and horizontal sections
102
,
104
, respectively, are separated by gaps
106
. Also part of the antenna
100
are meander lines
200
(FIG.
2
), which are typically connected between at least one of the vertical sections
102
and the horizontal section
104
at the gaps
106
.
The meander line
200
is designed to adjust the electrical (i.e., resonant) length of the antenna
100
. In addition, it is possible to switch lengths of the meander line
200
in or out of the circuit quickly and with negligible loss in order to change the effective electrical length of the antenna
100
. This switching is possible because the active switching devices (not shown) are usually located in the high impedance sections of the meander line
200
. This keeps the current through the switching devices (not shown) low and results in very low dissipation losses in the switches, thereby maintaining high antenna efficiency. Switching of sections of a meander line using microelectromechanical systems (MEMS) switches or the like are well known to those skilled in the antenna design arts.
The basic antenna of
FIG. 1
can be operated in a loop mode that provides a 360° coverage (i.e., radiation) pattern. Horizontal polarization, loop mode, is obtained when the antenna is operated at a frequency such that the electrical length of the entire line including the meander lines
200
is a multiple of full wavelength, as shown in FIG.
3
C. The antenna can also be operated in a vertically polarized, monopole mode by adjusting the electrical length to an odd multiple of a half wavelength at the operating frequency,
FIGS. 3B and 3D
, respectively. The meander lines
200
can be tuned using electrical or mechanical switches (not shown) to change the mode of operation at a given frequency or to switch frequencies using a given mode.
The invention of the meander line loaded antenna allowed the physical antenna dimensions to be significantly reduced in size while maintaining electrical lengths that were still multiples of a quarter wavelength. Antennas and radiating structures built using this design approach operate in the region where the limitation on their fundamental performance is governed by the Chu-Harrington relation:
Efficiency=FV
2
Q
where:
Q=Quality Factor;
V
2
=Volume of the structure in cubic wavelengths; and
F=Geometric Form Factor (F=64 for a cube or a sphere)
Meander line loaded antennas achieve the efficiency limit of the Chu-Harrington relation while allowing the antenna size to be much smaller than a wavelength at the frequency of operation. Height reductions of 10 to lower quarter wave monopole antennas are achieved while realizing comparable gain.
The existing MLA antennas are narrow band antennas. While the patch antennas have a greater bandwidth, they are too large to be useful in certain size constrained applications.
DISCUSSION OF RELATED ART
U.S. Pat. No. 5,790,080 entitled MEANDER LINE LOADED ANTENNA, describes an antenna that includes one or more conductive elements for acting as radiating antenna elements. Also provided is a slow wave meander line adapted to couple electrical signals between the conductive elements, wherein the meander line has an effective electrical length that affects the electrical length and operating characteristics of the antenna. The electrical length and operating mode of the antenna may be readily controlled.
A tunable microstrip patch antenna is described in U.S. Pat. No. 5,777,581. The patch is configured with numerous switchable microstrips. The resonant frequency of the patch is inversely proportional to the total effective patch length including the microstrip sections. Switching the microstrips changes the properties of the antenna allowing the resonant frequency to be manipulated. Other tunable microstrip patch antennas include U.S. Pat. No. 6,005,519 and U.S. Pat. No. 4,821,041.
U.S. Pat. No. 6,034,637 describes a double resonant wideband patch antenna that includes a planar resonator forming a substantially trapezoidal shape having a non-parallel edge for providing a substantially wide bandwidth. A feed line extends parallel to the non-parallel edge for coupling, while a ground plane extends beneath the planar resonator for increasing radiation efficiency.
U.S. Pat. No. 6,008,762 describes a folded quarter-wave patch antenna that includes a conductor plate having first and second spaced apart arms. A ground plane is separated from the conductor plate by a dielectric substrate that is approximately to the conductor plate. The ground plane is electrically connected to the first arm at one end and a signal unit is also electrically coupled to the first arm. The signal unit transmits and/or receives signals having a selected frequency band. The folded quarter-wave patch antenna can also act as a dual frequency band antenna. In dual frequency band operation, the signal unit provides the antenna with a first signal of a first frequency band and a second signal of a second frequency band.
Thus, the prior art inventions have been unable to produce small patch antennas. The small patch antennas are necessary in certain applications. What is needed is a small patch antenna with comparable characteristics and capabilities of the larger conventional patch antennas.
SUMMARY OF THE INVENTION
The antenna of the present invention differs from those of the prior art in that it includes a meander line allowing a physical length significantly smaller than its electrical length at its resonant frequency. This is accomplished by using switches to adjust the electrical length of the meander line. The invention also includes a fixed or switched capacitance to cancel meander line inductance. The net effect is that the size of the inventive antenna is significantly smaller than a conventional patch antenna at a particular operating frequency.
In accordance with the present invention there is provided a meander line loaded patch antenna where microelectromechanical systems switches or the like are used to electrically connect and disconnect sections of the meander line from the circuit. A capacitance value calculated for the resonant frequency provides compensation for inductive reac
Asmus Scott J.
BAE Systems Information and Electronic System Integration INC
Maine Vernon C.
Phan Tho G.
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