Communications: radio wave antennas – Antennas – Microstrip
Reexamination Certificate
1999-11-24
2001-01-30
Wong, Don (Department: 2821)
Communications: radio wave antennas
Antennas
Microstrip
C343S767000
Reexamination Certificate
active
06181281
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to patch antennas applicable mobile or wireless communications systems and more particularly, to a single-mode patch antenna for circularly polarized waves and a dual-mode patch antenna operable as a linearly polarized antenna at a frequency and a circularly polarized antenna at another frequency, which are capable of easy optimization in both impedance matching and axial ratio adjustment.
2. Description of the Prior Art
In the field of mobile communication, various types of patch antennas have been extensively used because of their advantage of compactness, which are equipped with a plate-shaped dielectric substrate and a conductor patch formed on the surface of the substrate.
FIG. 1
shows a prior art circularly polarized patch antenna, which is shown based on the paper written by P. C. Sharma et al., “Analysis and Optimized Design of Single Feed Circularly Polarized Microstrip Antenna”, IEEE TAP 1983, Vol., AP-31, No. 6, pp. 949-955.
In
FIG. 1
, a rectangular conductor patch
151
serving as a radiating element is formed on the surface of a rectangular dielectric substrate
150
. The two long sides of the patch
151
have a length of L
1
and two short sides thereof have L
2
. A plate-shaped grounding conductor
153
serving as a ground plane is formed on the opposite surface of the substrate
150
to the patch
151
. The reference numeral
152
denotes a feedpoint through which electric power is fed to the patch
151
.
The above-described prior-art patch antenna of
FIG. 1
has the following problem.
Specifically, with the prior-art patch antenna shown in
FIG. 1
, electric power is supplied to the patch
151
through the single feedpoint
152
and the antenna structure is very simple. Therefore, there is a problem that a degree of freedom to optimize both the axial ratio setting for circularly polarized waves and the impedance matching at a specific frequency is insufficient.
Also, a monopole or dipole may be additionally provided as an additional radiating element above the patch
151
in order to add another antenna function. In this case, to form a way of feeding electric power to the monopole or dipole thus added, a square or rectangular aperture needs to be formed in the patch
151
to expose the underlying surface of the dielectric substrate
150
. However, the aperture causes another problem that the location adjustment of the feedpoint
152
becomes more difficult. Also, it causes a further problem that unwanted shift of the axial ratio of elliptically polarized waves is generated and this shift cannot be fully compensated by simply changing the location of the feedpoint
152
.
FIG. 2
shows another prior-art circularly polarized patch antenna usable at two different frequencies of f
1
and f
2
, which is a dual-frequency antenna. This is shown based on the paper written by D. Sanchez-Hernandez et al., “Single-fed dual band circularly polarized microstrip patch antennas”, 26th EuMc 9-12 Sep. 1996, Prague, pp. 273-277.
In
FIG. 2
, a rectangular patch
257
serving as a radiating element is formed on the surface of a rectangular dielectric substrate
255
. The shape and size of the patch
257
are designed to have a resonant frequency at f
1
. A plate-shaped grounding conductor
253
serving as a ground plane is formed on the opposite surface of the substrate
255
to the patch
257
. The reference numeral
256
denotes a feedpoint through which electric power is fed to the patch
257
.
Unlike the prior-art antenna shown in
FIG. 1
, the patch
257
has a L-shaped slit
258
A formed near its long side
257
a
and a L-shaped slit
258
B formed near its short side
257
b
. The slit
258
A extends inwardly by a specific length from a point on the long side
257
a
and then, bends at a right angle and runs parallel to the side
257
a
by a specific length. The part of the slit
258
A which is parallel to the long side
257
a
is longer than that which is perpendicular thereto.
Similarly, the slit
258
B extends inwardly by a specific length from a point on the long side
257
b
and then, bends at a right angle and runs parallel to the side
257
b
by a specific length. The part of the slit
258
B which is parallel to the long side
257
b
is longer than that which is perpendicular thereto.
Here, it is supposed that the parts of the slits
258
A and
258
B, which are respectively in parallel to the long and short sides
257
a
and
257
b
, have a same width of SLSL and a same length of LSL. If the values of the width WLSL and the length LSL are suitably adjusted, a filter effect is generated due to the existence of the slits
258
A and
258
B, resulting in a resonant frequency at f
2
which is different from f
1
. Thus, the prior-art patch antenna shown in
FIG. 2
have two resonant frequencies at f
1
and f
2
, which means that it serves as a double-frequency antenna.
With the prior art patch antenna shown in
FIG. 2
, however, if a square or rectangular aperture is formed in the patch
257
a
in order to add a monopole or dipole over the patch
257
as an additional radiating element, the difficulty in location adjustment of the feedpoint
256
is increased due to existence of the slots
258
A and
258
B. Moreover, since the axial ratio adjustment and the impedance matching become more difficult than the prior art patch antenna shown in
FIG. 1
, the addition of a monopole or dipole above the patch
257
is extremely difficult to be realized.
To increase the ease in the axial ratio adjustment and impedance matching at different frequencies, several methods have been developed and proposed. However, all the proposed methods require to provide two arrays of patches on a same dielectric substrate. As a result, a large space of patches is necessary and the size of an antenna is increased, which is contrary to the advantage of compactness of patch antennas.
Furthermore, the difficulty in the axial ratio adjustment and the impedance matching is increased by the L-shaped slots
258
A and
258
B, because the addition of the slots
258
A and
258
B generates some deviation in the axial ratio and/or the matched impedance.
Actual communications systems require low-cost, small-sized circularly polarized antennas having a well-adjusted axial ratio and a well-matched impedance. However, as far as the inventors know, the prior-art antennas including the above-described antennas shown in
FIGS. 1 and 2
provide only one of a well-adjusted axial ratio and a well-matched impedance. This means that the prior-art antennas essentially requires a compromise between the axial ratio adjustment and the impedance matching.
On th other hand, in recent years, there have been the growing need for dual-mode patch antennas capable of operation as a linearly polarized antenna at a frequency and a circularly polarized antenna at another frequency. This need is one the basis of the intention to cope with several different communication systems, such as the ground wave communication systems using linearly polarized waves and the satellite communication systems using circularly polarized waves.
As explained previously, the prior-art patch antenna shown in
FIG. 2
is operable at the two different frequencies f
1
and f
2
. However, this antenna is dedicated to circularly polarized waves. Therefore, if it is applied to linearly polarized waves, it will produce a lot of cross polarization components. Thus, it is unable to be operated as a dual-mode patch antenna.
FIG. 3
shows a prior-art dual-mode patch antenna, which is equipped with two patches designed respectively for circularly and linearly polarized waves. This antenna is shown based on the same paper written by P. C. Sharma et al. as that cited with reference to FIG.
1
.
As shown in
FIG. 3
, a first rectangular patch
362
and a second parallelogrammic patch
363
are formed on the surface of a rectangular dielectric substrate
361
. These two patches
362
and
363
are apart from each other at a short distance. A plate-shaped grounding conductor
364
serving
Desclos Laurent
Madihian Mohammad
Clinger James
NEC Corporation
Ostrolenk Faber Gerb & Soffen, LLP
Wong Don
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