Microstrip array antenna

Details Microstrip array antenna Microstrip array antenna

2000-05-22

2002-07-23

Wimer, Michael C. (Department: 2821)

Communications: radio wave antennas

Antennas

Microstrip

Reexamination Certificate

active

06424298

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a planar array antenna formed of a microstrip conductor and capable of being used as a transmission/reception antenna of a radar mounted on a vehicle.
2. Description of the Related Art
U.S. Pat. No. 4,063,245 discloses a conventional planar array antenna formed of a microstrip conductor. As shown in
FIG. 18
, in the antenna disclosed in U.S. Pat. No. 4,063,245, a ground conductor layer
2
is formed on a reverse surface of a dielectric substrate
1
, and a plurality of straight feeder microstrips
3
are formed on the dielectric substrate
1
. The feeder microstrips
3
extend in parallel to each other and have first ends connected together and second ends of open-circuit termination (hereinafter referred to as “open ends”). A plurality of antenna elements
4
a
to
4
e
project transversely from each feeder microstrip
3
in the form of branches. Thus, a linear array is formed. The feeder microstrips
3
each forming a linear array are connected to a feeder microstrip
5
, and a composite signal is output from the center
6
of the feeder strip
5
. Thus, a two-dimensional array antenna is configured.
The antenna elements
4
a
to
4
e
are disposed at a pitch corresponding to the guide wavelength &lgr;
g
of electromagnetic waves that propagate within the feeder microstrip (hereinafter simply referred to as the “guide wavelength”), and the length of the antenna elements
4
a
to
4
e
is set to about half the guide wavelength &lgr;
g
; i.e., &lgr;
g
/2.
Since the excitation amplitude of each of the antenna elements
4
a
to
4
e
can be controlled through a change in the width thereof, the antenna can have desired directivity-related characteristics; i.e., gain and side lobe level, which are determined in accordance with the intended use (specifications). In the illustrated example, antenna elements nearer either end of each feeder microstrip
3
, such as
4
a
and
4
e
, are narrower than those nearer the center of the feeder microstrip
3
, such as
4
c
; and the antenna element
4
e
is connected to the feeder microstrip
3
at a point half the guide wavelength &lgr;
g
from the open end
7
of the feeder microstrip
3
. Thus, standing-wave excitation is enabled, and each linear array can have a peak-like amplitude distribution such that the amplitude increases toward the center of the feeder microstrip
3
. This amplitude distribution has the effect of shrinking side lobes.
FIG. 19
is a plan view showing the structure of another conventional array antenna. This array antenna comprises a straight feeder microstrip
53
as in the above-described conventional antenna, and a plurality of antenna elements
54
a
to
54
t
projecting transversely from the feeder microstrip
53
in the form of branches. One end of the feeder microstrip
53
is connected to an input/output port
56
, and the other end is connected to a matching termination element
58
a
, whereby traveling-wave excitation is realized. The antenna elements
54
a
to
54
j
in a first set project perpendicularly from one side of the feeder microstrip
53
at a pitch corresponding to the guide wavelength &lgr;
g
. Further, the antenna elements
54
k
to
54
t
in a second set project perpendicularly from the other side of the feeder microstrip
53
at a pitch corresponding to the guide wavelength &lgr;
g
. The positions at which the antenna elements
54
a
to
54
j
in the first set are connected to the feeder microstrip
53
are offset by &lgr;
g
/2 from the positions at which the antenna elements
54
k
to
54
t
in the second set are connected to the feeder microstrip
53
.
The above-described structure makes it possible to increase the number of antenna elements within a unit path length and to reduce the residual power reaching the terminal end, which residual power lowers the efficiency of an antenna which has a relatively short array length and is excited by traveling waves. Therefore, the structure can realize an antenna which operates efficiently even when the array length is relatively short (about 10&lgr;
g
in the antenna shown in FIG.
19
). Further, in the conventional antennas shown in
FIGS. 18 and 19
, the antenna elements
4
a
to
4
e
or the antenna elements
54
a
to
54
t
radiate electromagnetic waves mainly from their open ends and can therefore be considered to approximate magnetic dipoles. Therefore, radiated or received electromagnetic waves have a plane of polarization perpendicular to the feeder microstrip
3
or
53
.
Moreover, an antenna as shown in
FIG. 20
is known. In this antenna, antenna elements
74
a
to
74
e
are formed to incline with respect to a feeder strip
73
such that the antenna elements
74
a
,
74
b
, and
74
c
located on one side of the feeder strip
73
incline at an angle of about +45 degrees with respect to the feeder strip, and the antenna elements
74
d
and
74
e
located on the other side of the feeder strip
73
incline at an angle of about −45 degrees with respect to the feeder strip, whereby circularly polarized waves are produced. The antenna elements
74
a
and
74
d
are symmetrical with respect to a line A—A passing through the center of the feeder microstrip
73
and are disposed such that the distance between the antenna elements
74
a
and
74
d
becomes &lgr;
g
/4. In other words, an electric field Ea which is radiated from the antenna element
74
a
at an angle of +45 degrees relative to the feeder microstrip
73
and an electric field Ed which is radiated from the antenna element
74
d
at an angle of −45 degrees relative to the feeder microstrip
73
are composed with a phase difference of 90 degrees, so that circularly polarized waves are radiated mainly in the direction of a main beam.
Moreover, an array antenna having a structure as shown in
FIGS. 21A and 21B
is described in “Design of Low Cost Printed Antenna Arrays” (J. P. Daniel, E. Penard, M. Nedelec, and J. P. Mutzig, Proc. ISAP, pp. 121-124, Aug. 1985). On a dielectric substrate
101
(
201
) are disposed 10 square microstrip antenna elements
104
(
204
) which are connected to a feeder microstrip
103
(
203
) such that power is fed to the microstrip antenna elements
104
(
204
) from their corners. The plurality of microstrip antenna elements
104
(
204
) are disposed symmetrically along the longitudinal direction with respect to an input/output terminal
102
(
202
) formed at the center of the feeder microstrip
103
(
203
). In the antenna of
FIG. 21A
, the microstrip antenna elements
104
are connected to one side edge of the feeder microstrip
103
at a pitch corresponding to the guide wavelength &lgr;
g
of the feeder microstrip
103
, and an impedance transformer
105
having a length of &lgr;
g
/4 is formed on the upstream side (the side closer to the input/output terminal
102
) of each connection point. In the antenna of
FIG. 21B
, the microstrip antenna elements
204
are alternately connected to opposite side edges of the feeder microstrip
203
at a pitch corresponding to half the guide wavelength &lgr;
g
of the feeder microstrip
203
, and an impedance transformer
205
having a length of &lgr;
g
/4 is formed on the upstream side (the side closer to the input/output terminal
202
) of each connection point.
By virtue of the above-described structure, in the antenna of
FIG. 21A
, degenerated TM
01
, and TM
10
, modes perpendicular to the microstrip antenna elements
104
are excited, so that an electromagnetic wave polarized in a direction perpendicular to the feeder microstrip
103
is generated as a composite polarized wave. Similarly, in the antenna of
FIG. 21B
, an electromagnetic wave polarized in a direction perpendicular to the feeder microstrip
203
is generated. Further, in the antennas of
FIGS. 21A and 21B
, through adjustment of the conversion impedance of the impedance transformers
105
and
205
, the excitation amplitude of each of the microstrip antenna elements
104
and
204
can be controlled in order to attain desired directiv

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