Communications: radio wave antennas – Antennas – Balanced doublet - centerfed
Reexamination Certificate
2001-08-27
2002-07-30
Wong, Don (Department: 2821)
Communications: radio wave antennas
Antennas
Balanced doublet - centerfed
C343S893000
Reexamination Certificate
active
06426730
ABSTRACT:
This application is the national phase under 35 U.S.C. § 371 of PCT International Application No. PCT/JP00/09271 which has an International filing date of Dec. 26, 2000, which designated the United States of America and was not published in English.
TECHNICAL FIELD
The present invention relates to a multi-frequency array antenna that is used as a base station antenna in a mobile communication system, and is used in common for a plurality of frequency bands which are separated apart from each other.
BACKGROUND ART
Antennas such as base station antennas for implementing a mobile communication system are usually designed for respective frequencies to meet their specifications, and are installed individually on their sites. The base station antennas are mounted on rooftops, steel towers and the like to enable communications with mobile stations. Recently, it has been becoming increasingly difficult to secure the sites of base stations because of too many base stations, congestion of a plurality of communication systems, increasing scale of base stations, etc. Furthermore, since the steel towers for installing base station antennas are expensive, the number of base stations has to be reduced from the viewpoint of cost saving along with preventing spoiling the beauty.
The base station antennas for mobile communications employ diversity reception to improve communication quality. Although the space diversity is used most frequently as a diversity branch configuration, it requires at least two antennas separated apart by a predetermined spacing, thereby increasing the antenna installation space. As for the diversity branch to reduce the installation space, the polarization diversity is effective that utilizes the multiple propagation characteristics between different polarizations. This method becomes feasible by using an antenna for transmitting and receiving the vertically polarized waves in conjunction with an antenna for transmitting and receiving the horizontally polarized waves. In addition, utilizing both the vertically and horizontally polarized waves by a radar antenna can realize the polarimetry for identifying an object from a difference between radar cross-sectional areas caused by the polarization.
Thus, to make effective use of space, it is necessary for a single antenna to utilize a plurality of different frequencies, and in addition, the combined use of the polarized waves will further improve its function.
FIG. 1
is a plan view showing a conventional two-frequency array antenna disclosed by Naohisa Goto and Kazukimi Kamiyama, “Directivity of Dual Frequency Co-Planar Array Antenna” (Technical Report A.P81-40 of the Institute of Electronics, Information and Communication Engineers of Japan, Jun. 26, 1981).
FIG. 2
is a partial view of the array antenna seen looking normally to the A—A line of FIG.
1
. In
FIGS. 1 and 2
, the reference numeral
101
designates a ground conductor;
102
designates a dipole antenna that operates at a relatively low frequency f
1
;
103
designates a feeder for feeding the dipole antenna
102
;
104
designates a dipole antenna that operates at a relatively high frequency f
2
; and
105
designates a feeder for feeding the dipole antenna
104
. Thus arranging the dipole antenna
102
with a resonant frequency f
1
and the dipole antenna
104
with a resonant frequency f
2
on the same ground conductor
101
enables the two-frequency antennas to share the aperture. Here, although the description is made taking an example of the two-frequency array antenna for convenience sake, a multi-frequency array antenna, which is constructed by arranging three or more dipole antennas with different frequency characteristics on the same ground conductor, has an analogous configuration.
Next, the operation of the conventional antenna will be described.
The dipole antenna has a rather wideband characteristic with a band width of 10% or more. To achieve such a wide bandwidth, however, it is necessary for the height from the ground conductor to the dipole antenna to be set at about a quarter wavelength of radio waves or more. Besides, since the dipole antenna forms its beam by utilizing the reflection on the ground conductor, when the height to the dipole antenna is greater than the quarter wavelength, it has a radiation pattern whose gain is dropped at the front side. Therefore, it is preferable that the height from the ground conductor to the dipole antenna be set at about a quarter of the wavelength of the target radio waves. Furthermore, as the feeders
103
and
105
for feeding the dipole antennas, a twin-lead type feeder or coaxial line is usually used. Constructing the dipole antennas using a printed circuit board consisting of a dielectric board enables the twin-lead type feeder to be formed on the printed circuit board, offering an advantage of being able to obviate soldering and to facilitate its fabrication.
As for the foregoing array antenna comprising the dipole antennas
102
and
104
working at the frequencies f
1
and f
2
, respectively, the two dipole antennas
102
and
104
are disposed at the heights different from the ground conductor
101
: The dipole antenna
104
operating at the relatively high frequency f
2
is placed closer to the ground conductor
101
than the dipole antenna
102
operating at the relatively low frequency f
1
. Furthermore, it is necessary for the array antenna to have such element spacing that can prevent grating lobes at respective operating frequencies. Since the element spacing of the dipole antenna
102
working at the frequency f
1
differs from that of the dipole antenna
104
working at the frequency f
2
, their adjacent elements are disposed not to be overlaid on each other, to obtain the two-frequency characteristics.
With the foregoing configuration, the conventional array antenna has the following problems when it uses two frequencies. First, since the dipole antenna operating at the relatively low frequency f
1
is greater in size than the dipole antenna operating at relatively high frequency f
2
, the former hinders the operation of the latter. In addition, radio waves which are radiated from the latter will induce excitation current in the former when they are coupled with the former, thereby causing reradiation. Thus, another problem arises in that the radiation directivity of the dipole antenna operating at the frequency f
2
is disturbed by the effect of the dipole antenna operating at the frequency f
1
. Here, the disturbance of the radiation directivity of the dipole antenna operating at the frequency f
2
appears periodically depending on the spacing between the dipole antennas operating at the frequency f
1
. The periodic disturbance causes the grating lobes in the array radiation directivity as illustrated in FIG.
3
.
It is possible to reduce the disturbance of the radiation directivity of the dipole antenna operating at the frequency f
2
caused by the reradiation, by disposing the dipole antenna operating at the frequency f
2
over the dipole antenna operating at the frequency f
1
. In this case, however, since the height from the ground conductor becomes greater than a quarter of the wavelength of the radio waves of the operating frequency f
2
, there arises another problem in that the gain at the front of the antenna is reduced, and that null points, which are brought about by the reflection on the ground conductor in wide-angle directions, result in large distortion in the radiation directivity.
The present invention is implemented to solve the foregoing problems. Therefore an object of the present invention is to provide a multi-frequency array antenna that can reduce the degradation in the radiation directivity of the dipole antenna operating at the relatively high frequency when two frequencies share the aperture in common by weakening the effect of the dipole antenna operating at the relatively low frequency on the dipole antenna operating at the relatively high frequency.
DISCLOSURE OF THE INVENTION
According to one aspect of the present invention, there is provided a mult
Katagi Takashi
Nishimura Toshio
Nishizawa Kazushi
Ohmine Hiroyuki
Clinger James
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Wong Don
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