Communications: radio wave antennas – Antennas – With radio cabinet
Reexamination Certificate
2000-02-25
2002-02-12
Wong, Don (Department: 2821)
Communications: radio wave antennas
Antennas
With radio cabinet
C343S821000
Reexamination Certificate
active
06346916
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-051462, filed Feb. 26, 1999, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
The present invention relates to an antenna apparatus mainly used for a portable radio device and a radio device using the same.
A portable radio device, such as a portable telephone, a PHS terminal and a small radio base station, is often integral with an antenna (or a feed point is proximate to a housing). It is required to include an antenna in a main body of a radio device such as a plastic cover to prevent breakdown when the antenna of a portable telephone or a PHS terminal falls or to resist breakdown due to weather in case of the antenna at the radio base station.
Conventionally, an inverted F-type antenna is often used as one included in a portable radio device.
FIG. 1
shows the configuration of a portable radio device employing a conventional inverted F-type antenna. An inverted F-type antenna
103
is disposed (protruded) on a metal housing
101
which includes a radio device circuit
102
consisting of a radio circuit and a signal processing circuit, and which also serves as a shield. The metal housing
101
is disposed in a plastic cover which is not shown in
FIG. 1. A
feed point
103
a
is provided at the metal housing
101
. As can be seen in this example, the inverted F-type antenna
103
is low profile and small in size. It has an advantage in that good radiation characteristics can be obtained despite its proximity to the housing
101
.
Normally, the performance of the built-in antenna itself tends to deteriorate since the antenna is required to be smaller in size and thinner. As shown in
FIG. 1
, if the inverted F-type antenna
103
is employed, the metal housing
101
is used as part of the antenna, thereby making it possible to compensate for the deterioration of the antenna performance and to, therefore, enhance it.
In this way, while the inverted F-type antenna
103
is good in antenna performance, it has a disadvantage in that it tends to receive high frequency noise leaked from the radio circuit section or the signal processing circuit of the radio device circuit
102
. It is actually difficult to completely shield the radio device circuit
102
with the metal housing
101
with the configuration shown in FIG.
1
and leaked noise inevitably exists on the housing
101
. Besides, because of employing the metal housing
101
as part of the antenna in case of
FIG. 1
, the leaked noise is directly received by the inverted F-type antenna
103
, resulting in the great deterioration of communication quality. In recent years, in particular, the processing speed of the signal processing circuit increases and the difference between the radio communication frequency and the frequency of the leaked noise decreases, so that the deterioration of the communication quality due to the leaked noise from the signal processing circuit causes a grave problem.
To decrease such an influence of the leaked noise, there is proposed employing a dipole antenna
104
for a portable radio device as shown in FIG.
2
. As already known, the dipole antenna does not need a ground. It is, therefore, unnecessary to directly connect the dipole antenna
104
to the metal housing
101
serving as a ground. Owing to this, even if leaked noise exists on the metal housing
101
, it is possible to prevent the noise from directly flowing into the dipole antenna
104
.
Nevertheless, even with the dipole antenna, a problem inevitably arises if it is practically used as an antenna built in a portable radio device. If the dipole antenna is included in the plastic cover of the portable radio device, the antenna is disposed proximate to the metal housing
101
, thereby disadvantageously deteriorating antenna performance. Generally, the dipole antenna exhibits best performance when arranged in a free space where nothing is present around the antenna. Thus, if the metal housing
101
is provided near the antenna, the antenna performance deteriorates. This is because the radiation power of the antenna decreases, i.e., matching loss occurs if the dipole antenna
104
is put closer to the metal housing
101
.
FIG. 3
shows the calculation results of the matching loss of the dipole antenna made by the inventors of the present invention. In
FIG. 3
, the horizontal axis indicates a state in which an antenna is put and the vertical axis indicates matching loss. The matching loss is one which is generated when a feed line does not match with the antenna in impedance. If so, radiation power from the antenna decreases and communication quality deteriorates.
FIG. 3
shows calculation results of a case where the dipole antenna exists in a free space without a metal housing and a case where a metal housing is provided in the vicinity of the dipole antenna. If the dipole antenna exists in a free space, matching loss is as small as 0.2 dB and the antenna exhibits excellent characteristic. If the metal housing is provided in the vicinity of the dipole antenna, the matching loss increases to about 8.5 dB and the antenna characteristic clearly, greatly deteriorates.
Measures to improve the input characteristics of such a dipole antenna were already taken and an antenna known as a T-matching antenna was contrived (see “Antenna Engineering Handbook”, The Institute of Electronics, Information and Communication Engineers edition, pp. 114-115, 1980).
FIG. 4
shows a T-matching antenna. The T-matching antenna has a structure in which a short-circuit element
113
which causes a shortcircuit between quarter wavelength elements
111
and
112
is added to a dipole antenna
110
consisting of the two quarter wavelength element
111
and
112
. The short-circuit element
113
functions as an antenna impedance matching element, whereby even if the dipole antenna
110
is disposed proximate to the metal housing
101
as shown in
FIG. 5
, good antenna characteristics can be obtained.
The right of
FIG. 3
shows matching loss if the dipole antenna which has been T-matched as stated above in the vicinity of the metal housing. Although there is a metal housing in the vicinity of the antenna, the matching loss is as small as 0.5 dB and the antenna exhibits good characteristics.
The above consideration has been given to the characteristics of the dipole antenna which has been T-matched without regard to the influence of a feed line connecting the antenna to a radio device circuit. Actually, however, it is necessary to take account of the presence of the feed line. If the feed line exists, leaked noise is transferred from the radio device circuit to the feed line and finally to the antenna, possibly damaging communication quality. To prevent this, there is proposed a ferrite core for connecting a personal computer (PC) to a display. The ferrite core has, however, relatively high capacity and is not suited to be used as the feed line of a built-in antenna.
To prevent leaked noise from being transferred from the radio device circuit to the antenna without using such a ferrite core, there is proposed arranging the dipole antenna
110
which has been T-matched in the vicinity of the metal housing
101
, pulling out a feed line
114
from the surface of the metal housing
101
and putting the feed line
114
in parallel to the metal housing
101
in an electrically non-contact state as shown in FIG.
5
.
Upon so constituted, if the length of the feed line
114
which has been branched from the metal housing
101
is set at a quarter wavelength, two short-circuit parallel lines are formed by the feed line
114
on the metal housing
101
and the image of the feed line
114
as shown in FIG.
5
. The impedance of the two short-circuit parallel lines of a quarter wavelength viewed from a feed point is quite high. Thus, even if the current of the leaked noise flows on the feed line (particularly on the outer conductor of a coaxial feed line), the leaked noise curr
Odachi Noriaki
Sekine Syuichi
Kabushiki Kaisha Toshiba
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Tran Chuc
Wong Don
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