Communications: radio wave antennas – Antennas – With radio cabinet
Reexamination Certificate
1999-11-05
2002-09-17
Wimer, Michael C. (Department: 2821)
Communications: radio wave antennas
Antennas
With radio cabinet
C343S767000
Reexamination Certificate
active
06452554
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an antenna element suitable for radio systems such as portable radio communications apparatuses using microwaves, quasi microwaves or millimeteric waves, and particularly to an antenna element that is to be mounted in microwave radio communications apparatuses for an intelligent transport system (ITS), etc. such as vehicle information and communications systems (VICS) and electronic toll collection systems (ETC), and further to a radio communications apparatus comprising such an antenna element.
PRIOR ART
According to recent demand of miniaturization and cost reduction of microwave radio communications apparatuses, there is strong demand to reduce the size of antennas mounted in microwave radio communications apparatuses. For instance, used in cellular phones are generally monopole antennas that are retractable in bodies of cellular phones. From the aspect of improvement in portability, further miniaturization and reduction in weight of antennas and the mounting of antennas in bodies of phones are desired.
Widely used as built-in antennas are conventionally inverted F-type antennas and micro-strip antennas that are constituted by monopole antennas bent in parallel with a ground for miniaturization and reduction in thickness. However, because an antenna of this type utilizes a phone body as a ground, the dimension of a phone body affects the radiation directionality of an antenna, and part of electric current induced in a phone body by the radiation of electromagnetic waves from the antenna flows into a hand of a person holding the phone. Also, because sufficient bandwidth and gain are not obtained, the overall size of the antenna should be large to obtain bandwidth and gain almost comparable to those of the monopole antenna, whereby it cannot easily be installed in small radio communications apparatuses such as recent cellular phones.
Thus, pole antennas are disadvantageous in inconvenience and restriction in the freedom of design. Therefore, a coaxial resonant slot antenna having a structure in which a strip conductor is disposed in an internal space of a flat box-shaped conductor cubic in an insulated manner so that it is operable in a transverse electromagnetic mode (TEM) was proposed (U.S. Pat. No. 5,914,693). The structure of this slot antenna is shown in
FIGS. 50
(
a
) and (
b
). This slot antenna is constituted by bonding an insulating substrate
501
a
having a slot
503
formed by the pattern etching of a conductor layer
502
to an insulating substrate
501
b
having a strip conductor layer
504
formed by the etching of a conductor layer.
When transmission is carried out by this coaxial resonant slot antenna, a high-frequency signal supplied from a feeder flows through a strip conductor layer
504
to the slot
503
, from which it is radiated to the sky by a resonance phenomenon of the slot
503
. Also, in the case of reception, an electromagnetic wave (received signal) introduced into the conductor cubic through the slot
503
progresses through the strip conductor layer
504
in a direction opposite to the above direction and picked up by a feeder as a high-frequency signal.
However, because this coaxial resonant slot antenna has a structure in which an insulating substrate
501
a
having a slot
503
is bonded to an insulating substrate
501
b
having a strip conductor layer
504
, an electromagnetic coupling coefficient is susceptible to variation due to relative displacement between the slot
503
and the strip conductor layer
504
that is likely to occur in their bonding step, resulting in large variation in a resonance frequency and a voltage-standing wave ratio (VSWR) representing the condition of impedance matching. To reduce variation in VSWR, the insulating substrates
501
a
,
501
b
, the slot
503
and the strip conductor layer
504
should be formed at high precision, and the insulating substrates
501
a
and
501
b
should also be bonded precisely, resulting in complicated production processes.
In the case of mounting a slot antenna, an apparent impedance of the antenna varies by a floating capacitance between a ground pattern in contact with the slot antenna and a body of a microwave radio communications apparatus. Accordingly, impedance matching should be achieved between the slot antenna and the feeder system. However, because the above coaxial resonant slot antenna has a structure in which the strip conductor layer is disposed in a conductor cubic, the impedance matching cannot easily be achieved. In addition, because the coaxial resonant slot antenna should be designed in a manner matching with the shapes of the board and body of the microwave radio communications apparatus, there arises a problem of extremely increased production cost in cellular phones, etc. whose specifications are frequently modified.
In addition to the above coaxial resonant slot antenna, there is a rectangular hollow slot antenna having a shape shown in
FIG. 51
(see “Antenna Engineering Handbook,” page 89). This rectangular hollow slot antenna comprises a slot
3
on an upper surface of a first flat conductor
2
, and high-frequency electric power terminals OSCs at both ends of the slot
3
, and OSCs serve to receive a signal and emit radio waves.
The specifications required for the built-in antennas as described above depend on systems in which they are used. For instance, in US-PCS (the United States) or K-PCS (Korea) utilizing a cellular phone system of 1.9 GHz band (2 GHz band in the global standard CDMA system), frequency bandwidths shown in Table 1 below are necessary.
TABLE 1
Frequency
US-PCS
K-PCS
Specifications
(bandwidth)
(bandwidth)
Transmission
1850-1910
1750-1780
Frequency
MHz (60 MHz)
MHz (30 MHz)
Reception
1930-1990
1840-1870
Frequency
MHz (60 MHz)
MHz (30 MHz)
However, the rectangular hollow slot antenna as shown in
FIG. 51
does not meet the above specifications of antennas. Also, because a wider bandwidth is more effective to prevent the deterioration of performance due to the variation of use environment in a small antenna mounted in a cellular phone, it is important to expand the bandwidth of the antenna. The principle for expanding bandwidth is as follows. At a bandwidth Bw, the following equation is met.
Bw
∝−1
/Q
(1).
Thus, the smaller the Q, the more the bandwidth Bw expands. Also, at a radiation efficiency &eegr;, the following equation is met.
&eegr;=1/(1
+Qr/Qi
) (2).
wherein Qi=Qc+Qd, Qc and Qd are the values of Q by conductor loss and dielectric loss, respectively, and Qr is the value of Q by radiation. Therefore, when Qr is small, the radiation efficiency &eegr; is large.
Thus, the element should have a small Q to expand the bandwidth, and Qr should be small to increase the radiation efficiency &eegr;. For instance, in the case of an antenna for a cellular phone, the bandwidth of 20 MHz is needed.
Q∝&ohgr;C
(3),
C=a·&egr;
r
(4),
wherein &ohgr; is an angular frequency, C is capacitance, a is a constant determined by the antenna shape, and &egr; is a dielectric constant.
The following relation is satisfied from the equations (3) and (4).
Q∝&ohgr;·a·&egr;
r
(5).
It is known from the equation (5) that a material having a small dielectric constant should be used to keep the Q low.
Also, there is a relation between Qr and the thickness (height) of an antenna, which is
Qr
∝1
/t
(6),
wherein t is a thickness (height) of an antenna. Thus, the antenna should be made thick to decrease Qr.
Further, the slot antenna having a shape shown in
FIG. 51
is disadvantageous in that power of radio waves emitted from the antenna (radiation gain) is small. Thus, in the conventional slot antenna, a material having a small dielectric constant such as a glass-filled epoxy resin, Teflon, etc. is used to decrease the Q, thereby increasing the bandwidth. Here, assuming that the slot shown in
FIG. 51
has a length L, the following relations are satisfied.
L
=&lgr;/2&emsp
Aoyama Hiroyuki
Fukamachi Koji
Kawamura Toshimasa
Okabe Hiroshi
Takei Ken
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Hitachi Metals Ltd.
Wimer Michael C.
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