Flat antenna for circularly-polarized wave

Communications: radio wave antennas – Antennas – Microstrip

Reexamination Certificate

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Details

C343S895000, C343S846000

Reexamination Certificate

active

06369762

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a compact, flat antenna for a circularly-polarized wave which is suitable for use in surface mounting. More particularly, the present invention relates to a compact plane antenna for a circularly-polarized wave which enables transmission and reception of a circularly-polarized wave without use of a special feeding circuit.
A dominant-mode patch antenna is often used as a flat antenna of this type for a circularly-polarized wave. As shown in
FIGS. 7A and 7B
, the antenna of this structure comprises a ceramic substrate
21
serving as a dielectric substrate, and a patch antenna element
22
provided on the surface of the ceramic substrate
21
. Further, a ground conductor
23
is provided on the side of the ceramic substrate
21
opposite to the side where the patch antenna element
22
is disposed. A feeding pin
24
is connected to a feeding section
22
a
provided on the reverse side of the patch antenna element
22
, by way of a through hole formed in the ceramic substrate
21
and that formed in the ground conductor
23
. In principle, in the dominant-mode patch antenna, two sides A, which are orthogonal to each other within a plane, must be formed to an electrical length of substantially ½ wavelength. In order to make the dominant-mode patch antenna compact, a dielectric substrate having a large dielectric constant must be used as the dielectric substrate
21
. Provided that the relative dielectric constant of the dielectric substrate
21
is taken as &egr;
r
, the length of one side A can be shortened in proportion to &egr;
r
−½
. For example, a GPS vehicle-mounted receiving terminal has been reduced to about one-fifth the size of a receiving terminal which is embodied without use of a substrate of high dielectric constant (one side is reduced to a size of about 20 to 25 mm).
However, in applications involving use of a small communications device, such as a portable receiving terminal, demand has existed for a smaller and less weight antenna whose size is about one-quarter that of a recently-available antenna (which has a side of about 5 mm and is one-twentieth the size of an antenna not provided with a substrate having a high dielectric constant). Even when an attempt is made to embody such an antenna by means of increasing the dielectric constant of the substrate in the manner as mentioned above, difficulty is encountered in matching a resonance frequency, due to limitations imposed by manufacturing technology. Producing a high-dielectric substrate having &egr;
r
of 100 or more from low-loss material is difficult, because of limitations imposed by material.
An inverted-F-type antenna has hitherto been known as a related technique for miniaturizing a flat antenna for a linearly-polarized wave. The inverted-F-type antenna is formed as follows: Of two segments of an L-shaped conductive line (or plate), an open end of a shorter segment is grounded, and a longer segment is situated in parallel with the ground. Further, a feeding segment serving as a third conductor is placed substantially in parallel with and spaced apart from the shorter segment so as to satisfy requirements for impedance matching, as required. The feeding segment is formed from a conductive line (or plate) for connecting a power feeding point and the longer segment.
As shown in
FIGS. 8A and 8B
, a conductive film is formed so as to extend from one surface to a side surface of the substrate
21
, thereby forming an L-shaped conductor (radiator element
22
). The open end of the conductive film provided on the side surface (serving as a shorter segment) is connected to a ground conductor
23
provided on the reverse side of the substrate
21
. A feeding pin
24
is connected to a feeding section
22
a
of the radiator element
22
by way of a through hole formed in the substrate
21
and that formed in the ground conductor
23
. In this structure, a longer side B of the conductive film (i.e., the radiator element
22
) provided on one surface of the substrate
21
can in principle be formed so as to assume an electrical length of substantially ¼ wavelength. The F-shaped antenna can be reduced to half the length of the dominant-mode patch antenna.
As mentioned above, a very compact antenna for a circularly-polarized wave required for a recent portable terminal, such as a small communications device, cannot be formed from the dominant-mode patch antenna.
On the other hand, the inverted-F-type antenna is to be used for a linearly-polarized wave. If an attempt is made to construct an antenna for a circularly-polarized wave from an inverted-F-type antenna, two independent inverted-F-type antenna elements must be provided orthogonal to each other within a plane. Furthers there must be employed a special feeding circuit, such as a
3
d
B hybrid link, for making the amplitudes of feed signals to be sent to the antenna elements equal, as welt as for making the phase of one feed signal orthogonal to that of the other feed signal. As a result, the antenna cannot be made compact, which in turn imposes an impediment to productivity.
SUMMARY OF THE INVENTION
The present invention has been conceived to solve the drawbacks of the related antennas, and it is therefore an object of the invention to provide a very compact antenna for a circularly-polarized wave which can be mounted on a compact portable terminal, such as a compact communications device.
Further, it is another object of the invention to provide a compact antenna for a circularly-polarized wave to be used with a compact communications device, the antenna being able to transmit and receive both a right-handed circularly-polarized wave and a left-handed circularly-polarized wave.
In order to achieve the above objects, according to the present invention, there is provided an antenna for a circularly-polarized wave comprising:
a ground conductor plane;
an excitation electrode provided substantially in parallel with the ground conductor plane;
a pair of electrodes for radiating a linearly-polarized wave which are provided substantially in parallel with the ground conductor plane, with the excitation electrode interposed therebetween; and
a feeding section electrically connected to the excitation electrode,
wherein first ends of the respective radiation electrodes oppose to the excitation electrode, thereby constituting capacitive coupling; and wherein second ends of the respective radiation electrodes are connected to the ground conductor plane such that the directions in which electric fields are to be excited become substantially orthogonal to each other.
In this configuration, two electrodes for respectively radiating a linearly-polarized wave are constructed so as to cause excitation independently of the excitation electrode in a non-contact manner. Hence, the two radiation electrodes can be simultaneously excited so as to be electrically independent of each other. Hence, the compact antenna can transmit and receive a circularly-polarized wave without involvement of equal distribution of power or without use of a special feeding circuit for realizing a 90-degrees phase shift.
Preferably, an electrical length of each of the radiation electrodes should be substantially quarter wavelength of a desired frequency band. If each of the radiation electrodes is provided so as to extend from the first end to the second end in a meandering manner, a longer electrical length per unit dimension can be ensured, thereby rendering the radiation electrodes more compact.
Alternatively, each of the radiation electrodes extending from the first end may be folded at least once toward the first end so as to be able to cause resonation at two frequency bands. In this configuration, the antenna can transmit and receive signals of two frequency bands: that is, a signal of first frequency and a signal of second frequency which is about double the first frequency band.
Preferably, a dielectric member is interposed between the ground conductor plane, the excitation electrode, and the radiation electr

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