Communications: radio wave antennas – Antennas – Plural separate diverse type
Reexamination Certificate
2001-11-06
2003-02-18
Le, Hoanganh (Department: 2821)
Communications: radio wave antennas
Antennas
Plural separate diverse type
C343S725000, C343S866000, C343S797000
Reexamination Certificate
active
06522302
ABSTRACT:
TECHNICAL FIELD
The present invention relates to circularly polarized antennas.
BACKGROUND ART
With proliferation of the use of communications satellites in recent years, there is a growing demand for circularly polarized antennas having good axial ratio characteristics and a hemispherical radiation pattern.
Conventionally, cross dipole antennas as shown in
FIG. 21
have been used as one typical form of circularly polarized antennas.
Referring to
FIG. 21
, designated by the numerals
12
a
,
12
b
,
12
c
,
12
d
are elements of cross dipoles. The elements
12
a
,
12
b
are fed from an excitation source
13
a
while the elements
12
c
,
12
d
are fed from an excitation source
13
b
, wherein there is a 90° difference in exiting phase between the two excitation sources
13
a
,
13
b
. The directions of the elements
12
a
-
12
b
and the elements
12
c
-
12
d
intersect each other at right angles. Accordingly, this cross dipole antenna produces circularly polarized waves in a direction perpendicular to a plane containing the two dipoles.
The aforementioned cross dipole antenna produces circularly polarized waves in its frontal direction (the direction perpendicular to the plane containing the two dipoles). However, the waves gradually become elliptically polarized waves toward sideways directions, and become linearly polarized waves on the plane containing the two dipoles.
As another typical circularly polarized antenna, a four-wire fractional winding helical antenna as shown in
FIG. 22
is also used conventionally. Referring to
FIG. 22
, designated by the numerals
22
a
,
22
b
,
22
c
,
22
d
are elements of the four-wire fractional winding helical antenna, and designated by the numerals #
1
to #
4
are feeding points at the terminal ends of the elements. In this example, the number of turns is 0.5, which means that each element is wound, with its length from one end to the other, around a cylindrical surface as much as half its circumference. With the four-wire fractional winding helical antenna having such a structure, left-handed circularly polarized wave is obtained when the elements are wound clockwise as viewed from their feeding points to terminal ends, whereas right-handed circularly polarized wave is obtained when the elements are wound in the opposite direction (counterclockwise). Further, the direction of radiation is determined by winding method of the elements and the relation of phase of feeding to the four feeding points.
Although the structure of the four-wire fractional winding helical antenna of this kind is more or less complicated as compared to the cross dipole antenna, it is possible to obtain a favorable axial ratio over a wide angle.
One typical example of a circularly polarized antenna is a conical log spiral antenna. This antenna has spiral-shaped elements arranged on a conical surface. A four-wire conical log spiral antenna, for example, has a number of parameters due to its structure and can create various forms of radiation directivity by the choice of these parameters. As such, the four-wire conical log spiral antenna exhibits almost the same characteristics as the aforementioned four-wire fractional winding helical antenna.
In the aforementioned four-wire conical log spiral antenna and conical log spiral antenna, however, the hand of polarization (right-handed or left-handed) of the circularly polarized wave is determined by the winding direction of the elements unlike the cross dipole antenna, so that it has been extremely difficult to electrically switch the hand of polarization.
When transmitting and receiving circularly polarized waves having different hands of polarization at the same or nearby frequencies, for example, it has been necessary to provide separate antennas dedicated exclusively to the right-handed and left-handed circularly polarized waves.
Also for a recent satellite-based mobile communications antenna, a compact antenna smaller than the currently available four-wire fractional winding helical antenna and conical log spiral antenna is required.
Although it is possible to decrease the overall physical size of either the four-wire fractional winding helical antenna or the conical log spiral antenna by reducing the number of turns, there has been a problem that an angular range in which a specific axial ratio can be maintained decreases in exchange for the reduction in antenna size.
It is an object of the invention to provide a circularly polarized antenna having a favorable axial ratio over a wide angle despite its compactness.
It is another object of the invention to provide a circularly polarized antenna which makes it possible to electrically switch the hand of polarization.
DISCLOSURE OF THE INVENTION
A circularly polarized antenna of this invention comprises a loop-shaped element whose perimeter is approximately equal to the wavelength of radiated radio wave, and four elements extending upward from the loop-shaped element whose terminal ends or points near the terminal ends at one side are connected to the loop-shaped element at its four equally dividing points, feeding points being provided at the opposite terminal ends of the four elements, and the length of each of the four elements being approximately equal to half the wavelength of the radiated radio wave.
FIGS. 1A-1C
are diagrams showing an example of the aforementioned circularly polarized antenna. In
FIG. 1A
, designated by the numeral
1
is a loop-shaped element and designated by the numerals
2
a
-
2
d
are first to fourth elements. Also, designated by #
1
-#
4
are feeding points of the individual elements
2
a
-
2
d
. As depicted in
FIG. 1B
, excitation sources
3
a
and
3
b
are connected to the feeding points #
1
-#
2
which serve as first balanced feeding points and to the feeding points #
3
-#
4
which serve as second balanced feeding points, respectively. There is a phase difference of approximately 90° between currents fed from these excitation sources
3
a
,
3
b
. In this example, the upper ends of the elements are used as the feeding points.
FIGS. 1A and 1B
show examples in which one end of each of the first to fourth elements
2
a
-
2
d
is connected to a corresponding one of four equally dividing points of the loop-shaped element and the feeding points are provided at the other ends.
FIG. 1C
shows an example in which the loop-shaped element
1
is connected points close to ends of the elements.
The circularly polarized antenna having such a structure exhibits characteristics generally equivalent to a four-wire fractional winding helical antenna or a conical log spiral antenna due to its operational effects described below.
Specifically, by exhibiting the operational effects equivalent to the four-wire fractional winding helical antenna or the conical log spiral antenna, the present invention achieves antenna characteristics equivalent to those antennas and, yet, solves drawbacks of the four-wire fractional winding helical antenna or the conical log spiral antenna.
FIGS. 2A-2C
show current distributions on two elements of four-wire fractional winding helical antennas, of which
FIG. 2A
is a current distribution diagram showing a state in which paired two of the four elements fed by one excitation source are extended in a linear form. Here, the one element is expressed by 0.75&lgr; where &lgr; is the wavelength of the radiated radio wave.
FIG. 2B
is a side view showing a state in which the elements shown in
FIG. 2A
are wound in a helical form and
FIG. 2C
is a top view of the same. In this example, the number of turns is set to 0.5, which means that each element is wound as much as half the circumference of a cylindrical surface.
This invention configures a new antenna which exhibits approximately the same current distribution as that on the paired two elements as they are wound in a helical form.
Let us now focus on a current distribution on portions beneath the feeding points. In the current distribution on helically wound portions of the two elements, maximum current is observed at approximately
Furuno Electric Co. Ltd.
Le Hoang-anh
Pillsbury & Winthrop LLP
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