The present invention relates to circularly polarized antennas.
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 12a, 12b, 12c, 12d are elements of cross dipoles. The elements 12a, 12b are fed from an excitation source 13a while the elements 12c, 12d are fed from an excitation source 13b, wherein there is a 90xc2x0 difference in exiting phase between the two excitation sources 13a, 13b. The directions of the elements 12a-12b and the elements 12c-12d 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 22a, 22b, 22c, 22d 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.
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 2a-2d are first to fourth elements. Also, designated by #1-#4 are feeding points of the individual elements 2a-2d. As depicted in FIG. 1B, excitation sources 3a and 3b 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 90xc2x0 between currents fed from these excitation sources 3a, 3b. 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 2a-2d 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 0.75xcex where xcex 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 the middle of each element and the current flowing in these portions is considered to be important for antenna characteristics. Although the helically wound portions of the two elements are spaced at some distance apart, the distance between them (or the diameter of the helical shape) is sufficiently small compared to the wavelength and, therefore, it is assumed that the current on the portions beneath the feeding points can be approximated by the vector sum of currents flowing through the proximity of the maximum current points in the helically wound portions of the two elements. (To obtain a vector sum, the initial points of its two constituent vectors need to coincide with each other in principle.)
Therefore, to configure an antenna equivalent to the helical antenna formed by these two elements, it is preferable to provide an object through which a current of the same phase as the excitation source flows in the same direction as the current fed from the excitation source in the proximity of the maximum current points.
While the foregoing discussion has dealt with a case in which an antenna equivalent to the helical antenna is configured by the two elements, it is also possible to obtain a four-wire fractional winding helical antenna by approximation by providing two pairs of elements in such a way that the two pairs intersect each other at 90xc2x0 and by feeding them currents with a phase difference of 90xc2x0. What is important here is how to configure the aforementioned object. In this invention, the inventor considered a cross dipole antenna excited by excitation sources 3a, 3b as shown in FIG. 3A, and studied how to configure an object through which a current of the same phase as a current fed from the excitation source 3a flows in the same direction as the current flowing near the feeding point from the excitation source 3a, and through which a current of the same phase as a current fed from the excitation source 3b flows in the same direction as the current flowing near the feeding point from the excitation source 3b, at positions spaced downward from the feeding points by a specific distance and spaced horizontally by a distance equal to the radius of the helical shape.
The inventor has consequently reached an extremely simple structure for providing the aforementioned object. Specifically, as shown in FIG. 3B, a loop-shaped element whose perimeter is equal to wavelength xcex is employed and the feeding points are connected to four equally dividing points of the loop-shaped element by elements whose length is approximately equal to xcex/2 (half the wavelength).
FIG. 4 shows to which points of the aforementioned two elements 2a, 2b the loop-shaped element 1 should be connected.
In this Figure, the length of each of the four elements is made equal to 0.75xcex in the same way as in the case of the four-wire fractional winding helical antenna, where in current distribution on each element is shown by a thin line and voltage distribution is shown by a broken line. As can be seen from the Figure, points on the elements separated by 0.5xcex from the feeding points become equivalent short-circuit points. Since the input impedance of the loop-shaped element 1 is low, it is possible to achieve impedance matching if the loop-shaped element 1 is connected to the points separated by approximately 0.5xcex from the feeding points.
As the elements 2a, 2b constituting one element pair are not wound in a helical form but two points located opposite each other of the loop-shaped element 1 are connected to terminal parts of the elements 2a, 2b, a current flows through the loop-shaped element 1 in the same direction as the current flowing near the feeding point from the excitation source 3a. Moreover, since the distance from the feeding points of the elements 2a, 2b to the connecting points of the loop-shaped element is made approximately equal to half the wavelength, a current of the same phase as the current fed from the excitation source 3a flows in the loop-shaped element.
FIG. 4 shows to which points of the elements 2a, 2b the loop-shaped element 1 should be connected. Portions of the elements 2a, 2b from their points connected to the loop-shaped element 1 up to their extreme ends are not necessary for feeding currents to the loop-shaped element 1. Since the currents flowing in the aforementioned portions are oppositely directed, these portions are rather useless for the antenna. Thus, the elements 2a, 2b would be long enough if their length corresponds to the extension from the excitation source 3a to the points connected to the loop-shaped element 1 (approximately 0.5xcex).
In the case of the four-wire fractional winding helical antenna, the element length from the feeding point to the terminal end of each element is approximately 0.75xcex. In contrast, the element length from the feeding point to the terminal end of each element is approximately 0.5xcex in the aforementioned structure, so that the element length is reduced to about two thirds compared to the four-wire fractional winding helical antenna, resulting in a reduced overall antenna size.
While FIG. 4 shows how the elements 2a, 2b constituting one element pair are connected to the loop-shaped element, two points on the loop-shaped element offset from the aforementioned connecting points by 90xc2x0 in terms of rotational angle and electrical phase angle are connected to the terminal ends of the elements 2c, 2d constituting the other element pair as shown in FIG. 3B. As a result, a current having approximately the same phase as the phase of the current fed from the excitation source 3b flows in the same direction as the current flowing near the feeding point from the excitation source 3b. 
FIG. 5 shows changes with time of the direction of the current flowing through the aforementioned loop-shaped element. While distribution of the current flowing through the loop-shaped element whose impedance is matched to that of the aforementioned four elements is not necessarily clear, it is expected that the direction of the current cyclically varies with time according to cycles of a transmitting signal as illustrated in FIG. 5.
The circularly polarized antenna of the invention further comprises a reflector plate provided at a position separated from the aforementioned loop-shaped element by a specific distance, the reflector plate being disposed parallel to the loop-shaped element.
With this structure, radio wave having an opposite rotating direction radiated from the feeding points toward the loop-shaped element is reflected by the reflector plate and radiated back as a circularly polarized wave having a specific rotating direction. This helps eliminate directivity in undesired directions and increase gain in a specific direction.
The circularly polarized antenna of the invention further comprises baluns connected to the aforementioned feeding points for performing mode conversion between unbalanced transmission mode and balanced transmission mode. With this structure, electric power can be fed by use of the baluns.
In the circularly polarized antenna of the invention, the aforementioned baluns are formed on a reverse side of the aforementioned reflector plate. This makes it easier to, configure the baluns in a broad area separated from the four elements and to feed electric power to the feeding points in the balanced transmission mode.
The circularly polarized antenna of the invention further comprises a first substrate on which a conductor pattern constituting parts of the aforementioned four elements is formed, a second substrate disposed parallel to the first substrate with a conductor pattern constituting the aforementioned loop-shaped element formed on the second substrate near its outer periphery, and a cylindrical substrate joining the first and second substrates to each other with a conductor pattern constituting the remaining parts of the aforementioned four elements formed on the cylindrical substrate. Alternatively, the aforementioned loop-shaped element is provided on the aforementioned cylindrical substrate and not on the second substrate.
By configuring the aforementioned individual elements by the first and second substrates and a cylindrical substrate, it becomes easier to configure the individual elements and to retain them in specific shapes.
In the circularly polarized antenna of the invention, the aforementioned baluns are provided on the aforementioned first substrate. This serves to facilitate the manufacture of the baluns and decrease variations in their characteristics.
The circularly polarized antenna of the invention may comprise a plurality of substrates standing in approximately a vertical position and intersecting one another with the aforementioned four elements configured by conductor patterns formed on these substrates. This makes it easier to configure the four elements and retain them in specific shapes.
The circularly polarized antenna of the invention may comprise a flexible substrate on which a beltlike conductor pattern is formed or a beltlike metal plate for sequentially joining edges of the aforementioned multiple substrates and configuring the aforementioned loop-shaped element. This makes it easier to configure the loop-shaped element and simplifies the structure for keeping it in a specific shape.