JP2004254168A - Helical antenna - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a helical antenna which has solved the problems that generally the antenna is two wavelengths or higher, and that there are restrictions on the height of the antenna and its mechanical strength is weak, has reduced its height to lower its posture, and has a radiation characteristic of a good axial ratio. <P>SOLUTION: To make the height and posture of the antenna low, its pitch angle α is reduced so as to satisfy 4°<α<9°. An axial ratio property deteriorates by making the pitch angle smaller. To improve this point, the tip of the helical antenna is made to be in the shape of a tapered cone, keeping the pitch angle small. Consequently, a trouble of a conventional antenna that the pitch angle α is required to be 12°<α<14° is prevented to radiate a wave having a circularly-polarized wave property of an axial mode. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an axial mode helical antenna that emits circularly polarized waves.
[0002]
[Prior art]
Helical antennas are widely used as antennas for radiating circularly polarized waves, such as antennas mounted on satellites and antennas for mobile communication terminals. As the helical spiral, there are one-wire, two-wire, and four-wire windings, and there are various excitation modes. As a basic configuration of a helical antenna, there is a helical antenna in which power is fed from one wire end of a single spiral wire and a spiral pitch angle is α, and several turns are wound at a constant pitch α. This configuration is an axial mode helical antenna. It is known that in order to efficiently radiate axial mode circularly polarized waves, it is necessary to set one circumference to about one wavelength and 12 degrees <α <14 degrees. (For example, see Non-Patent Document 1).
[0003]
[Non-patent document 1]
H. Nakano, Y .; Samada, and J.M. Yamauchi, "Axial Mode Helical Antennas", IEEE Trans. Antennas Propagagat. , Vol. AP-34, pp. 1144 1986.
[0004]
[Problems to be solved by the invention]
In order to radiate circularly polarized waves with a good axial ratio and obtain high gain with a helical antenna, it is necessary to increase the number of helical turns. Therefore, compared with other circularly polarized antennas, there are problems that the antenna height is relatively high, the antenna volume is large, and the mechanical strength is weak.
[0005]
An object of the present invention is to solve the above-described problems and to obtain a helical antenna with a reduced attitude.
[0006]
[Means for Solving the Problems]
A helical antenna according to the present invention includes a spirally wound first helical conductor having substantially the same diameter and a tapered spirally wound first helical conductor connected to one end of the first helical conductor. 2. A helical antenna comprising a helical conductor of No. 2 and a metal ground plate provided with a power supply connector, wherein the spirally wound helical conductor has a pitch angle α of 4 <α <9 and has the tapered shape. The number of turns of the spirally wound helical conductor is three or more.
[0007]
BEST MODE FOR CARRYING OUT THE INVENTION
Embodiment 1 FIG.
FIG. 1 is a schematic configuration diagram showing Embodiment 1 of the present invention.
There are various types of circularly polarized wave excitation antennas other than the helical antenna, but generally a separate feed circuit for circularly polarized wave excitation is required, which complicates the antenna configuration. The helical antenna does not particularly require a feed circuit for exciting circularly polarized waves, and thus has a feature that the antenna configuration can be extremely simplified.
[0008]
The helical antenna can excite circularly polarized waves by making a wire conductor constituting a helical spiral, and appropriately selecting the spiral wire specifications in a certain excitation mode so as to obtain a desired radiation pattern. The excitation mode changes with the diameter D of the helical antenna, the pitch angle α, and the number of turns N. An axial mode is a mode for radiating circularly polarized waves along the central axis of the helical antenna toward the tip.
[0009]
The axial mode antenna configuration will be described below.
A power supply connector 11 for forming a power supply unit is provided on a certain finite metal ground plane 1. The power supply connector 11 uses a coaxial connector, and the inner conductor 2 of the connector extends substantially vertically from the ground plane. .
[0010]
A line conductor is connected to the inner conductor 2, the ground plane 1 is used as a ground, and a transmission line 3 is formed with the line conductor. A wire conductor 4 constituting a helical antenna is connected to a tip of the transmission line 3. The transmission line 3 is a transmission line for transmitting power from the connector to the helical wire, and also serves as an impedance converter from a coaxial connector having a characteristic impedance of 50 ohms to a helical antenna having a characteristic impedance of about 180 ohms.
[0011]
Assuming that the pitch angle is α, the wire conductor 4 is spirally wound N turns with substantially the same diameter at an inclination of α. In the axial mode, in order to excite circularly polarized waves, it is known that good circular polarization in the axial mode is emitted by setting α to 12 degrees <α <14 degrees. Increasing the number of turns increases the gain and improves the axial ratio. The number of turns is generally about 10 to 15 turns.
[0012]
At this time, an axial ratio of 1 dB or less and a gain of 12 dBi or more are obtained.
However, there is a problem that the antenna height becomes about 2 to 3 wavelengths, and the antenna height becomes high.
For this reason, there is a problem that the height of the mounting location is restricted as compared with other circularly polarized antennas. Further, since the diameter D is as small as about 0.3 wavelength as compared with the height, there is a problem in that it is weak in mechanical strength and vibration. In particular, in the case of an antenna mounted on a satellite, the vibration conditions are severe, and reinforcement of the strength leads to an increase in weight.
[0013]
As a means for lowering the attitude, it is conceivable to reduce the number of turns or reduce α. Reducing the number of turns decreases the gain and the axial ratio, so that the characteristics of the helical antenna deteriorate. On the other hand, when α is reduced, the gain does not decrease so much, but the axial ratio is extremely deteriorated.
[0014]
FIG. 2 shows the axial ratio characteristics when α is a parameter and the number of turns is changed. As typical examples of α, three examples of α = 4, 7, and 13 degrees are shown. In the case of α = 13 degrees, which is a normal pitch angle, the axial ratio has about 10 turns, and a good axial ratio characteristic of approximately 1 dB or less is obtained.
[0015]
On the other hand, when α is reduced to 7 degrees or 4 degrees, it is understood that the axial ratio characteristics are not so much improved even if the number of turns is increased. In other words, this indicates that the operating conditions of the circularly polarized antenna in the axial mode are deviated.
[0016]
As a method for improving this point, in the present embodiment, the wire conductor is connected to the wire conductor 5 with substantially the same radius, and the wire conductor near the tip is wound into a cone-shaped tapered shape 6. The factor that degrades the axial ratio is reflection at the end of the wire conductor. Since this reflected component contributes to the radiation in the opposite direction to the radiation, it radiates a circularly polarized wave in reverse rotation, and the axial ratio is degraded.
[0017]
Therefore, the reflection from the tip can be reduced by making the tip end smooth and tapered. This taper shape has conventionally been studied for widening the frequency characteristics in the axial mode of 12 degrees <α <14 degrees, but no improvement in the axial ratio characteristics when α is small has been studied. .
[0018]
FIG. 3 shows the axial ratio characteristics when the tip is tapered. This is an axial ratio characteristic when the number of turns N1 = 5 at the same radius is fixed and the number of turns N2 in a tapered shape is changed. Since α = 7 degrees, N2 = 0, that is, when there is no taper, the axial ratio characteristic is as bad as about 5 dB.
[0019]
However, it can be seen that the axial ratio is improved by increasing N2, and the axial ratio can be reduced to 1 dB or less when N2 = 4 turns. It is important to smooth this taper when α is small. When the normal angle is 12 degrees <α <14 degrees, the effect of improving the axial ratio by the taper is small. Even when there is no taper, an axial ratio of 1 dB or less can be achieved, and even if the number of turns is set to 1 turn or 2 turns, the improvement effect is small, and 2 turns are sufficient.
[0020]
However, when α is reduced (here, when 4 <α <9 as described below), the number of turns N2 of this tapered shape is effective as a means for improving the axial ratio, and more than 3 turns (3 turns in FIG. 3). If the axial ratio is set to 1.5 dB and the number of turns is increased to 4 or more, it is understood that a good result can be obtained with an axial ratio of about 1 dB.) That is, when α is small, the point of improving the axial ratio is to form a gentle taper shape of three turns or more.
[0021]
As a result of obtaining the frequency bandwidth (1 dB or less) of the axial ratio when α is changed by a helical having a tapered tip of the wire, about 1% is obtained when α = 2 degrees, less than 10% when α = 4 degrees, and α = The bandwidth increases as α increases to about 30% at 7 degrees and about 45% at α = 13 degrees. When considering an application for communication, a bandwidth of less than 10% is often required in consideration of a transmission / reception band. From this point, α needs to be 4 degrees or more.
[0022]
On the other hand, if α increases, the bandwidth increases, but the antenna height also increases. As a simple calculation, since the height of one turn is about tan (α) wavelength, when α = 13 degrees, one turn is about ４ wavelength. If the total length is 10 turns, the antenna height will be as high as about 2.5 wavelengths. Therefore, the total length of the helical with a tapered tip when α was changed was calculated. As a comparison condition, the comparison is made over the entire length at which a helical gain of about 12 dBi is obtained, and the number of turns differs depending on α.
[0023]
As a result, although 0.8 wavelength was obtained at α = 2 degrees, 1.2 wavelengths were obtained at α = 7 degrees, 1.5 wavelengths were obtained at α = 9 degrees, and 2.7 wavelengths were obtained at α = 13 degrees. Since the helical diameter is about 0.3 wavelength, when the height / diameter ratio increases, mechanical conditions such as vibration conditions become severe. If this ratio needs to be suppressed to about 5, it must be set to α = 9 degrees or less.
[0024]
Therefore, it is appropriate to set 4 <α <9 for a helical with a tapered tip portion due to restrictions on the bandwidth and the antenna height. Considering that the normal helical satisfies 12 <α <14 degrees, it can be understood that the attitude can be reduced.
[0025]
By making the tip of the wire tapered as described above, it is reasonable to set 4 <α <9 degrees. Further, for example, even when α is set to α = 7 which is about half of the normal value, the axial ratio can be improved from FIG. 3 by making the tip of the wire tapered. The axial ratio can be improved to 0.5 dB. Therefore, it can be understood that the tapered tip of the wire operates as a circularly polarized antenna even when α is set to about half of the normal value, and there is an effect that the height of the antenna can be reduced.
[0026]
Embodiment 2 FIG.
FIG. 4 is a schematic configuration diagram showing Embodiment 2 of the present invention.
The helical antenna needs a finite ground plane 1 to radiate in one direction. When the base plate 1 becomes smaller, radiation to the back surface increases, and the gain decreases. Therefore, in order to increase the gain, it is necessary to increase the size of the ground plane 1. However, the size of the ground plane 1 is generally about two wavelengths, and there is a problem that the dimensions of the ground plane 1 are increased.
[0027]
Therefore, a bent rim 7 is provided at the tip of the base plate 1 to form a cavity structure. With this cavity structure, radiation to the back surface is reduced, and radiation to the back surface can be reduced even if the diameter of the ground plane is reduced. Further, the gain can be varied by changing the size and size of the cavity and changing the height of the rim 7.
[0028]
By providing the base plate 1 with the rim 7 to form a cavity structure, a high gain can be obtained while keeping the diameter of the base plate 1 small, and the gain can be varied by changing the height of the rim 7.
[0029]
Embodiment 3 FIG.
FIG. 5 is a schematic configuration diagram showing Embodiment 3 of the present invention. The rim 7 at the front end of the main plate 1 has a double structure to form a double cavity, and includes an inner rim 8 and an outer rim 9. The double-structured cavity is conductive at the bottom surface, thereby forming a double-structured cavity.
[0030]
Assuming that the depth of the double-structured cavity is L, by setting L to about 1/4 wavelength, the upper end of the cavity can be made electrically open, and thus operates as a choke. That is, since the electric field component is reduced at the upper end of the cavity, radiation to the back surface of the ground plane is reduced. Here, the heights of the inner rim 8 and the outer rim 9 are the same. However, even if the heights of both are changed to some extent, they can operate as a choke and can control the radiation level to the side surface and the back surface.
[0031]
Also, here, the circular cavity is used, but even a square, hexagonal, elliptical, or polygonal cavity operates as a choke by adopting a double structure, and thus does not depend on the shape. Although an example in which a double cavity is used has been described, the function as a choke can be further increased by using a triple, quadruple, or multiple cavity.
[0032]
By thus forming the cavity in the double configuration and the choke, there is an effect that radiation to the back surface can be suppressed.
[0033]
Embodiment 4 FIG.
FIG. 6 is a schematic configuration diagram showing Embodiment 4 of the present invention.
Consider a helical array in which two or more helical antennas are arranged. It is known that the coupling between the helical arrays is generally large, and the coupling between the helicals deteriorates the radiation characteristics. That is, the reduction of the gain and the deterioration of the axial ratio due to the coupling. This coupling has a large coupling in the helical power supply part because the current flowing particularly near the power supply part is strong.
[0034]
Therefore, a choke is formed between the two arranged cavities.
That is, assuming that the height of the rims 10a and 10b of each helical cavity is L, this L is set to about 1/4 wavelength, and a choke is formed using the two cavities. Thereby, the coupling between the two helices can be reduced.
[0035]
Here, an example of two arrays has been described, but a similar choke can be formed with three or more arrays. In addition, although a hexagonal cavity is used, a square, polygonal, circular, or elliptical cavity may be used.
[0036]
By forming a choke between the helical cavities, there is an effect that the coupling between the helical antennas can be reduced.
[0037]
Embodiment 5 FIG.
FIG. 7 is a schematic configuration diagram showing Embodiment 5 of the present invention. Consider a helical array in which two or more helices are arranged and two or more arrayed helical antennas are arranged. The cavity has a double structure, and the shapes of the inner rim 15a and the outer rim 16a are changed. The inner cavity is strongly responsible for the helical radiation properties.
[0038]
That is, when the size and height of the inner cavity are changed, the gain and the axial ratio change. Therefore, by changing the cavity specifications, the helical radiation characteristics can be changed. On the other hand, by setting the height L of the rim to about 1/4 wavelength in the outer cavity, a choke is formed between the helical cavities, and the coupling between the helical antennas can be reduced.
[0039]
Here, the shape of the inner cavity is elliptical, and the shape of the outer cavity is hexagonal, but may be quadrangular, circular, or the like, and is not limited to this shape. Although an example of two arrays has been described, it goes without saying that three or more arrays may be used.
[0040]
As described above, by independently changing the shapes and specifications of the inner cavity and the outer cavity, it is possible to control the helical radiation characteristics and adjust the coupling between the helical antennas.
[0041]
Embodiment 6 FIG.
FIG. 8 is a schematic configuration diagram showing Embodiment 6 of the present invention. A cavity 18 is provided as a helical conductor 17 having a pitch angle α of 4 degrees or less in order to further lower the helical attitude.
[0042]
As a single helical, reducing α decreases the circular polarization characteristics. In order to improve the radiation characteristics, the diameter of the cavity and the height of the rim are controlled to form a cavity antenna, and the helical is operated as its exciter. Since the height of the antenna can be reduced, it is less likely to be affected by restrictions on the installation location, and when an array is used, coupling between antennas can be reduced.
[0043]
By using a small helical having α <4 degrees as the exciter for the cavity antenna, it is possible to reduce the height of the antenna and to reduce the coupling when an array is used.
[0044]
Embodiment 7 FIG.
FIG. 9 is a schematic configuration diagram showing Embodiment 7 of the present invention.
Reference numeral 20 denotes a skirt provided below the helical base plate. As the helical ground plane is made smaller, the radiation to the back increases and the gain decreases.
[0045]
That is, the purpose of this embodiment is to have a broad radiation pattern. When the base plate 1 is a disc, the scattering of the radiation pattern at the edge of the base plate 1 causes ripples or nulls in the radiation pattern. In order to control the shape of the radiation pattern in a broad-angle direction with a broad radiation pattern, a metal skirt is provided under the main plate, and the height and the opening angle of the skirt are adjusted to control the radiation pattern. it can.
[0046]
Therefore, by providing a skirt portion below the main plate, there is an effect that the radiation pattern shape can be controlled.
[0047]
Embodiment 8 FIG.
FIG. 10 is a schematic configuration diagram showing Embodiment 8 of the present invention.
Since the impedance of a helical antenna is generally as high as about 180 ohms, an impedance converter is required to achieve impedance matching with a 50 ohm line. For this impedance matching, the transmission line 3 formed with the ground plane 1 can be used.
[0048]
That is, by changing the impedance of the transmission line, impedance matching between the feed line and the helical is achieved. However, the length and width of the straight transmission line 3 alone are limited, so that the impedance matching cannot always be achieved, or the shape of the transmission line becomes complicated, and there are variations such as manufacturing errors. Effects also occur.
[0049]
Therefore, the stub 21 is provided near the connection between the transmission line and the helical conductor. Since this stub has a reactance component, impedance matching can be achieved by adjusting the stub length. Further, since the stub is on the outer periphery of the helical, its length can be easily adjusted, and even if the impedance varies due to a manufacturing error, the stub length can be easily cut and adjusted after assembling.
[0050]
Therefore, by providing a stub on the outer periphery of the helical diameter and adjusting the stub length, impedance matching can be easily achieved, and there is an effect that a variation error caused by manufacturing can be easily adjusted and absorbed.
[0051]
Embodiment 9 FIG.
FIG. 11 is a schematic configuration diagram showing a ninth embodiment of the present invention. When the helical antenna is arrayed, strong coupling between the antennas occurs, and as a means for reducing the coupling, there is a choke structure provided between the cavities as shown in the fourth and fifth embodiments. Further, in this embodiment, the radiation characteristics are adjusted by changing the coupling phase between the helices. When each helical antenna is used independently, since it is circularly polarized, the polarization characteristics do not change even if the feeding unit is rotated.
[0052]
In order to change the coupling phase, the position of the feeder, that is, the length direction of the transmission line 3 is changed for each helical. As a result, the coupling phase changes, so that the coupling can be reduced at a specific frequency, the radiation pattern shape can be changed, and the axial ratio can be finely adjusted. In this embodiment, it is also possible to make the position of the power supply unit random, and it is possible to suppress deformation of the radiation pattern at a specific frequency by randomization.
[0053]
Here, an example of three arrays has been described, but any two or more arrays are effective.
[0054]
Therefore, by changing the position of the power supply unit for each helical, there is an effect that the coupling phase between the helicals is changed and the radiation characteristics can be adjusted.
[0055]
【The invention's effect】
As described above, the present invention makes it possible to reduce the antenna height while maintaining the circularly polarized wave radiation characteristics by reducing the pitch angle and making the tip portion tapered.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram showing Embodiment 1 of a helical antenna according to the present invention.
FIG. 2 is a diagram showing axial ratio characteristics when a pitch angle α and the number of turns are changed.
FIG. 3 is a diagram showing an axial ratio characteristic when the number of taper turns is changed.
FIG. 4 is a schematic configuration diagram showing Embodiment 2 of a helical antenna according to the present invention.
FIG. 5 is a schematic configuration diagram showing Embodiment 3 of a helical antenna according to the present invention.
FIG. 6 is a schematic configuration diagram showing Embodiment 4 of a helical antenna according to the present invention.
FIG. 7 is a schematic configuration diagram showing Embodiment 5 of a helical antenna according to the present invention.
FIG. 8 is a schematic configuration diagram showing Embodiment 6 of a helical antenna according to the present invention.
FIG. 9 is a schematic configuration diagram showing Embodiment 7 of a helical antenna according to the present invention.
FIG. 10 is a schematic configuration diagram showing Embodiment 8 of a helical antenna according to the present invention.
FIG. 11 is a schematic configuration diagram showing a ninth embodiment of a helical antenna according to the present invention.
[Explanation of symbols]
1 ground plane, 2 inner conductor, 3 transmission line, 4 wire conductor, 5 spiral helical conductor, 6 tapered helical conductor, 7 rim, 8 inner rim, 9 outer rim, 10 helical cavity rim, 11 power supply connector, 15 inner Rim, 16 outer rim, 17 helical conductor, 18 cavity, 20 skirt, 21 stub.

Claims (9)

A helically wound first helical conductor having substantially the same diameter;
A helically wound second helical conductor having a tapered shape connected to one end of the first helical conductor;
A helical antenna composed of a metal ground plate provided with a power supply connector,
The pitch angle α of the spiral wound helical conductor is set to 4 degrees <α <9 degrees,
A helical antenna, wherein the spirally wound helical conductor having the tapered shape has three or more turns. 2. The helical antenna according to claim 1, wherein a cavity is formed by a bottom portion to which the other end of the first helical conductor is connected, and an upright portion erected on an edge of the bottom portion. 3. The cavity according to claim 1, wherein the erected portion provided on the edge of the bottom portion is a cavity having a double structure, and the depth of the cavity is set to about １ ／ wavelength. A helical antenna according to any one of the preceding claims. A helical array in which two or more helical arrays are arranged,
4. The cavity according to claim 1, wherein the cavity formed by the upright portion provided at the edge of the bottom portion forms a choke with the upright portion of another helical antenna. The helical antenna according to the paragraph. 5. The method according to claim 4, wherein the erected portion provided on the edge of the bottom portion is a cavity having a double structure, and the inner erected portion and the outer erected portion have different shapes. Helical antenna. The helical antenna according to claim 2, wherein a pitch angle α of the spiral wound helical conductor is set to α <4 degrees. The helical antenna according to any one of claims 1 to 6, wherein an upright portion is provided at an edge of the bottom surface portion in a direction opposite to a radiation direction of a helical axis. 8. The circuit according to claim 1, wherein a matching circuit is provided on a helical outer periphery of a connection portion between the first helical conductor and the transmission line configured with the metal ground plane as a ground. The helical antenna described. The helical antenna according to any one of claims 1 to 8, wherein the lengths of the transmission lines are arranged randomly in a horizontal plane on which a ground plane is formed.

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