JPH0231883B2 - - Google Patents

Description

Detailed Description of the Invention In a spiral antenna formed by two conductors on an insulating support, which is used as an ultra-wideband antenna, the length of these two conductors and the position of the outer peripheral end of the conductor are as follows: The present invention relates to a two-wire spiral antenna characterized in that the circular polarization coefficient (hereinafter referred to as the axial ratio of circularly polarized waves) of the circularly polarized waves radiated from these conducting wires is improved in a low frequency band by selecting the following.

A conventional two-strand spiral antenna (hereinafter simply referred to as an antenna) consists of two conductive wires of equal length spiraling from the center to the outer periphery, and the outer shape is often conical or planar.
There are many shapes of these spirals, such as logarithmic spirals, Archimedean spirals, and special square spirals, all of which are used as ultra-wideband antennas. The operating principles of these antennas are all the same, and will be explained using an Archimedean spiral antenna as an example.
FIG. 1 is a plan view showing an embodiment of a conventional double-strand spiral antenna. In FIG. 1, 8 is a disk shape and an insulating support. 1 and 2 are conductive wires formed on an insulating support 8, with ends 6 and 6' near the center 7, respectively.
Starting from , it flares out in a helical manner towards the peripheral ends 5 and 5', the ends 6 and 6' and the peripheral ends 5 and 5' being respectively symmetrically located with respect to the center 7.
Further, the conductive wires 1 and 2 have the same overall length. 3 is a power supply line connected to ends 6 and 6' of conductors 1 and 2;
Currents 4 and 4' are supplied to conductors 1 and 2, respectively. A portion of the intermediate portion between conductive wires 1 and 2 is shown by a broken line and omitted. 30 is a circle with radius γ=λ/2π, where λ is the wavelength of electromagnetic waves radiated from conductive wires 1 and 2.
Reference numeral 31 indicates a group of circles corresponding to radii that differ slightly in wavelength λ, and is called a current band region. The electromagnetic wave with the wavelength λ is particularly strongly radiated at a portion where the circumference 2πγ of the circle 30 is equal to the wavelength λ, and the polarization plane of the radiated electromagnetic wave exhibits circular polarization. Currents 4 and 4' from the power supply line 3 are hardly attenuated inside the current band region 31 and proceed outward, and in the current band region 31, radiation is strongly performed and the currents are significantly attenuated. After passing through the current band region, it reaches the outer peripheral end portions 5 and 5' with small attenuation and is reflected. The outer diameter of the antenna is sufficiently large.
When the current band region corresponding to the operating frequency band has sufficient margin up to the outer diameter of the antenna, the current reaching the outer peripheral end portion is extremely small and is almost ignored. In a frequency band where such conditions are met, the antenna's radiation characteristics remain constant with respect to frequency changes, so it is used as a wideband antenna for various purposes. However, when this antenna is used in an aircraft or the like, the outer diameter of the antenna is restricted due to the requirement for small size and light weight. The outer diameter of this antenna limits the lower limit of the operating frequency band as described above. That is, as the current band region 31 gradually moves toward the outer circumference and approaches the outer diameter of the antenna, the current that reaches the outer circumferential ends 5 and 5' and is reflected cannot be ignored. To explain further, the reflected current generates a standing wave on the outer periphery of the antenna, and this standing wave generates a linearly polarized wave component, which deteriorates the axial ratio of the circularly polarized wave. Fortunately, this reflected current does not affect other characteristics of the antenna, such as gain, radiation directivity, and input impedance, to the same extent as the axial ratio characteristics. If it is possible to attenuate the reflected current that causes a worsening of the axial ratio of circularly polarized waves, the lower limit of the operating frequency band can be expanded, and several ideas have been proposed in the past. For example, attempts have been made to provide non-reflective terminators at the outer ends of the conductors 1 and 2, to load absorbers, or to create a zigzag structure. However, these attempts have had the drawback of not being able to produce significant effects despite the high manufacturing costs, and of affecting other properties other than the axial ratio.

The present invention achieves a low frequency band in the operating frequency band by keeping the lengths of the two conductive wires that constitute the spiral antenna at different lengths and arranging the outer peripheral ends asymmetrically with respect to the center. An object of the present invention is to provide a two-strand spiral antenna that can improve the axial ratio characteristics of circularly polarized waves radiated by the antenna and expand the operable frequency band of the antenna.

According to the present invention, circularly polarized waves with significantly improved axial ratio characteristics in the above-mentioned low frequency band can be obtained, and compared to conventional antennas having the same frequency and the same axial ratio of circularly polarized waves, the antenna is smaller and lighter. A double spiral antenna can be obtained.

Next, an embodiment of the double spiral antenna according to the present invention will be explained based on the drawings in comparison with a conventional one. FIG. 2 is an explanatory diagram showing the standing wave distribution on the conductor of a conventional two-strand spiral antenna. As mentioned above, the electromagnetic waves radiated from the antenna are generated by combining the circularly polarized wave component radiated while traveling in the current band region, the reflected wave reflected at the outer end of the conductor, and the traveling wave toward the outer end. There is a linearly polarized wave component radiated by the current in a standing wave state. The waves whose amplitudes are shown in FIG. 2 in correspondence with the lengths of the arrows are such standing waves. 2A and 2B show standing waves in conductors 1 and 2, and the same numbers as in FIG. 1 are used. waves 10, 11 and 12 and wave 1
0', 11' and 12' are the antinodes of the standing waves, respectively. Since the conductors 1 and 2 have the same length and their outer peripheral ends are at symmetrical positions with respect to the center 7, the positions of the antinodes of the standing waves are also symmetrical with respect to the center 7. Due to the nature of the standing wave antenna, the radiation waves from the antinode of the standing wave are all in phase, and when combined, they become linearly polarized components with polarization planes in the directions shown by arrows 20 and 20'. In other words, the linearly polarized components radiated from conductors 1 and 2 are vector-added,
Since the linearly polarized wave component has twice the amplitude, the axial ratio of the circularly polarized wave significantly deteriorates. In this way, in conventional antennas, the axial ratio of circularly polarized waves deteriorates because the standing wave current generates linearly polarized components.
Deterioration of the axial ratio of the circularly polarized wave can be avoided by using means that makes the radiated electromagnetic wave due to the standing wave a circularly polarized wave having the same rotational direction as the radiant circularly polarized wave due to the traveling wave in the current band region. This is the ultimate point of the invention.

FIG. 3 is an explanatory diagram of the standing wave distribution on the two-strand spiral antenna conductor according to the present invention. FIG. 3A is the same drawing as FIG. 2A, and shows the standing wave distribution of the conductor 1, and is shown for comparison with FIG. 3B. In FIG. 3B, the length of the conductor 2 is shortened so that the standing wave of the conductor 2 shown in FIG. This shows the standing wave distribution of the conductor 2' in a state where the conductor 2' is positioned asymmetrically with respect to the center 7 compared to the outer peripheral end 5 of FIG. 3A. Waves 10, 11 and 12 indicating the part are wave 1
0'', 11'', and 12'' are all located asymmetrically with respect to the center 7. The radiation waves from the conductors 1 and 2' have polarization planes in the directions indicated by arrows 20 and 20'' as described above. They become linearly polarized components that differ temporally and spatially from each other. Now, if we choose orthogonal coordinates with the center 7 as the origin and the direction of the arrow 20 of the conducting wire 1 as the x-axis, and the radiation direction as the z-axis, the electric field at an infinite point on the z-axis can be expressed by the following equation.

〓 _x =〓 _x {A・cos (ωt+φ ₁ )+cosθ・Acos (ωt+φ ₂ )} …(1) 〓 _y =〓 _y {sinθ・Acos (ωt+φ ₂ )} …(2) Here, 〓 _x , 〓 _y is an electric field component, 〓 _x , 〓 _y is a unit vector in the xy-axis direction, A is the amplitude, ω is the angular velocity φ ₁ and φ ₂ are the excitation phases, and θ is the angle shown in FIG. 3B.
From equations (1) and (2), the axial ratio AR of the circularly polarized wave formed by the electric field components 〓 _x and 〓 _y is calculated as follows.
The problem lies in the relative phase difference between the electric field components _x and _y , which is expressed as φ ₁ =0 and φ ₂ =△φ.

AR=(1+cosθ)(1+cos△φ)/(1−cosθ)(
1−cos△φ)...(3) If we set AR=1 in equation (3), we obtain the equation cosθ+cos△φ=0...(4) Equation (4). It is explained that in Equation (3), when θ=0, that is, when the arrangement is symmetrical, the axial ratio becomes infinite, and when θ≠0, the axial ratio improves rapidly. Furthermore, by solving Equation (4), it is possible to obtain almost perfect circular polarization at a certain frequency. Therefore, even if there are standing waves in the conductors 1 and 2', the asymmetric arrangement allows the conductors 1 and 2' to
By combining the linearly polarized wave components radiated from and 2', an electromagnetic field having the same rotational direction as the circularly polarized wave radiated from the current band region can be obtained. In addition, the high-frequency current flowing through the conductors 1 and 2' is reflected at the outer peripheral end portions 5 and 5'', and the phase difference of the reflected currents approaches in the opposite direction, resulting in a closed circuit between the conductor 1 and the adjacent conductor 2'. Qualitatively, it can be seen that harmful radiation is less likely to occur because the number of lines of electric force increases.In other words, due to the asymmetrical arrangement of conductors 1 and 2', the standing wave amplitude on the conductors becomes smaller. For this reason, the axial ratio of circularly polarized waves can be greatly improved at the lower limit of the operating frequency band.Up to now, we have explained the situation in which the antenna is used when operation in a wide band is desired. By selecting the asymmetrical arrangement of 1 and 2, narrow band operation is also possible and the best axial ratio can be achieved at the desired frequency.

FIG. 4 is an explanatory diagram showing the axial ratio and frequency characteristics of circularly polarized waves radiated from a double spiral antenna. In Figure 4, the vertical axis shows the axial ratio, and the horizontal axis shows the frequency (G
Hz). Graph (a) shows the characteristics of the conventional antenna, graph (b) shows the characteristics of the antenna according to the present invention configured so that the axial ratio is minimized around the frequency of 2 GHz, and graph (c) shows the characteristics of the antenna according to the present invention. This shows the characteristics of an antenna configured to have a flat axial ratio so that it can operate in a wide frequency band. graph (c)
As shown, the operable frequency range is expanded by nearly 100% in this operating frequency range. Furthermore, as a result of many experiments, it has been confirmed that the antenna according to the present invention has almost no effect on other characteristics of the antenna, and the angle θ shown in equations (1) and (2) is from 40 degrees to
The optimum value exists between 150 degrees.

In addition, in a two-strand spiral antenna, for the purpose of measuring disconnection of the antenna conductor wires, a short-circuit portion is often provided at the outer peripheral ends of both conductor wires. In such a case, the state of reflection of the high frequency current at the end of the outer periphery will change compared to the previously described open case. However, even in this case, it has been confirmed that the axial ratio of circularly polarized waves can still be improved by making the arrangement of the outer peripheral end portion asymmetrical with respect to the center as described above.

As described above, the present invention can be applied regardless of whether the outer peripheral end portion of the antenna conducting wire is open or short-circuited.

As explained above, the present invention makes it possible to adjust the operating frequency by simply keeping the lengths of the two conductive wires that make up the spiral antenna at different lengths and arranging the outer end portions at asymmetric positions with respect to the center. It is possible to provide a double-strand spiral antenna that can improve the axial ratio characteristics of circularly polarized waves radiated in a low frequency band and expand the operable frequency band of the antenna.

Incidentally, it is also possible to use the present invention in combination with conventionally used means for improving the axial ratio of circularly polarized waves, and an even better axial ratio can be obtained. Further, the present invention can be applied not only to planar logarithmic spiral antennas and planar rectangular spiral antennas, but also to all spiral antennas having two conductive wires, such as conical spiral antennas.

[Brief explanation of drawings]

FIG. 1 is a plan view showing an embodiment of a conventional two-strand spiral antenna, FIG. 2 is an explanatory diagram of standing wave distribution on a conventional two-strand spiral antenna conductor, and FIG. FIG. 4 is an explanatory diagram of the standing wave distribution on a line, and is an explanatory diagram showing the axial ratio and frequency characteristics of the circularly polarized wave radiated from the double spiral antenna. 1 and 2 and 2'... conductor wire, 3... feeder line, 4 and 4'... current, 5, 5' and 5''... outer peripheral end, 6
and 6'... end, 7... center, 8... insulating support,
10 and 11 and 12 and 10' and 11' and 12' and 1
0″, 11″ and 12″… waves, 20, 20′ and 20″
...Arrow, 30...Circle, 31...Current band region.

Claims (1)

[Claims] 1. In a spiral antenna composed of two conductive wires that extend spirally from the center toward the outer periphery, each of the two conductive wires has a different overall length, and the position of the outer peripheral end of the conductor wire is symmetrical with respect to the center. A double spiral antenna characterized by being moved by a predetermined center angle from a fixed position.