JPH0352245B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an antenna device such as an offset antenna using a rotationally non-symmetrical mirror surface.

A conventional device of this type is shown in FIG. In the figure, 1 is the mirror surface of a non-rotationally symmetrical parabolic reflecting mirror, the center is M, the aperture diameter is d, and the chain double-dashed line indicates the parabolic surface with axis 2 as the axis of rotational symmetry.
10 (parabola diameter D, parabola focal length F), 3 is the focal point of the parabola 10, and 4 is a power feeding horn. Reference numeral 5 denotes the horn axis of the power feeding horn 4, and the focal point 3 is set to coincide with a line drawn at the center M of the mirror surface 1.

Note that X, Y, and Z indicate three orthogonal axial directions provided for convenience of explanation, θ ₀ is the angle between axis 1 and horn axis 5, and θc and θ′c are when looking at mirror surface 1 from focal point 3. Indicates the angle of elevation. Further, x, y, and z indicate the sizes in the x, y, and z directions measured from the center of the aperture surface of the mirror surface 1. The operation of this type of antenna is well known, for example in the document 1 “DePolarization Properties of
offset Reflector Antennas，TA−SHING
CHU etal, IEEE TRANS.AP−21, No. 3,
MAY, 1973, P.339-345". An example of the characteristics of this antenna is shown in Figure 2. Figure 2A shows the radiation pattern in the YZ plane, and Figure 2B shows the radiation pattern in the XZ plane. In the figure, the solid line indicates the radiation pattern of right-handed circularly polarized waves, the dashed-dotted line indicates the radiation pattern of left-handed circularly polarized waves, and ΔG ₀ indicates the amount of gain reduction in the front direction of the antenna.

Next, the operation will be explained. When the feeding horn 4 shown in Fig. 1 is fed with circularly polarized waves, due to cross-polarized components generated by the offset antenna, the radiation pattern on the YZ plane shifts ( (hereinafter referred to as beam shift) occurs. Since this beam shift is different for right-handed circularly polarized waves and left-handed circularly polarized waves, so-called beam separation occurs, and when this type of antenna is applied to a satellite communication antenna, for example, the following drawbacks occur. That is, in satellite communications, downlink and uplink generally use different frequency bands and circularly polarized waves that are orthogonal to each other. Intelsat (International Commercial Satellite Communications Organization) uses right-handed circularly polarized waves in the 4GHz band for downlinks, and left-handed circularly polarized waves in the 6GHz band for uplinks. In such a system, when the antenna described above is used, the maximum gain direction in the 4 GHz band is different from the maximum gain direction in the 6 GHz band. Generally, a slight angular deviation in the antenna pointing direction△
The relative gain reduction ΔG of the antenna due to θ is expressed by the following equation.

△G=-12 (△θ ₂ /Θ ₃ ), dB Here, θ ₃ is 3 dB width (so-called beam width for the target frequency) The amount of beam shift is determined by the cross-polarized wave component generated by the offset antenna. According to Document 1, for example, θ ₀ in Fig. 1 is 51.3°, θc is 51.3°, F/D
When is 0.4, it becomes 13% of the beam width. Now, assuming that the antenna is pointing at the center of the right-handed/left-handed beam, the gain reduction corresponding to △G ₀ in Figure 2 is calculated using the above formula, and the value is 0.2 dB.

This invention was made in order to eliminate the drawbacks of the conventional ones as described above, and utilizes circularly polarized waves in which the frequency bands of the transmitted and received waves are different and the polarizations of the transmitted and received waves are orthogonal to each other. As an antenna for a communication system, by configuring an offset antenna using a primary radiator consisting of a feeding horn dedicated to each band and a reflector with frequency selectivity, it is possible to reduce the gain due to the beam shift inherent in the offset antenna. The purpose of the present invention is to provide an antenna device that can avoid such problems.

An embodiment of the present invention will be described below with reference to FIG.

In the figure, 4a is a power supply horn for frequency _F1 ;
4b is a feeding horn for frequency _F2 ; 6 is a frequency-selective planar reflector that reflects _F2 and passes _F1 ; and 30 is an image focal point of focal point 3 generated by the reflection of reflector 6. Other symbols are the same as or equivalent to those in Fig. 1.

Generally, as shown in FIG. 4, if the feeding horn 41 placed facing the reflecting mirror 11 is offset from the focal point 31 by Δy in a plane perpendicular to the mirror axis 21 (the horn axis in this case is 22). , reflecting mirror 11
The beam 23 from is shifted from the mirror axis 21 by an angle Δθ determined by the following equation. (Literature; Microwave
Antenna Theory and Design: Siven) △θ=k tan ^-1 △y/F The one in FIG. 4 is an example having a rotationally symmetrical mirror surface 11. Here, k...proportionality coefficient △y...offset amount from the focal point 31 in the y-axis direction F...focal length of the main reflecting mirror D...diameter.

Note that the beam shift direction is in the same plane as the offset direction of the feeding horn 41 (in this case, in the yz plane).

Taking advantage of the fact that the direction of the beam can be shifted by shifting the horn axis from the mirror axis as described above, the deviation between the radiation pattern of the right-handed radio wave and the radiation pattern of the left-handed radio wave in the yz plane shown in Figure 2 can be adjusted. , that is, the beam separation is corrected as follows.

As shown in FIG. 3, a right-handed feeding horn 4a and a left-handed feeding horn 4b are arranged with a reflection plate 6 in between, so that the radiation beams from the mirror surface 1 generated by the respective feeding horns 4a and 4b coincide. The center of the power supply phase of each power supply horn 4a, 4b is
The focal point 3 and the image focal point 30 are offset by a required amount with respect to the xy plane. The reason for arranging the two feeding horns 4a and 4b through the reflector 6 is that the offset amounts △y ₁ and △y ₂ of each feeding horn necessary to correct the beam shift are calculated as shown below. is not a large value, so without the reflector 6, the two feeding horns 4a and 4b would have to be placed one on top of the other, which would be physically impossible, but with the reflector 6, the _F1 wave is reflected. and its feeding phase center is near the image focus 30, and
If the _F2 wave is allowed to pass through as is and its feeding phase center is brought to the vicinity of the focal point 3, the two feeding horns 4a and 4b can be physically placed at predetermined positions that will not cause beam separation. I can do it.

The calculation results of the required offset amount (see FIG. 3) are shown below.

When θ ₀ = 51.3°, θ ₂ = 51.3°, F/D = 0.4,
Δy=0.1 wavelength.

In the case of an offset parabolic antenna with diameter (d) = 4 m, at F = 4 GHz, △y = 7.5 mm, at F = 6 GHz, △y = 5 mm. Here, the right-handed feed horn and the left-handed feed horn are the same with respect to the xy plane. offset in the direction,
This is because the frequency-selective reflector converts the incident circularly polarized wave into anti-rotation for the _F2 frequency reflected and reflects it.

Note that the offset direction of the feeding horn with respect to the xy plane is uniquely determined by the direction of rotation of the circularly polarized wave used by the feeding horn.

As mentioned above, by using the frequency selective reflector, 2
Since the feeding phase centers of the two feeding horns are formed at separate positions, the two feeding horns 4a and 4b can be placed at desired positions and beam separation can be eliminated.

Another advantage of the antenna device provided herein is that the feeding horn can be designed specifically for each band, so that an inexpensive horn that emits a beam with rotational symmetry, such as a dual mode horn, can be used.

In the embodiment shown in FIG. 3, a planar plate-shaped frequency-selective reflector 6 was explained, but in consideration of application to a so-called offset cassegrain type antenna, a curved-shaped frequency-selective reflector was used. A similar effect can be obtained.

As described above, according to the present invention, it is possible to add frequency selectivity to an antenna device that uses a non-rotationally symmetrical mirror surface, such as a so-called offset parabolic antenna in a satellite communication system, which uses two circularly polarized waves that are orthogonal to each other. A primary radiator consisting of a pair of feeding horns with a reflector interposed is fed with offset power, and each of the pair of horns is offset (polarized) with respect to a plane of symmetry including the antenna mirror axis (xy plane in Fig. 3). The structure is such that the beam shift caused by the horn offset (deviation) corrects the beam shift that occurs due to the non-rotationally symmetric mirror surface, so a high-performance antenna device that does not suffer from loss of gain due to beam shift can be achieved. There are benefits to be gained.

[Brief explanation of drawings]

FIG. 1 is a diagram for explaining a conventional antenna device, FIG. 2 is a diagram for explaining the radiation pattern characteristics of the antenna in FIG. 1, and FIGS. 3 and 4 are diagrams for explaining an antenna device according to an embodiment of the present invention. This is a diagram. 1 is the mirror surface of the non-rotationally symmetric offset parabolic antenna, 2 is the rotationally symmetric axis of the parabolic antenna, and 3 is the mirror surface of the non-rotationally symmetric offset parabolic antenna.
The focal point 30 of the parabolic antenna is also the image focal point, 4a and 4b are the feeding horns, 5 is the horn axis,
6 indicates a frequency selective reflector. Note that the same reference numerals in the figures indicate the same or equivalent parts.

Claims (1)

[Claims] 1. A non-rotationally symmetrical mirror surface having a curved surface formed by a part of a rotationally symmetrical parabola, which exhibits a passing characteristic for radio waves of a first frequency, and exhibits a transmission characteristic for radio waves of a second frequency. A horn axis is set toward the center of the non-rotationally symmetrical mirror surface through a frequency selective reflector showing reflection characteristics, and a horn axis is set toward the center of the non-rotationally symmetrical mirror surface, and the phase center of the power supply is determined from the focus of the rotationally symmetrical parabola. a first feeding horn that emits a radio wave having the first frequency and whose plane of polarization rotates in a first direction, which is disposed at a distance; The reflected horn axis is set toward the center of the rotationally symmetrical parabola, and the focal point of the rotationally symmetric parabola is placed at a predetermined distance apart, so that the plane of polarization rotates in a second direction opposite to the first direction. An antenna device comprising: a second feeding horn that radiates radio waves having a second frequency. 2. The antenna device according to claim 1, wherein the frequency selective reflector has a planar structure. 3. The antenna device according to claim 1, wherein the frequency selective reflector has a curved structure. 4. The antenna device according to claim 1, wherein the frequency selective reflector has high-pass characteristics. 5. The antenna device according to claim 1, wherein the frequency selective reflector has low-pass characteristics.