JPH046908A - Offset dual reflecting mirror antenna - Google Patents

Abstract

PURPOSE:To suppress the gain reduction based on phase distortion and the deterioration in the paraxial directivity by making a focus of a main reflecting mirror with an apparent wave source of a reflecting wave when viewing a reflecting wave of a subreflecting mirror from a main reflecting mirror side. CONSTITUTION:A radiating wave from a primary radiator 1 is reflected in a subreflecting mirror 21 and reaches a main reflecting mirror 3 without convergence to a focus F12 and when viewing a radio wave reflected in the subreflecting mirror 21 from the main reflecting mirror 3, since the wave is regarded as a spherical wave taking a point apart from a focus F12 of the subreflecting mirror 21, the focus F3 of the main reflecting mirror 3 is made coincident with the apparent wave source of the spherical wave. Through the constitution above, the reflecting wave in the subreflecting mirror 21 is a spherical wave taking the focus F3 of the main reflecting mirror 3 as the apparent wave source and reaches the main reflecting mirror 3, then the reflecting wave from the main reflecting mirror 3 is a planer wave having no phase distortion.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application This invention relates to an offset double reflector antenna used for terrestrial microwave communications, satellite communications, and the like.

BACKGROUND ART FIG. 3 is a diagram showing an example of a conventional offset double reflector antenna, in which 1 is a primary radiator, 23 is a sub-reflector made of an ellipsoid of revolution, and 3 is a main reflector made of a paraboloid of revolution. Since the sub-reflector 23 is made of an ellipsoid of revolution, it has two focal points F31 and F3°, and the phase center of the -order radiator 1 is the focal point F3□.
The focal point of the main reflecting mirror 3 is arranged so as to overlap the focal point F3□.

That is, the sub-reflecting mirror 23 and the main reflecting mirror 3 are arranged so as to be confocal.

FIG. 4 is also a diagram showing an example of a conventional offset double reflector antenna, in which the focal point F of the sub-reflector 24 made of a hyperboloid of rotation is shown.
The phase center of the primary radiator 1 coincides with 41, and the imaginary focus F4□
is arranged so as to coincide with the focal point of the main reflecting mirror 3.

Problems to be Solved by the Invention In the conventional offset double reflector antenna shown in FIG. Furthermore, it was thought that the wave becomes a spherical wave from the focal point F3□, enters the main reflecting mirror 3, is reflected by the main reflecting mirror 3, and is converted into a plane wave 4.

The focal point F4° of the sub-reflector 24 in the conventional offset double-reflector antenna shown in FIG. The reflected wave from 24 becomes a spherical wave from the focal point F4°, and is reflected by the main reflector 3 to become a plane wave 4.
It was thought that it would be converted into

However, this way of thinking is based on the premise that the frequency of radiated radio waves is extremely high and can be treated as if it were light. In the case of an antenna with an aperture diameter of less than 100 wavelengths, it is not necessarily correct to regard the radiated radio waves as light, and in such cases, the inventors have As a result of various studies, we were able to confirm the following facts.

That is, in the conventional offset double reflector antenna shown in FIG. is equivalent to a spherical wave whose wave source is not a spherical wave whose wave source is at the focal point F3°, but a spherical wave whose wave source is a point F33 which is distant from the main reflecting mirror 3 by Δf from the focal point F3°.

Even in the conventional offset double-reflector antenna shown in FIG. 4, the radio waves reflected by the sub-reflector 24 and reaching the main reflector 3 are not spherical waves with the source at the focus F4□, but from the focus F4°. Point F43 that is away from the main reflecting mirror 3 by △f
It is equivalent to a spherical wave whose source is .

Note that the above-mentioned wave properties of radio waves within a reflecting system configured with a plurality of reflecting mirrors can be analyzed using a beam mode expansion method.

Although the source of the wave incident on the main reflector 3 can be considered to be F33 (Fig. 3) or F43 (Fig. 4), in the conventional offset double reflector antenna, the focal point of the main reflector 3 is is made to coincide with the focus F,2 of the sub-reflector 23 (FIG. 3) or the focus F43 of the sub-reflector 24 (FIG. 4),
The reflected wave from the main reflecting mirror 3 is not a plane wave 4 as shown in FIG. 3 or 4, but a wavefront 5 having spherical aberration is actually formed.

FIG. 5 is a curve diagram showing examples of the relationship between the used frequency and the focal point movement amount Δf and the relationship between the used frequency and the maximum phase distortion amount Δφ at the aperture end of the main reflecting mirror 3, and the horizontal axis ( The logarithmic scale) is the operating frequency (GHz), and the vertical axis (equally spaced scale) is the movement amount Δf (mm) of the focal point or the maximum phase distortion amount Δφ (deg) at the aperture end of the main reflecting mirror 3.

In addition, in FIG. 5, the opening diameter of the main reflecting mirror 3 in FIG. 3 or 4 is 3600 mm, and the sub-reflecting mirror 23 or 24 is shown.
The maximum width of 720mm, 5-F32 in Figure 3
The length between 5-F4□ in Figure 4 is 400m.
m, between F3□-M in Figure 3 or 6 in Figure 4.
This is the result obtained by calculation using the beam mode expansion method for the case where the length between F4□ and M is 2200 mm.

As is clear from FIG. 5, as the operating frequency increases, the geometric optical state approaches, so the focal point movement amount Δf and the maximum phase distortion amount Δφ rapidly decrease.
When the aperture diameter of the main reflecting mirror 3 is 3600 mm and the operating frequency is 10 GHz or less, these values cannot be ignored.

The above-mentioned spherical aberration has a negative effect on the gain and paraxial directivity of the antenna, and Figure 6 shows the amount of gain reduction based on calculations under the same conditions as explained in Figure 5. be.

The horizontal axis (logarithmic scale) in Figure 6 indicates the operating frequency (GHz
), the vertical axis (equally spaced scale) is the amount of gain reduction (dB), and as shown in the figure, the amount of gain reduction approaches zero as the frequency used increases, but it is The amount of decrease is not negligible in a frequency band of 10 GHz or less used in relatively small-diameter satellite communications earth station antennas.

Means for Solving the Problems This invention was made based on the fact that the present inventors were able to confirm through theoretical analysis as described above. The present invention attempts to eliminate the drawbacks of the conventional reflector by arranging the main reflecting mirror and the sub-reflecting mirror so that their focal points are shifted from each other, without maintaining the same state.

Effect As mentioned above, when the focal points of the main reflecting mirror and the sub-reflecting mirror are arranged at positions shifted from each other, the radio waves incident on the main reflecting mirror are reflected from the spherical surface from the focal point of the main reflecting mirror itself. The reflected wave from the main reflecting mirror is almost a plane wave.

Embodiment FIG. 1 shows an embodiment of the present invention, in which 1 is a primary radiator, and 21 is a sub-reflector, which is formed of an ellipsoid of revolution. 3 is the main reflecting mirror, which is made of a paraboloid of revolution.

Fll and F12 are sub-reflectors 2. F3 is the focal point of the main reflecting mirror 3.

The primary radiator 1 is arranged so that its phase center coincides with one focus Fil of the sub-reflector 21, as in the conventional case, but in this invention, the focus F3 of the main reflector 3 is 2, but not at the other focal point F1□, but at a point that is an appropriate distance away from the main reflecting mirror 3 from the focal point F12.

In other words, the radiation wave from the -order radiator 1 is reflected by the sub-reflector 21 and then reaches the main reflector 3 without converging at the focal point F1°, but the radio waves reflected by the sub-reflector 21 are reflected by the main reflector 21. When viewed from the mirror 3 side, the point away from the focal point F1□ of the sub-reflector 2□ appears to be a spherical wave with the wave source above.
In this invention, the spherical wave is constructed so that the focal point F3 of the main reflecting mirror 3 coincides with the upper wave source.

When configured in this way, the reflected wave from the sub-reflector 21 reaches the main reflector 3 as a spherical wave whose wave source appears to be above the focal point F3 of the main reflector 3. The reflected wave becomes a plane wave with no phase distortion.

FIG. 2 is a diagram showing another embodiment of the present invention, in which 1 is a primary radiator, and 2 □ is a sub-reflector, which is made of a hyperboloid of revolution. 3 is the main reflecting mirror, which is made of a paraboloid of revolution.

F2□ and F2□ are the focal points of the sub-reflector 2□, and F3 is the focus of the main reflector 3.

In this embodiment as well, the phase center of the primary radiator 1 is arranged to coincide with the focal point F21 of one of the sub-reflecting mirrors 22, as in the conventional case, but the focal point F3 of the main reflecting mirror 3 is As in the previous embodiment, the appearance of the radio waves reflected by the sub-reflector 2□ is made so that the focal point F3 of the main reflector 3 coincides with the wave source above, without making it coincide with the other focus F2□ of □. It is composed.

Also in this embodiment configured in this way, the sub-reflector 2
Since the reflected wave of □ becomes a spherical wave with the focal point F3 of the main reflecting mirror 3 as the apparent upper wave source, the reflected wave from the main reflecting mirror 3 becomes a plane wave without phase distortion.

Effects of the Invention In this invention, regardless of whether the sub-reflector is made of an ellipsoid of revolution or a hyperboloid of revolution, the appearance of the reflected wave of the sub-reflector when viewed from the main reflector side is By aligning the focus of the main reflector with the wave source of Deterioration of paraxial directivity can be suppressed.

[Brief explanation of the drawing]

1 and 2 each show an embodiment of the present invention, FIGS. 3 and 4 show a conventional antenna, and FIGS. 5 and 6 show a conventional antenna, respectively. Primary radiator, with a curve diagram to explain the shortcomings
2. ．． 2□, 23 and 24: Sub-reflector, 3: Main reflector, 4 and 6: Wavefront of radiation wave, Fil, Fl□,
F21, F2□, Fs□. F3□, F41 and F4□: Focus of sub-reflector, F3:
Focal points of main reflecting mirror 3, F1a and F43: The appearance is the upper wave source. References for analysis of wave dynamic properties of radio waves using the method of beam mode expansion (1) Mr. Chubu, Shimanuki, Ryōbu, and Furuno 4: ``4 and 5.6 GHz bands with excellent wide-angle directivity and cross-polarization characteristics. "Shared Offset Antenna" Transactions of the Institute of Electronics and Communication Engineers (B) J67-B
, 2. pp, 194-201 (Sho 59-02) (
2) H, Kogelnik and T, Lj = “1, aserBeams and Re5onat
orsJ Proe, IEEE, 54.10. p
p.

Claims (1)

[Claims] 1. Consists of a main reflecting mirror made of a paraboloid of rotation, a sub-reflecting mirror made of an ellipsoid of revolution or a hyperboloid of revolution, and a primary radiator, and the sub-reflecting mirror and the primary radiator reflect radiation waves from the main reflecting mirror. 1. An offset double-reflector antenna arranged so as not to block a radio wave path, wherein the main reflector and the sub-reflector are arranged so as to be shifted from a confocal state.