JPH0218582Y2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION This invention relates to the structure of an opening in a radar antenna, particularly in an antenna horn.

The microwaves radiated by a radar antenna include vertically polarized components and horizontally polarized components.

Among these, the vertically polarized component causes side lobes in the horizontal directivity direction of the transmitted pulse. Since the sidelobes sense return signals from unnecessary directions in the horizontal plane, they degrade the azimuth resolution of the detection function.

As a countermeasure for removing these side lobes, a common method is to suppress the vertically polarized wave component by arranging a flat plate formed with a metal vertical grating in the antenna horn opening. The suppression effect can be thought of in the same way as a filter effect on vertically polarized waves when a vertical grating is placed inside a so-called microwave waveguide, and the degree of vertically polarized waves suppression S (db) can be expressed by the following equation. .

In the above equation, α=a/λ, where a represents the dimension when vertical gratings with a grating width a are arranged at grating width intervals, and λ represents the wavelength of the radar signal.

By the way, the existence of this vertical grating is necessary from the viewpoint of suppressing side lobes in the horizontal direction, but it does not contribute in any way to the request for improving the directivity in the vertical direction, and is rather unnecessary.

If the vertical grating is arranged in the shape of a flat plate, it will not affect the directivity in the vertical direction, but if the vertical grating is formed in the shape of a curved surface, various effects will occur.

On the other hand, radar antennas are required to have horn openings shaped into curved shapes in order to improve aerodynamic characteristics due to the need to save power in antenna rotation motors.

Based on the above requirements, this invention realizes an antenna horn that does not deteriorate the directivity in the vertical direction even when the horn opening is formed into a curved surface.

This will be explained below using the drawings.

FIG. 1 shows a grating film 1, in which a is a front view and b is a plan view. In the figure, 2 is a flexible resin film with a thickness of about several tens of microns, and 3, 3, ... are metal vertical grids formed by etching a metal thin film previously pasted on one side of the film 2. be. FIG. 2 is a side sectional view showing an example of the structure of a horn opening using the grating film 1 described above. Reference numeral 4 denotes a 3cm wave radar antenna horn having a width of 40°, and 5 a radio wave transmitting antenna provided in the opening of the antenna horn 4, which is integrated with the grating film 1 interposed between them to prevent corrosion. Low loss diameter
It is a 100mm semicircular resin plate. Incidentally, the grating film 1 is placed at the center of the resin plate 5.

Figure 3 shows the vertical polarization suppression characteristics and vertical directivity characteristics (beam width) when a metallic vertical grating formed on the above-mentioned grating film is placed in the horn opening. indicates the grating length l of a vertical grating in a state where a grating film is attached to a semicircular resin plate with a diameter of 100 mm.

As mentioned above, it is desirable for the radar antenna to suppress vertically polarized components as much as possible from the viewpoint of suppressing side lobes in the horizontal plane. Furthermore, empirically, it is desirable that the vertical directivity angle (half angle, ie, 3 db beam width) of the transmitted pulse be about 3 degrees.

From this point of view, the optimum lattice length l is determined from Figure 3 as follows: 50≦l≦85 (mm).

That is, when the grating length l becomes 50 mm or less, the degree of suppression becomes extremely poor, as is clear from the orthogonal polarization suppression ratio curve. For example, when the grating length l is set to 25 mm, the directivity characteristics in the horizontal plane are as shown in Fig. 4 (b).
As can be seen from this characteristic curve B, the entry angle ±
An extremely large sidelobe occurs at around 40°.

Furthermore, as can be seen from the 3db beam width curve, the beam width increases rapidly when the grating length l exceeds 100 mm, and according to the experimental results, the grating length l must be 85 mm or less in order to prevent the beam width from exceeding 3 degrees. It was confirmed that it was necessary to do so.

Next, Figure 4 shows the received wave directivity characteristic curve showing the relationship between the incident angle θ and the gain. Curve A is for the case without the grating film 1, curve B is for the case where the film size l = 25 mm, and curve H is for the case where the grating film 1 is not used. is the film size l
= 157mm (full opening area). From the figure, the peak level is high and the directivity width (angle) is narrow in curve B (l = 25 mm). On the other hand, curve C (l=
157mm), the peak level is low and the directivity width (angle)
is wide. The relationship between the peak level and the directivity width tends to correspond to the dimension l, and the 3 dB beam width curve in FIG. 3 shows this in detail. That is,
From the same figure, in order to obtain a sharper directional beam than when no grating film is used, l85
It turns out that mm is preferable. Therefore, from the above, the condition that the film size l should satisfy is 50l85
Find mm.

Based on the above points, when determining the optimal grating length l, it is found that when the shape of the antenna horn 4 is semicircular in cross section, it is practically preferable to have a grating processing size of approximately 1/3 to 2/3 of the total length. I was able to confirm it.

Although this embodiment has been described with reference to the case where the cross section is semicircular, substantially the same results can be obtained even when the cross section is arcuate, parabolic, or hyperbolic. (However, the larger the curvature, the smaller the dimension l tends to be.)

[Brief explanation of the drawing]

FIG. 1a is a front view of the grating film, and FIG. 1b is a plan view. FIG. 2 is a side sectional view of an antenna section showing an embodiment of the present invention. FIG. 3 is a relationship diagram showing the relationship between the film dimension l, the orthogonal polarization suppression ratio, and the 3 dB beam width. FIG. 4 is a diagram showing received wave directivity characteristic curves for each film size l. 1... Grating film, 2... Resin film, 3... Metal grid, 4... Antenna horn, 5... Resin plate.

Claims (1)

[Scope of Claim for Utility Model Registration] Double plywood made of an insulating plate with a streamlined side cross-sectional shape with low radio wave transmission loss and a bendable resin film on which a metallic vertical grid is etched is used in the antenna horn opening. 1. An antenna horn opening structure for a closed radar antenna, characterized in that a side cross-sectional bending length of the resin film is approximately 1/3 to 2/3 of a side cross-section bending length of the insulating plate.