1. Field of the Invention
The present invention relates to an antenna device which can be used in an on-vehicle radar device for monitoring the driving direction of cars.
2. Background Art
An on-vehicle radar device has a radar function using millimeter waves, for example, and improves the driving safety of a car, so the development of a device with higher performance and lower price is under way for its dissemination. Such an on-vehicle radar device performs digital beam forming (DBF), for example.
The radar device performing DBF includes a plurality of columns of receiving antennas arrayed in the transverse direction and generates scanning beams by converting receiving signals from each receiving antenna into digital data, a giving phase difference to each receiving signal equivalently by arithmetic processing, and synthesizing the receiving signals. The radar device does not need driving parts or operating mechanisms, and can scan beams at a high speed and with a high degree of precision.
A field of view of about 20° in the transverse direction is necessary to monitor preceding cars or intercepting cars on the own driving lane or the adjacent lane in front. As a radar antenna, the waveguide slot array antenna can form beam characteristics of a fan shape suitable for this, and further a high gain is obtained since the reduction in power supply is small. The whole of this antenna is composed of a metal flat plate, so it has characteristics suitable to a small on-vehicle radar device, such as almost no performance variation or deformation due to heat and the ability to obtain a heat radiation function or the like.
Here, a conventional waveguide slot array antenna is disclosed, for example, in JP-A-2010-103806. The outline and principle are described in pp. 112 to 119 of “New Millimeter Wave Technology” written and edited by Tasuku Teshirogi/Tsukasa Yoneyama, Nov. 25, 1999, Ohm Co., Ltd.
The waveguide slot array antenna is a traveling-wave antenna which can obtain a high gain by forming a plurality of slots on the wall surface of sufficiently long waveguides and arranging the waveguides periodically such that the phases of the electric fields radiating sequentially from each slot match one another in a predetermined direction. By having the radiation electric fields of the respective slots match one another, a main beam is obtained in the straight direction with respect to the antenna surface (the waveguide wall surface having slots).
In a high gain single beam antenna used in communications or the like, a plurality of linear arrays are arranged in the transverse direction and power is supplied thereto such that the radiation electric fields of all slots become the same phase by a power supplying waveguide.
As a general structure, a simple manufacturing method, in which a metal thin plate (a slot plate) which has slots punched therein is placed on a metal flat plate (a base) which has waveguide slots processed therein and the peripheries of the plates are screw-fixed, is known.
Here, it is difficult to dispose a partition for separating waveguides and the slot plate without having any gap therebetween; however, a method of suppressing the leakage of radio wave between waveguides by supplying power to the neighboring linear array in a reverse phase is known. This method is to offset by making the wall surface current flow backward on both sides of the partition; therefore, it is very effective in a plane array antenna using a plurality of linear arrays. However, the offsetting effect cannot be obtained from the outermost waveguide, and other measures are necessary. For instance, forming choke grooves on the periphery is disclosed in “The 2000 IEICE General Conference, B-1-134”.