CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is based on PCT filing PCT/JP2018/032822, filed Sep. 5, 2018, which claims priority to JP 2017-170247, filed Sep. 5, 2017, the entire contents of each are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a small low-profile antenna device suitable for applications such as telematics.

BACKGROUND ART

In recent years, there has been increasing demand for telematics for vehicles carrying communications equipment. Telematics is a combination of the words “telecommunication” and “informatics”, and is a technique for providing information and services in real time to communications equipment of a vehicle using a mobile communications system and the like.

As a technique for responding to such demand, for example, Patent Literature 1 discloses an antenna device that conducts MIMO communication using a frequency band of LTE (Long Term Evolution) communication. The LTE communication is a communications mode that speeds up the third generation (3G) communication. The MIMO (Multiple-Input Multiple-Output) communication is a communications mode that uses plural antennas, transmits different data from each antenna, and receives data simultaneously by the plural antennas.

The antenna device disclosed in Patent Literature 1 includes plural antennas housed in a shark fin antenna housing with length 100 mm, width 50 mm, and height 45 mm. One of the antennas is an unbalanced antenna, i.e., a monopole antenna, which determines the height of the antenna device. Not only the antenna device disclosed in Patent Literature 1, but also antenna devices mounted on vehicles use a vehicle roof as a ground plane, and thus monopole antennas are used often.

PRIOR ART DOCUMENTS

Patent Literature

Patent Literature 1: National Publication of International Patent Application No. 2016-504799

SUMMARY OF INVENTION

Problems to be Solved by the Invention

Preferably the antennas used for LTE communication and MIMO communication have high gain in the horizontal direction (direction parallel to the ground) orthogonal to the zenith direction (upward in the vertical direction). Also, antenna devices mounted on vehicles are required to be small and low-profile.

However, if a monopole antenna is made low-profile as with the antenna device disclosed in Patent Literature 1, the antenna size (height) in the zenith direction decreases, resulting in deterioration of a VSWR (Voltage Standing Wave Ratio) and shortage of gain in the horizontal direction. The monopole antenna can be made low-profile to some extent by loading an antenna coil and the like to satisfy a resonance condition or interposing an impedance matching circuit, but it is difficult to reduce deterioration of VSWR of the antenna itself or gain in the horizontal direction. Also, to conduct MIMO communication using an antenna device for a vehicle, it is necessary to mount plural antennas, and thus there is a limit to downsizing.

An object of the present invention is to provide a small low-profile antenna device that can properly transmit and/or receive signals in a wide frequency band without providing an antenna coil and increase gain in the horizontal direction.

Solution to Problem

The present invention provides an antenna device for a vehicle, the antenna device being fixed to a predetermined part of the vehicle and comprising at least one metal surface, wherein: a slot is formed in the metal surface with a slit provided at a part of edges of the slot; the slot faces a direction parallel to the ground; and a power feed unit is provided on inner edges between either slot end of the slot and the slit.

Advantageous Effects of Invention

When a slot is used as an antenna element, a direction orthogonal to the antenna element corresponds to main polarization. Also, gain in an opening direction of the slot becomes high. In the antenna device according to the present invention, since the slot facing a direction parallel to the ground is formed in the metal surface, the gain in the direction parallel to the ground becomes high. In the metal surface, since the slit is provided at a part of edges of the slot and the power feed unit is provided on inner edges between either slot end of the slot and the slit, types of available frequency bands increase compared to when there is no slit. That is, bandwidth can be widened.

