This invention relates to an antenna unit and, in particular, to an antenna unit for use in a digital radio receiver for receiving an electric wave from an artificial satellite (that may be also called a “satellite wave” hereinafter) or an electric wave on the ground (that may be also called a “terrestrial wave” hereinafter) to listen to a digital radio broadcasting.
In recent years, a digital radio receiver, which receives the satellite wave or the terrestrial wave so as to listen to the digital radio broadcasting, has been developed and is put to practical use in the United States of America. Specifically, two broadcasting stations called XM and Sirius provide radio programs on 250 or more channels in total. The digital radio receiver is generally mounted on a mobile object such as an automobile and is adapted to receive a radio wave having a frequency of about 2.3 gigahertz (GHz) as a received wave to listen to the digital radio broadcasting. In other words, the digital radio receiver is a radio receiver capable of listening to mobile broadcasting. Inasmuch as the received wave has the frequency of about 2.3 GHz, a reception wavelength (resonance frequency) λ thereof is equal to about 128.3 mm. It is noted here that the terrestrial wave is a radio wave obtained by receiving the satellite wave at a ground station, slightly shifting the frequency of the satellite wave, and retransmitting the linear polarized wave. Thus, the terrestrial wave is the linear polarized wave exhibiting linear polarization while the satellite wave is a circular polarized wave exhibiting circular polarization.
An XM satellite radio antenna apparatus normally serves to receive circular polarized radio waves from two stationary satellites and, in an insensitive zone of the circular polarized waves, receives a radio wave by using a terrestrial linear polarization portion of the radio antenna apparatus. On the other hand, a Sirius satellite radio antenna apparatus normally serves to receive circular polarized radio waves from three orbiting satellites (synchronous type) and, in the insensitive zone, receives a radio wave by a terrestrial linear polarization portion of the radio antenna apparatus.
As described above, the radio wave having the frequency of about 2.3 GHz is used in the digital radio broadcasting. Therefore, an antenna for receiving the radio wave must be located outside as known in the art. If the digital radio receiver is mounted in the mobile object such as the automobile, the antenna unit is must be attached to a roof of the mobile object (car body). It is therefore necessary for such an antenna unit to be highly resistant to weathering and to render waterproof.
The antenna unit comprises an antenna and an antenna case for covering the antenna. The antenna case comprises a dome-like top cover and a bottom plate. The antenna comprises an antenna element, a circuit board, a shield case, and a shield case. For example, the antenna element may comprise a patch antenna for receiving the satellite antenna. The circuit board is provided with a circuit (hereinafter will be called a signal processing circuit) for performing various kinds of signal processing, such as signal amplification, upon a signal received by the antenna element. The shield case serves to shield the signal processing circuit.
In a case where the antenna unit is attached to the roof of the car body, it is necessary to match color of the top cover in the antenna unit with cover of the car body. It is therefore necessary to coat the top caver of the antenna unit with paint having color which is similar to that of the car body on which the antenna unit is mounted. The car bodies have color variations in abundance. For instance, it will be assumed that there are ten vehicle types each having the color of ten. In this event, it is necessary to support 100 models of the car bodies.
It is therefore in prior art disadvantageous in that types of the antenna units are increased, management thereof is complicated, and it results in degrading production efficiency.
In addition, as an antenna unit related to this invention, an antenna unit capable of surely prevent water from penetrating is known, for example, in United States Patent Application Publication No. 2004/0246196 A1 which will later be called a patent document 1 hereinafter. According to the patent document 1, a gasket for covering at least a periphery of the bottom plate is arranged and the gasket comes into contact with the inner surface of the cover member thereby sealing the inner space. However, the antenna unit disclosed in the patent document 1 has the challenge to surely prevent water from penetrating thereto and no mention is made of how to color of the top cover in the antenna unit.