The present invention relates to a multi-frequency antenna capable of operating in two different mobile radio bands and FM/AM radio bands.
There are known various types of antenna that are installed on vehicles, but conventionally, roof vehicles which are installed on the vehicle roof have been preferred since they enable reception sensitivity to be improved by means of the antenna being installed of the roof which is the highest position on the vehicle. Moreover, since an FM/AM radio is generally fitted in a vehicle, it is convenient to use an antenna capable of receiving both FM and AM radio bands, and hence roof antennas which are capable of receiving two radio bands conjointly have been widespread.
If a mobile telephone is mounted in a vehicle, then an antenna for the mobile telephone is fitted to the vehicle. In this case, if the number of usable frequencies for mobile telephones has become insufficient due to an increase in the number of subscribers, then there may be cases where two frequency bands are allocated for mobile telephone use, namely, a frequency band which can be used in all regions, and a frequency band which can be used in urban areas. For example, in Europe, mobile telephones using the 900 MHz band Global System for Mobile communication (GSM) can be used in all regions of Europe, but in urban areas, in order to compensate for the insufficiency of usable frequencies, mobile telephones using the 1.8 GHz Digital Cellular System (DCS) can also be used. If corresponding antennas are fitted respectively and independently in a vehicle, then design problems arise and maintenance and installation tasks, and the like, become more complex, and hence multi-frequency antennas which can receive two frequency bands for mobile telephones, and FM/AM radio bands, in a single antenna, have been proposed.
A multi-frequency antenna disclosed in Japanese Patent Publication No. 06-132714 is known as one example of this type of multi-frequency antenna. This multi-frequency antenna is constituted by a retractable rod antenna forming a combined three-wave antenna for receiving a mobile telephone band, FM radio band, and AM radio band, a planar radiating element forming a GPS antenna for receiving GPS signals, and a loop radiating element forming a keyless entry antenna for receiving keyless entry signals.
These antennas are installed on the upper face of a main body, and a metal plate is provided in the upper portion of the main body, the planar radiating body and the loop radiating body being formed on this plate via an inductive layer. Since the plate forms a ground plane, the planar radiating element and the loop radiating element operate as microstrip antennas. Furthermore, a protective cover is formed over the planar radiating element and loop radiating element.
Since a multi-frequency antenna of this kind comprises a retractable rod antenna, it is necessary to provide a space for accommodating the rod antenna when it is installed. Therefore, whilst it is possible to install the multi-frequency antenna on the boot lid or wing of the vehicle where such space can be formed, it cannot be installed on the roof, which is the optimum position for situating an antenna, since this does not have the required accommodating space.
Therefore, a multi-frequency antenna designed to resolve this problem is disclosed in Japanese Patent Publication No. 10-93327.
This multi-frequency antenna is constituted by an antenna element designed to resonate at multiple frequencies by being provided with a trap coil, and a cover section having a built-in matching circuit board, or the like, on which this antenna element is installed. By fixing this cover section to the roof, the multi-frequency antenna can be installed on the roof.
With increase in the number of mobile telephone users, a plurality of frequency bands have been allocated for mobile telephone use. For example, in the PDC (Personal Digital Cellular telecommunication system) used in Japan, the 800 MHz band (810 MHz-956 MHz) and 1.4 GHz band (1429 MHz-1501 MHz) are allocated. In Europe, the 800 MHz (870 MHz-960 MHz) GSM (Global System for Mobile communications) and the 1.7 GHz (1710 MHz-1880 MHz) DCS (Digital Cellular System) are employed. To operate an antenna in a plurality of operating frequencies of this kind, antennas which operate in the respective frequency bands are provided, but generally, two antennas are connected by means of a choke coil so that they do not mutually affect the operation of the other.
However, in a choke coil, such as a trap coil, or the like, it is difficult to separate signals across a broad frequency range. In other words, even if a choke coil is provided between antennas operating in respective frequency bands, if the frequency bandwidths are large, as in mobile telephone bands, then it is not possible to make the respective antennas work independently across these frequency bands, and hence there is a problem in that the antennas affect each other and cannot be made to operate satisfactorily.
Moreover, a problem also arises in that the antenna increases in size due to the inclusion of a choke coil.
Therefore, it is an object of the present invention to provide a compactified multi-frequency antenna which operates across at least two broad frequency bands.
In order to achieve the aforementioned object, the multi-frequency antenna according to the present invention is a multi-frequency antenna comprising: an antenna circuit board, on which are formed an antenna pattern and a passive element pattern, in the proximity of the antenna pattern; an antenna case section for accommodating the antenna circuit board; and an antenna element, wherein a choke coil is disposed between an upper element and a lower element, the lower end of the lower element being connected to the upper end of the antenna pattern formed on the antenna circuit board when the antenna element is installed on the antenna case section; wherein antenna means comprising the lower element, the antenna pattern and the passive element pattern is able to operate in a first frequency band, and a second frequency band, which is approximately double the frequency of the first frequency band.
Moreover, in the multi-frequency antenna according to the present invention described above, the first frequency band and the second frequency band may be mobile radio bands.
Furthermore, in the multi-frequency antenna according to the present invention described above, the whole of the antenna including the upper element and the choke coil may be able to operate in a third frequency band, which is lower than the first frequency band.
Moreover, in the multi-frequency antenna according to the present invention described above, frequency dividing means for dividing the first frequency band and the second frequency band from the third frequency band may be incorporated into a circuit board accommodated inside the antenna case section.
Furthermore, in the multi-frequency antenna according to the present invention described above, the frequency dividing means may include a matching circuit for the first frequency band and the second frequency band.
According to the present invention, antenna means comprising a lower element, and an antenna pattern and passive element pattern formed on an antenna circuit board, is able to operate in a first frequency band and a second frequency band, which is approximately double the frequency of the first frequency band, without using a choke coil, and hence the multi-frequency antenna can be compactified.
Moreover, FM/AM broadcasts can be received by the whole antenna including an upper antenna connected via a choke coil to the lower element. The multi-frequency signal received by the multi-frequency antenna is divided by frequency dividing means into a mobile radio signal and an FM/AM signal. In this case, a matching circuit can also be incorporated into the section for dividing the mobile radio bands, and since the frequency dividing means is accommodated inside the antenna case section, a more compact composition for the multi-frequency antenna can be achieved.