1. Field of the Invention
The invention relates in general to an antenna, and more particularly to a multi-band antenna.
2. Description of the Related Art
In wireless communication system, antenna serves as a medium for the transmission and reception of electromagnetic signals, and the electrical characteristics of an antenna influence the quality of telecommunication. When in service, ordinary antennae are always bothered by multi-path interference problem. To solve this problem, one of the solutions is to improve the quality and performance of signal transmission/reception by means of antenna diversity structure. When the system is operating under a single frequency band, the user may use two or more sets of single band antenna to form an antenna diversity system. For example, the 5 GHz frequency band used in WLAN 802.11a or the 2.4 GHz frequency band used in WLAN 802.11b, a master antenna and a slave antenna are provided to achieve antenna diversity. The master antenna transmits and receives signals, while the slave antenna can only receive signals. Thus, one of the antennae can be selected to receive signals according to the signal intensity. Besides, WLAN 802.11 g operated in the 2.4 GHz frequency band is equipped with two antennae, both of which have transmitting and receiving functions but which one is to be selected depends on the quality of the signals so as to transmit/receive electromagnetic waves coming from different directions.
When the system adopts a dual-band or even a multi-band operation, most antenna systems will adopt a design of using plural sets of independent antennae or using a combined antenna set to achieve antenna diversity so that the excellent characteristics of signals in various bands may be maintained. Therefore at least four sets of antennae are required to meet the operating frequency ranges needed for the operation of the WLAN 802.11a/b/g, namely, 2.4˜2.4835 GHz, 5.15˜5.35 GHz, 5.47˜5.725 GHz and 5.725˜5.825 GHz. Obviously, such a design will largely increase the complexity of the radio frequency system (RF system), reduce operation reliability, and increase manufacturing costs.
Unlike the above design, the design of multi-band antenna uses the second harmonic generation (SHG) effect of a resonant structure to create several resonant modes whereby the object of multi-band operation is achieved. However, such a design has inherent restrictions, i.e., a multiple relationship exists among the central frequency of each resonant mode and that all of the frequency bands are narrow whose bandwidth is hard to expand. For example, in the dual-band antenna of 2.4 GHz and 5 GHz frequency bands used in ordinary WLAN, the designer simply adjusts the structural parameters of the double frequency resonant mode, whose frequency band is 4.8 GHz, to be used for the transmission/reception of 5 GHz electromagnetic signals. Consequently, the transmission efficiency of electromagnetic waves in high frequency range is normally poor, affecting signal quality greatly. Due to the restriction of the multiple relationship among resonant modes, the above design cannot be applied in WLAN 802.11a/b/g whose operating frequency ranges are 2.4˜2.4835 GHz′5.15˜5.35 GHz′5.47˜5.725 GHz and 5.725˜5.825 GHz because multiple relationship does not exist among the bands of 5 GHz frequency ranges. Furthermore, the overall bandwidth, which is near 1 GHz, is too wide. With regard to the application in WLAN 802.11a/b and WLAN 802.11a/g under these circumstances, how to develop an antenna covering the operating characteristics of various frequency bands and having the advantages of small size at the same time has become a hard-to-break-through bottleneck for designers.