1. Field of Invention
The invention relates to an ultra-wideband monopole antenna structure and, in particular, to an omnidirectional ultra-wideband monopole antenna that provides good omnidirectional radiation patterns for frequencies across a very wide operating bandwidth.
2. Related Art
With the continuous development and advance of digital audio/video (AV) and mobile communications in wireless local area network (WLAN), there have been demands for higher data transmission rate.
The IEEE 802.15 WPAN (Wireless Personal Area Network) put forward by the Institute of Electrical and Electronics Engineers is a standard for ultra-wideband operation with a high data transmission rate. For practical design considerations of the antennas for such an ultra-wideband operation, in addition to providing a wide operating bandwidth with a frequency ratio over 1:3, the antenna has to maintain stable omnidirectional radiation patterns over its operating bandwidth to achieve wide coverage and good communication performances. Thus, whether the ultra-wideband antenna can provide the required stable and omnidirectional patterns over the operating bandwidth is the main factor that determines whether the antenna structure is suitable for practical applications.
Among the currently known ultra-wideband antenna structures, the planar metal-plate monopole antenna has the highest application values. Although this type of antennas can provide an ultra-wide operating bandwidth, their radiation stability and omnidirectional property become worse as the operating frequency increases. Therefore, they cannot satisfy practical needs.
To improve the omnidirectional radiation patterns, the U.S. Pat. No. 6,339,409 discloses a thin, long cylinder structure for the antenna. A rectangular metal plate is coiled into a spiral shape to control the radiation patterns produced by the antenna, thereby satisfying the omnidirectional requirement. However, the drawback of this structure is its complicated structure, which makes good yield difficult to obtain.
Another known wideband antenna structure, such as the one disclosed in the U.S. Pat. No. 4,466,003, makes use of a combination of metal rod with different lengths. Although such a structure can generate many different resonant frequencies, its drawback is also its complicated structure and high production cost. The whole antenna is too large in size. The antenna structure disclosed in the U.S. Pat. No. 5,828,340 cannot satisfy the requirement of omnidirectional radiation patterns and provide a sufficiently wide operating bandwidth.
Therefore, how to design an antenna structure with an ultra-wide operating bandwidth, omnidirectional radiation patterns, and with the characteristics of simple structure, easy fabrication, and low cost is the most important research direction in the field of ultra-wideband monopole antennas.