1. Field of the Invention
The present invention relates to an antenna, more particularly to a miniature monopole antenna for dual-frequency printed circuit boards.
2. Description of the Related Art
In general, a so-called Lecher wire is usually used in the traditional parallel antenna structure of televisions. Please refer to FIG. 1. When the parallel radiation metal pipe 14 (such as a copper pipe) of such antenna is nearby, it can sense that the current flows in an opposite direction (as indicated by the arrow in FIG. 1), and thus causing an electromagnetic field in opposite directions to offset with each other without producing a radiation. Therefore, in order to let an antenna maintain an effective radiation of electric waves in a narrow space, the front end of a Lecher wire is generally divided into two ends and bent 90 degrees in opposite directions with each other as shown in FIG. 2, such that the current can flow in the same direction (as indicated by the arrow in FIG. 2) to constitute a so-called dipole antenna. Such dipole antenna uses a transmission line of the balanced structure as a feed line 24, and two signal line terminals 242 of the transmission line of such balanced structure is extended separately to the same length in opposite directions. The length of such signal line terminal is about a quarter of the resonance wavelength (λ), and thus the total length is about half of the resonance wavelength (λ). The dipole antenna can thus use two sections of such signal line terminal with a length equal to a quarter of the wavelength as the radiator. Therefore, such antenna is also called half-wavelength dipole antenna, which is generally adopted in a mono-frequency design.
To make the antenna lighter, thinner, shorter, and smaller, some manufacturers build the antenna in a printed circuit board. Please refer to FIGS. 3 and 4 for the printed antenna, which comprises a dielectric printed wire board; a printed wire 34 built on one side of the printed wire board 37, and one end of the printed wire 34 is used as a signal feed terminal 341, and the other end of the dielectric printed wire board 37 comprises a metal grounding surface 38 corresponding to the position of the printed wire 34, and the other end of the printed wire 34 extends an inverted L-shaped radiator 342 from a position other than that corresponding to the metal grounding surface 38 to define a monopole antenna. Such monopole antenna bases on the image theory to map images of the unbalanced structure of the printed wire 34 and the inverted L-shaped radiator 342 on the metal grounding surface 38, and thus forming a radiator structure equivalent to the aforementioned dipole antenna, and such monopole antenna is generally adopted in a mono-frequency design.
In recent years, since the demand of mobile communication products in the market has been increasing drastically, it expedites the development of wireless communications. Among so many wireless communication standards, the most eye-catching one is the IEEE 802.11 wireless local area network protocol established in 1997, such protocol not only provides unprecedented functions for wireless communications, but also offers a solution for mutual communications between different branded wireless products. Therefore, such protocol opens up a new mileage to the development of wireless communication. However, the IEEE tried to combine the IEEE/ANSI and the ISO/IEC into a joint standard in August of 2000 and further revised the specification. The contents of such revision include two important protocols: the IEEE 802.11a protocol and the IEEE 802.11b protocol. According to the rules of these two protocols, the bandwidths of an extended standard physical layer must be set to 5 GHz and 2.4 GHz respectively. Therefore, when a wireless communication product wants to use both wireless communication protocols, the aforementioned traditional antenna no longer can satisfy such requirement, but has to install additional antennas according to the bandwidth requirements. However, such arrangement not only increases the component cost, complicates the installation procedure, but also requires more space for installing such antennas on the wireless communication product. As a result, the volume of the wireless communication product cannot be reduced to comply with the trend of a compact design.