1. Field of the Invention
The present invention relates to a dipole antenna, which can be applied to an ultra-high frequency (UHF) band.
2. Description of Related Art
The radio frequency identification (RFID) tag is widely used nowadays, for example, passports, transportation payments, and product tracking. The function of the RFID tag is to transmit data to the remote terminal. Some of the RFID devices are applied in the UHF band (860-930 MHz). Usually, an RFID tag consists of an antenna and a chip IC. The most common type of the antenna is the dipole antenna.
Design of the dipole antenna is diversified. FIG. 1 is a schematic diagram illustrating a structure of a conventional dipole antenna. Referring to FIG. 1, the conventional dipole antenna includes a radiation metal line 100, wherein a size thereof corresponds to a required operation frequency. The conventional dipole further includes a rectangular loop 102. A distance between a middle area of the rectangular loop 102 and the radiation metal line 100 is d. One end of the rectangular loop 102 has an opening 104 served as a feeding terminal. The radiation metal line 100 and the rectangular loop 102 can be formed on a circuit board, for example, a printed RFID tag, which can be easily fabricated.
FIG. 2 is a schematic diagram illustrating an equivalent circuit of the dipole antenna of FIG. 1. Referring to FIG. 2, the feeding terminal 106 is connected to an external chip. The radiation metal line 100 is inductively coupled to the rectangular loop 102. A signal can be input through the feeding terminal 106, and can be sent out through the radiation metal line 100. Conversely, the radiation metal line 100 can receive the signal, and the feeding terminal 106 can output the signal.
Generally, the impedance of the chip IC is capacitive. In order to deliver the maximum power to the antenna, the impedance of the antenna must be designed to be inductive for conjugate matching. For different operating frequencies and the size reduction requirement, various antenna designs and matching techniques were developed.