Original Publication Date: 2001-Mar-01
Included in the Prior Art Database: 2003-Jun-19
It is possible to construct a multi-tuned antenna to place a small metallic object near by the original antenna which has a fundamental resonant frequency. The size of the metallic object ( parasitic element) is much smaller than the original antenna. The intrinsic resonant frequency of the parasitic element is higher than the original antenna. The antenna with the parasitic element has two resonant frequencies. One is the original resonant frequency and other is the parasitic resonant frequency. This type of antenna can be operated at 2.4 GHz and 5.2 GHz ISM bands wireless LAN application and also suitable for a software wireless equipment by single antenna system without any switching device.
Theory of Operation
A ntenna w ith P arastic Elem ent
As shown in the Figure above, the feeding point voltage and current can be expressed in the form of an impedance matrix as below.
Driven A ntenna Z11
Parasti A ntennaX22
= Z11 Z12
If one of the antenna elements is parasitic, then the voltage induced at its terminal is zero, because the terminal point is shorted. The matrix can be as following.
= Z11 Z12
The driving input impedance is
22Z2(1IV (3) Zin =1 = Z11 -Z12 ) For simplicity, assuming both antennas are dipole antennas, the Fig. 2 shows driving point input impedance (Zin), mutual impedance (Z12, Z21) and self impedance (Z11,Z22) versus frequency by theoretical calculation. There is a frequency range where Z12 is larger than Z22. In this range, absolute value of the 2nd term in Eq.(3) become larger than the 1st term. The Z11 is reduce by the 2nd term. But outside this range, the 2nd term is very small. So it does not affect the Zin. The total impedance Zin is a...