In this dissertation, various techniques for small and broadband antennas have been proposed. To implement the small and broadband antennas, the composite right/left handed (CRLH) transmission line theory, planar inverted F antenna (PIFA), lumped capacitor, and varactor diodes are utilized. All designed antennas have utilized the various capacitors and can be applied for mobile applications except for the CRLH resonant antenna.
A new size reduced CRLH resonant antenna consists of the mushroom type resonators, interdigital capacitors, and defected ground structures (DGSs). The proposed antenna has the left-handed (LH), zeroth-order (ZOR), and right-handed (RH) resonant modes due to the two cells mushroom type resonator. Especially, LH resonant mode is the lowest resonance and depends on a series capacitance occurred by interdigital capacitors. The capacitance of interdigital capacitors can be increased by tight coupling made by the DGS placed on the ground plane. As a result, the LH resonant mode is moved to the lower frequency.
In spite of the merit of a small size, applications of a CRLH transmission line antenna is limited because of the narrow bandwidth. A compact dual-band antenna with a lumped capacitor for wireless local area network (WLAN) applications is designed to secure a wide band. The loop strip with a lumped capacitor of a top plane generates resonances of 2.4 GHz band and 5 GHz band is caused by a folded monopole resonance which is made by top and bottom stripline. The equivalent circuit and electric fields are simulated. An antenna size is only 5 × 9 mm2. To consider a practical circumstance, the experiments about the effect of LCD and the ground size have been conducted.
The frequency reconfigurable antenna is very helpful to obtain the broadband characteristic. The proposed model is utilized by controlling the capacitance of the varactor diodes on a dual-band PIFA. The capacitance of the varactor diode is varied by the applied voltage. As changing the capacitance, the frequency is changed by 665 ~ 965 MHz and 1343 ~ 2181 MHz. Therefore, it covers the long term evolution (LTE), code division multiple access (CDMA), global system for mobile communications (GSM), distributed control system (DSC), personal communications service (PCS), and wideband code division multiple access (WCDMA) bands. In addition, the PIFA size is only 35 × 11 mm2.