Dual-band and frequency-reconfigurable monopole antennas

Abstract

The designs of three compact dual-band monopole antennas for wireless-local-area-network (WLAN)applications are presented. In these designs, an L-or U-shaped monopole element with microstrip-fed is used to generate a high-frequency band at around 5.5 GHz to cover the high WLAN bands at 5.2/5.8GHz for the IEEE 802.11a standard. An E-shaped element, loop element or meander-microstrip ground stub element with coupled-fed through the monopole element is used to generate a low-frequency band at around 2.4 GHz to cover the low WLAN band for the IEEE 802.11b/g standards. With such arrangements, the three antenna shave very compact radiators of only 11.3×8 mm2,12.6×9 mm2and11.8×9.4 mm2. To investigate the performances for practical uses, these antennas are also designed on a mobile-phone printed-circuit board and studied using computer simulation and measurement. Dual-band antennas with reconfigurable

Dual-band antennas with reconfigurable lower band, higher band and dual-band are designed in this thesis. The dual-band antenna consists of two radiating branches generating the frequency bands at around 2.4 GHz and 3.5 GHz for the WiMAX system. Varactors are placed on the corresponding branches for continuously tuning of the operating bands for different WiMAX standards. For frequency tuning of the lower band or higher band, simple and novel DC biasing circuits without requiring any soldering wire are proposed to bias the varactor on a radiating element. While for simultaneous frequency tuning of the two individual bands, simple and novel DC biasing circuits requiring two soldering wires are proposed to bias the varactors on the radiating elements. Both simulation and measurement results show that the DC biasing circuits designed have very little affects on the antennas performances.

The design of a monopole ultra-wide band (UWB)antenna with a reconfigurable notch band is presented. The antenna employs a vertical-ellipse radiator to achieve an UWB. A compact defected-ground structure (DGS)is used to create a notch band for the antenna. To frequency tune the notch band, a varactor is placed on the DGS to control the resonance frequency. The tuning performance, in terms of reflection coefficient, radiation pattern, efficiency and gain, of the antennais studied using simulation and measurement. Results show that the notch band can be tuned continuously from 5.2 to 6.32 GHz for the WLAN bands.

In the measurement of a monopole antenna with a small ground plane, the feeding cable used to connect the antenna to the measurement equipment Satimo Starlab system causes discrepancies between the simulated and measured radiation patterns, efficiencies and peak gains at lower frequencies. In the designs of antennas in this thesis, the cable effects are studied by modeling the feeding cable using the EM simulation tool CST. Results show that, by using the cable model, the simulated and measured results agree very well.