King Saud University

Small size antennas usually suffer from bandwidth limitations. Bandwidth can be increased by adding lossy elements but, they significantly affect the efficiency of the antenna. One method to solve the efficiency problem without increasing the antenna size is to use the tunable antenna concept. Recently, tunable antennas attract much attention in mobile communications. The objective of this project is to develop compact dual-band electronically tunable microstrip antennas for operation in GSM/DCS-1800 system. Tunable antenna, is a small size antenna that would not cover all bands simultaneously, but provides narrower instantaneous bandwidths that are dynamically selectable at higher efficiency than conventional antennas. Bandwidth selection can be achieved by electronically change the reactive loading of the resonator by means of PIN diode or varactor diode. The main characteristics of the PIN and varactor diodes that are useful for tunable microstrip antenna design are described. The loading effect of varactor diode on microstrip circuits is investigated and design curves that relate the biasing voltage to the effective electrical line length for different line widths are presented. The analysis of a compact dual-band microstrip antenna suitable for this application is, then, described. A dual-band antenna is designed and implemented to operate at the GSM/DCS-1800 bands when connected to the varactor diode SMTD3100. The design and implementation is carried out on Duroid dielectric substrate with εr = 2.2 and thickness 1.57 mm. The simulations are performed using IE3D simulator. Simulation results show that the required bandwidth can be easily covered with voltage changes from 0 V to 3 V, which is suitable for mobile hand phones. Frequency shift between simulations and measurements due to the limited accuracy of the varactor diode capacitance is observed. This frequency shift is compensated by adding a small piece of copper foil on the element that resonates at the lower band. However, the circuit layout is difficult to support such modification at the higher band. Computed radiation patterns show consistency for different bias conditions and show omni-directional shape.