Multi-band and reconfigurable antennas for wireless devices

Abstract

This thesis presents the designs of a multi-band slot antenna, a frequency-reconfigurable (FR) monopole antenna, a frequency-reconfigurable multiple-input-multiple-output (FR MIMO) antenna, a polarization-reconfigurable (PR) monopole antenna, a new metasurface (MS) and a circular-polarization-reconfigurable metasurface (CPR MS) monopole antenna.

The multi-band slot antenna consists of a rectangular slot, a T-shaped feeding patch, an inverted T-shaped stub and two E-shaped stubs and has a compact area. The rectangular slot and the inverted T-shaped stub together generate the 1st band. The two E-shaped stubs operating as monopole radiators generate the 2nd band. By using a double-folded stub in the T-shaped feeding patch, two resonant frequencies, i.e., 3rd and 4th bands, can be generated and tuned independently. With this method, the slot antenna can have four operating bands and a size smaller than the previous works. The effect of the feeding cable used in measurement and of the cover used in wireless devices is also investigated.

In the FR planar monopole antenna, the radiator consists of two sections, section I and II, which are connected together using a PIN-diode. Section I alone generates a high-frequency band, while section II, when electrically connected together with section I using the PIN-diode, generates a low-frequency band. The radiator of the antenna is meandered for compact size. Continuous tuning of the two operating bands is accomplished by placing a varactor on section I of the radiator, leading to a wide tuning range of 2.39-3.0 GHz. With the use of this technique, a FR monopole antenna and a FR MIMO monopole antenna with wide-continuous tuning are designed. In the FR MIMO monopole antenna, a reconfigurable defected ground structure (DGS) employing two PIN diodes is designed to switch isolation across the whole tuning band. A simple and novel biasing circuit using only three direct current (DC) wires is designed to bias the diodes.

The PR monopole antenna consists of two meandered radiators placed perpendicular to each other, a feeding network using the Wilkinson power divider, two switchable 900-phase shifters with PIN-diode switches for control, and a DGS. The antenna is more compact and has a wider axial-ratio bandwidth than those of the previous PR antennas. A phase shifter and a double-pole double-throw (DPDT) switch are used to reduce the complexity of the biasing circuit, and a metallic reflector is placed at the back of the antenna to increase the boresight gain.

The dual-cap mushroom-like metasurface (MS) is a new design, with each unit cell having a simple patch with a centered via on one side of the substrate and a meandered slot on the other side. The meandered slot introduces inductive effect to lower down the operating frequency and increase the operating bandwidth. The dual-cap mushroom-like MS is used in the design of a compact CPR MS monopole antenna to enhance the boresight gain and bandwidth.