Study On Ultra-wideband And Frequency Reconfigurable Antennas For Cognitive Radio Systems

Rapid development of modern communication systems has caused significant spectrum congestion. To raise spectrum usage efficiency, cognitive radio(CR) is receiving increased attentions. Through sensing the spectrum, CR system can detect the unused frequency band and change system parameters to use this frequency band to communicate. In the aspect of hardware, CR system needs a broadband antenna with omnidirectional radiation patterns for monitoring and a frequency reconfigurable antenna for communicating. In this dissertation, a thorough research is presented on ultra-wideband(UWB) and frequency reconfigurable(FR) antennas, which are essential components of a CR system. Several novel UWB and FR antennas are proposed, fabricated and measured.Specifically, the major contribution of this dissertation are summarized as follows:1. Three compact UWB antennas are proposed. By adding a folded strip to the ground plane, the current path is enlarged. Thus, the first two antennas demonstrate fairly compact dimensions. The third antenna has modified ground plane and radiator to achieve miniaturization as well as enhanced impedance bandwidth.2. Five UWB antennas with multi-band-notched characteristics are proposed. By cutting slots or/and adding parasitic strips to the UWB antenna, dual-, triple and even quadruple notched bands can be achieved. To better comprehend the mechanism of the band-notched properties, distributed-element and lumped-element equivalent circuit models of the dual band-notched UWB antenna are carried out. Besides, a UWB monopole antenna with extra GSM, Bluetooth, and dual-notched bands is presented. All these antennas have compact sizes and the notched bands can be controlled independently.3. Four UWB antennas with sharp band-notched characteristics are proposed. Most UWB antennas have wide notched bands, which causes wasting of the useful frequencies, especially the band between lower and upper WLANs(5.35–5.725 GHz). Based on some novel resonant structures, such as interdigital capacitance loading loop resonator(IDCLLR), complimentary interdigital capacitance loading loop resonator(CIDCLLR) and mushroom-type electromagnetic-bandgap(EBG) structures, four compact UWB antennas with sharp band-notched characteristics are presented. Due to their high Q values, these resonant structures can achieve lower notched frequencies and narrower notched bandwidths,thus the present designs effectively save many more useful frequencies.4. A UWB antenna with tunable band-notched characteristic is proposed. Once a traditional UWB antenna is fabricated, the notched band is also fixed. However, the existing undesired radio signals vary from time to time and from place to place. Therefore, a compact UWB antenna with tunable band-notched characteristic is proposed. The center frequency of the notched band can be electronically tuned by employing varactor diodes on the open-loop resonators. Such an antenna can adapt itself to the changing environment.5. Frequency reconfigurable designs based on bow-tie antenna are proposed. By using PIN diodes and/or varactor diodes, two FR antennas and a FR array are presented. The first antenna achieves a continuous wide tuning range from 3.04 to 5.89 GHz(51%) using a pair of PIN diodes and a pair of varactor diodes. The second antenna enables discrete tuning using PIN diodes to operate in either Bluetooth, Wi MAX or WLAN band. The novel biasing circuit used in this design eliminates the need for extra bias lines, and allows the biasing network to be located far away from the radiator, thus avoids distortion of the radiation patterns. Besides, a FR array using the novel biasing circuit is proposed. The array has compact size, simple biasing circuit and is easy to integrate. Moreover, similar radiation patterns are achieved across the entire tunable frequency range for all the antenna elements and array.