Antenna design for ultra wide band communications and frequency selective surfaces

In narrow band systems, the use of multiple antennas at the transmitter and the receiver, implementing what is traditionally known as Multiple Input Multiple Output (MIMO) system, has been shown to provide increased channel capacity as compared to a single antenna system.;Whereas traditional MIMO systems employ arrays of antennas that are separated by half a wavelength, MIMO systems implemented using orthogonal co-located loops and dipoles have been shown to be able to deliver the same capacity of traditional systems. In 2002, the Federal Communications Commission (FCC) approved the use of ultra wide band (UWB) devices operating in the frequency range 3.1--10.6 GHz. Since then efforts have been made to design ultra wide band systems for communications purposes. Ultra wide band systems promise increased data rates over short distances compared to traditional narrow band systems. Such systems can find applications in home networks, corporate wireless networks and also military communications. Since MIMO systems employing co-located antennas in the narrow band case deliver increased channel capacity as compared to a single antenna system, the design of co-located antenna systems for ultra wide band communication can potentially provide similar increases in capacity over a wide band.;The current work reports the design of such a vector antenna for an ultra wide band communications system. The vector antenna system consists of a loop antenna and two orthogonal bowtie antennas. It is shown that these antennas have a broad bandwidth, transmit and receive narrow pulses and have low mutual coupling. The performance of the antenna system is evaluated in a rich scattering environment and capacity calculations show that the channel capacity obtained is approximately equal to those employing a linear array of UWB antennas. It is also shown that the co-located vector antenna provides approximately a three fold increase in capacity as compared to a linear array of UWB antennas. An alternate design of a UWB vector antenna which is not co-located but distributed in space is also described. This antenna is smaller compared to the co-located antenna and provides similar increases in capacity.;Vector antennas can not only be used as active arrays but also as individual elements in passive arrays which is also investigated in this work. These arrays produce reflections at certain frequencies and allow transmission at others forming Frequency Selective Surfaces (FSS). Preliminary results including the reflection co-efficient curves for different geometries are reported. A vector antenna design of an FSS is described in detail which resonates at multiple frequencies producing a distinct signature.