Low-frequency Antennas, Transparent Ground Planes, and Transponders for Communication Enhancement in Unfavorable Environments

The communication environment has a major influence on the performance of wireless networks. Unlike antennas, receivers, processors, and other components of a typical wireless system, the designer has almost no control over the communication channel. Therefore, it is imminent that the adverse effects of the communication channel such as path-loss, multi-path, lack of a clear line of sight, and interference are among the most limiting factors in designing and operating wireless networks. Recent investments in infrastructures such as cell-phone towers, communication satellites, routers, and networking devices have been aimed at reducing the aforementioned adverse effects. However, wireless ad hoc networks (WANET) cannot rely on pre-existing infrastructures such as access points or routers. In this thesis, a number of solutions are presented to enhance communication and navigation in harsh environments. 1) At lower frequencies, the defects of the communication channel are less prominent, which has led militaries to use UHF and VHF frequency bands for communication. A number of optically transparent UHF antennas are developed and embedded in the windows of military vehicles to reduce their visual signature. 2) Direction finding at low frequencies using baseline method results in an exorbitantly large array of sensors. However, a vector sensor consisting of three orthogonal two-port loop antennas can be used. A simple and accurate circuit model for the two-port loop antenna is developed for the first time that can be used for direction of arrival estimation over a wide range of frequencies and angles. 3) Using a conventional radio repeater with ad-hoc systems requires a communication protocol and decreases the throughput by a factor of two for every repeater in the chain. A full-duplex repeater, capable of simultaneously transmitting and receiving at the same frequency, is developed for the 2.4 GHz ISM band.PHDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/143898/1/manikafa_1.pd

Electromagnetic Wave Transmission through Sub-wavelength Channels and Bends Using Metallic Wires

Techniques and technologies to transfer electromagnetic energy through sub-wavelength channels have been researched extensively in the past few years because their application in different areas such as sub-wavelength imaging, telecommunication, increasing the storage capacity, and confinement and transmission of electromagnetic energy. Common ways of achieving such transmission includes exciting surface plasmon polaritons on both sides of the cannel or using double negative metamaterials. Recently a mechanism to squeeze the electromagnetic energy through sub-wavelength channels using materials with extremely small permittivity was introduced. Such materials may be found naturally at some limited frequencies in the infrared and optical frequency ranges, but they are commonly fabricated for a desired frequency as engineered metamaterials by by embedding metallic inclusions in a dielectric medium. The main problem with the engineered materials is that they have relatively large losses at their low permittivity frequency. In this thesis,I have presented a novel structure consisting of arrays of metallic wires that can be used to squeeze electromagnetic energy through sub-wavelength channels and junctions with negligible loss. The theory of transmission through such array is derived and design methods to tune the transmission frequency is provided. The structure is also tested numerically and experimentally in several geometries and results are compared with previous methods

Dielectric Sensors Based on Electromagnetic Energy Tunneling

We show that metallic wires embedded in narrow waveguide bends and channels demonstrate resonance behavior at specific frequencies. The electromagnetic energy at these resonances tunnels through the narrow waveguide channels with almost no propagation losses. Under the tunneling behavior, high-intensity electromagnetic fields are produced in the vicinity of the metallic wires. These intense field resonances can be exploited to build highly sensitive dielectric sensors. The sensor operation is explained with the help of full-wave simulations. A practical setup consisting of a 3D waveguide bend is presented to experimentally observe the tunneling phenomenon. The tunneling frequency is predicted by determining the input impedance minima through a variational formula based on the Green function of a probe-excited parallel plate waveguide