Date of Award
Doctor of Philosophy (PhD)
Electrical & Computer Engineering
Shirshak K. Dhali
The biological effects of intense sub-nanosecond pulses on tissues or cells are in the dielectric domain and not based on thermal loading as in the conventional microwave radiation, which may lead to an entirely new approach of modifying cell functions. Moreover, the resulting cell functional change may be detected with higher resolution by broadband, sub-nanosecond pulses than conventional narrowband systems. The delivery of intense sub-nanosecond pulses to near-field biological tissues, however, has not been studied, not mentioning the focal depth and volume. In this dissertation, for the first time, an impulse radiating antenna with a balanced feed structure is studied for focusing electromagnetic fields in the near-field for the purpose of therapy and target detection. This antenna has the potential of radiating sub-nanosecond pulses up to 100 kV. It is a travelling wave antenna with the conical transmission lines as the wave launcher. The electric field distribution is studied both through experiment and simulation studies. Results show a close agreement between experimental and simulated results. The antenna focal spot is found to be 32cm wide in axial direction and 10cm wide in lateral direction near the focal point, which is 16cm from the aperture plane. Enhancement of focal spot size and increase of field at the focal point is studied with a dielectric lens. The use of a dielectric lens to match the waves to the target medium increases coupling between the antenna and the target medium, thereby increasing the field strength at focus and decreasing the focal spot size. Experimental study shows an increase in electric field at focus by a factor of 3 and an increase in resolution by a factor of 1.5. The delivery of sub-nanosecond pulses to tissues is studied with the antenna and the combination of the lens and antenna. While a lossless lens may enhance the coupling of the radiation to the tissue, the trend of decreasing in intensity as the wave penetrates remains the same as the case where only an antenna is used. However, the trend can be reversed or modified by a lossy lens, which contains some resistive materials as part of its structure. With a lossy lens, a local maximum forms in the deep region (6cm in depth) of the tissue. The design of such lossy lens is novel and provides an extra means to control the electric field distribution in the target.
"Characterization of an Impulse Radiating Antenna in the Near Field"
(2014). Doctor of Philosophy (PhD), Dissertation, Electrical & Computer Engineering, Old Dominion University, DOI: 10.25777/g43z-nr61