Date of Award

Spring 1-1-2014

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Electrical, Computer & Energy Engineering

First Advisor

Dejan S. Filipovic

Second Advisor

Neill W. Kefauver

Third Advisor

Zoya Popovic

Fourth Advisor

Edward Kuester

Fifth Advisor

Scott Palo

Abstract

Frequency Independent (FI) antennas have impedance and far-field properties that remain largely invariant with frequency making them an excellent choice for wideband applications. The need for polarization diversity prompts the use of 4-arm sinuous or 4-arm log-periodic FI apertures that can radiate dual-CP over the full bandwidth. The 4-arm sinuous is almost always backed with carbon loaded absorber limiting its use to receiving or low-power transceiver use. Similarly the 4-arm log-periodic antenna suffers from twice the size requirement as compared to the spiral or the sinuous. The sinuous and log-periodic antennas that are commercially available or reported in literature are rated only for low powers (2 - 10 W) and are not low-profile especially in the UHF band. The need for low profile antennas extends to the HF band also where antennas capable of transmitting with large instantaneous bandwidths are needed. It is this need for transmit capable low profile antennas in HF/UHF bands that this thesis addresses.

The development of low-profile antennas for transmit applications in the parts of UHF (500 MHz - 3 GHz) and HF (2 MHz - 10 MHz) bands is presented. In the UHF band, 4-arm sinuous and 4-arm planar log-periodic (LP) antennas are chosen for their wideband dual circular polarized (CP) FI performance. Various techniques like dielectric lens-loading, lossless cavity-backing with integrated matching network, ferrite-tiled cavity-backing, and ferrite-tiled ground plane backing are incorporated to convert the inherently bidirectional sinuous aperture into low-profile, flush-mountable, efficient, unidirectional antennas with dual CP capability. The developed prototypes are fabricated, characterized, and shown to be capable of transmitting 10 times higher power than conventional sinuous antennas while maintaining thermal and electromagnetic stability.

A novel method to halve the size of a 4-arm planar log-periodic aperture for dual CP performance is introduced. The size reduction is achieved by coupling the currents from the booms of the LP to an annular ring resulting in a ring-turnstile-LP composite aperture. This novel aperture is fabricated, characterized and is shown to be a highly desirable antenna for dual CP applications due to its small size, simple beamformer, and dual CP capability over the same bandwidth. The proposed concept is implemented in slot configuration and the resulting ring-turnstile-slot LP is developed and fabricated. Through both low-power and high power characterizations, the antenna is shown to be a low profile bidirectional antenna capable of transmitting hundreds of Watts of CW power.

A reconfigurable antenna configuration of a horizontal dipole and a vertical loop is proposed to achieve bandwidths close to 24 KHz for NVIS communication in the HF band. The antenna is designed to be used with the Amphibious Assault Vehicle (AAV) and is low-profile with NVIS compatible radiation patterns throughout the frequency band. The vertical loop is operated in the lower frequencies and the horizontal dipole at higher frequencies in the HF band and the effects of real grounds, wire diameter and the AAV's tracks on the achieved bandwidth, gain, efficiency, and input resistance are illustrated. Various studies involving the effect of the grounds, position of the antenna above the roof of the AAV, and effect of the AAV's presence on the performance of horizontal dipole and horizontal loop antennas is also detailed.

The results presented in this thesis and the antennas developed herein should pave the way for use of sinuous antenna in efficient transmit applications for the first time, and the use of the log-periodic antenna for dual-CP transmit applications alongside the spiral or the sinuous in its novel reduced size form, while maintaining hundreds of Watts of CW power capability in the UHF band for both the antennas. The reconfigurable antenna in the HF band should provide an excellent starting configuration for achieving large bandwidths for NVIS transmit applications requiring high data rates.

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