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You searched for +publisher:"University of Colorado" +contributor:("Neill W. Kefauver"). Showing records 1 – 3 of 3 total matches.

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University of Colorado

1. Samson, Timothy W. Characterization of Alternative Mounting of Vehicular Antenna Systems Near Real Grounds.

Degree: MS, Electrical, Computer & Energy Engineering, 2015, University of Colorado

Vehicular antenna systems for jamming and communications are frequently mounted on either the roof or rear bumper of the vehicle. While this facilitates easier mounting and maintenance of the antenna systems, it reduces the maneuverability and increases the visual profile of the vehicle. This thesis researches alternative mounting locations such as underneath the vehicle or on the sides of the vehicle. For either mounting location, the presence of the ground electrically close to the antenna system may drastically modify its performance and characteristics. Therefore, it is vital to understand how the presence of the ground affects the antenna's patterns, impedance, efficiency, and axial ratio when the antennas are designed to be circularly polarized. Once the effects of the grounds on the antenna systems are understood, steps can be taken to mitigate potential adverse effects and capitalize on any benefits. To better understand propagation close to the ground, both with and without the vehicle model, ideal sources were initially studied. More conductive soils offer higher efficiencies, but cause a greater amount of depolarization for circularly polarized sources. For sources under the vehicle, the source is not just having a stronger coupling with the ground, but the vehicle body redirects energy back towards the ground. To mitigate this, the parallel plate source is proposed for HF and VHF frequencies. Small modifications to the geometry can achieve significantly higher efficiencies for sources under the vehicle. Other antennas are considered for operation in the UHF region. For antenna systems mounted on the side of the vehicle, two mounting approaches (for spiral antennas) are considered. First, a cavity-backed aperture is mounted flush with the vehicle's body. The second configuration has the spiral antenna aperture offset from the vehicle thus requiring minimum changes to the vehicle's body. While the first approach is more commonly used, the latter provides greater gain and efficiency at the lower end and comparable gain and efficiency at higher frequencies while maintaining good axial ratio. For side mounted sources, the four arm spiral provides a lower and more stable axial ratio when compared to the two arm spiral. Both antennas offer similar efficiency, but the four arm spiral maintains higher quality performance with the introduction of a cavity backing. The performance of side mounted sources can be further improved with the use of the spiral-helix that improves both the low and high frequency performance. Advisors/Committee Members: Dejan S. Filipovic, Neill W. Kefauver, Maxim Ignatenko.

Subjects/Keywords: Antennas; Computational electromagnetics; Jamming; Spiral antennas; Vehicle mounting; Wide band; Electromagnetics and Photonics; Systems and Communications

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APA · Chicago · MLA · Vancouver · CSE | Export to Zotero / EndNote / Reference Manager

APA (6th Edition):

Samson, T. W. (2015). Characterization of Alternative Mounting of Vehicular Antenna Systems Near Real Grounds. (Masters Thesis). University of Colorado. Retrieved from https://scholar.colorado.edu/ecen_gradetds/105

Chicago Manual of Style (16th Edition):

Samson, Timothy W. “Characterization of Alternative Mounting of Vehicular Antenna Systems Near Real Grounds.” 2015. Masters Thesis, University of Colorado. Accessed December 02, 2020. https://scholar.colorado.edu/ecen_gradetds/105.

MLA Handbook (7th Edition):

Samson, Timothy W. “Characterization of Alternative Mounting of Vehicular Antenna Systems Near Real Grounds.” 2015. Web. 02 Dec 2020.

Vancouver:

Samson TW. Characterization of Alternative Mounting of Vehicular Antenna Systems Near Real Grounds. [Internet] [Masters thesis]. University of Colorado; 2015. [cited 2020 Dec 02]. Available from: https://scholar.colorado.edu/ecen_gradetds/105.

Council of Science Editors:

Samson TW. Characterization of Alternative Mounting of Vehicular Antenna Systems Near Real Grounds. [Masters Thesis]. University of Colorado; 2015. Available from: https://scholar.colorado.edu/ecen_gradetds/105


University of Colorado

2. Sammeta, Rohit. Low-Profile Antennas for Wideband Transmit Application in HF/UHF Bands.

Degree: PhD, Electrical, Computer & Energy Engineering, 2014, University of Colorado

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… Advisors/Committee Members: Dejan S. Filipovic, Neill W. Kefauver, Zoya Popovic, Edward Kuester, Scott Palo.

Subjects/Keywords: antennas; high power; log periodic; low profile; sinuous; wideband; Electromagnetics and Photonics

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APA · Chicago · MLA · Vancouver · CSE | Export to Zotero / EndNote / Reference Manager

APA (6th Edition):

Sammeta, R. (2014). Low-Profile Antennas for Wideband Transmit Application in HF/UHF Bands. (Doctoral Dissertation). University of Colorado. Retrieved from https://scholar.colorado.edu/ecen_gradetds/92

Chicago Manual of Style (16th Edition):

Sammeta, Rohit. “Low-Profile Antennas for Wideband Transmit Application in HF/UHF Bands.” 2014. Doctoral Dissertation, University of Colorado. Accessed December 02, 2020. https://scholar.colorado.edu/ecen_gradetds/92.

MLA Handbook (7th Edition):

Sammeta, Rohit. “Low-Profile Antennas for Wideband Transmit Application in HF/UHF Bands.” 2014. Web. 02 Dec 2020.

Vancouver:

Sammeta R. Low-Profile Antennas for Wideband Transmit Application in HF/UHF Bands. [Internet] [Doctoral dissertation]. University of Colorado; 2014. [cited 2020 Dec 02]. Available from: https://scholar.colorado.edu/ecen_gradetds/92.

Council of Science Editors:

Sammeta R. Low-Profile Antennas for Wideband Transmit Application in HF/UHF Bands. [Doctoral Dissertation]. University of Colorado; 2014. Available from: https://scholar.colorado.edu/ecen_gradetds/92


University of Colorado

3. Elmansouri, Mohamed Ali. Joint Time/Frequency Analysis and Design of Spiral Antennas and Arrays for Ultra-Wideband Applications.

Degree: PhD, Electrical, Computer & Energy Engineering, 2013, University of Colorado

Ultra-wideband (UWB) systems transmit and receive extremely short pulses, permitting the corresponding antennas to distort their shape. Thus the design of an antenna for a UWB system plays an important role for the reliability and quality of communication. A UWB antenna design coalesces both the determination of conventional frequency domain parameters and the analysis of time domain response into a single overarching system requirement. While the former is needed to ensure system’s sensitivity, the later is critical to minimize pulse distortion. Well-designed spiral antennas are known for their almost frequency independent characteristics; thus they are viable candidates for UWB systems from the frequency-domain side. However, due to their fundamental principles of operation, they are dispersive and arguments were made they should not be used for pulsed UWB applications (time-domain side). The presented research unequivocally proves that spiral antennas and various derivatives thereof, including arrays, can be excellent candidates for multifunctional time/frequency domain systems. A complete framework for joint frequency and time domain characterization of planar spiral antennas in UWB communication systems is developed first. By utilizing theory, simulations, and experiments, all essential to the analysis frameworks, the various hypotheses are comprehensively treated and relevant conclusions are established. The dispersion and pulse distortion of the conventional spiral antennas are characterized in the radiation and system modes and conclusions regarding the effects of geometrical parameters such as number of arms, mode of operations, etc., on time- and frequency-domain performance are derived for the first time. A method based on controlling the spiral’s growth rate and input pulse shape is demonstrated as an effective approach to reduce the pulse distortion. Theoretical pre-distortion compensation method based on a frequency-dependent delay removal technique is employed and performance enhancement of spiral antennas as pulse radiators is successfully demonstrated. A novel spiral antenna topology, named combined power spiral, is derived from first principles to have simultaneously excellent time- and frequency-domain performances without any auxiliary hardware and/or pre-distortion compensation. The role of the reflective cavity backing on the performance of spiral antennas in time and frequency domains is investigated in order to achieve an efficient unidirectional UWB radiation. Resistively-loaded cavity-backed spirals are designed as a compromise for achieving simultaneously good time and frequency domain performances while maintaining high efficiency over the most of operating bandwidth. The lens loading approach is used as a way to further improve the spiral’s gain and reduce the amplitude distortion associated with a typical communication channel. UWB spiral arrays based on the derived good time/frequency two- and four-arm spiral antennas are developed and analyzed in time and frequency… Advisors/Committee Members: Dejan S. Filipovic, Neill W. Kefauver, Edward Kuester, Zoya Popovic, Douglas Sicker.

Subjects/Keywords: Spiral Antenna; Spiral Antenna Arrays; Time Domain; Ultra-Wideband; Electrical and Computer Engineering; Electromagnetics and Photonics

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APA · Chicago · MLA · Vancouver · CSE | Export to Zotero / EndNote / Reference Manager

APA (6th Edition):

Elmansouri, M. A. (2013). Joint Time/Frequency Analysis and Design of Spiral Antennas and Arrays for Ultra-Wideband Applications. (Doctoral Dissertation). University of Colorado. Retrieved from https://scholar.colorado.edu/ecen_gradetds/60

Chicago Manual of Style (16th Edition):

Elmansouri, Mohamed Ali. “Joint Time/Frequency Analysis and Design of Spiral Antennas and Arrays for Ultra-Wideband Applications.” 2013. Doctoral Dissertation, University of Colorado. Accessed December 02, 2020. https://scholar.colorado.edu/ecen_gradetds/60.

MLA Handbook (7th Edition):

Elmansouri, Mohamed Ali. “Joint Time/Frequency Analysis and Design of Spiral Antennas and Arrays for Ultra-Wideband Applications.” 2013. Web. 02 Dec 2020.

Vancouver:

Elmansouri MA. Joint Time/Frequency Analysis and Design of Spiral Antennas and Arrays for Ultra-Wideband Applications. [Internet] [Doctoral dissertation]. University of Colorado; 2013. [cited 2020 Dec 02]. Available from: https://scholar.colorado.edu/ecen_gradetds/60.

Council of Science Editors:

Elmansouri MA. Joint Time/Frequency Analysis and Design of Spiral Antennas and Arrays for Ultra-Wideband Applications. [Doctoral Dissertation]. University of Colorado; 2013. Available from: https://scholar.colorado.edu/ecen_gradetds/60

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