Date of Award

2016

Document Type

Open Access Dissertation

Department

Electrical Engineering

Sub-Department

College of Engineering and Computing

First Advisor

Mohammod Ali

Abstract

Wireless devices such as smartphones, tablet computers, smartwatches etc. have become ubiquitous. With that, the demand for high speed data has increased tremendously. Designing antennas for such applications is challenging because of limited availability of space, shadowing or blockage from the human body, and signal loss from multipath fading. Conventional broad, fixed beam low gain antennas result in poor reception, faster battery drainage, and low data rate. Compressed footprint high gain pattern reconfigurable antenna arrays can solve these problems which is the focus of this dissertation. Two innovative high gain pattern reconfiguration techniques, the switched beam parasitic array and the varactor controlled series-fed phased array are studied and developed.

First, by taking advantage of the controlled coupling between closely spaced driven and parasitic dipoles, a compressed footprint beam steering array is developed for handheld devices. By optimizing the interelement spacing and the ON/OFF states of the RF switches located at the input of the parasitic dipoles, beam steering in the azimuth plane is achieved. Furthermore, a collinear arrangement of subarrays allows narrow elevation plane beamwidth and gain of up to 11 dBi. By contrast, typical handheld device antennas have about 3 dBi gain and little or no steering ability. System level analysis shows about 59% improvement in signal-to-interference-plusnoise ratio level over traditional omnidirectional antennas.

Second, a high gain switched beam parasitic array is proposed based on fabric materials which can be integrated within the clothing or uniforms of first responders. Material sensitivity analyses considering various conductive and nonconductive fabrics are performed. Studies of the array near a multilayered human body phantom reveal that a minimum distance from the body is required for the array to allow beam steering and high gain. For example, with 10 mm spacing from the body −300 to 300 steering is achieved with 10 dBi peak gain which are excellent for high throughput communication.

Third, a novel concept to design ultrathin directional broadband antennas using a nonuniform aperiodic (NUA) metasurface is introduced. By employing a decreasing taper for both the metasurface patch and their interelement spacing, broad impedance and pattern bandwidths are attained. Experimental results show that, with a total thickness of 0.04 free-space wavelength at the lowest frequency of operation, an octave bandwidth can be obtained, which is significantly larger compared with existing designs on uniform mushroom electromagnetic band-gap structures. Based on the NUA metasurface, a thin switched beam (00, 250, and 3350) parasitic antenna array is presented which with a thickness of 0.04 wavelength can attain high gain (8.4 dBi) and very high front-to-back ratio.

Finally, to overcome the challenges of wide and overlapping beams with parasitic arrays, and the space constraint and circuit complexity required by phased arrays, a new varactor controlled series-fed phased array is proposed for wearable applications. At the center of the design is a varactor controlled phase shifter, where varactor capacitance is changed by applying different bias voltages which alters the progressive phase between series-fed antenna input currents and allows array pattern to be reconfigured. Low return loss, high gain, and beam steering with nulls between two consecutive beams are achieved. It is observed that the choice of substrate and varactors are critical to minimize loss. While the works presented here reflect the 5 GHz frequency band the design and ideas are likely scalable and adaptable for next generation mm-wave systems operating at 28, 38, and 60 GHz.

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