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Metamaterials are subwavelength-structured materials designed to exhibit tailored electromagnetic properties. Metamaterials have allowed extreme control over constituent material parameters (i.e. permittivity, permeability, and chirality), which has enabled a myriad of counterintuitive physical phenomena. However, metamaterials typically suffer from high losses, difficulties in fabrication, and are bulky. This has led to the development of metasurfaces, which are the two dimensional equivalent of metamaterials. Metasurfaces can impart abrupt discontinuities on electromagnetic wavefronts, allowing electromagnetic fields to be tailored across subwavelength length scales. The building blocks of metasurfaces are subwavelength textured, polarizable particles. Near resonance, these particles support strong currents, which makes them excellent small antennas. In this thesis, a circuit model is developed that can model an arbitrary small antenna based on its frequency dependent polarizability. In addition, a direct transfer patterning process is developed that allows metallic patterns to be printed onto arbitrarily contoured substrates. This work will find immediate applications in a number of emerging technologies resulting from the rapid expansion of the mobile electronics industry. Next, extreme control of the polarization and profile of a wavefront is demonstrated using two-dimensional arrays of polarizable particles (i.e. metasurfaces). A new class of metasurfaces, referred to as metamaterial Huygens' surfaces, is shown to have a significantly improved efficiency over the state of the art. Metamaterial Huygens' surfaces utilize polarizable particles that exhibit both an electric and magnetic response, which allows for reflectionless wavefront control. Next, it is shown that simply cascading patterned metallic sheets can also provide high transmission and complete phase control. To demonstrate the design methodology, several different metasurfaces are developed that deflect incident Gaussian beams to a stipulated angle or convert an incident Gaussian beam into a vector Bessel beam. Further, utilizing sheets with anisotropic patterns provides additional magneto-electric coupling, which enables complete control of a wavefront (i.e. amplitude, phase, and polarization control). The experimental verification at frequencies ranging from microwaves to optics highlights the versatility of this work.

As technology progresses, mobile devices such as laptops, tablets, cell phones, and two-way radios have become smaller in size. Consequently antennas become electrically small to fit inside aggressive packaging requirements with rapidly changing real and imaginary impedances. As such, these antennas are very narrow in bandwidth with high-Q and input impedance which is very sensitive to environmental effects. The radiation efficiency of the device is drastically decreased as the antenna is detuned and signal quality is degraded. As the number of mobile devices we use increases, adaptive impedance tuners have and will become a bigger necessity, especially as more radios are integrated into a single device. This dissertation presents novel improvements to closed loop tuning topologies from a system level perspective addressing impedance tuners, sensing techniques, and how they apply to different antennas. The biggest design hindrance to impedance tuners are losses due to small signal resistance, and loss due to circuit resonances and radiation. A detailed explanation of these loss mechanisms is developed, providing designers with the knowledge to minimize the impact of said losses and improve system efficiency. By exploiting loss mechanisms, a novel small and low cost VHF impedance synthesizer is presented to characterize impedance tuners in load pull measurements.With full consideration of circuit loss mechanisms, a new directional coupler based tuning topology is presented. Traditional tuning topologies aim to minimize |S11| of the matching network. As demonstrated in this work, such a method has the potential to maximize losses in the circuit, especially in multi-stage tuners. Alternative directional coupler based topologies are presented which maximize the system transducer gain. Furthermore, a novel method of sensing a tuned state through the use of a near field probe that detects far field radiated power is introduced. A design guide is detailed with several examples for use with different types of antennas. Concepts developed in this dissertation are demonstrated in an adaptive tuning system where mechanical means of tuning is applied as a low loss tuner. An electrically small monopole is tuned using the power sensor to provide feedback over a 2.2:1 bandwidth (180 to 400MHz) where at the lowest tunable frequency the antenna is 1/11.3 wavelengths in size.

Nowadays miniaturized low-power electronics and wireless devices are ubiquitous in everyday life. However, there is still significant interest to further reduce size, lower the power and improve data rate required by such systems. For such goals, antennas still constitute a major bottleneck in terms of size, efficiency, polarization and radiation pattern. Also for many near ground applications, low profile small antennas with vertical polarization are needed to achieve much lower propagation path loss. Conventional approaches to reduce the height of monopole antennas cause significant drop in efficiency and polarization purity. Two novel miniaturization techniques are presented for different levels of size reduction (λ/300 < h < λ/45 where h is antenna height). For h ≈ λ/45, a novel inductively coupled capacitively loaded antenna topology is presented, which enables up to 9dB higher gain than the conventional inverted-F antenna with a small ground plane. For extremely short monopole antennas with h ≈ λ/300, an in-phase two-element monopole antenna topology is demonstrated, which exhibits 16.5 dB higher gain than the conventional inverted-F antenna having the same dimensions. Also, radiation pattern deformation caused by low profile configurations on a small ground plane is addressed and, to solve this problem, novel designs using vertically and horizontally balanced architectures are introduced. For GPS or mobile platforms where the aforementioned miniaturized antennas with vertical polarization cannot provide reliable communication connectivity, a new miniaturization technique is presented to reduce the size of circularly polarized (CP) patch antennas. 75% size reduction is achieved compared to a conventional CP patch antenna and it is 25% smaller than the smallest CP patch antenna reported in the literature. Lastly, a novel approach to combine the miniaturized monopole and patch antenna elements into a compact common aperture diversity antenna is presented for MIMO applications. This achieves a compact size of 0.27λ X 0.27λ X 0.09λ and low envelop correlation (

There is strong interest to combine many antenna functionalities within a single, wideband aperture. However, size restrictions and conformal installation requirements are major obstacles to this goal (in terms of gain and bandwidth). Of particular importance is bandwidth; which, as is well known, decreases when the antenna is placed closer to the ground plane. Hence, recent efforts on EBG and AMC ground planes were aimed at mitigating this deterioration for low-profile antennas.In this dissertation, we propose a new class of tightly coupled arrays (TCAs) which exhibit substantially broader bandwidth than a single patch antenna of the same size. The enhancement is due to the cancellation of the ground plane inductance by the capacitance of the TCA aperture. This concept of reactive impedance cancellation was motivated by the ultrawideband (UWB) current sheet array (CSA) introduced by Munk in 2003. We demonstrate that as broad as 7:1 UWB operation can be achieved for an aperture as thin as λ/17 at the lowest frequency. This is a 40% larger wideband performance and 35% thinner profile as compared to the CSA.Much of the dissertation's focus is on adapting the conformal TCA concept to small and very low-profile finite arrays. Three particular designs are presented. One is a 6x6 patch array occupying a λ/3 x λ/3 small aperture (mid-frequency is at 2.1 GHz). Remarkably, it is only λ/42 thick yet delivers 5.6% impedance bandwidth (|S11| < -10dB), 4.4dB realized gain (87% efficiency) and 23% gain bandwidth (3dB drop). The second finite TCA consists of 4x2 patches and occupies a λ/3.2 x λ/3.2 aperture on a λ/26 thick substrate (mid-frequency is at 2 GHz). This antenna delivers 17.3% impedance bandwidth, 4.8dB realized gain (95% efficiency) and 30% gain bandwidth. That is, more than twofold impedance bandwidth is delivered as compared to a single patch antenna of the same size on conventional or EBG substrate.The third array being considered consists of 3x2 patches occupying a λ/3.2 x λ/3.2 aperture and situated on a λ/22 thick substrate (mid-frequency is at 1.95 GHz). Although of very low-profile and of small size, this TCA achieves a 26% wideband performance with 4.5dB realized gain (97% efficiency) and 40% gain bandwidth. It therefore covers DCS, PCS and UMTS bands (1.7 GHz - 2.2 GHz) for mobile communications. As compared to a conventional patch antenna that can cover the same frequency bands, the TCA is 85% smaller in size while also being 60% thinner.This dissertation demonstrates that the tightly coupled array concept is quite versatile for miniaturization, bandwidth enhancement and applicability to a diverse range of next generation antennas.

With the rapid growth of wireless communications and associated demand for high data-rates, many future antennas must be ultra-wideband (UWB). A single antenna is certainly a most attractive solution to handling the large bandwidth requirements. Designing such an antenna that is concurrently of low-profile is necessary for mounting on a variety of grounds and airborne vehicles. This thesis presents a new low-profile UWB antenna concept. The concept is based on a shorted low-profile patch strategically fed on the open side to obtain wide bandwidth. For miniaturizing the proposed antenna and widening its bandwidth, it was loaded with ferrite bars placed between the patch plate and the ground plane. Much of the design effort for this antenna is focused on the location and shape of the ferrite bars. Minimization of antenna weight is also performed. The developed ferrite-loaded shorted-patch concept is compared with a low-profile monopole to demonstrate its superior bandwidth and gain performance over the frequency range from 30 MHz to 400 MHz. Prototype versions of various ferrite-loaded shorted-patch antennas were fabricated and measured for validation. A version of these was the design that has a planar patch as small as λ/16 in diameter and a height of only λ/200 at the lowest operational frequency.We note that the ferrite loading design led to 5 to 14 dB gain improvement in the low frequency range below 50 MHz. The resulting gain was actually close to the optimal Fano-Bode limit. An important aspect of the ferrite loading design was the use of commercially available materials. Although these were lossy at high frequencies, their strategic placement resulted in minimal gain reduction.