Minas H. Tanielian

Simulation and testing of a graded negative index of refraction lens

A gradient index (GRIN) lens using a negative index of refraction material (NIM) has been designed and tested. The GRIN lens was fabricated using a NIM slab with a variable index of refraction perpendicular to the propagation direction. Ray tracing calculations based on the isotropic Eikonal equation determined the index of refraction gradient required for a given focal length. An electromagnetic code was then used to design the required ring and wire unit cells. Finally, the index of refraction was approximated using ten discrete steps in an effective medium simulation for the GRIN lens that agreed with the experimental measurements.

Experimental determination and numerical simulation of the properties of negative index of refraction materials

Negative index of refraction materials have been postulated for many years but have only recently been realized in practice. In the microwave region these materials are constructed of rings and wires deposited on a dielectric substrate to form a unit cell. We have constructed, experimentally characterized and simulated several of these structures operating in the 10 - 15 GHz range. Our simulations using Maxwells Equations solvers have included wire arrays, ring arrays and assemblies of unit cells comprised of rings and wires. We find good agreement between the numerical simulations and experimental measurements of the scattering parameters and index of refraction. The procedure was to first model ring and wire structures on the unit cell level to obtain scattering parameters from which effective å, ì and n were retrieved. Next an assembled array of unit cells forming a 12 degrees wedge was used for the Snells Law determination of the negative index of refraction. For the structure examined the computed value of n is within 20% of the one experimentally measured in the Snells Law experiment from 13.6 to 14.8 GHz.

Experimental Verification of Z Antennas at UHF Frequencies

Both 300- and 570-MHz versions of an electrically small, coaxially fed Z antenna were designed and tested. These two cases demonstrate the ability to change the resonance frequency of the Z antenna by changing the value of its lumped element inductor. The numerically predicted and measured results are in good agreement.

Origin of dissipative losses in negative index of refraction materials

Negative index of refraction materials have been postulated for many years but have only recently been realized in practice. In the microwave region these materials are constructed of rings and wires deposited on a dielectric substrate to form a unit cell. We have constructed, experimentally characterized, and simulated several of these structures operating in the 10–16 GHz range. The origin of the dissipative losses has been identified and effective schemes to reduce them devised and implemented. Numerical simulation and experimental verification shows that losses in negative index of refraction materials can be significantly reduced by minimizing the underlying losses of the constituent materials.

Design and Experimental Verification of a 3D Magnetic EZ Antenna at 300 MHz

Several variations of a 300-MHz version of the electrically small coax-fed three-dimensional (3D) magnetic EZ antenna were designed and tested. The final version of this low-profile antenna had an electrical size that was ka ~ 0.437 at 300.96 MHz. Nearly complete matching to the 50-Omega source and high overall efficiency (nearly 100%) were achieved. The measured fractional bandwidth was approximately 1.66%. The numerically predicted and the measured results were in good agreement. Comparisons to similar-sized loop antennas that were matched to the source with both custom-made and commercially available, general purpose external matching networks confirm the performance enhancements achieved with this metamaterial-inspired, near-field resonant parasitic antenna.

An efficient, low profile, electrically small, three-dimensional, very high frequency magnetic EZ antenna

A very high frequency version of the electrically small, coax-fed, three-dimensional magnetic EZ antenna was designed and tested. The fabricated antenna was formed by integrating a capacitively loaded loop element with a coaxially-fed, electrically small, semicircular loop antenna. This low profile antenna (height ∼ λ / 25 ) had an electrical size that was k a ∼ 0.46 at 105.2 MHz (where a is the radius of the minimum enclosing hemisphere). Nearly complete matching to the 50 Ω source and a high overall efficiency (nearly 95%) were achieved. The numerically predicted and the measured results were in good agreement.

Steering Phased Array Antenna Beams to the Horizon Using a Buckyball NIM Lens

In this paper, we present the design, optimization, fabrication, and measurement of a negative index metamaterial (NIM) buckyball shell lens to steer phased array antenna (PAA) beams to the horizon. The conformal mapping technique of transformation optics is utilized in the design process to facilitate with lens fabrication. A new dual polarization unit cell is designed to avoid the issues associated with short cut wires, were a split ring resonator (SRR) and wire design to be used. The lens is measured using an actual PAA and it demonstrates to-the-horizon scanning as designed, although the material loss is high. An improved unit cell design is proposed to reduce several known loss mechanisms. A new lens design methodology using Bezier curves as seed surfaces is also described.

Development of Negative Index of Refraction Metamaterials with Split Ring Resonators and Wires for RF Lens Applications

Metamaterials are engineered ring and wire composites whose response to an incident electromagnetic wave can be described by an effective negative dielectric permittivity ε and magnetic permeability μ. Simultaneous negative ε and μ within a given frequency band of a metamaterial gives rise to a negative index of refraction n. This has been demonstrated via a Snell’s law experiment. The electromagnetic properties of many metamaterial structures in the microwave region are investigated through numerical simulations and experiments. A negative index of refraction, n, allows lenses with reduced primary (Seidel) aberrations compared to equivalent positive index lens. This is demonstrated both for cylindrical lenses and spherical lenses, as well as for the gradient index lenses. Detailed field maps of the focal region of the metamaterials lenses are made and compared to a comparable positive index of refraction lens.

Experimental confirmation of negative phase change in negative index material planar samples

We use far-field range measurements to determine and confirm the negative phase change through a planar negative index material as a function of frequency. The metamaterial is composed of wires and split ring resonators. At frequencies for which the surface impedance Z∕Z0=1, we determine the index (n) from the measured phase change (relative to a vacuum) and via numerical simulation. In addition to confirming the simulated negative phase change at the frequency where n=−1, we find good agreement with prior Snell’s law measurements from n=−2.5 to −0.5. This illustrates that measuring the phase change of the transmitted signal can be a practical means of identifying the existence of negative index in planar test samples.

Negative Index Metamaterial Lens for the Scanning Angle Enhancement of Phased-Array Antennas

The unique electromagnetic properties of negative index metamaterials (NIM) provide enormous flexibility in lens design resulting in lenses with better performance and lighter weight. This work presents the design and full wave simulations of a NIM lens to enable a Phased-Array Antenna (PAA) to scan to the horizon. Simulation and optimization techniques are discussed in detail with fabrication issues considered along the way. Simplified 2D effective medium simulations are used to explore the design space and optimize lens geometry and material makeup. More computationally intensive effective medium simulations in 3D are then performed for a better assessment of the antenna performance. Material an-isotropy is then explored to find the lowest order material for easier fabrication while maintaining desired beam characteristics. The conformal mapping technique of transformation optics is used to convert lens shapes into contours that are easier to fabricate. We also discuss on going investigations of alternative NIM materials with higher level isotropy.