Ryan Quarfoth

Experimental Validation of Performance Limits and Design Guidelines for Small Antennas

The theoretical limit for small antenna performance that was derived decades ago by Wheeler and Chu governs design tradeoffs for size, bandwidth, and efficiency. Theoretical guidelines have also been derived for other details of small antenna design such as permittivity, aspect ratio, and even the nature of the internal structure of the antenna. In this paper, we extract and analyze experimental performance data from a large body of published designs to establish several facts that have not previously been demonstrated: (1) The theoretical performance limit for size, bandwidth, and efficiency are validated by all available experimental evidence. (2) Although derived for electrically small antennas, the same theoretical limit is also generally a good design rule for antennas that are not electrically small. (3) The theoretical predictions for the performance due to design factors such as permittivity, aspect ratio, and the internal structure of the antenna are also supported by the experimental evidence. The designs that have the highest performance are those that involve the lowest permittivity, have an aspect ratio close to unity, and for which the fields fill the minimum size enclosing sphere with the greatest uniformity. This work thus validates the established theoretical design guidelines.

Artificial Tensor Impedance Surface Waveguides

A surface waveguide structure is studied using a theoretical model and simulated using Ansoft HFSS. A tensor impedance surface is surrounded by two lower impedance surfaces on a plane. Surface waves are guided losslessly within the inner region as long as it has higher impedance than the outer region. A theoretical model is proposed which predicts the dispersion relation of the waveguide using a ray optics method. Our tensor impedance surface theory can also predict the dispersion of scalar impedance surfaces. The theory and simulation show agreement for scalar and tensor impedance surfaces. Two applications of tensor impedance surface waveguides are presented.

Surface Wave Scattering Reduction Using Beam Shifters

A surface wave beam shifter is designed using ring-mushroom unit cells. The unit cell consists of a rectangular ring that surrounds an interior patch and plated via. The unit cell is anisotropic, and surface waves propagate with different directions of phase velocity and power flow. Two adjacent regions are designed such that the shift direction in each region is mirrored. The result is that an incident beam is split apart, and power does not radiate in the original direction of the beam. It is shown that an object placed in line with a source scatters surface waves in multiple directions from the object. By using a beam-splitting surface, the incident beam avoids the object, and scattering is significantly reduced. When the wave is incident in series with the two beam shifters, the surface shifts the wave in one direction followed by a shift in the opposite direction.

Nonscattering Waveguides Based on Tensor Impedance Surfaces

A tensor impedance surface waveguide is built using anisotropic unit cells. The waveguide can propagate a confined waveguide mode along its axis while waves incident to the guide at an orthogonal direction pass through as if the waveguides were not present. Both straight and curved implementations are demonstrated. Surface waves incident at an angle to the waveguide show reflection and refraction at the impedance interface. A theoretical model for tensor impedance surface waveguides is generalized to include dispersive unit cells and bending loss around curves. Dispersion results for modes propagating in the waveguide show agreement between the theory, simulation, and experimental measurements. A curved waveguide is also constructed which guides surface waves around a curve and is transparent to surface waves incident at an orthogonal angle.

A 2-D Multibeam Half Maxwell Fish-Eye Lens Antenna Using High Impedance Surfaces

This letter describes the design and measurement of a planar multibeam half Maxwell fish-eye (HMFE) lens antenna with seven beams in azimuth based on variable high impedance surfaces (HISs). The desired refractive index profile is controlled by using slotted square mushroom metallic cells of different sizes on a grounded dielectric slab. The lens antenna, 6.2λ×3.1λ in lens plane and 0.65 λ total height, is fabricated and characterized in TM mode at 13 GHz for both single- and multiple-beam configurations. The optimized lens for single-beam operation achieves 14.1 dBi measured gain, 67 ° and 12 ° half-power beamwidth for E-plane and H-plane, respectively, and 55.3% impedance bandwidth for -10-dB return loss. This lens can be used to launch multiple beams by implementing an arc array of seven-element short monopoles at the periphery of the lens. The measured overall scan coverage is up to 45 ° with gain drop less than 2.4 dB. Its low profile, light weight (only 136 g), and ease of manufacturing makes it well suited for Local Multipoint Communication Systems (LMCS).

Nonlinear Power-Dependent Impedance Surface

Impedance surfaces are artificially designed periodic structures that support surface waves. This paper presents a novel concept of a nonlinear impedance surface, whose impedance increases with the surface wave power. The proposed surface consists of a lattice of modified slotted mushroom-like cells loaded with varactor diodes and lumped circuits. The circuits are designed to sense the incoming surface wave power and feed back the signal to control the bias of the varactors, which eventually tunes the surface impedance. Such proposed power-dependent impedance surface has been demonstrated by a prototype consisting of 4 × 10 cells. It is observed from the experiments that as the power varies from 16 to 27 dBm, the surface exhibits a surface impedance range as much as j385 to j710 Ω. This characteristic enables the surface to be potentially useful in self-trapping surface-wave waveguide, power-dependent beam-steering antenna reflector, and other nonlinear applications.

Broadband Unit-Cell Design for Highly Anisotropic Impedance Surfaces

A unit-cell design for highly anisotropic impedance surfaces is simulated and experimentally demonstrated. The unit cell consists of a grounded dielectric substrate with a metal patch and plated metal via. The design has a ring of metal removed from the patch so that the resonant effect of the via is reduced. The reduced resonance prevents backward-wave modes which are difficult to excite experimentally. On grounded dielectric substrates, the transverse-magnetic mode is fundamental and the transverse-electric (TE) mode is bound above a cutoff frequency. It is shown that highly anisotropic surface impedance can only be achieved below the TE cutoff frequency. The proposed unit cell is constructed using printed-circuit-board fabrication technology, and the result is a unit cell that obtains highly anisotropic impedance over a broader frequency range than traditional patch or mushroom unit cells.

Anisotropic surface impedance cloak

The applicability of coordinate transformation methods are investigated for surface structures. It is shown that in-surface transformation implementations cannot perform the full range of applications achieved with volume media. Anisotropic impedance surfaces are used to split a surface wave beam and then recombine it in phase. An interior region is cloaked from the incident surface beam so that minimal energy enters. The beam shifting and combining is performed by using a linear coordinate transformation method.

Simulation of anisotropic artificial impedance surface with rectangular and diamond lattices

Infinite lattices of patches covering grounded dielectric slabs were simulated using Ansoft HFSS. Both rectangular and diamond unit cells were studied. For waves propagating along rectangular unit cells, the transverse cell dimension has no effect on the surface impedance or dispersion characteristics. For diamond unit cells, the surface impedance and dispersion characteristics vary significantly for different transverse cell lengths when the wave propagates along the shorter unit cell dimension. For waves propagating over the longer dimension, surface impedance and dispersion remain similar. Results show that at a given frequency, the impedance can be varied by changing the length of the cell in the propagating direction. For rectangular cells, impedance in orthogonal directions is independent of each other, and an anisotropic impedance tensor can be created based on the size and orientation of the cell.

Impedance surface waveguide theory and simulation

A novel waveguide structure is studied using a theoretical model and simulation. An impedance surface is surrounded by two lower impedance surfaces in a plane. Waves are guided losslessly within the higher impedance region. A theoretical model is proposed which predicts the dispersion relation using a ray optics method. The dispersion is predicted for each of the modes of the structure. Simulation shows similar results to the theoretical model for the lowest five modes of the structure. These results hold for a wide variety of impedance ratios and guide sizes.