Jonathan Ethier

Antenna Shape Synthesis Without Prior Specification of the Feedpoint Locations

An antenna shape synthesis method is proposed that allows shaping of the antenna geometry prior to specification of the feed location and type. This reduces the constraints placed on the optimization process and can lead to potentially new designs due to the increased degree of freedom afforded. An appropriate feedpoint is easily chosen after shape optimization by selecting a location on the resulting structure for best impedance matching. The procedure is made possible through the use of characteristic mode concepts. Examples show that the antenna-Q values of the resulting shaped radiators closely approach the fundamental bounds.

Loss Reduction in Reflectarray Designs Using Sub-Wavelength Coupled-Resonant Elements

A novel design approach is proposed for designing high performance reflectarrays using low-cost, lossy substrates. Using sub-wavelength coupled-resonant elements, in particular sub-wavelength loops, one can dramatically reduce losses in reflectarrays and not incur significant gain drop due to the use of the lossy, low-cost substrate. It is further shown that the sub-wavelength loop achieves sufficient phase variation in a single layer design with a modest requirement in etching tolerance, making the loop superior to both the single and double-layer sub-wavelength patch elements.

Observations on computational outcomes for the characteristic modes of dielectric objects

Some computational aspects for the characteristic modes (CM) of dielectric objects are examined. Volume integral equation (VIE) formulations are reliable but computationally burdensome. The use of surface integral equation (SIE) formulations results in a lighter computational load but, we show here, obtrudes certain non-physical modes in addition to the physical ones. We show that the non-physical ones are easily identified using a radiated power check, and can be discarded, so allowing use of the SIE for finding the true CMs of dielectric objects. The SIE can be used with fine meshing with relatively modest computational resources; this is of practical importance.

An Interpretation of Mode-Decoupled MIMO Antennas in Terms of Characteristic Port Modes

If N ports are defined on the moment method model of a radiating structure, so-called characteristic port modes may be computed, using the moment method operator matrix, with respect to these ports. This results in N port excitation vectors (each of length N). Each vector excites a radiated field which is orthogonal to the fields excited by any of the other N excitation vectors. It is shown that this may be usefully employed in the design of antennas for multiple-input multiple-output (MIMO) communications, and that recent multimodal/multipolar approaches for MIMO antenna design may be interpreted in terms of such port modes.

The Use of Generalized Characteristic Modes in the Design of MIMO Antennas

An extended polarization-dependent generalized characteristic-mode theory for a radiating structure is given. We show how these modes can be computed and employed in the design of closely located multiple-input multiple-output multiantenna sets with low interantenna envelope correlation and good mean effective gain under realistic incoming power distribution scenarios found in mobile communications.

Reflectarray Design With Similarity-Shaped Fragmented Sub-Wavelength Elements

A new technique for synthesizing reflectarray antennas is presented. It utilizes fragmented elements in a manner that allows the elements of the reflectarray to be shaped-optimized so that a high degree of geometrical similarity is maintained between adjacent elements. The implication is that in a reflectarray of such elements each element will see an electromagnetic environment that more closely emulates the infinite periodic one used to compute the element properties. We show experimentally that this indeed results in aperture efficiencies closely approaching the upper bounds achievable for some selected feed system, and offers a significant improvement over that obtained with conventional reflectarrays (that is, those not using similarity-synthesized fragmented elements). It is also shown that a reflectarray surface that uses fragmented elements can be simultaneously patterned with a visual image while still closely maintaining the desired reflection phase from its surface to yield a high-gain antenna.

Through the looking glass: A characteristic mode view of electromagnetic modeling & design

In recent years there has been a surge of interest in various disciplines within applied electromagnetics, including electrically small antennas, MIMO antennas, periodic structures and metamaterials. Though a multitude of interesting applications have arisen, there still remain numerous fundamental questions regarding these burgeoning areas of research. We show how a characteristic mode analysis can provide insights into some of these questions.

Multiband antenna synthesis using characteristic mode indicators as an objective function for optimization

The theory of characteristic modes has been given in [1] for perfectly conducting objects. If we use a moment method formulation such as [2] to model a conducting structure, with N of the RWG expansion functions used, the EFIE is converted into a matrix equation [Z][J]=[V]. Column vector [J] contains the coefficients of the expansion functions for the unknown electric current density on the structure, [V] is the excitation vector, and [Z] the impedance matrix. Reference [1] derives the matrix eigenvalue equation [X][Jn] = λn[R][Jn] for the currents [Jn] of the n-th characteristic mode and its eigenvalues λn with [Z]=[R]+j[X] separating [Z] into its real and imaginary parts. For a perfectly conducting structure the λn are real, and have the physical interpretation of being proportional to the ratio of the reactive to radiated power associated with the n-th mode. Characteristic modes are independent of any specific excitation of the conducting body, and when such a mode is externally resonant its eigenvalue approaches zero. In this paper we use the characteristic mode concept in the shape synthesis of antennas. Some of the material in this paper has already been described in preliminary abstract form [3]. The present paper will present these developments more completely and present multiband results for the first time.

On the classification of characteristic modes, and the extension of sub-structure modes to include penetrable material

The characteristic modes of perfectly conducting objects computed using modified Greens functions are classified and demonstrated to be the same as sub-structure characteristic modes. We indicate the advantage of the sub-structure viewpoint, and that the idea is easily extended to include lossless dielectric objects as the “rigid” member of the structure.

Numerical experiments in tracking the characteristic modes of dielectric objects

Existing characteristic mode tracking algorithms experience difficulties when used for dielectric objects due to the presence of non-physical modes when surface integral equation models are used. It is shown how these problems can be circumvented using a pragmatic approach.