A new sub-structure characteristic mode concept for electromagnetic systems is defined and used to extend the applicability of an existing antenna shape-synthesis technique that does not require the feedpoint location to be specified prior to shaping. Application of the procedure is successfully illustrated by a realistic example, with experimental results.Introduction: The theory of characteristic modes (CMs) for conducting objects was given in [1]. A moment method formulation [2], which uses RWG expansions functions [3] to model a conducting structure, converts the electric field integral equation 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 and [Z] is the operator matrix. Excitation vector [V ] accounts for one or more antenna feedpoints on the structure, modelled as gap-sources [3] at appropriate 'edges' with respect to which RWG functions are defined. Reference [1] gives the associated matrix eigenvalue equation [Z][J n ] = (1 + jl n )[R][J n ] for the current density [J n ] of the nth CM and its eigenvalue l n , with [Z] = [R] + j[X ]. We showed [4,5] that when an antenna only has a single CM that is of any significance (low magnitude eigenvalue) at its operating frequencies, the resonance frequency and smallest possible Q (at this resonance frequency) of such a singlemode antenna can be predicted without specifying the location of the feedpoint(s) beforehand. This leads to a novel approach for the shape synthesis of a wide class of antennas that are sufficiently small in terms of wavelength that they are singlemode, and that does not require the feed location to be specified a priori.
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.
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