Sub-structure characteristic mode concept for antenna shape synthesis

Abstract

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.

Antenna Shape Synthesis Without Prior Specification of the Feedpoint Locations

“…However, situations of practical importance arise where the latter may not be true, as when one has an antenna proper mounted close to some electrically larger chassis. We showed in [45] that the shape synthesis method described here (but not described in [45]) can be used without alteration if the substructure characteristic modes introduced there are utilized. These substructure characteristic modes view such a situation as follows: The structure is considered to consist of Portion A and Portion B.…”

“…For the matrices on the left-hand sides of (41) and (42) we thus have (43) and (44) The lossless case expression (43) was quoted in [24], [25] without derivation; the lossy case (44) has not been stated previously. If a single dominant characteristic mode is present (this is the single-mode antenna situation), with all other modes having eigenvalues of large magnitude eigenvalues, namely for , the above expressions become (45) and (46) So if the dominant mode is resonant , the matrix traces become purely real. Notice that this prediction of resonance does not require knowledge of the excitation (that is, feed location or type), true to its origins in eigenanalysis.…”

“…Notice that this prediction of resonance does not require knowledge of the excitation (that is, feed location or type), true to its origins in eigenanalysis. Thus using result (45) or (46) one can detect whether an electrically intermediate or small structure could potentially be used as an antenna that is near resonance, without actually having to excite the antenna or compute input reactance. Instead of using the trace we could compute the eigenvalues and observe the magnitude of the dominant characteristic mode eigenvalue directly, but the trace approach is the computationally simpler one.…”

Contribution to the physical interpretation of characteristic mode resonances. Application to dielectric resonator antennas

“…Moreover, the TCM gives the possibility to know the contribution of these modes when the excitation is considered. In addition, the TCM can be applied to a certain substructure part of a more complex structures demonstrated in [7,9,20,43,106]. Such feature is very appealing for DRA design methods since it separates the DR from the slot or coaxial probe through which the DR is fed.…”

“…It is possible to circumvent this problem using the concept of characteristic modes of substructures that was first proposed in [43]. Here, it is applied to DRA design.…”