Three-dimensional effects for radio frequency antenna modeling

Abstract

Electromagneticfield calc_ationsforradio frequency(rf)antennas in two dimensions (2-D) neglectfinite antenna lengtheffects as wellas thefeedersleadingto the main current. strap.Comparisons with experimentsindicate that these 2-D calculations can overestimate the loadingof the antenna and fail to givethe correctreactive behavior.The 2-D calculationsalsopredictthat the returncurrentsin the sidewalls of the antenna structuredepend stronglyon plasma parameters,but thisprediction isalsosuspectbecause of experimental evidence. To study the validity of the 2-D approximation,the MultipleAntenna Implementation System (MAntIS) has been used to perform 3-D modeling of the power spectrum, plasma loading,and inductancefora relevant loop antenna design.Effects on antenna performance caused by feedersto the main currentstrap,conductingsidewalls, and finite phase velocity are considered.The plasma impedance matrix for the loadingcalculation is generatedby use of the ORION-ID code. The 3-D model isbenchmarked with the 2-D model in the 2-D limit. For finite-length antennas,inductance calculations are found to be in much more reasonable agreement with experiments for 3-D modeling than for the 2-D estimates. The modeling shows that the feeders affect the launchedpower spectrum in an indirect way by forcingthe drivenrf currentto returnin the antenna sidewalls ratherthan in the plasma as in the 2-D model. Thus, the feedershave much more influence than the plasma on the currentsthatreturnin the sidewall. It has alsobeen found thatpoloidaldependenciesin the : plasma impedance matrix can reducethe loadingfrom that predictedin the 2-D model. For some plasma parameters,the combined 3-D effects can lead to a reductionin the predicted loading by as much as a factorof 2 from that given by the 2-D model.