محدوديت هاي فيزيكي ساختارهاي تشعشي و طراحي آنتن هاي بهينه

چكيده انگليسي :

Physical Limitations in Radiating Structures and Design of Optimum Antennas Behrooz Semnani b semnani@ec iut ac ir Date of Submission 2011 13 3 Department of Electrical and Computer Engineering Isfahan University of Technology Isfahan 84156 83111 Iran Degree M Sc Language FarsiSupervisor Dr mir Borji aborji@cc iut ac ir AAbstractA considerable amount of research has been dedicated to the miniaturization of antennas in recent years Emergingbroadband and mobile wireless applications with market pressure for miniaturized communication devices haveencouraged the use of electrically small antennas ESA Reduction of antenna size and miniaturization of radiatingstructures gives rise to some unavoidable limitations on their radiation performance The need for small portabledevices has motivated many engineers and scientists to study these fundamental limits Physical bounds on thebandwidth realized gain directivity gain bandwidth product scattered power and absorption efficiency have beendiscussed in this thesis The concept of antenna quality factor relating the antenna realizable bandwidth to theequivalent current distributions induced over the antenna structure plays an important role in this matter Numerouspapers have been published trying to define the minimum possible radiation quality factor of the antenna as afunction of its size Many of these derivations assume a circumscribing sphere and deal only with the fields externalto this sphere As a result the lower limits tend to be overly optimistic and the actual quality factor of the antenna isusually considerably higher than the lower bound predicted by these theories In this thesis a general expression forthe quality factor of an arbitrary current distribution is derived based on the concept of observable stored energy The given expression closely predicts the actual impedance bandwidth of the antenna which is derived from itsresonance model In order to design antennas with optimal radiation performance a novel scheme based on thetheory of characteristic modes is developed The new design methodology is applicable to arbitrary topologies and itdoes not rely on the assumption of a canonical volume such as a sphere enclosing the antenna The particulardefinition of characteristic modes as proposed in this thesis substantially reduces the number of basis functionsrequired to expand a realizable current distribution over an arbitrary topology By developing the novel notion of generalized Chu s theorem it has been shown that only a finite number of eigencurrents contribute in radiation andthis considerably reduces numerical calculations in the optimization procedure Furthermore a general modaltechnique is established to design spherical antennas with maximum gain bandwidth product In addition to thephysical limitation of small antennas a fundamental limitation on the scattering behavior of receiving antennas hasbeen introduced in this thesis for the first time Specifically a lower bound on the total power scattered from alossless and matched receiving antenna for a given incident wave is presented The vector spherical waveexpansion is used to describe the scattering and power absorption mechanisms Scattering is the main reason behindantenna absorption This poses a fundamental limit on minimization of antenna scattering In other words if we areto extract any power from the incident wave the receiving antenna cannot be completely invisible and scattering isan inevitable consequence of the power capturing process Finally a universal limit on the absorption efficiency ofarbitrary receiving antennas in terms of their normalized gain is derived which is based on the modal analysis of thescattering process in receiving mode Key WordsAntenna quality factor Impedance bandwidth Characteristic modes Antenna scattering Absorption efficiency

سمناني، بهروز

محدوديت هاي فيزيكي ساختارهاي تشعشي و طراحي آنتن هاي بهينه

ضريب كيفيت , انرژي راكتيو , سنتز , مدهاي ويژه , پراكندگي , بازدهي جذب