The generalized system function, H(s), directly associated with the radiated or scattered fields is presented in this paper, which is constructed by applying the model-based parameter estimation (MBPE) technique combined with the complex frequency theory. A complex frequency ω̃ relating the real resonant frequency with radiated or scattered Q factor is introduced to antenna and scattering systems. By analyzing the characteristics of complex poles and zeros of H(s) in a finite operational frequency band, and combining with adaptability of MBPE, we can determine the resonant frequency and Q-value of the antenna and scattering systems effectively. The intensity of resonance can be estimated in terms of Q-value and residues at the complex resonant frequencies. Some examples of the practical antenna arrays and scattering systems are given to illustrate the application and validity of the proposed approach in this paper.
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