Circuit Modeling and Parameter Extracting of a Filtering Series-Fed Antenna

This article presents a new method to model and extracts the equivalent circuit of a filtering series-fed antenna. The series-fed antenna usually suffers from spurious mode radiation excited by its long feeding line. The radiation strongly interferes with the antenna performance at the adjacent operating frequency bands. The filtering antenna technique is a potential method to solve this problem. However, there are rare reports about how to characterize a filtering series-fed antenna using the circuit model. To address this issue, a kind of series-fed antenna is investigated in this work. The antenna resonance characteristic is analyzed, and its equivalent circuit is constructed accordingly. By using the circuit transformation method and mathematical proof of this circuit, the series-fed antenna is proven to be equivalent to a second-order bandpass filter circuit. Then, as the antenna input admittance is approximated by the polynomials, the extraction method can be used to calculate the filter parameters. With these extracted filter parameters, the filtering series-fed antenna can be easily designed. Moreover, a notch circuit is introduced to further suppress the spurious mode radiations. For demonstration, a filtering series-fed array antenna with three elements is designed at the V2X band. The simulation and measurement results validate the proposed filtering circuit model and extraction method.