A Wideband Equivalent Circuit Electric Dipoles

The problem of obtaining a wideband equivalent circuit for dipole antennas is addressed backward; the four-element circuit topology is taken as given; its three-element part was suggested by Chu and its extension to four elements by Stuart. The goals here are to obtain the element values with the least effort and to explore the available bandwidth. Based on the impedance spectrum, rather than on radiation or current eigenmodes, this approach identifies elements of the circuit from their behavior with frequency over a wideband impedance spectrum that includes both first and second resonances. Element values are derived from their impedance behaviors at three frequencies: zero (dc), first (series or short-circuit), and second (parallel or open-circuit) resonance. An Argand diagram is introduced to determine R and L at the first resonance; the two capacitances, C and C_t, at first and second resonances, respectively, complete the circuit. Optimization follows. Several examples are given and limitations discussed.