Based on the equivalent circuit of Foster canonical pattern, the physical level of a low profile, ultra-wideband and tapered antenna used for wireless communication is analyzed. The antenna is composed of a traditional taper antenna, a parasitic ring and a sleeve. The coverage of the antenna is 747MHz~6000MHz, and the physical height of the antenna is only 0.0625λ of low-frequency (747MHz). In the paper, an equivalent model of the lumped parameter is designed for simulating the changes on input impedance of the antenna and explaining the antenna working principle and the functions of its main components. Comparing the result of equivalent circuit with the full-wave electromagnetic simulation results, we can find out that they have an excellent consistency.