ABSTRACTThis article presents the investigation on the applicability of variational technique for the estimation of input impedance of a coaxial line fed probe in a circular cylindrical waveguide based on thin current approximation. In this work, the lower limit of diameter of the probe for which thin current approximation is valid has been established. The variation of input impedance has been studied as a function of probe height, short length from the probe as well as diameter of the probe. The convergence of the solution based on variational technique is checked in terms of the number of higher order modes of circular cylindrical waveguide excited at the asymmetrical junction of probe. The limitation of the existing technique has been ascertained using the finite element method based simulator high frequency structure synthesizer (HFSS) and experimental validation. The simulated result based on HFSS is in excellent agreement with the experimental results. Bandwidth performance with variation in diameter has also been studied using HFSS simulator with varying probe diameter. © 2013 Wiley Periodicals, Inc. Microwave Opt Technol Lett 55:2652–2656, 2013