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The impedance of an electrical coil wound over a fluorescent tube is found experimentally both when the tube is energized and de-energized. This paper reports results showing a significant change in the resonance characteristics of the coil due to the influence of the plasma core. The effects are described with the help of an equivalent circuit consisting of an inductance with a series resistance and a parallel capacitance. This paper demonstrates that by energizing the core, the series resistance and shunt capacitance in the equivalent circuit increase significantly. The experiment is performed both for alternating current and direct current excitations. From the observed results, fractional changes in the resonant characteristics are found and explained using a perturbational viewpoint. This enables us to indirectly estimate the plasma density profile and model the beam loading effects in high-energy radio frequency sources. The equivalent circuit model presented here can be employed to explain the macroscopic effect of low-temperature plasma on circuit elements within close proximity.