This paper presents a method to locate a wideband perturbation along a fiber link using a fiber Fox-Smith interferometer (FSI). With an aluminum film coated at the end plane of fiber, phase of each interfering beam is modulated twice by the perturbation. An impact on the fiber by a bar connected with a motor is simulated as a wideband perturbation. The test point is acoustically and spatially isolated from the mass of the FSI. A short-time fast Fourier transform (FFT) shows a series of odd-multiple nulls in the averaged power spectrum from which the position of the impact can be determined. An experiment of 50 impacts at a distance of 22.85 km achieves a standard deviation of 0.041 km, corresponding to 0.18% relative error. The method achieves adjustable sensitivity, higher dynamic range, simplicity of installation, and low cost.