When measuring soil water content by time domain reflectometry (TDR), several methods are available for determining the related apparent dielectric constant (Ka) from the TDR waveform. Their influencing factors and effective frequencies have not been extensively investigated and results obtained from different methods have not been critically compared. The purpose of this study was to use numerical simulations to systematically investigate the effects of electrical conductivity, cable length, and dielectric dispersion on Ka and the associated effective frequency. Not only does the dielectric dispersion significantly affect the measured Ka, it also plays an important role in how Ka is affected by the electrical conductivity and cable length. Three methods for determining Ka were compared, including the dual tangent, single tangent, and derivative methods. Their effective frequencies were carefully examined with emphasis on whether the effects of electrical conductivity, cable length, and dielectric dispersion can be accounted for by the estimated effective frequency. The results show that there is no consistent trend between the change in Ka and the change in effective frequency as the influencing factors vary. Compensating the effects of electrical conductivity, cable length, and dielectric dispersion by the effective frequency seems theoretically infeasible. To improve the accuracy of TDR soil water content measurements in the face of these influencing factors, future studies are recommended toward TDR dielectric spectroscopy or developing signal processing techniques for determining the dielectric permittivity near the optimal frequency range.