Ultra high frequency (UHF) partial discharge sensors are widely used for condition monitoring and defect location in the insulation systems of high voltage equipment. Designing sensors for specific applications often requires an iterative process of manufacture, test, and mechanical modifications. This paper demonstrates the use of finite-difference time-domain (FDTD) simulation as a tool to predict the frequency response of a UHF sensor design. Using this approach, the design process can be simplified and parametric studies can be conducted in order to assess the influence of component dimensions and material properties on the sensor response. The modeling approach is validated using a broadband UHF sensor calibration system, which uses the step response of the sensor to determine its frequency-domain transfer function. The use of a transient excitation source is particularly suitable for modeling using FDTD, which is able to simulate the step response output voltage of the sensor, from which the frequency response is obtained using the same post-processing applied to the physical measurement. Comparisons between simulation and measurement are made for three different sensors, demonstrating sensitivity agreement to within about 10%. Some examples of simple parametric studies carried out using the FDTD model are also presented.