The high-altitude electromagnetic pulses (HEMP) are commonly described by the double exponential waveforms. To detect the HEMP waveforms without any distortion, the highest frequency, fc, is the critical parameter for the sensor design. In the engineering applications it is estimated roughly by the rise time of pulses, tr, i.e. fc0=0.35/tr. However, after carefully investigation, a more precise coefficient is recommended in this paper, i.e. fc=0.75/tr. Moreover, it has been deduced that if β≫α, two turning frequency points f3dB, fb are related to α, β with simple equations, 2πf3dB=α, 2πfb=β respectively.