We present in this paper a field-grade method for the determination of the electric permittivity of the explosives used in improvised explosive devices found in Colombia. This paper presents a direct simplified method used to calculate the permittivity of dielectric materials. The efficiency of the direct method is analyzed using the computer simulations of known low-loss and lossy materials. In addition, the field-deployable system and measurement setup are explained and their performance for field measurements is compared against reference measurements performed by a vector network analyzer in the frequency range from 0.3 to 3 GHz. Finally, the measurements of complex permittivity of improvised explosives made of ammonium nitrate fuel oil are presented.