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Given the increased likelihood of dangerous situations in Ukraine and abroad related to the intensification and scale of terrorist acts, it is important not only to quickly locate and eliminate the terrible consequences, but also to prevent, including prevent destabilizing factors. That is why remote detection of explosives and drugs is relevant and promising in the direction of strengthening the fight against terrorism and increasing national security. A promising method for detecting the vast majority of explosives and narcotics that contain nitrogen (14N) or chlorine atoms (35Cl, 37Cl) is the nuclear quadrupole resonance method. In the presence of a gradient of high-frequency field strength inside the sample in the process of forming the response signal is “blurring” the vector of the total spin magnetization, which leads to the expansion of the resonant signal and reduce its amplitude. Increasing the homogeneity of the high-frequency field is especially important for the detection of weak signals observed in the study of substances with a low natural prevalence of magnetoactive nuclei. The paper presents the results of research on the topology of the electromagnetic field strength of a helical coil designed for use as a transmitting and receiving antenna of a portable detector of explosives and drugs, the principle of which is based on the registration of  14N isotope signals by nuclear quadrupole resonance. To determine the topology of the electromagnetic field, a computational domain was created, which is a model of the physical structure of the spiral coil. The finite element method performed numerical simulations in COMSOL Multiphysics. The simulation was performed at a frequency of 3.4 MHz, which is close to the value of the detection frequency of the attenuation signal of free induction from sodium nitrite 3.41 MHz. The best results were shown by the model of the coil with five turns, internal radius of 25 mm, external radius of 225 mm. The obtained magnetic field diagrams indicate better resistance to radio frequency interference when working in a real experiment. Based on the data obtained during the simulation, a multilayer (Multislice) model of the electric field and the isosurface of the electromagnetic field of the studied model were created, which demonstrate the dynamics and density of the electromagnetic field around the coil. From the obtained graphical dependences it is possible to draw a conclusion about the normalized attenuation of the intensity of electric field radiation from the geometric center of the coil, which, in turn, makes it clear exactly how to place the sample.

INVESTIGATION ABOUT ELECTROMAGNETIC FIELD TOPOLOGY OF THE SPIRAL COIL FOR NQR DETECTOR OF EXPLOSIVE AND NARCOTIC SUBSTANCES