EN
A detailed computer analysis of the electron spin echo envelope modulations of SeO_{3} radical in a suitable orientation of (NH_{4})_{3}H(SeO_{4})_{2} single crystal is presented. It was found that the modulations are due to a weak dipolar coupling with nitrogens and protons of the only two neighbouring NH_{4} groups among the five NH_{4} groups surrounding SeO¯_{3} center. Isotropic dipolar coupling constant is 1.7 MHz for nitrogens and 0.9 MHz for protons. It was shown that thermal reorientations of NH_{4} groups observed by NMR have a negligible effect on the electron spin echo envelope modulation pattern but can be responsible for the same value of a^{H}_{iso} for all protons in a NH_{4} group. A good fit obtained between experimental spectra and theoretical calculations assuming nondisturbed crystal geometry indicates a small damage of the crystal by X-rays during the radical center formation.