We present a method of determination of numbers of gyroresonance layers most effectively radiating in the transition
region of the active solar atmosphere. It is based on determination of frequency in the spectrum of an extraordinary wave
of an active region, at which the gradient of this spectrum abruptly rises. Observations of 29 active regions carried out
at the RATAN-600 radio telescope have been analyzed. The magnetic field in the transition region was determined from
the frequency of bending point in the spectrum of antenna temperatures in an extraordinary mode under condition of
emission in the third harmonic of the gyrofrequency. The relation between the photospheric magnetic field strength and
the magnetic field strength in the transition region (1.52–2.28) is substantially greater than previously obtained, namely,
the field is decreased only by 10–20% in the transition region. The heights in the transition region were determined from
the reconstructed magnetic field in the nonlinear force-free approximation, and are in the range of 1.00–3.57 Mm.