Electromagnetic waves of VLF range that propagate along the Earth’s magnetic field lines are called whistlers. It is known that using whistlers, it is possible to calculate the electron concentration of the plasmasphere by knowing the nose frequency and absolute group delay time (limiting variance). However, such calculations are complicated by the fact that at mid latitudes, whistlers are usually recorded within the frequency range below the nose frequency, which is typical for Kamchatka. Moreover to determine the delay time, it is necessary to know initiating atmospherics (lightning discharge), and it is practically impossible to do on a spectrogram. Therefore, methods based on the extrapolation of a whistler trace from the available part of a spectrum are typically used. The article proposes a method of linear extrapolation of whistle spectra recorded in Kamchatka.