The plasma resonance phenomena observed near the plasma frequency ƒN through active experiments of relaxation sounding have been experimentally investigated within the range 100-500 km in the ionosphere by means of the three rocket launches of the EIDI payloads (Experiment on the Impedance of a Dipole in the Ionosphere). The design of the EIDI relaxation sounders has been improved with respect to that of previous topside sounding experiments (Alouette and ISIS programs) in that it records both the envelope and the waveform of the resonance signals in a large dynamic range. This allows the study of spectral features of the plasma resonance phenomena. The data of EIDI 3 are first used to point out the accuracy and the reliability of relaxation sounding experiments for the measurement of the ionospheric electron density; the electron density profile obtained is typical of a midlatitude, winter, nighttime ionosphere. Secondly, the spectral features of the ƒN resonance signals are investigated. Their comparison with those predicted by the theory of oblique echoes for the ƒN resonances leads to a disagreement, although the waves received are shown to be the electrostatic slow ones used in this theory. However, a correlation processing between the Fourier spectra of the ƒN resonance signals observed in sequence allows us to point out the existence of small-scale irregularities of the electron density. This result is inconsistent with the theoretical hypothesis of a linear density variation versus altitude. By taking these irregularities into account, the features of the resonance signals can then be qualitatively explained. The properties deduced from the data regarding the size and the amplitude of the density irregularities are in agreement with those obtained under comparable ionospheric conditions through another kind of experiment for measuring the irregularities (Retarding Potential Analyzer on the OGO 6 satellite). Moreover, the unexplained phenomena of beat frequency oscillations observed on the Alouette and ISIS ƒN resonance data are tentatively interpreted in a similar way through the existence of sinusoidal irregularities of the electron density. Finally, it is pointed out that the relaxation sounding experiment on-board the GEOS satellite has been designed to provide an effective measurement of the electron density irregularities in the magnetosphere.