We present measurements of stimulated electromagnetic emission (SEE) excited in the ionosphere by a powerful high-frequency ordinary mode radio wave vertically injected from a ground-based transmitter, at frequencies ${\mathit{f}}_0$ near high harmonics of the ionospheric electron cyclotron frequency ${\mathit{f}}_{\mathit{c}}$. A prominent lower-sideband SEE feature was found to be absent in an extremely narrow ${\mathit{f}}_0$ range of Δ${\mathit{f}}_0$/${\mathit{f}}_0$≊2×${10}^{\mathrm{-}5}$. This absorption resonance width allows an estimate of ${\mathit{f}}_{\mathit{c}}$ with an accuracy of a few tens of Hz, corresponding to a magnetic field magnitude accuracy of about 1 nT.