(a) Measurements (symbols) and calculations (solid lines) of Rydberg EIT-AT spectra as a function of the coupling laser detuning Δ_c for fixed probe/coupling laser Rabi frequency Ω p = 2 π × 3.99 MHz and Ω c = 2 π × 2.06 MHz and an indicated microwave power P_RF = 0.32 mW (blue) and 1.99 mW (red). The RF electric field with frequency 15.21 GHz couples the nearby Rydberg states ∣ 66 S 1 / 2 〉 and ∣ 65 P 1 / 2 〉 . The AT splitting γ_AT is obtained by multipeak Lorentz fittings to the Rydberg EIT-AT spectra. The RF field-free EIT signal (black) is obtained with a 6 S 1 / 2 − 6 P 3 / 2 − 66 S 1 / 2 three-level scheme, and a Lorentz fitting to the data yields an EIT linewidth γ EIT = 2 π × 4.68 MHz . (b) The residuals R between experiment and theory. The RMS values of the residuals are 0.052 and 0.079 for P_RF = 0.32 mW and 1.99 mW, respectively, showing good agreement with the experimental data. The small fluctuations near the center of the line are attributed to the broadening that is mainly induced by the inhomogeneity of the RF field, see text.