We present a comprehensive microwave spectroscopic investigation on bulk Nb:SrTiO₃ in the mixed state. By measuring the complex optical conductivity σ as a function of temperature T, frequency f, and magnetic field B, we extract the energy gap Δ, the penetration depth λ_eff, the GL coherence length ξ_GL, and the upper critical field B_c2 across the superconducting dome.

Our field-dependent measurements reveal flux-flow consistent with the Coffey–Clem model of the mixed state. This includes a strong peak in σ₁(B), which we dub flux-flow peak. More importantly, we observe a distinct characteristic field B* next to B_c2. We interpret B* and B_c2 as the upper critical fields of two superconducting bands. Their ratio remains constant across doping and matches the known ratio of effective masses of the lower and middle bands, establishing a direct link between the critical fields and the band structure. Furthermore, we spectroscopically track the energy gap Δ(B) in the mixed state. We find that Δ(B) closes at B ≈ B* well below B_c2. We observe only a single superconducting energy gap.