We present a microwave spectroscopic study of the vanadium-based kagome superconductors AV₃Sb₅ with A=Rb, K, Cs, using multimode superconducting stripline resonators. These resonators offer experimental access to the electromagnetic properties of metals and dielectrics in a broad frequency range from 100 MHz to 50 GHz and down to mK temperatures.

From the temperature dependence of the complex microwave transmission coefficient Ŝ₂₁(f, T) we extract the superconducting transition temperature T_c, the normalized complex optical conductivity σ(T)/σ_n , and the normalized superfluid density ρ_s(T)/ρ_s0 of the three different kagome superconductors. This allows us to determine the zero-temperature energy gap ∆₀, the superconducting gap structure, and the number of gaps.

Previous studies report conflicting results, particularly regarding the pairing symmetry. Using cavity pertubation theory, we obtain σ(T)/σ_n and calculate the superfluid density_. At low temperatures, ρ_s(T)/ρ_s0 remains nearly constant, consistent with an s-wave superconducting state in all three samples. The data are best described by a two-gap s-wave model, with one gap clearly dominating. The larger gap is approximately consistent with the BCS ratio 2∆₀ /(k_B T_c) = 3.53.