Transmission measurements of 35 GHz microwaves through two 200 nm YBaC
uO films are performed in the temperature range between 7 and 100 K. T
he films are facing each other parallel in such a way that resonant Fa
bry-Perot transmission is made possible. From the amplitude and the fr
equency width of the resonance, die values of the London penetration d
epth and the Ohmic losses are obtained independently for the first tim
e in an absolute way over the whole temperature range below T-c.