EN
We present new experimental evidence indicating the importance of magnetic field component of microwave field (f=9.4 GHz) for magnetoresistive properties of polycrystalline La_{0.7}Ca_{0.3}MnO_3 films. The microwave measurements revealed a different character of the temperature-dependent electrical resistance of polycrystalline La_{0.7}Ca_{0.3}MnO_3 films placed in the centre (maximal amplitude of H_{10} wave vector) and at a narrow wall of the wave-guide (reduced H_{10} amplitude). Theoretical estimations of the influence of substrate onto distribution of microwave electric and magnetic fields in the waveguide were performed using the finite-difference time-domain method.