The anomalous decrease in microwave surface resistance, R-s, of superconduc
ting YBa2Cu3O7-d (YBCO) thin films in the presence of a low de magnetic fie
ld is studied using a microstrip resonator technique. We have done a de fie
ld hysteresis measurement of R-s to study the effects of vortex penetration
on the anomalous effect. It is shown that the anomaly happens at a field l
evel far below the low critical field, H-c1,H-strip of the superconducting
microstrip and that the vortex (Abrikosov) penetration would eliminate the
anomalous effect observed at low field. This implies that the anomalous eff
ect does not originate from vortices.