  The anomalous decrease in microwave surface resistance, R_{s}, of
superconducting YBa_{2}Cu_{3}O_{7-d} (YBCO) thin films in the presence of a low
dc magnetic field is studied using a microstrip resonator technique. We have
done a dc field 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 level far below the low critical field, H_{c1,strip}, of the
superconducting microstrip and vortex (Abrikosov) penetration would eliminate
the anomalous effect observed at low field. This implies that the anomalous
effect is not contributed by vortices.
