We measured radio frequency electromagnetic noise in a cylindrical magnetic shield using a developed monopole antenna RF noise entering the shield through its open ends affects the SQUID operation. In flux-locked mode, it causes the feedback circuit to unlock or increases the noise floor. First, we examined the effect of RF noise reduction using dc-SQUID first-order gradiometers and copper mesh shield. RF noise was also measured using a conical log spiral antenna and a developed monopole antenna. Second, we investigated the reduction of RF noise in a cylindrical magnetic shield by filling its open ends with metal-coated cloth using a fabricated antenna. The RF noise level was decreased inside the shield; as a result, the low-frequency signal-to-noise performance of the SQUID sensors were improved.