In order to eliminate the influence of the large-amplitude magnetic field noise that has complicated magnetocardiographic studies since October 2009, we have performed high-accuracy measurement of the environmental magnetic field noise in the vicinity of Beijing Subway Line 4 using a three-component height Tc radio frequency (rf) superconducting quantum interference device (SQUID). By analysing the spatial form and other characteristics of the time and the frequency domains and by calculating the circumferential magnetic field distribution based on a duel-end feeding system model, we reach the following conclusions: (i) the main source of magnetic field noise is the magnetic field generated by the subway trains, and (ii) the magnetic field interference results mainly from the imbalance between traction current and return rail current that is caused by the leakage current.

Antenna is a key component of radio system. The existing low-frequency (refering to 3Hz-30KHz) receiving antenna has low efficiency, high level noise, and low sensitivity.We propose using HTc rf SQUID magnetometer as low frequency receiving antenna to improve the receiving system's performance. Therefore, we introduce a typical low-frequency radio system to demonstrate the effectiveness of HTc rf SQUID antenna.