Schwarzbeck HFRA 5149 Circular Transmitting Loop Antenna

In the VLF-HF frequency range 9 kHz to 30 MHz the magnetic field strength is measured preferably, but often expressed in the unit of the electric field strength as the “fictive E fieldlevel” ( dBμV/m). In the undistorted far-field both units are linked by the characteristic impedance of free space 120 πΩ = 377 Ω. Practical EMC/EMI measurements however are carried out in the near-field zone (D <0.1 λ). For that reason defined magnetic field sources are required. For Immunity Tests powerful H fields might be needed, but also general tests and measurements at an open site, in screened rooms and in absorber-lined rooms require well-defined powerful H field sources. Magnetic fields in the near-field zone decay with the inverse 3rd power of distance (ca. 18 dB at twice the distance). Even at 1 m distance at 30 MHz the transition from pure near-field to far-field begins, the exponent of degradation gradually reduces from 3 to 1 in the undistorted far-field.