EMC Question of the Week: May 9, 2022

Magnetic field probes

For a given magnetic field strength, the voltage detected by a magnetic-field probe (electrically small loop or coil) connected to a 50-Ω spectrum analyzer is

  1. independent of frequency
  2. proportional to frequency
  3. (a) at low frequencies and (b) at higher frequencies
  4. (b) at low frequencies and (a) at higher frequencies


The best answer is “d.” At low frequencies, where the inductive reactance (ωL) of the pick-up loop is less than 50 Ω, the voltage detected is proportional to frequency. However, when the inductive reactance is greater than 50 Ω, the voltage detected is independent of frequency. 

Probes with higher inductance have lower cut-off frequencies. For example, a 1-turn loop with a 1-cm diameter and 10 nH of inductance would output a voltage that was proportional to frequency up to about 800 MHz. On the other hand, a 5-turn coil with the same diameter and 260 nH of inductance, would have a flat frequency response at frequencies above about 30 MHz.

Of course, at very low frequencies (e.g., 0 Hz), loop probes cannot detect a magnetic field. And at very high frequencies (e.g., when the loop is no longer electrically small), the voltage is a more complex function of frequency. 

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