Electronic – What makes a magnetic field probe insensitive to E-fields


Consider a magnetic field (B-field) probe such as this one:

field probe


I understand that as as the magnetic flux through the loop formed by the coax shield changes, the charge carriers in the loop will be shoved around the loop by Faraday's law of induction. Since the loop is small relative to the wavelength of interest, we can pretend that the current in it is constant. The high impedance formed by the gap at the top means this induced current will be accompanied by a voltage difference at the gap, and that voltage difference will travel down the transmission line formed by half of the hoop and the feedline to \$Z_L\$, where it's measured. In this way, the probe measures the B-field.

But, what about the E-fields? The point of this probe is to be maximally sensitive to B-fields and minimally sensitive to E-fields. Why is it not sensitive to E-fields?

Best Answer

It is sensitive to E-fields, but since it is shielded the only reasonable place for the E-Fields to interact with the conductor is at the very top and the wavelength must be of the same order as the size of the gap for it to couple in any energy.

That is presumably several orders of magnitude difference in wavelengths and your circuit that is analyzing the output will be looking at the larger (presumably) return signal form the dominant magnetic coupling. The E-Field that is of the same wavelength as the H-Field will not "see" that gap.

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