Short H Field Probe

near-field H-field probe is a small shielded loop held just above a circuit, used to find
where high-frequency noise current is flowing. It is a Faraday’s-law instrument: the voltage it
develops follows the *rate of change* of the magnetic flux through the loop, not the static field
strength, so it responds to changing current and not to the voltage on a trace. Because the flux
depends on the area the loop presents to the field, the reading is strongly orientation-dependent —
turning the loop through ninety degrees can take a strong indication down to a null. Loop size sets
a trade: a larger loop intercepts more flux and finds weaker signals, a smaller one localises
better. Readings are relative and qualitative, a pre-compliance troubleshooting aid rather than a
compliance measurement.

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