Extending Penetration Depth with a Carrier Frequency
A carrier-frequency method that combines the fine resolution of high frequencies with deep penetration.
Due to the skin effect, as the frequency of an alternating current rises it flows less in the depth of a conductor and concentrates near the surface.
Detecting small defects requires high frequencies, but as shown below, high frequencies do not penetrate deeply. The way to detect small defects while also ensuring penetration depth is to use a carrier frequency.
In radio communication, the human voice is about 0.2–2 kHz. Because low frequencies don't travel far, the voice is carried on a carrier wave of around 1 MHz.
Eddy current testing uses a similar approach: the inspection frequency is carried deep into the material on a carrier frequency.
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