The model F-81 is optimized to measure low frequency currents on large pipes, bundles of cables, or architectural conductive structures. The aperture is 127 mm. The usable frequency range is 100 Hz to 10 MHz with a typical transfer impedance of 1W (± 2dB) from 10 kHz to 10 MHz. The F-81 can be used without saturating up to 350 amperes of DC to 400 Hz, 100 amperes of CW, or 100 amperes of pulse current.