PRODYN's magnetic field B-Dots are small to medium sized, free-space and surface, high frequency sensors that measure time rate of change of magnetic fields.  Probes with sensitive areas of up to 0.1m2 or upper 3dB points of 11GHz are available. 

To aid sizing of the probes a ready reckoner has been provided to help you estimate the output of a device based on rise time and field strengths B-DOT output calculator.

The voltage output from the probe proportional to the RATE OF CHANGE of the incident magnetic field.  Examples of how to interpret the output from these sensors can be found here:

Further reading on the devices can be found here:

B-DOTs are also available in Radiation Hardened configurations.