B-DOT AND D-DOT FIELD SENSORS

A B-dot sensor measures the time derivative of a magnetic flux density (dB/dt) and is typically constructed as a specialized conductive loop. Conversely, a D-dot sensor measures the time derivative of an electric displacement field (dD/dt) and is typically constructed as an asymptotic conical dipole. Both are passive analog sensors used to characterize fast-changing electromagnetic environments.

Standard field probes often struggle with ultra-fast, high-intensity transient signals due to bandwidth limitations and saturation risks. These standard field probes are designed to measure the rms value of the field, not the real time-domain waveform. Derivative sensors (B-dot and D-dot) are specifically engineered for pulsed electromagnetic fields, such as EMP (Electromagnetic Pulse) or HPM (High-Power Microwave). They offer sub-nanosecond rise times, exceptionally high upper cutoff frequencies, and the ability to withstand extreme field strengths without signal distortion.

Yes. Because B-dot and D-dot sensors output a voltage that is proportional to the rate of change of the field, the raw signal must be mathematically integrated over time to reconstruct the actual magnetic (H) or electric (E) field waveform. This is typically achieved using a dedicated passive or active hardware integrator placed in line before the oscilloscope, or by using numerical integration after the signal acquisition.

These high-precision sensors are critical for demanding applications involving high-voltage pulsed power. Common use cases include:

• NEMP (Nuclear Electromagnetic Pulse) and EMP testing
• Lightning strike simulation and measurement
• HPM (High-Power Microwave) characterization
• Advanced EMC (Electromagnetic Compatibility) immunity testing
• Particle accelerator diagnostics
• Any EM study where the shape of the field matters (not only the rms value)

Sensor selection requires a trade-off between sensitivity and high-frequency bandwidth. A larger sensor (greater equivalent area) provides a higher voltage output, making it suitable for lower-intensity fields, but it will have a lower upper-frequency limit. A smaller sensor provides less signal output but offers a higher bandwidth, which is essential for measuring ultra-fast rise times.

Unlike electronic probes, these passive sensors do not “drift” over time. As long as the mechanical geometry remains unchanged (no dents or structural damage), the sensitivity factor remains valid. We recommend a simple Time Domain Reflectometry (TDR) test to check electrical continuity and a visual inspection of the sensor’s dimensions to confirm its integrity.

Montena’s B-dot and D-dot field sensors are specifically designed to perform reliable measurements in environments where electromagnetic fields change extremely fast and can reach very high amplitudes.

These sensors are passive derivative probes: they naturally handle high-intensity transient signals without the saturation issues often encountered with conventional field probes. Their design enables very fast rise-time measurements and wide bandwidth, making them well suited for capturing rapid electromagnetic phenomena.

Montena sensors are also engineered with precise geometry and high-quality materials to ensure stable sensitivity and predictable frequency response. This is essential for accurate measurements in demanding testing environments.

When combined with appropriate baluns and fiber optic links, the sensors can operate with excellent immunity to electromagnetic interference and complete galvanic isolation, ensuring reliable signal transmission even in harsh test conditions.

These characteristics make Montena B-dot and D-dot sensors particularly suitable for applications such as EMP and NEMP testing, lightning simulation, pulsed power experiments, and advanced EMC immunity testing.