How does an RF Sensor work?

All materials have four distinguishing electromagnetic parameters, namely the real and imaginary parts of the electrical permittivity (ε’ and ε’’) and the magnetic susceptibility (χ’ and χ’’).

The RF sensor operates by defining a sensitive volume and interrogating the values of these parameters for whatever materials invade this volume. Any or all of the parameters may be simultaneously measured by either a single sensor or a sensor pair and the sensor deduces the nature and behaviour of the invading items from the resulting signature.

The sensitive volume is defined by a radiofrequency antenna which is tuned to the sensor electronics. The size and nature of the antenna and the nature of the material detected govern the optimum operating frequency of the RF sensor, which can be a few Kilohertz for large systems or tens of Gigahertz for very small ones.

An inductive antenna is used for susceptibility detection and a capacitive antenna is used for permittivity detection. The size and shape of the antenna determines the sensitive volume and choice of construction materials of the antenna determines permissible operating temperature.

The sensor operates by exciting the antenna at the specified operating frequency. Changes in the real part of the electromagnetic parameter being measured change the operating frequency (FM). Changes in the imaginary parameter produce amplitude variations in the carrier (AM). The frequency and amplitude modulations are then processed to produce the information needed, such as speed or position or composition.

ε’ and ε” versus frequency