Re-Evaluating Radiated Emissions and Exposure for Potted Transmitter Housings
Encapsulating transmitters alters dielectric loading, detuning antennas and shifting RF exposure boundaries, demanding re-evaluation of emissions and SAR compliance.
Near Field Exposure
Meaning
Electromagnetic field interaction occurs within a distance of less than one wavelength from the radiating structure of a device. In cases of near field exposure, the electric and magnetic fields are not yet coupled into a plane wave. This region is characterized by complex energy patterns that do not follow the inverse square law.
Energy Density
Reactive components of the field dominate the environment very close to the antenna. Measuring near field exposure is difficult because the presence of a person or a sensor changes the distribution of the energy. The energy is stored and recirculated without being radiated away.
This results in high local absorption rates that are not visible in far field measurements. Probe calibration is essential to maintain measurement accuracy in these high gradient areas.
Measurement Zone
Boundaries between the reactive and radiating regions depend on the frequency and the size of the antenna. Determining near field exposure is particularly relevant for mobile devices where the user is within millimeters of the transmitter. Precise positioning of the device during testing is required to capture the peak values.
Small changes in distance lead to large variations in the measured signal strength.
Safety Margin
Regulations set limits on the amount of power that can be absorbed by human tissue in this region. Evaluation of near field exposure ensures that the localized heating from a high gain antenna remains within safe levels. Compliance is often demonstrated through numerical simulation or physical phantom testing.
Shielding or antenna placement is used to reduce the peak exposure for the user.