Meaning
Physical interaction between a measurement sensor and the radiator under test alters the field that the system is trying to characterize. The presence of probe loading occurs because the physical structure of the probe reflects energy back toward the antenna and absorbs a portion of the transmitted signal.
Field Interaction
Proximity of the probe to the antenna aperture creates a mutual coupling effect similar to that found between elements in an array. When the distance between the two components is small, the probe loading becomes more pronounced as the reactive fields interact. This can lead to standing waves between the scanner and the antenna, causing ripples in the measured data.
Small, minimally invasive probes with low radar cross sections are preferred to reduce these unwanted reflections.
Error Mitigation
Mathematical techniques can remove some of the predictable errors introduced by the measurement hardware during the data processing stage. By characterizing the scattering properties of the probe beforehand, software can apply a correction factor to the raw data to account for probe loading effects. This probe compensation is a standard step in modern near field to far field transformations.
It ensures that the final radiation pattern represents the antenna in isolation.
System Sensitivity
Residual errors from uncompensated interactions define the lower bound of measurement uncertainty for the test facility. Even with advanced software, some level of probe loading remains, particularly when testing high frequency devices where the probe size is comparable to the wavelength. Selection of the correct probe for the frequency range and antenna type is a main task for the test engineer.
Documenting these interactions is necessary to support the validity of the final certification data.
