8.7 MEASUREMENT PROBES AND ANTENNAS
The sensor is the part of a measurement system that interacts directly with the electromagnetic field. A field probe is generally designed to indicate a field quantity at the probe location, while a calibrated measurement antenna converts the incident field into a received signal for analysis by a receiver or spectrum analyzer. The terms are sometimes used loosely, but the distinction is important when selecting equipment and interpreting results.
Sensor performance can dominate the uncertainty of an RF measurement. Relevant characteristics include frequency response, sensitivity, dynamic range, isotropy or directivity, polarization, dimensions, spatial resolution, calibration factor, linearity, and the degree to which the sensor, support, cable, or operator perturbs the field.
8.7.1 Measurement Probes
Isotropic field probes are commonly used with broadband meters for exposure surveys. Three approximately orthogonal sensing elements are normally combined to estimate the resultant field independently of probe orientation. Separate probes are used for electric-field strength, magnetic-field strength, and, in some frequency ranges, power density. No practical probe is perfectly isotropic at every frequency and orientation. The calibration certificate and manufacturer data should be used to determine the usable frequency range, isotropy error, linearity, dynamic range, response time, and applicable correction factors. Probe dimensions must be small enough to resolve the spatial variation relevant to the assessment, particularly in nonuniform near fields.
Electric-field probes commonly use electrically short dipole sensing elements. High-impedance, resistive, or optically coupled arrangements are used to reduce loading and cable interaction. The probe should have adequate sensitivity and bandwidth while remaining sufficiently small to avoid excessive spatial averaging or disturbance of the field. Electric-field probes are used in both near-field and far-field assessments, but their readings must be interpreted according to the field region. In the reactive near field, an electric-field measurement does not determine magnetic-field strength or incident power density.
Magnetic-field probes generally use small loop sensing elements. Shielded and balanced designs reduce unwanted response to the electric field. Loop dimensions, orientation, frequency response, and the proximity of conductive structures affect the measured value. These probes are especially important at lower RF frequencies and near induction heaters, wireless power-transfer systems, loop antennas, and other sources for which the magnetic field may dominate. An approximately isotropic magnetic-field probe combines multiple loop elements; a single-axis loop may instead be used where field direction or a particular component is required.