Meaning
Conducting surfaces that form a semi-enclosed volume can trap radio frequency energy and cause severe field distortions. Ground plane cavity resonances are electromagnetic standing waves that occur between a raised ground plane and the floor or walls of a test chamber. These resonances can create severe localized field variations, corrupting emission and immunity measurements.
Physical Source
Geometric dimensions of the conductive cavity dictate the specific frequencies at which standing waves develop. When the distance between the raised ground plane and the underlying floor equals a multiple of a half-wavelength, ground plane cavity resonances are excited by the test signals. This results in significant amplification of the local electric field.
Measurement Impact
Uncontrolled variations in field strength degrade the repeatability of compliance testing across different laboratories. When ground plane cavity resonances occur, the measured emissions from a product can be artificially inflated or reduced by several decibels depending on the exact position of the test antenna. This instability makes it difficult to determine whether a device actually meets the regulatory limits.
The effect is particularly problematic in the lower frequency bands where the dimensions of the cavity align with the wavelengths of interest.
Damping Strategy
Acoustic-style electromagnetic absorbers or resistive loading materials are placed inside the cavity to dissipate the trapped energy. By strategically positioning ferrite tiles or conductive foam beneath the raised platform, ground plane cavity resonances are suppressed. This damping ensures a more stable and predictable test environment.
