Meaning
Broadband measurements often employ conductive elements with a double-cone geometry to cover wide frequency ranges. Biconical antenna loading refers to the change in feedpoint impedance and radiation pattern that occurs when the antenna is positioned close to conductive surfaces or absorbent materials during testing. This effect alters the calibration factor of the antenna, leading to measurement errors if uncorrected.
Structural Influence
Proximity to the ground plane or chamber walls introduces capacitive coupling to the antenna elements. In radiated emission testing, biconical antenna loading varies as the mast height changes, causing the antenna factor to fluctuate. These fluctuations must be accounted for to ensure measurement repeatability.
Signal Correction
Numerical corrections or fixed attenuators are applied to the measurement path to stabilize the input impedance. When biconical antenna loading occurs, the mismatch between the antenna feedpoint and the coaxial cable creates standing waves that distort the measured signal level. Inserting a ten-decibel attenuator at the antenna connector reduces the standing wave ratio and improves the accuracy of the measured field strength.
This technique minimizes the impact of the loading on the test results.
Calibration Consistency
Standardized test procedures dictate the minimum separation distances between the antenna and surrounding objects. By controlling these distances, the biconical antenna loading is kept within acceptable limits defined by international standards. This control ensures that the test data remains comparable across different test sites.
