The general method for predicting radio noise in the AM radio frequency band near high-voltage transmission lines (HVTL) caused by corona discharges on the conductor surface of HVTL is to first predict the horizontal magnetic field and then multiply it by a conversion factor of 120π (=the wave impedance in free space) to obtain the vertical electric field. The same method is adapted in CISPR/TR18, the only document on corona noise near HVTL published by IEC. This paper examines the wave impedance near a single-phase HVTL with an infinitely long horizontal conductor at 15m above the ground through theoretical and numerical electromagnetic field analysis, and it is found that at the CISPR position and a frequency of 0.5MHz, as specified in TR18, the wave impedance is 148πΩ instead of 120πΩ. This represents a difference of about 2dB in the vertical electric field level.
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Tanabe et al. (2024) studied this question.
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