Predictability of radiation in vertical directions based on in situ measurements close to the ground at frequencies above 30 MHz

National and international standards specify limits for the radiated disturbances created by industrial, scientific, and medical (ISM) radio-frequency equipment. The in situ E-field limits they specify above 30 MHz apply at heights above ground of 6 m or less. The international standard CISPR 11 specifies limits at a fixed height of 3 m. The US FCC specifies 1-4 m or 26 m height scans, subject to measuring distance. Specified measuring distances vary. For protection of aeronautical safety of life services, CISPR 11 specifies in situ limits 10 m from the exterior wall of the building housing the ISM equipment. The actual distance from the ISM equipment is not specified. This paper considers the predictability of radiation in vertical directions based on in situ measurements using the CISPR and FCC methods. The paper shows that the ISM fields measured by those methods are very poor guides to the fields at elevated angles. In consequence, the specified in situ limits cannot deliver the protection they are assumed to provide for aeronautical safety services. Recommendations are provided to improve the predictability of the fields at elevated angles.