SAE ARP958D

This SAE Aerospace Recommended Practice (ARP) outlines a standard method for the checkout and calibration of electromagnetic interference measurement antennas. Its primary application is for use when measuring a source 1 m from the antenna in a shield room versus a source at a greater distance (far field). This is the typical distance used in performing military EMC testing. Thus, this is a method of calibration. Shield room characteristics are not considered. It does not address an unknown distributed source. Yet it is close to reality since it is based on another antenna that represents a distributed source. This document presents a technique to determine antenna factors for antennas used primarily in performing measurements in accordance with 2.1 and 2.2. The purpose of Revision B is to include the calibration of other antennas, such as small loop antennas that are also specified for use in these same references. Revision D includes a specific procedure for loop antennas that are separated by 1 m from the device under test. For an antenna to be useful in measuring EMI, an antenna factor (AF) must be specified which permits converting voltage at the input of a receiver (V) to field strength (E) in volts per meter (V/m) or into units suitable for comparison with radiated emission limits (dBµV/m) of reference 2.1. Thus, where: E = V/m V = volts where: AF is the antenna factor based upon power gain of the antenna (it is derived from the square root of the power density (W/m2)). Converting to dB: where: E = dBµV/m V = µV AF is the antenna factor based upon an antenna calibration similar to gain, henceforth referred to as 1 m gain, but performed under conditions characteristic of the actual use (1 m from the source) of the antenna for component-level EMC specification compliance testing. Numeric antenna gain (G) and wavelength (λ) (in meters) in a 50 Ω system express the antenna factor (see Appendix A) as follows: Antenna gain is the ratio of the radiated power density in a certain direction to the average radiated power density. The average radiated power density is the isotropic radiation intercepted by a surface area of a sphere of unit radius (see 2.3 and 2.4).