Electromagnetic Interference Measurement Antennas; Standard Calibration Method
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.
Rationale: Suggested improvements of SAE ARP 958
1. Use near-field formulation to derive AF from the site insertion loss between a pair of antennas.
2 Investigate the sensitivity of AF to the orientation of the biconical elements, in which the extremities can be anywhere between 0.48m and 0.55m apart. Investigate the sensitivity to the orientation of the asymmetrical cross bar
3 Include the three-antenna method and compare with the two “identical” antenna method, particularly where the antenna is of the same type (e.g. biconical) but not the same manufacturer or model. At least two 1:1 (50 Ω) baluns should be paired, and two 4:1 (200 Ω) baluns.
4. Specify the orientation of the antennas as vertical. If there is some reason that horizontal is required, use the Smith formulation to remove the ground ray, though the veracity of this needs to be checked if the near-field expression for separation also has to be used.
5. Try using smaller antennas to calibrate the AUC in order to reduce the coupling effects.
6. Quantify the phase centre errors for LPDA antennas. Make a stand on whether hybrid biconical log antennas are allowable.