Electromagnetic Interference Measurement Antennas; Standard Calibration Method
|Publication Date:||1 March 1999|
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.
The intent of Revisions A and B is to make this document applicable to passive and active antennas that may be used to measure signals from a source 1 m distant. Typical antennas being considered are the following:
b. Resonant dipole
c. Log periodic dipole
d. Log spiral (200 MHz to 1 GHz)
e. Log spiral (1 GHz to 10 GHz)
f. Double ridged horn
g. Log periodic
h. Standard gain horns
i. Loop antennas
j. Vertical monopole
Most present day "104 cm" (41 in) rod antennas are of the active type and not calibratable per the previous issue of ARP958. The theoretical behavior of rod antennas is well understood. Therefore, the calibration really involves verifying the gain of the electronics in the antenna base. These antennas are calibrated by the use of a signal substitution source as defined in Revision B.
A separate section is included to cover the "RE01/RE101" loop and "RS01/RS101" loop even though they are used much closer than 1 m from the equipment under test.
General Background and Limitations:
This document originally was limited to determining antenna factors for conical logarithmic spiral antennas. It has been expanded to cover other antennas as indicated in 1.2. Antenna factors can be determined and calculated for the far field condition independent of ground refections. The method, described in this document, of moving from the far field to a 1 m distance results in changes in antenna factors of a few decibels. The primary conditions which influence the antenna factors are the antenna separation, the height of the antenna above the ground plane, the orientation of the antenna relative to the ground plane, and the conductivity of the ground plane.