Assessment of field uniformity in a GTEM cell using a novel non-invasive scanning positioning system

Summary form only given, as follows. To assess field uniformity within a GTEM cell, a set of field strength measurements have to be made over a 3D mesh of points inside the cell. To minimise the disturbance of the field structure during the measurement of the probe, its support and the scanning system should be minimally perturbing. The problem of how to repeatedly position the probe element in space without disturbing the field has presented a significant challenge. The requirements of the positioning system are that the probe can move in all three dimensions, over a reasonably large volume, preferably near to the testing zone in the GTEM and under computer control. NPL has developed a novel automated positioning system consisting of a platform supported at its four corners by strings running to a set of motors mounted outside of the cell. Connected to each motor drive is a drum around which the strings are would and by winding in or out certain lengths of string it is possible to move the platform around in three dimensions. The exact angular rotation required by each motor to obtain a given position in space is calculated by a computer program and implemented under IEEE bus control. This program is also used to coordinate the data acquisition from whichever field detection system is used.