The introduction of electronic systems and microprocessors in measuring instruments that were initially mechanical in nature (balances, flow meters, etc) and in many other forms of instrumentation has introduced new problems of reliability. The electronic part can produce parasitic signals or wrong signals under the effect of electrical perturbations conducted in the mains (voltage peaks or voltage interruptions) or even more so under the influence of electromagnetic radiation (such as produced by antennae of portable radio transmitters, radar, etc) or by high frequency instrumentation in the vicinity.
The specifications for military applications are well defined but they are hardly applicable for commercial instruments because they would make them uneconomical. The task of the project consisted in investigating the main parameters of the testing procedure for commercial instruments in an anechoic test chamber.
The investigation covers the range of 80 MHz to 1 GHz. The final product of the project should be a detailed description of a test procedure, with appropriate instrumentation and calibration procedure.
Both a vector (IFS type EFS1) and an isotropic sensor (made by Holladay Instr.) have been used to monitor the level of the radiated EM field.
The conclusions of the project are that, in the case of a shielded room without absorbers, the measured field strength depends greatly on the type of sensor, with readings differing by as much as a factor 4. Indeed, at constant antenna power in the 80-200 MHz range, the vector and the isotropic sensor have been found to display across the room variations at 28 and 17 dB respectively. However, these variations due to standing waves can be stongly reduced (to +/- 3 dB over the full volume, and to +/- dB over the test area) by the presence of absorbers. In this case the results have been found to be essentially independent of the field sensor used.