Traditionally, measurements have been made using an open area test site, or a semi-anechoic chamber (both incorporating a ground plane). Fully anechoic rooms were not economically feasible (at least for the frequencies relevant to EMC testing) until ferrite tiles became more readily available. Ferrite is a much more compact absorber in the 30MHz to 1GHz range than foam absorbers. (The latter only come into their own at higher frequencies, as they would be excessively large at lower frequencies). Advantages of the fully anechoic chamber include the avoidance of variation in results caused by interaction between the direct and reflected signal, elimination of the need for scanning at different heights, and the use of the chamber for emission and immunity testing without changes.
The dimensions of the EMC Test Chamber are: 4.8m wide x 6.0m long and 2.4m high, and it is configured as a standard 3 metre test field. The equipment is set up to perform standard EMC tests to 1GHz, but if more specialised tests are to be performed, the Club’s test chambers can be used for other tests which require a screened or anechoic environment.
Any item of equipment to be tested must be capable of being carried through a standard 750mm doorway and down a couple of steps, A maximum peak power capacity of 3kW single phase (13A at 230VAC) is available with the standard wiring setup, but this will be adapted to suit members’ needs (including 3 phase or 110V if required).
The scan of a calibrated emission reference source (ERS) is used to create a correction file which is automatically applied to all future runs. Any possible errors due to inaccuracies in the antenna factor, cabling, room characteristics or synthesiser are automatically nulled out, as the complete test system is included in the calibration loop.
The accuracy of the whole test is then precisely related to the calibration of the ERS. The Test Club’s ERS has been calibrated against a ‘transfer standard’ whose radiation at a distance of three metres (the test distance used by the EMC Test Club) has been precisely measured by the leading test site in the UK, NPL (National Physical Laboratory).
This obviates the need to maintain individual calibration standards for all the separate items in the chain. The overall calibration can be checked at any time by doing a scan of the ERS, and comparing this with the results of the ‘transfer standard’ ERS on the NPL site at Teddington. This site has become one of the ‘master’ sites in Europe, and the calibration of the ERS involves the absolutely ‘correct’ procedures and the most accurate instrumentation standards.
A copy of the NPL Calibration Certificate and report for the ‘transfer standard’ ERS (against which the Club’s ERS is calibrated) is available for members to view at the Club premises.