Determination of new radiated E field limits for electric field measurements of ‘medium sized’ test objects at a distance of 3 m and 5 m

In 2011 IEC-CISPR-B² decided to start a project to investigate the possibility of introducing two new measurement distances (3 m and 5 m)¹ in CISPR 11 [1] for radiated emission measurements on medium sized test objects (devices under test) in the frequency range 30 MHz-1 GHz. In this context `medium sized´ test objects are objects that fit within an imaginary cylindrical test volume of 3 m diameter and 1.8 m height. This paper describes how 3 m and 5 m electric field emission limits have been determined by the authors of this paper, giving the same radio protection as compliance with the established 10 m measurement method. The Numerical Electromagnetic Code and Matlab has been used to determine the statistical properties of the interference potential of a large set of random test objects which exactly fulfill the limits for the established 10 m measurement method. Each random test object consisted of a single straight wire with random location within an imaginary `medium sized´ test volume. The direction, length and position of voltage source within the wire were randomly chosen. Limits for 3 m and 5 m methods were derived in such a way that, provided theoretical compliance with these new limits, the statistical properties of the interference potential of the test objects matched as good as possible (using integer values for the limits) to the statistical interference properties of the 10 m measurement distance. Following this approach it is shown that for the (simplified, random, medium sized) test objects, compliance with the proposed new 3 m and 5 m limits give the same radio protection as compliance with the established 10 m limits. In 2013 a proposal to introduce these limits in CISPR 11 was sent by CISPR-B to its members in a document for comments CISPR/B/560/DC [2].