Measurement distance and separation distance
By J. M. Woodgate B.Sc.(Eng.) C.Eng. MIET SMIEEE FAES Hon FInstSCE MIOA
WARNING – may contain mathematics
Measurement distance and separation distance
These two distances are crucial in understanding emission limits and immunity requirements. For measuring emissions, it's obviously necessary to specify the distance between the EUT (Equipment Under Test) and the measurement antenna, because the field strength of the emission varies with distance from the EUT) and its cables, if any). That, in itself, presents problems that we will address later. The standards committees then determine the immunity test levels, making then higher than the emission limit at the measurement distance. (The process might work the other way, for a new type of EUT and existing immunity levels.)
But when the EUT and some other equipment are actually in use, the separation distance between them may well be different from the measurement distance. If it's greater, there is clearly no problem, but it might be much less. In that case, the immunity may not be sufficient and detectable interference results. This is completely unavoidable, and can only be combated by making the immunity levels as high as possible, and likewise the emission limits as low as possible, without either being economically unsupportable. The criterion for determining whether a solution works in practice is that the number of complaints of interference is acceptably low, allowing for the fact that only a very small fraction of the actual number of interference cases results in a formal complaint.
Measurement distances specified in standards are mostly 3 m, 10 m and 30 m. These are quite long; pieces of electronic equipment are often separated in use by 1 m or even much less (put you phone on top of your DAB radio!). Why don't the standards specify 1 m? It couldn't be much less, because the measurement antenna maybe around 1 m long.(There is a way round this, but it doesn't work in all cases.)
To understand this, we have to study the way electromagnetic emissions behave. Anything that emits is an 'antenna', although it may be a very inefficient one, because its physical dimensions are incompatible with the frequencies being emitted. Emissions are of two types, propagating, which means that electromagnetic waves are generated and travel to a long distance from the EUT, and ephemeral, whose strength falls off very rapidly with distance from the EUT. Most standards don't cover ephemeral emissions.
Propagating emissions do not 'spring fully-formed' from the EUT. Classically, two regions of space around the EUT were identified, the 'near field' and the 'far field', with an essentially blurred boundary between them, but these days, three regions are identified. There is a good explanation in Annex A of IEC/EN 62311:2008, but it has not been carried over into the current edition. This is an IEC TC106 standard, so many people working on EMC may not have seen it. It is a very big subject; the Annex is 4½ pages, but is necessarily highly condensed. Unfortunately, there are also some typos in the mathematics.
Reactive near-field region
This is closest to the EUT (or cable). All 'antennas' produce both electric and magnetic fields, but one is dominant. For anything that looks like a wire or cable, the electric field is dominant, so it is an 'electric antenna', but for a loop or solenoid, the magnetic field is dominant, so it is a 'magnetic antenna'. During one half-cycle of an emission frequency, energy is emitted into space, but it goes back into the source before the next half cycle starts. Nevertheless, we can measure both fields and we find that the field strengths vary a lot in amplitude, and the phase difference between them is 90°, at points around the EUT. This means that measurements in this region cannot normally be specified in standards because they would be repeatable only with very great difficulty.
The outer boundary of this region is not clear-cut (no discontinuity is space), and is variously considered to be ?/2p, ?/4 or 0.62v(D3/?), depending on whether a particular 'antenna' of length D can be identified.
Radiating near-field region
This region, also called the Fresnel region, is too close to the EUT to allow the 'antenna' to be treated as a point source. This means that the signal from one end of the antenna is not in phase with that from the centre of the antenna at any distant point, because of the different path distance. Both the magnitude and phase of the resultant field strength thus vary with distance, so it's not a good region in which to set emission limits. Conventionally, the outer boundary of this region is set at a distance from the EUT of 2D2/?, where D is the largest dimension of the antenna projected in the direction of the measurement point and ? is the wavelength.
This region, also called the Fraunhofer region, is where conventional electromagnetic radiation occurs. The 'antenna' is considered small enough to be treated as a point source, i.e. D << ?. The energy radiated in one half-cycle doesn't have time to get back to the source before the polarity reverses, so it propagates outwards towards distant points. In this region, the electric and magnetic fields are in-phase in time, but orthogonal in space, and their ratio is constant and equal to 120p ? (377 ?). It is (fairly) easy to make repeatable field strength measurements in this region. so emission limits can be applied.
Unfortunately, there is a problem. We want to measure, if we can, radiated emissions at 150 kHz or at even lower frequencies. At 150 kHz, ? is 2000 m, so the outer boundary of the reactive near-field region is around ?/4 = 500 m. We will need a VERY big test site! The only way out is to measure electric and magnetic field strengths separately. Luckily, one of them is usually too small to take into account, so we can measure just the other one.
This three-region situation is the reason why it is often not permitted to measure field strengths at any other distance than that specified in the standard, or, alternatively, there may be specified conversion factors which do not coincide with the expected inverse-square law that applies to free-space emissions.
New Stability Dates for some standards.
IEC SC77B set new dates for some standards at their recent meeting. The Stability Date is widely misunderstood, and is the earliest date at which an amendment ro revision is expected to be published.
IEC 61000-4-2, the EDS standard, is proposed to be improved, but this is controversial. On the one hand, the basic physics is not well respected — the way the energy flows around the set-up is not well controlled, but on the other hand, EUTs that pass the tests do not fail to a significant extent in the field, so the tests are effective (but might be too stringent), and changes to the test equipment can be costly.
 How far? Until the signal is too weak to receive.