I also wish you great luck. Two major issues will confound you: 1) To convert a near field measurment to the far field, by definition you need to know the wave impedance of the source. We know that in the far field, the wave impedance will have become 377 ohms, but in the near field it may thousands of ohms from an electric field source, or it may be tens of ohms from a magnetic field source. With measurments from a loop probe, you are measuring only the magnetic field component, and you will have trouble doing the extrapolation to the far field. Once you have solved this problem, its closely related cousin lurks for you in the darkness.
2) The cousin: Antennas like dipoles and biconicals produce a voltage that is related to the electromagnetic field in the vicinity of the antenna. That field in turn is generated by the EUT, and the strength of that field is determined by two factors, the current flowing in the radiating element, and the geometry of the radiating element (its length/area). A small loop probe produces a voltage that is only proportional to the current flowing in the radiating structure. Thus to calculate the farfield effects of the current you need to be continually moving the loop probe and doing line integrals and area integrals. PS If those two dont get you, I've always had a sneaking suspicion that the process of making nearfield loop probe measurements can affect the emissions coming from an EUT, but I've never actually tried to prove it. Anyways, good luck is in order, but I won't be holding my breath. Paul Cook NARTE Certified EMC Engineer Alpha EMC Inc 8540 West River Rd Minneapolis, Minnestoa 55444 Tel # (612)-561-2844 Fax #(612)-561-3400 E-mail paulc...@skypoint.com Specialty - EMC Consulting -----Original Message----- From: Arun Kaore <kao...@sg.adi-limited.com.au> To: 'emc-p...@majordomo.ieee.org' <emc-p...@majordomo.ieee.org> List-Post: emc-pstc@listserv.ieee.org Date: Wednesday, June 16, 1999 11:58 PM Subject: NEAR/ FAR FIELD CORRELATION ISSUES > > >Greetings and Salutations! I was wondering if this could be mailed out via >the epc-pstc channels. > >I want to know if anyone is doing any work in "near/ far field correlation >to commercial EMC standard limits" area and possibly correspond with them >with a view to exchanging notes. > >Brief Follows: > >Brief: > >"Application of Near Field and possibly Surface probe techniques in >Evaluating Emissions from source equipment and correlating/quantifying this >data to an OATS, LISN, Absorbing Clamp etc based measurement.." > >Details: > >We assume that the current EMC compliance regime around the world has >quantified limits of compliance to which if every source equipment adheres >to, then it has a reasonable chance of performing as intended in a real life >situation, noting that the immunity threshold test levels are much more >stringent than EM emission limits. > >Current EMC measurements are very cumbersome, require large expense in >setting up and maintaining (calibrating) OATS, LISNS, Absorbing Clamps, >Ferrite tiled lined semi- anechoics etc. Despite this expense, the >measurement uncertainities are still of the order of 6 to 10dB (inherent). > >Every newly released EMC standard by IEC CISPR or CENELEC has potentially >new transducers and new headaches from point of view of sourcing and >maintenance, calibration. EMC today is where Safety was 10 years ago. I am >of the opinion that EMC testing should be simplified and reasonably >accessible to all end users. > >Continued progress and urban development has led to increasing levels of >broad and narrowband noise to the extent that ambient profiles sometimes >swamp out the limits; this has led to most test houses in the EU to opt for >GTEMS or semi-anechoics (referred to as "alternative all weather test >sites") at considerable expense. > >Current techniques such as emission E Field prescanning in shielded rooms >prior to OATS based testing with biconilog antennae have the drawbacks of >"peaking" the emissions and reflections/standing waves. > >Hence: > >I propose to develop near or surface probe H (inverse of E) field >techniques which actually senses the emission profiles and correlate them to >an Absorber clamp, or OATS or LISN (Common mode fix) or whatever. Cables, >panels, slots etc could be "sniffed" with a Loop and if it is possible to >correlate this near or induction field data to compliance limits then it >becomes very easy for individuals and organisations to do precertification. >Transducers could be simple and light weight, rugged, and physically defined >so that minimal calibration is required. EMI receivers will still need >calibration. > >With these techniques, you measure actually what comes off the source and >not the "peak" value of the bounced and direct rays within say the Fresnel >ellipse, or move the Absorbing clamp up and down the rail and peak the >field. > >Conducted emissions could be scanned by common mode techniques and radiated >emissions by surface or near scans. > >Currently, these techniques are used only qualitatively for precompliance at >board levels and more work needs to be done to bring them of age and >reliability. (Am I right?) > >What I am proposing has a corollary with the bulk current or damped sinusoid >(NEMP- Nuclear EM Pulsing) or lightning injection techniques (CS 114, 115 >and 116 of 462D). With these methods, it was possible to achieve identical >or several orders of magnitudes higher levels of RF injection power at a >fraction of the cost of say an RS03 OR RS05(at least for the bulk cable >loom!). What resulted was a cheap, powerful and a more repeatable test. > > > >Arun Kaore >EMC Engineer > >ADI Limited >Systems Group >Test & Evaluation Centre >Forrester Road, St Marys, NSW 2760 >P O Box: 315, St Marys NSW 1790 > >Tel: 61 2 9673 8375 >Fax: 61 2 9673 8321 >Email: kao...@sg.adi-limited.com.au > > >--------- >This message is coming from the emc-pstc discussion list. >To cancel your subscription, send mail to majord...@ieee.org >with the single line: "unsubscribe emc-pstc" (without the >quotes). For help, send mail to ed.pr...@cubic.com, >jim_bac...@monarch.com, ri...@sdd.hp.com, or >roger.volgst...@compaq.com (the list administrators). > > > --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to majord...@ieee.org with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to ed.pr...@cubic.com, jim_bac...@monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators).
