In my opinion... The right limit is the limit which protects reception of broadcast signals in licensed bands. At low frequencies where the "antenna" is either a loop or whip, the limit would be ascertained using a typical loop/whip connected to a typical receiver. The most accurate limit would be the rfi level at the receiver antenna port that allows a specified quality of reception in the presence of the rfi. The measurement would be made at a specified distance >from the rfi source. The limit would then be so many dBuV or dBm in a certain bandwidth, using a particular loop/whip. The loop/whip construction and transducer factor would have to be specified.
This is the only technically proper way of doing things until you are at a frequency/distance from rfi source such that a far field measurement is possible. A field intensity limit allowing the use of any suitable antenna is only appropriate if a far field measurement is possible. From: "Iain Summers" <[email protected]> List-Post: [email protected] Date: Thu, 23 Jun 2005 21:44:27 +0100 To: <[email protected]> Subject: Re: ??? RE: ??? RE: Near field H-field measurement. Ken very succinctly put. I believe it was me that you responded to in an earlier posting regarding Broadband over PowerLine (PowerLine Telecommunications in the UK and Europe). I have been trying to explain the point that you have made to those that write reports estimating the likely effects of BPL. The problem in the BPL world is both of accuracy and repeatability. Accuracy because authors are quoting figures to 2 decimal places of an E field that was derived >from a loop in the near-field in an uncalibrated test site. Repeatability because there is no agreed way to measure emissions from BPL yet. As part of my research I am measuring both E and H and then conducting subjective mutual interference tests on a radio receiver to attempt to correlate the effects to a measured limit. The answer will not be "an acceptable limit for BPL is ?dBuv/m". It may all be academic because politicians don't do EMC. Iain ----- Original Message ----- From: Ken Javor <mailto:[email protected]> To: [email protected] Sent: Thursday, June 23, 2005 9:05 PM Subject: Re: ??? RE: ??? RE: Near field H-field measurement. There is a significant over-generalization here. An electrically small probe does not have to convert rf into heat and thereby measure total broadband pickup. There are any number of electrically short whips, dipoles and loops whose transducer factors as a function of frequency are well characterized. Thereof ere the electric or magnetic field intensity as a function of frequency can be accurately measured. What cannot be characterized is a single number that describes the field intensity anywhere except the point at which it is measured. To quote myself >from a previous post on this very same subject. here there was interest in measuring rfi from broadband communications over power-lines: "I don't know how BPL really works in detail but there are a few basic physical principles that apply regardless: 1) If the measurements of interest are really in the near field, especially in the induction field, then the only valid measurement is to use the same type "antenna" as the likely victim, and place it the same distance from the source as the victim will occupy. The idea of measuring a field intensity in the near field is worthless. What you are measuring is the noise potential delivered to a receiver connected to a representative victim "antenna" or more accurately, field probe. This is very close to the old idea of an "antenna-induced" limit, except that concept was based on open-circuiting the probe output, whereas here we are talking about power delivered into a 50 Ohm receiver front end. 2) The probe must look like the victim to be protected by the limit. If an electrically short whip is being used, you don't use a loop and try to correlate the two. And vice versa. And the whip or loop being used needs to be the same size as the likely victim - the probe delivers a potential that is proportional to the average field impinging over its physical aperture. Unless the field is close enough to a plane wave that over the physical dimension of the probe it is homogenous, you cannot correlate the output of two different size probes, even if they are both whips or both loops. Further, you cannot extrapolate the field intensity to another distance other than that measured unless you have a priori a complete understanding of the current distribution on the transmitting structure and are only using the probe to get an amplitude data point." From: [email protected] List-Post: [email protected] Date: Thu, 23 Jun 2005 18:56:33 +0000 To: [email protected] Subject: RE: ??? RE: ??? RE: Near field H-field measurement. By measuring the E and H components separately, using near-field probes, which are, in principle, electrically short dipoles and loops. The biggist price to pay is the sensitivity, since they are electrically short. Also, they pick up the broadband field that gets detected in the detector circuit (DC output proportional to the field strength), so the frequency information is usually lost. Using isotropic probes can help, since it reduces the need for alignment of polarization. Unwanted pickup by the probe leads and the readout electronic, as well as the field perturbation by the probe and the operator sometimes cause considerable issues here. Neven -------------- Original message -------------- > How would one go about accurately measuring low frequency fields say at 10 > kHz where the far-field even at lambda/6 is 5000 meters? > > Bob Heller > 3M EMC Laboratory, 76-1-01 > St. Paul, MN 55107-1208 > Tel: 651- 778-6336 > Fax: 651-778-6252 > ========================= > > > > "Price, Ed" > > om> To > Sent by: [email protected] > owner-emc-pstc@ie cc > ee.org > Subject > RE: ??? RE: ??? RE: Near field > 06/23/2005 11:24 H-field measurement. > AM > > > > > > > > > > > > > > -----Original Message----- > > From: Bob Richards [mailto:[email protected]] > ! t; Sent: Thursday, June 23, 2005 7:06 AM > > To: [email protected] > > Subject: Re: ??? RE: ??? RE: Near field H-field measurement. > > > > Ken, > > > > I believe you are correct. There should be factors for this > > loop antenna from Emco that convert the raw reading (in dBuv) > > to a magnetic field level (like dBuA/m). > > > > The spectrum analyzer does NOT read in dBuv/m as he seems to > > suggest, it only reads in dBuv or dBm, but some analyzers do > > allow factors to be entered so the display will be in useful > > units. I prefer not having the analyzer apply factors. Less > > chance for errors that way, IMHO. > > > > Bob Richards, NCT. > > > > > > > EMCO (ETS) shows generic correction factors for the 6502 at: > > > http://www.ets-lindgren.c! om/product age.cfm/model/6502/producttype/Antennas > > > They have a pair of charts for correcting the antenna output (in dBuV) to > units of electric field (dBuV/m) or magnetic field (dBuA/m). > > > I don't like the practice of a factor for converting loop output voltage > directly to dBuV/m, because it is making the assumption of far-field > conditions (377 ohms, and an H-field to E-field conversion of +51.5 dB). > The 6502 is a shielded loop, and responds to the H-field, no matter where > you place it. You can accurately derive the E-field from the H-field only > when you are confident of far-field conditions. > > > Ed Price > [email protected] WB6WSN > NARTE Certified EMC Engineer & Technician > Electromagnetic Compatibility Lab > Cubic Defense Applications > San Diego, CA USA > 858-505-2780 (Voice) > 858-505-1583 (Fax) > Military & Avionics EMC Is Our Specialty > > > > ---------------------------------------------------------------- This > message is from the IEEE Product Safety Engineering Society emc-pstc > discussion list. Website: http://www.ieee-pses.org/ > > > To post a message to the list, send your e-mail to [email protected] > > > Instructions: http://listserv.ieee.org/listserv/request/user-guide.html > > > List rules: http://www.ieee-pses.org/listrules.html > > > For help, send mail to the list administrators: > > > Scott Douglas [email protected] Mike Cantwell [email protected] > > > For policy questions, send mail to: > > > Richard Nute: [email protected] Jim Bacher: [email protected] > > > All emc-pstc postings are archived and searchable on the web at: > > > http://www.ieeecommunities.org/emc-pstc ! > > -------------------------------------------------------- -------- > This message is from the IEEE Product Safety Engineering Society > emc-pstc discussion list. Website: http://www.ieee-pses.org/ > > To post a message to the list, send your e-mail to [email protected] > > Instructions: http://listserv.ieee.org/listserv/request/user-guide.html > > List rules: http://www.ieee-pses.org/listrules.html > > For help, send mail to the list administrators: > > Scott Douglas [email protected] > Mike Cantwell [email protected] > > For policy questions, send mail to: > > Richard Nute: [email protected] > Jim Bacher: [email protected] > > All emc-pstc postings are archived and searchable on the web at: > > http://www.ieeecommunities.org/emc-pstc ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: http://listserv.ieee.org/listserv/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas [email protected] Mike Cantwell [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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