Interesting thread; educational. There's the pure science of the issue and there's the regulatory expedient solution. Interestingly, the FCC mandates using a loop for frequency measurements below 30 MHz, but gives the limit in dBuV/m. The antenna factors of the loop antennas are typically given for both E and H field related by far field factor of 51 dB. As indicated before, 50 kHz is anything but far field, and one will not be able to measure the H field of a loop source of 50 kHz in the far field.
So as Cortland pointed out, precise and inaccurate it is... Don Umbdenstock Manager Compliance Engineering Tyco Safety Products / Sensormatic 6600 Congress Avenue Boca Raton, FL 33487 USA 561.912.6440 [email protected] From: Bob Richards [mailto:[email protected]] Sent: Friday, June 24, 2005 8:05 AM To: [email protected] Subject: Re: ??? RE: ??? RE: Near field H-field measurement. Ken, I'm no expert, but as I understand it, making measurements at low frequencies, measured in the near field, it is possible to have a very high E-field and very low H-field if the radiating source is high impedance. The opposite is true if the radiating source is low impedance, ie: high H-field, low E-field. Also, if the coupling mechanism of the susceptible EUT is primarily inductive, (low impedance) then a high-impedance radiator will be less likely to cause interference than a low-impedance radiator. If my understanding is correct (tell me if I am wrong) then I can see the case for making low-frequency radiated emissions measurements with both E-field whips and H-field loops, with a limit specified for each, such as is done for GR1089 testing. There also needs to be some insurance that the immunity tests cover these two scenarios as well. I wonder just how well the conducted immunity tests relate. The 61000-4-3 test (with CDNs) is with a fixed source impedance of 150 ohms. Theoretically there can only be a 6dB increase in either current or voltage if the EUT port has a very low or very high impedance. Bob Richards, NCT. --- Ken Javor <[email protected]> wrote: > 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. > 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
