I'm trying to understand the purpose of the measurements...it seems to me that the purpose is to determine the emissions from power lines??? I'm not sure what the signal is that would emit from the power lines...whether it's some sort of high frequency/low level signal superimposed on the AC for transmission purposes (i.e. ethernet via power lines)...or if the purpose is to try and measure the radiation of high frequency "noise" from power lines. The only reason that I'm fixating on power lines is that I believe "PLT" in the thread stands for "Power Line Transmission"
Now, if any of my assumptions above are correct...the near field/far field boundary of these emissions would be extremely hard to predict. The only reason I say that is that the length of the "antenn" (i.e. power line) could be long relative to a wavelength. It also seems to me that the boundary may change from one situation to another...depending upon the configuration and/or routing of the power lines. In the extreme case, consider an infinitely long, straight power line. My intuition (which is often wrong) would tell me that the field would be more cylindrical than spherical...which would make it hard to get the 1/r^2 field strength relationship typical of a far field. In this situation...it almost seems as though you would have to just measure the E field without making too many assumptions about the H field (or vice versa). I also have to agree with Robert's caveat about reflections at these frequencies. These frequencies will be very reflective and reactive with relatively large objects, and or widely spaced arrays of metallic objects (Robert's example of the roofing nails of a house makes sense to me). In the extreme...think of the AM radio bands. Your measurement range overlaps them a little bit. AM transmissions disappear under bridges and fade in and out as you go over hills and into valleys. Wow...I've rambled quite a bit. I have to get back to work...at any rate; I hope that I've at least helped by making you step back and see why the answer that you're looking for may be hard to nail down. Best regards, Chris Maxwell Critical Imaging >> I have conducted measurements from 1-30MHz in a densely >> populated housing estate, in a semi rural college and at >> a semi rural house that has no objects to reflect the >> field strength back. > > The house was built without the use of nails? There is no > plumbing in the home, no contiguous metal piping? No metal > flashing along the roof line? > > Even at 1 MHz enough nails like roof, or a wall, make a > "phased array" reflector. > > > It is my understanding the formula is based upon the > normalization of the spatial Fourier transform - distance > normalized using lambda. The 2 *(D/lambda) is the "width" > of the sync function, or the sin(x)/x. > > [opinion] > At low frequencies I trust the magnetic field to be better > behaved. Whatever that means. Measuring HField and then > relating back to free field EFields (seem to me) would more > accurately relate to what you want. Those loop antennas > may have already taken that into account though in their > calbration curves. > > - Robert - > > On Wed, 2 Aug 2006 13:31:35 +0100 > "Iain Summers" <[email protected]> wrote: >> John thanks, >> >> I am aware that the boundary is a smooth transition and >> the main reason for wanting to know is to do with some >> subjective analysis of powerline telecommunications >> measurements that I have completed. >> >> There are various papers that state that the attenuation >> of E with distance is 1/distance(d) in the far field and >> some combination of 1/d, 1/d^2 and 1/d^3 in the near (or >> very near) field. >> >> I have conducted measurements from 1-30MHz in a densely >> populated housing estate, in a semi rural college and at >> a semi rural house that has no objects to reflect the >> field strength back. I have plotted all of the graphs and >> included curves for 1/d^0.5, 1/d, 1/d^2, 1/d^3 and I am >> now in the process of correlating them. >> >> What I'd like to know is how large the measurement errors >> are. eg I have used a loop (which is the recognised >> method) and then converted to E for comparison to others >> results by using E/H=377 which is only true in the far >> field. I though it would be useful to determine what the >> results might be if E/H approached 3000 or 30 say and for >> that thought I'd use the point source formula >> lambda/2*pi and also 2*D^2/lambda to determine how near >> or far the boundary might be. >> >> Sorry that is a bit of a ramble but that's the background >> to it. >> >> You mentioned that D might be an area which was my >> initial thoughts as it is quoted for use as an aperture >> and therefore may not be wholly appropriate to use in the >> instance of the source being a large loop in the >> horizontal plane. >> >> The overall aim of my PhD is not to come up with any >> answers per se but to explain to others who quote EMC >> theory in relation to PLT measurement. There are lots of >> assumptions but no one aver states "the answer is x and >> by the way that is + or - 50%". There are lots of people >> measuring E in dBuV to 2 decimal places with no mention >> of the errors in their measurements. This is fine for >> compliance if everyone does the same thing but some >> people are trying to predict interference based upon >> these results. >> >> Thanks, >> Iain >> >> >> >> ----- Original Message ----- >> From: "John Woodgate" <[email protected]> >> To: <[email protected]> >> Sent: Wednesday, August 02, 2006 8:59 AM >> Subject: Re: Near field/Far field what is D. >> >> >> In message <001c01c6b604$c0ed2810$6401a8c0@homepc>, dated >> Wed, 2 Aug >> 2006, Iain Summers <[email protected]> writes >> >There are various references and opinions as to what >> constitutes the >> >near field far field boundary. The common one from >> antenna theory is >> >2D^2/lambda where D is defined as the largest aperture >> of the antenna. >> >> There isn't a sharp boundary. What you take as the >> boundary depends on >> why you want to know where it is, and you don't tell us >> that. >> >> For example, you might decide that you want to know where >> the phase >> angle between E and H is only 1 degree. Or is 45 degrees. >> Or more. >> > >> >Has anyone researched what the limitations of this >> formula are? I am >> >interested in the far field boundary between 1-30MHz for >> large loops of >> >wire. >> >> Large compared with the wavelength at 1 MHz? That's large >> indeed! >> >> >In terms of a loop if this formula is applicable is D >> the circumference >> >or diameter of the loop. >> >> Or maybe the area? >> -- >> OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and >> www.isce.org.uk >> 2006 is YMMVI- Your mileage may vary immensely. >> >> John Woodgate, J M Woodgate and Associates, Rayleigh, >> Essex UK >> > > - > ---------------------------------------------------------------- > 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/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: > > Jim Bacher: [email protected] > David Heald: [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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