Iain, For the PLC case you are interested in -- and I am, too! -- the fields drop off at different.
First, there's the near-field/far-field transition. The crossover distance between near and far fields is traditionally taken to be where the phase error due to different distance from points on the source is 1/16 of a wavelength, or about 22.5 degrees, or less. By geometrical methods, this is at the distance from the source antenna of twice the square of the source antenna's largest radiating dimension, divided by the wavelength. A two kilometer wire excited with a 3 MHz (100 meter) signal could conceivably have a far field transition distance of 80 km. Ouch! Moreover, this distance is still not that at which the ratio of E-field to H-field has reached the "far field" value of 377 (ohms). For an exact far field one must look to a greater distance. NASA mentions four times the distance given by the above formula. (NASA PREFERRED RELIABILITY PRACTICES, GUIDELINE NO. GT-TE-2402, NEAR FIELD MEASUREMENT FOR LARGE APERTURE ANTENNA PATTERN DETERMINATION) It seems that without other factors, all our measurements will be taken in the near field. Luckily, there ARE other factors. Most of the RF is radiated from within about a wavelength or so of the generator. This reduces the far field transition distance we need to worry about to around four wavelengths from the source, and the actual far field to 12 wavelengths or so. But for a 3 Mhz signal we are still talking about approximately 400 meters, and about 40 meters at 30 MHz. If the RF is more or less constant along the line (perhaps due to use of repeaters) the loophole above goes away and the far field can REALLY reach out there. Where IS the far field? Here's how to find out: "When a measurement is made in free space, a good check to ensure that it was performed in the far field is to repeat the measurement at twice the distance. The power should decrease by exactly 6 dB." https://ewhdbks.mugu.navy.mil/ANTNRFLD.HTM For a visual, see the holographic representations of a 10 wavelength wire at http://www.antennex.com/docs/2.pdf . Also, there s a PowerPoint(tm) presentation at http://www.ecse.monash.edu.au/ucourses/ece4203/L4AntIntro.pdf Miscellaneous stuff: "The extensive near field of an AM antenna further complicates the problem. Near-field effects may extend to two miles or more, compared to dozens of feet at VHF, and measurements used to determine the station antenna pattern may extend out as far as 20 miles. " There's that long reach again -- but AM broadcasting is at wavelengths up to 556 meters. http://www.lbagroup.com/associates/lbatn102.php Then there's the vertically polarized surface wave. There are two orthogonally oriented fields generated by an elevated transmission line excited by RF. One is horizontally polarized. The other is a vertically polarized component due to a difference in potential between the elevated source, caused by common-mode signals, and Earth. In a balanced system, one would not expect a common-mode voltage -- but it takes very little difference between wires to convert some of the balanced RF drive to an unbalanced, common-mode signal. This propagates differently than a horizontally polarized wave. The vertically polarized component creates a surface wave. In effect, and where ground characteristics permit a surface wave to exist, the wave propagates as if inside an open waveguide and does not fall off as it would in free space. And while we assume that "ground wave" is only a factor for Medium Wave transmissions, it may (depending on ground characteristics) be noted well past the 75 meter band. Some comments filed in response to the FCC's recent Notice of Inquiry mention using single-wire transmission lines fed against Earth, which would produce a *rather* higher vertically polarized field than parallel wire transmission lines carrying the same RF power. See http://murray.newcastle.edu.au/users/staff/eemf/ELEC351/SProjects/Fitzsummo ns/growav.htm and http://www.tpub.com/content/neets/14182/css/14182_76.htm And finally, if someone tells you that a balanced line won't radiate, show them "Transmission lines as antennas" at http://rfdesign.com/ar/radio_transmission_lines_antennas/ . Widely spaced transmission lines such as are used for HV power may radiate quite a bit of the incident RF, even if balanced, and at surprisingly low frequencies, too. Cortland This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ieee-pses.org/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: [email protected] Dave Heald: [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
