In 1987 the FCC and EPA performed a study, Radiofrequency Electromagnetic Fields and Induced Currents in the Spokane Washington Area (see below). The study was available from the Government Printing Office and I obtained a copy.
The study found areas near some of the FM and television antenna sites where hazardous levels could be expected and were in fact present. It also raised the issue of field enhancement due to conductive objects: "... an area of fairly uniform 10V/m electric field was found near the base of the fire tower on Mount Spokane. Then a 1.46 meter electrical conduit pipe was held horizontally in that field by one investigator while another person measured the electric fields near the rod using a Holaday HI-3001. At one end of the rod 62 V/m was measured; at the other end over 80 V/m was found. ...” (page 24) Unfortunately for our purposes it didn't go into the field strengths beyond areas where hazards might exist. It also looked into possible hazards from AM broadcasting. Quoting from the study conclusions: "KGA-AM is among the stations in the United States that have been granted a license by the FCC to operate at the maximum allowable power of 50 kW. Despite the high power at which KGA operates, the electric fields along 63rd street, only 100 to 200 feet away, are well below* the ANSI guideline of 632 V/m. Electric field values throughout the southern Spokane AM antenna cluster are rarely over 5% of the ANSI guideline. This holds true for measurements made inside the Mullen Road School and inside a residence near the KGA antennas. Similarly, the magnetic fields measured in the school and residence were almost always less than 5% of the ANSI guideline of 1581 mA/m. EPA found no electric or magnetic field values, even in localized areas, that exceeded the ANSI guideline at AM radio frequencies in publicly accessible areas. However, levels far below the ANSI guide can cause annoying RF shocks/burns and can interfere with the operation of electronic equipment." Note that last sentence. Masters of understatement, they are. As the investigators seemed more concerned with RF levels inside dwellings and schools and other places where people might be exposed to potentially hazardous levels, there is not much explicit information on the distance between emitters and measurement points. However, Figure 2 does show in map form, to scale, the location of eight of the ten AM station antennas and 19 measurement points. If the scale is correct, the distance between station KGA and the measurement point furthest from it** is 1.75 miles or about 2.8 km. * 43.7 to 6.0 V/m **FS here about 1 V/m. Figure 6 is a similar map of the Mica Peak (FM and TV) antenna site. As one who has seen telco equipment installed at the edge of an AM station's radial field, and extensive elevated cable plants running near high-power AM stations, I can testify that levels of induced current far beyond those for which Telcordia GR-1089 requires immunity are encountered all the time in the "real world." And then problems are always the vendors fault! Document referenced: 520/6-88-008 Radiofrequency Electromagnetic Fields and Induced Currents in the Spokane Washington Area (PB88 244 819/AS) http://www.epa.gov/radiation/docs/pubslist_02.pdf Cortland Richmond 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
