On the issue of broadcast field strengths, I can provide some input based on a TV/FM field strength study I completed last December for a local municipality.
The Town of Brighton and the City of Rochester New York are somewhat unique in having the main broadcast tower cluster located on a hill in the middle of a populated residential/commercial area. In most towns in the US, the TV towers are outside of highly populated areas. I know there are other exceptions - no need to point them out to me. The residents have long experienced interference problems with their consumer electronics, garage door openers, and who knows what else because of the high field strength levels in the vicinity of the towers. The Town of Brighton established a task force on the issue, which I participated in as a volunteer, to give some technical moderation to the proceedings. You can find my contribution to that group on the Town's website at www.townofbrighton.org/interference.html. The Town also commissioned me and a colleague to do a formal field strength survey of signals in the area surrounding the towers. The results of interest here are that with 240 individual measurements made (12 stations at 20 locations) there were only two instances of field strength levels exceeding 2 V/m. Both were for a UHF station on channel 31 (573.25 MHz visual carrier frequency). At one location 957 meters from the tower, the signal was 2.483 V/m. At another, 3040 meters from the tower (and clear line of sight) the level was 2.630 V/m. The station runs several megawatts erp. The closest site measured was 221 meters from the towers. There, the channel 31 station measured 741.3 mV/m. However, an FM station at 96.5 MHz measured 1.762 V/m by comparison, and the FM runs at 50 kW erp (possibly 50 kW horizontal and 50 kW vertical polarization).] All measurements were made with Potomac Instruments field strength meters and tunable dipole antennas. Everything was calibrated by the manufacturer. Summing up, it would appear, from this study at least, that the contention that signal levels in excess of 2 V/m at the broadcast frequencies are unlikely to be found is justified. Getting back to the interference problem, there's a lot of consumer electronics that does not like 2 V/m or anything even approaching it. Manufacturers, as you are all aware, don't spend an extra cent if it is not absolutely required or they would not get caught - (many, perhaps not all). The FCC does not have any immunity requirements to speak of. One of the reasons signal levels don't get too excessive near a tower (besides that the antennas are a hundred or several hundred meters up as was already mentioned by someone) is a matter of simple geometry. There is not much downtilt to broadcast antenna vertical radiation patterns. So nearby, the radiation is already well down the dB curve of the main lobe. I spent many midnight hours years ago under the tower of two FM stations (I was Director of Engineering for one of them) and I could operate an oscilloscope and a 1948 RCA FM radio with no problem at all. I hope this lengthy response was enlightening. Regards, Jacob Z. Schanker, P.E. Consulting Engineer 65 Crandon Way Rochester, NY 14618 USA Tel: 585 442 3909 Fax: 585 442 2182 [email protected] This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ 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] Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
