Sorry, but I must be missing something.... Transmitting antennas are designed to radiate power, and the field strength of any signal being radiated will be at it's highest close to the antenna.... in fact, depending on frequency and distance, one might even be in the near field effects....
Antennas may be made directional, which results in power being concentrated in one direction, but if you're talking broadcast antennas, they are generally omni-directional arrays that achieve gain by keeping fields concentrated at low angles so power radiated towards the sky is minimized. Such antennas would clearly produce their highest levels of radiation close to the antenna structure itself; however, directly above or below the antenna fields would be less. I cannot guess why the radiated levels for these frequencies would be lower than for other frequencies..... The only thing I can think of is that maybe it isn't expected that one would be close enough to a broadcast antenna at these frequencies for it to be an issue (antennas are mounted atop very tall buildings and towers) -- on the other hand, a piece of lab equipment or control equipment in a process plant could certainly be very close to other sources or radiation from, say, 5W walkies used by security personnel -- how much of a field can you get a 150MHz a foot from a 5W transmitter?? Mike Hopkins Thermo KeyTek From: John Woodgate [mailto:[email protected]] Sent: Monday, March 03, 2003 12:41 PM To: [email protected] Subject: Re: RADIATED IMMUNITY REQUIREMENTS FOR EN61326 I read in !emc-pstc that Gordon,Ian <[email protected]> wrote (in <E1BA0362B28ED211A1E80008C71EA3060206FB33@z- 160-100-30-252.est.ibm.com>) about 'RADIATED IMMUNITY REQUIREMENTS FOR EN61326' on Mon, 3 Mar 2003: >Furthermore, I have never been able to work out why there is a relaxation at >these freqs. The wording from table 1 of EN61000-6-2 is ....10V/m........ >"Except for the ITU broadcast frequency bands (given above) where the level >shall be 3V/m". >Can anyone provide the "logic" behind this clause? (I'm sure there is >some!). The transmitter antennas are designed not to produce high field strengths even close to the antennas, because receivers would be severely overloaded and maybe damaged. I did some measurements on portable radios and, scaling up from what you get at the base of the first transistor with say 100 mV/m, you would get 1 to 2 V at 10 V/m, which could even damage the transistor. -- Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk Interested in professional sound reinforcement and distribution? Then go to http://www.isce.org.uk PLEASE do NOT copy news posts to me by E-MAIL! 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 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
