Hello Ralph: That's a good question. At one time, I pondered the same question myself. There are obviously plenty of communication systems operating under 30 MHz. I suppose there are reasons why CISPR or CISPR 22 does not specify radiated emissions below 30 MHz. I can suggest one possibility. Perhaps others here will come up with more. For a fixed cable of length L, the ratio of L/lambda gets progressively small for frequencies much less than 30 MHz with most commercial EUTs. If we consider the cable part of dipole antenna, the reduction in frequency has a diminishing effect on the antenna's radiation resistance. Given a constant current, the radiated power would decrease with decreasing radiation resistance. At 550 KHz (bottom of the AM broadcast band in North America), the 1/4 wavelength is 136 meters. Even if the antenna's reactance is ignored, one would need very long cables driven by a significant CM noise voltage at this frequency to radiate much energy.
-- ------------------------------------------------------------------------ Daniel Kwok Principal EMC Engineer Intetron Consulting, Inc. Vancouver, Canada Phone (604) 432-9874 Email [email protected] Web http://www.intetron.com"; Ralph Cameron wrote: > > Ken: > > I like the idea of setting a limit to common mode currents on attaching > cables but mI wonder why CISPR has chosen to start such measurements at > 30Mhz when most of the common mode currents are the result of switching > products and are generated harmonically from the fundamental and as such > propagate from the low Khz range up through 30Mhz. is there no consideration > for those who occupy the spectrum below 30Mhz? > ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: [email protected] Michael Garretson: [email protected] For policy questions, send mail to: Richard Nute: [email protected]
