Too bad if it is ignored; for FCC it can make the difference of correlating to field calculations and hence compliance for limit knockers. For EU it is moot, you measure at 10 meters.
When doing 2 point roll-off calculations for inductive loop systems, one would expect 3rd order roll-off for simple, non-canceling loops in the very near field such as with 50 kHz. If one were to measure on galvanized wire mesh at distances greater than about 15 meters, the mesh effects produce a roll-off that can be 2 or 2.5 factor instead of 3, thus eating up your margin of compliance. The purpose of ground planes for higher frequency is for repeatability. For inductive loop systems, we have established over several years and various sites that repeatability and correlation to calculations are achievable with a clear ground (no utilities, ground plane, etc.) Don Umbdenstock Manager Compliance Engineering Tyco Safety Products / Sensormatic 6600 Congress Avenue Boca Raton, FL 33487 USA 561.912.6440 djumbdenst...@tycoint.com From: rehel...@mmm.com [mailto:rehel...@mmm.com] Sent: Wednesday, June 29, 2005 2:23 AM To: emc-p...@ieee.org Subject: Re: Radiated emission measurement below 30MHz I agree with you but Derek is correct in his statement. The standard leads you to believe that the ground plane is prohibited. The exact statement is: "For measurements at frequencies below 30 MHz no artificial ground plane shall be used." I could be wrong but I suspect that this is often (if not always) ignored. Bob Heller 3M EMC Laboratory, 76-1-01 St. Paul, MN 55107-1208 Tel: 651- 778-6336 Fax: 651-778-6252 ========================= Ken Javor <ken.javor@emccom pliance.com> To Sent by: EMC-pstc <emc-p...@ieee.org> owner-emc-pstc@ie cc ee.org Subject Re: Radiated emssion measurement 06/28/2005 11:21 below 30MHz PM Strictly conjecture on my part; I have no specific knowledge of the standard cited. 1) Below 30 MHz, there is not much opportunity for constructive/destructive interference, because path length differences between direct and reflected rays are a small fraction of a wavelength (on a 3 meter site). Also measurements below 30 MHz are often made with an electro-statically shielded loop, and that measurement is supposed to be less site dependent than an electric field measurement. That means a ground plane is unnecessary, not prohibited. 2) The size of an OATS or indoor site is tied to the antenna-EUT separation. For an OATS, the clear area is an ellipse whose major diameter is twice the antenna-EUT separation, and the minor diameter is (if memory serves) รท3*antenna-EUT separation. I don't know of a reason for the 80 MHz cut-off unless this is a really old standard from the days of pyramidal foam absorber instead of ferrite tile. Then it would be obvious why 80 MHz was the lower frequency. 3) Below 30 MHz, and using a loop I don't think the ground plane should make that much difference, but laying absorber on the ground plane also won't make much of a difference: ferrite tile absorber is only rated down to 20 or 30 MHz. At lower frequencies its performance drops off below the nominal 20 dB it is rated from 20/30 - 1000 MHz. From: Y W Leung <leungderek2...@yahoo.com.hk> Date: Wed, 29 Jun 2005 11:40:20 +0800 (CST) To: EMC-pstc <emc-p...@ieee.org> Subject: Radiated emssion measurement below 30MHz Dear all, In EN300330 radiated emission measurement, Annex A 1.1 mentioned about for measurement (on an outdoor test site) of frequencies below 30MHz, no artificial ground plane shall be used. But for 30MHz or above, a conducting ground plane shall be used. Is anyone know the reason why the ground plane is not needed for frequency below 30MHz? There are two other options test site for the test, one is the indoor site of min. 6m x 7m with a barrier of absorbent material in front of the wall, but this site is only for frequency over 80MHz.Why the sized needed so big and the frequency is over 80MHz (not 30MHz)? Another option is use anechoic chamber, but just for frequency over 30MHz. I have an idea how about I use a fully anechoic chamber (absober placed on the floor between the EUT and receiving antenna) instead of the outdoor test site without ground plane, any difference between them? Thanks advance for any comment or idea. Regards, Derek. ???????? ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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