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) *3*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 <[email protected]> List-Post: [email protected] Date: Wed, 29 Jun 2005 11:40:20 +0800 (CST) To: EMC-pstc <[email protected]> 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. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: http://listserv.ieee.org/listserv/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas [email protected] Mike Cantwell [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 ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: http://listserv.ieee.org/listserv/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas [email protected] Mike Cantwell [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
