Hello Cecil, Although it is common practice to extrapolate test distances using a linear scale (20 dB/dec) you need to be warned about the theory behind it that suggest different.
Using an OATS type of measurement , the reflected wave by the ground plane does at 10 meters distance fully cancel the direct wave at low antenna heights. Therefore one needs to rise the antenna to find a point where the direct and indirect wave sum up to get twice the amplitude. This is never the case however in normal test set ups. Normally antenna height goes to 4 meters max height and at this point both reflected and direct wave still attenuate. At 30 Mhz this is -25 dB approx. This is valid for horizontal polarization. Vertical the situation is much less critical. At 3 meters however, the situation if fully different. Of course this analysis is theory and valid for small radiators only and for using dipole antennas instead of wideband antenna's, but one must make sure to well understand that this effect will heavily impact the measurement results in many cases. For those who have MathCad , I can send you a Matcad sheet that shows the required test height distance per frequency / OATS size / EUT height. Regards, Gert Gremmen Manager Ce-test, Qualified Testing ce marking and more .. EMCD LVD R&TTED MDD MD www.ce-test.nl Electrical / Electronic Equipment -----Original Message----- From: [email protected] [mailto:[email protected]]On Behalf Of [email protected] Sent: Sunday, April 21, 2002 8:12 PM To: [email protected] Subject: FCC Class measuring distance 3 meters vs 10 meters Hi All, While reading subpart B I saw that section 15.31 Measurement Standards section (f) (1)states: "At frequencies at or above 30 Mhz, measurements may be performed at a distance other than what is specified provided: measurements are not made in the near field... When performing measurements at a distance other than that specified, the results shall be extrapolated to the specified distance using an extrapolation factor of 20dB/decade..." Is 3 meters distance testing acceptable for testing to FCC Class A? Cecil ------------------------------------------- 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] All emc-pstc postings are archived and searchable on the web at: http://ieeepstc.mindcruiser.com/ Click on "browse" and then "emc-pstc mailing list" ------------------------------------------- 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] All emc-pstc postings are archived and searchable on the web at: http://ieeepstc.mindcruiser.com/ Click on "browse" and then "emc-pstc mailing list"
