Wendy, Antenna factors vary as a function of frequency, distance, polarization, and height above the ground. The asymptotic value as distance and height get large is the free-space factor (because this is the free-space geometry). The theoretically most useful antenna factor for emissions measurements is a free-space calibration because (1) this is close to, if not perhaps equal to, the mean of all the antenna factor values over a varying range of distance/polarization/height and (2) this is theoretically the most repeatable measurement because there is no mutual coupling. However, creating true free space conditions is a challenge. Thue most useful practical antenna factor for emissions measurements is a 10 meter horizontal calibration with the receive antenna fixed at a 2 meter height above a large ground plane. According to the experts this will yield an antenna factor value within approximately 0.5 dB of the free-space value, thus it has been termed the "near free-space". It has a practical advantage of good repeatability, and is specified by ANSI C63.5-2004 as the calibration geometry to be used for emissions measurements in accordance with ANSI C63.4. Many people (years ago I was among these) reach the conclusion that better accuracy will result from using four separate factors: 10mHor / 10mVert / 3mHor / 3mVert. The problem is that when emissions are maximized at some antenna height that is different from the height at which the antenna was calibrated, there is still a significant residual error. In fact, this error can actually be greater than the deviation from the free-space value or the near free-space value because the calibration geometries that have more mutual and/or ground plane coupling (Closer distances / Vertical polarization / Lower heights above the ground plane) will exhibit wider deviations from the free-space value at a more frequencies. Calibrations for NSA measurements are a different story, and this topic is covered in Annex F of ANSI C63.5-2004. It is recommended that you recalibrate your antenna at the standardized near free-space condition. Best Regards, Mike
Michael Heckrotte Engineering Manager, CCS (408) 463-0885 ext. 121 fax: (408) 463-0888 [email protected] From: [email protected] [mailto:[email protected]]On Behalf Of wendy Sent: Thursday, January 26, 2006 6:41 PM To: [email protected] Subject: Calibrated Antenna Factor I have just purchased a new bilog antenna. It was calibrated at a 3m site and gave only horizontal Antenna Factor. Can I use the AF for a 10m measurement? And do I assume the horizontal and vertical AF is the same? Any quick advice? Regards, Wendy Nya LabOne Singapore - ---------------------------------------------------------------- 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/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: Jim Bacher: [email protected] David Heald: [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/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: Jim Bacher: [email protected] David Heald: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
