I agree that this is a problem, but one that is unfortunately not improved much (if at all) by the OATS scan height at a 3m distance. This is why CKC proposes above 1 GHz (not 500 MHz) to use a scan arc over the top of the EUT at 1m distance in free space. (a hemisphere type scan, not a straight vertical scan).
The problem on an OATS at 3m distance is that the distance (hypotenuse) from the EUT in the 1m - 4m scan increases from 3m to 4.24m at the top of the scan mast. At the same time the directional characteristics (beamwidth) of the antennas typically used (horns) above 1 GHz are narrowing. Due to the increasing distance as the antenna is raised, an EUT would need >3 dB E-field directional gain just to maintain the same signal level as the antenna is raised. Still more gain is needed to overcome any off center beamwidth attenuation of the antennas used, which can easily be >10 - 15 dB above a few GHz. In my estimation, a straight vertical scan of the antenna that does not maintain a boresight on the emissions source or account for the changing test distance as the antenna is raised seems equally if not more unlikely to capture any increased "upward" emissions from the EUT as a fixed height measurement boresight on the EUT since the amount of EUT gain required to overcome the effects mentioned is more than many antenna designers could hope for at these frequencies. Our principal argument for using the fixed height free space methods in the 30 MHZ - 1 GHZ range is that they provide greater convenience at equal to lower uncertainty than the OATS and therefore should be acceptable. We have verified this claim by three separate methods as have others. Above 1 GHz, I believe that improved methods, such as a scan arc or rotation of the EUT (as in ETS testing)is necessary. Since there are no commercial ITE standards yet above 1 GHZ (besides ETS which are all free space), I believe this presents an excellent opportunity to get it right when >1 GHz methods are incorporated into the ITE standards. Sent by Mark Darula on behalf of Clark Vitek EMC Staff Engineer CKC Laboratories, Inc. -----Original Message----- From: [email protected] [mailto:[email protected]]On Behalf Of Hans Mellberg Sent: Saturday, June 19, 1999 7:10 AM To: Roman, Dan; '[email protected]' Subject: Fully Anechoic Chambers (was NEAR/ FAR FIELD CORRELATION ISSUES) The single most biggest problem with a fully anechoic chamber with fixed antenna height as proposed by various groups, is, the inability to detect directed beam emissions especially at higher frequencies (over 500 MHz) Such emissions are, for example, emissions out of drive bays from computers. Most EMC engineers have seen those GHz harmonic emission when processors of 400MHz and higher are used. === Best Regards Hans Mellberg EMC Consultant _________________________________________________________ Do You Yahoo!? Get your free @yahoo.com address at http://mail.yahoo.com --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to [email protected] with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to [email protected], [email protected], [email protected], or [email protected] (the list administrators). --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to [email protected] with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to [email protected], [email protected], [email protected], or [email protected] (the list administrators).
