Mark, In context with the boresighting issues, I think that you are probably referring to the hemispherical arc (with a rotating centre table), which is generally used to model ship borne HF antennae.
The other technique could be to boresight the antenna (with a pneumatic cylinder) once the turntable and the mast have been "peaked". However, while testing EPIRBS (locator beacons) to COSPAS/SARSAT Type Approval Standard C/S T.007 Issue 3 December 1992 I came upon a fairly interesting math derivation. Such tests are normally performed at 406 MHz with the pristine DIPOLE Antenna (and not the general broadband stuff). Depending on the mast height, the horizontal emissions would generally remain ok, but the verticals could get really crook by as high as 6 dB. Hence for the VERTICAL emissions they propose a: Corrected Antenna Factor AFC = AF (nominal, the 10 metre factor) corrected by a factor P. P= (COS (90*SIN (THETA)))/COS(THETA) THETA= INV TAN (ANTENNA ELEVATION OFF THE TURN TABLE/TEST DISTANCE) Thus, at least mathematically, if not practically (because broadband antennae are generally used in commercial measurements), your measurement uncertainity comes down due to better mathematical boresighting and you get a better vertical result. Arun Kaore EMC Engineer ADI Limited Systems Group Test & Evaluation Centre Forrester Road, St Marys 2760 P O Box: 315, St Marys NSW 1790 Tel: 61 2 9673 8375 Fax: 61 2 9673 8321 Email: kao...@sg.adi-limited.com.au -----Original Message----- From: Mark Darula [SMTP:mdar...@ckc.com] <mailto:[SMTP:mdar...@ckc.com]> Sent: Wednesday, 23 June, 1999 11:45 To: emc-p...@majordomo.ieee.org; <mailto:emc-p...@majordomo.ieee.org;> Roman, Dan; Hans Mellberg Cc: CKC - Clark Vitek Subject: RE: Fully Anechoic Chambers (was NEAR/ FAR FIELD CORRELATION ISSUES) 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: owner-emc-p...@majordomo.ieee.org <mailto:owner-emc-p...@majordomo.ieee.org> [mailto:owner-emc-p...@majordomo.ieee.org] <mailto:[mailto:owner-emc-p...@majordomo.ieee.org]> On Behalf Of Hans Mellberg Sent: Saturday, June 19, 1999 7:10 AM To: Roman, Dan; 'emc-p...@majordomo.ieee.org' 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 _________________________________________________________ --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to majord...@ieee.org with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to ed.pr...@cubic.com, jim_bac...@monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators).
