I have also heard this, but haven't seen any evidence in our 3m weather protected site. This site, with shingled roofing and vinyl siding has never had a deviation wider than 1.5 dB in NSA measurements in its 8 years of use.
Best Regards, Mike Fahrion [email protected] >Related to the issue of reflections on an OATS is contaminants. I have >heard from a few sites that have all weather test capabilities, that they >need to periodically pressure clean their shelters -- 6 dB was suggested as >a variation due to contaminants. If the effects of contaminants apply to >shelters, it seems only logical that the effects of contaminants would apply >to other obstructions such as curbs and posts; who knows, maybe even to >vegetation! (Anybody pressure clean their trees lately?) > >Don Umbdenstock > >Sensormatic > > > >> ---------- >> From: Robert Bonsen[SMTP:[email protected]] >> Reply To: Robert Bonsen >> Sent: Thursday, November 12, 1998 9:15 PM >> To: [email protected]; [email protected] >> Subject: ANSI C63.4-- OATS construction issues >> >> First of all, expanding the width is a Good Idea. Generally speaking, the >> larger the ground plane, the better it will approach a "perfect OATS" and, >> hence, deliver better performance. This is particularly important for a >> smaller sized OATS. >> >> The minimal size described in CISPR22 is 1m beyond the boundary of EUT >> (quiet zone) and the largest antenna to be used. The metallic ground plane >> must cover the entire area between the EUT and the antenna. >> >> The ellipse of 2Rx1.73R (R=test distance) is a recommended obstruction >> free >> zone. Buildings, fences etc. qualify as obstructions if they are of a >> conductive nature. Chances are you will get away with a few hedges etc. >> within this ellips. The site attenuation measurements will determine >> whether the site qualifies or not. It is however smart to keep metallic >> objects outside this ellips. >> >> Definitely put the screen on the top of the concrete. Choose wire mesh >> with >> a sufficiently small mesh size (smaller than 10% of smallest >> wavelength-to-be-measured) and solder at the seams of the individual >> overlapping sheets every 3-5 inches or so (recommended is 10% of smallest >> wavelength but experience has shown that is excessively stringent most of >> the time). >> >> If you cannot bury the power cables or the signal cables run them >> perpendicular to the measurement axis on top of the ground plane. Make >> sure >> to bond these cable to the ground plane since any metal above the metallic >> ground plane may influence measurements and, hence, should be present in >> the same configuration the site attenuation qualification measurements are >> done. >> >> One additional issue is the ground plane termination into the surrounding >> earth. In case of an abrupt termination at the edges of the ground plane, >> eg. in the case of a rooftop OATS, the reflection from the edges of the >> ground plane may have significant impact on measurements. Even on a very >> large OATS the reflections from the ground plane edges will show up in >> measurements, when the edges are not properly terminated. This effect can >> be significantly reduced by providing a smooth transition between the >> ground plane and the surrounding earth, eg. by running the mesh wire out >> beyond the concrete and burrying it in the surrounding dirt and by >> shoveling up the dirt to the level of the ground plane. Note that >> reflections from the ground plane terminations do not necessarily >> disqualify an OATS in the site attenuation measurements. The net effect >> depends on a variety of factors, including the size of the ground plane >> and >> the measurement range layout. >> >> Sometimes an OATS does not meet site attenuation requirements, which >> happens more often in the case of a smaller OATS than with larger ones. By >> doing proper diagnostic measurements it is possible to indicate where >> reflections due to site imperfectness or obstructions are originating from >> and eliminate the problem. >> >> Regards, >> Robert Bonsen >> EMC facility consultant >> >> Robert Bonsen >> Principal Consultant >> Orion Scientific >> email: [email protected] >> URL: http://www.orionscientific.com >> phone: (512) 347 7393; FAX: (512) 328 9240 >> >> >> --------- >> 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). > --------- 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).
