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).
