We have a second story, weather protected 3m site. Ground plane measures 6.1m x 7.3m. NSA measurements always were close to the 4 dB limit at 30MHz in horizontal polarization, all higher frequencies had no problems in vertical or horizontal polarizations. The 30MHz trouble was attributed to marginal ground plane area.
However - when we started using the volumetric method of SA measurements problems were found. Using this method, the antenna is moved to various locations encompassed by the swept volume of the EUT when rotated on the turntable. This brought our antenna (now broadband instead of dipole) much closer to the edges of the ground plane. Note the discontinuity at the edges of our ground plane were huge since it was about 3 meters above ground/earth level. This led to a current, expensive project of constructing a new weather protected OATs. New ground plane dimensions are 9.75m x 17m. I have hopes of this site meeting NSA at both 3m and 10m. This is still much smaller than 17 x 20m dimension recommended by someone else here, but exceeds the freznel ellipse and is much bigger than listed in CISPR16 (although CISPR doesn't to my knowledge call for volumetric testing, which seems more stringent). The new site ground plane will be within 4 inches of ground level. I don't plan to try to bond the interior ground plane to earth unless there is trouble meeting NSA. I will know the outcome of this project in 4-6 weeks. For the $ involved, it had better be successful! Best Regards, Mike Fahrion mfahr...@bb-elec.com --------- 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, j...@gwmail.monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators).
