Hello Tony, Your thoughts on the effect of the ground plane are correct. The equation you gave for the E-field approximates radiation from an isotropic source in free space. It is an ideal scenario that ignores reflections from boundaries and obstacles. If you are using a ground plane in an OATS, the usual practice is to add an extra 6 dB to account for ground plane reflection.
--------------------------------------------------------------------- Dan Kwok, P.Eng. Principal Engineer Electromagnetic Compatibility Intetron Consulting, Inc. Ph (604) 432-9874 E-mail [email protected] Internet http://www.intetron.com ----- Original Message ----- From: Tony To: [email protected] Sent: Thursday, October 17, 2002 6:45 AM Subject: Effects of the Ground Plane Hi group, I am making a fundamental field strength measurement at 608MHz of a spread spectrum device in accordance with Part 95.639(g) & 95.11159(a). The device is failing by about 7dB. The client says that is not possible citing the equation given in the alternative test procedure for determining peak conducted power in Appendix C of R&O 97-114 as given below: We measured the conducted power to be 11.29dBm. He states his antenna has 0dBi of gain and we are measuring at 3 meters on an open area test site. If you plug these variables in to the equation above, the E-field at 3 meters should be 211 mV/m which is still over the limit, but no the issue here. However, my measurement shows it is failing by much more than that at 501mV/m. That equates to about 7.5 dB over the limit. I have made the measurement in my 3 meter semi-anechoic chamber and was able to repeat my result from the OATS. As a sanity check, I have the unit at another site to repeat the measurement. I am confident in my result since I have just finished my yearly NSA and it was fine, especially in this area. My thoughts are that the above equation does not take into account the reflectivity of the ground plane causing an additive effect. I have heard this reflection could add as much as 5 or so dB to a measurement, which would come close to explaining he difference between the measured reading and the calculated result. Questions are: 1) Am I correct that the GRP reflectivity can add to the signal strength? If so, by what amount is typical? 2) How can I prove to my client that this is so? 3) What documentation exists that explains this? 4) Or is my client correct and I have some unexplained problem? I am conducting several experiments now to prove or disprove the GRP effect. I intend to make measurements off of the GRP outside as well as lay out some ferrite tile on the floor in my chamber. Any help or guidance is appreciated. Tony Rayman Compliance Engineer Advanced Compliance Solutions 5015 B.U. Bowman DR Buford, GA 30518 Tele (770) 831 8048 FAX (770) 831 5898
<<image001.gif>>
