Hi Tony, Do not forget to emphasize that the limist heven been set taking into account the ground reflection. of 5-6 dB. Do not try this test at frequecies belo2 200 Mhz as their is no way of finding the maximum +5 dB at no height of the antenna. The difference in path length is way to small to get a sum of the two waves as (when using hor polarized antenna's) the ground plane creates a phase shift of 180 degrees. I have made a model in Mathcad that shows graphs of the attenuation as fucntion of antenna heigth and frequency. This is surprising as OATS limits are made up using the 5-6 dB presumption and make no difference between HOR and VERT limits At vertical polarisation a 0 degrees shift occurs so a full sum is easy to obtain.
FAR (fully anechoic rooms) do not have a ground reflection and the limits in a FAR should be 5.21 dB lower. Products therefore tend to fail in a FAR at hor polarisation and lower frequencies. Gert Gremmen -----Original Message----- From: [email protected] [mailto:[email protected]]On Behalf Of Tony Sent: donderdag 17 oktober 2002 15:45 To: [email protected] 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
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