The easiest way to think of this problem is that if the reflection is in phase, you can get double the original signal (double = 6dB), if out of phase you get a deep null. If you have scanned the height of the antenna, you will not observe the nulls and the maximum you see will be up to double the reading in free space.
To get a better estimate of the number in your test, if you assume the EUT height is 1m and the antenna height is 1m you can draw a right angled triangle to find out the length of the reflected path compared with the direct path between EUT and antenna. You should come up with the answer sqrt(13) = 3.61m for the reflected path. The amount of signal you get along each path is 1/r, so you next do (1/3) + (1/3.61) = 0.61. This is 1.83 times larger than the signal you would get from the EUT in free space. This corresponds to 5.25dB, so the 6dB rule of thumb wasn't far off. If the antenna were at a different height for the maximum of the height sweep, is probably drops to a bit less than 5dB. The other problem that your customer's calculation has is that I think it is unlikely that a real antenna has 0dBi gain. It may be an omnidirectional antenna in that it is a short dipole. I think that the gain of a Hertzian dipole is 1.64dBi, but maybe someone else can confirm this. This would get you somewhere near the 7.1 dB discrepancy. Contact me offline if this description in text is a little hard and I can enlighten things with diagrams. ---------------------------------------------- Dr Luke Turnbull Principal EMC Engineer TRW Conekt Statford Road Solihull B90 4GW Tel: +44 (0)121.627.3966 email: [email protected] ---------------------------------------------- >>> "Tony" <[email protected]> 10/17/02 02:45pm >>> 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 ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: [email protected] Dave Heald: [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://ieeepstc.mindcruiser.com/ Click on "browse" and then "emc-pstc mailing list"
