Hello, There is a huge difference between FAR and OATS measurements in practice AND theory, but for the difference in distance. Within construction limits, results in 3 versus 10 meter FAR should be similar, with reliability in favour of to the smaller room, as the quality factor of small cavities (=room) is always lower then a big room, so dampening using ferrites is more effective in a small room. (I believe that volume - surface ratio is what counts) The advantage of a larger room however, is that close field effects (antenna - EUT, antenna-wall and EUT-wall) will be smaller !
The limits for EMI measurements were based on the fact that the receive antenna on an OATS would receive both the direc wave AND the ground plane reflected wave, and that they would sum up the received amplitude to be 6 dB higher as the actual field strength of the direct wave only. Any OATS, in spite of being the standard for EMC measurements, is blind for lower frequencies (say < 200 MHz) at horizontal polarization . This is caused by the fact that a OATS ground reflected wave at HOR polarization is reflected with 180 degrees phase shift (counter phase) and actually cancels out the directly received wave. Of course the designers of the OATS found out that this was the case, and tried to cover the effect by the height scan of the receive antenna. The idea was that by rising the height of the antenna the increase in length of the direct wave would be smaller then the increase in way length for the reflected wave so as the actual phase shift would reach 0 degrees at the right point. A simple calculation using Pythagoras will show you that at for example 100 MHz the required difference would be 180 degrees at lambda 3 meters = 1.5 meters and that this difference cannot be achieved within the 1-4 meters of height scan for lower frequencies. It can be at higher heights, but then the way length of both waves will be so long that the sum would not be 6 dB compared to 3/10 meters of test distance. At 30 MHz it will never be possible to compensate 5 meters of way length anymore regardless the height of the receive antenna. This is easy to understand if you treat the reflected wave as coming from an EUT mirrored in the ground plane, the difference in way length for both waves for an infinitely high antenna would be twice the EUT height, and this is thus by definition the maximum phase shift expressed in meters that can be compensated for. Weather the designers of the OATS were aware of this effect or took it for granted or judged this effect irrelevant for EMI measurements is unknown to me. (someone please ?) The effect also appears for vertical polarization ( no 180 degrees of phase shift on refelection), creating notches in receive amplitude at a few places somewhere at 350 and 600 MHz (if I remember well). The effect is less visible using large receive antennas and larger EUT's , as the way length will be somewhat diffused as there are multiple paths that contribute to the measurement result. Anyhow, a well designed FAR does not include any ground reflected wave so there is no attenuation for lower frequencies. I hope I made clear that FAR and OATS measurements cannot be compared at all, but for strictly theoretical isotope receivers and transmitters that behave as single points. It is clear however, that EUT's will have more difficulties acquainting compliance in FARS then on an OATS, however. For those having Mathcad on their PC, I can provide a simple worksheet allowing to graph the effect. Let me know by private mail if you are interested. It is easy however to model the effect yourself also, and definitely will contribute better to your comprehension of the problem ! Gert Gremmen ce-test, qualified testing www.cetest.nl From: [email protected] [mailto:[email protected]] On Behalf Of robert Macy Sent: zondag 21 maart 2004 19:23 To: [email protected]; [email protected] Subject: Re: What is wrong in my 3 meters FAC measurement I'm impressed with how closely those two sets agree. My experience, measuring at 3m and "adjusting" the signal level for 10m is not the same as MEASURING at 10m. When the FCC changed the rules from 3m to 10m many of my clients' marginal equipment went right of compliance. Increase in signal level? Around 3.5dB In other words, there should have been a drop of 9.5 dB when moving the antenna further away. Unfortunately, the signal only dropped 6dB. - Robert - Robert A. Macy, PE .. [email protected] 408 286 3985 . . . .. . . fx 408 297 9121 AJM International Electronics Consultants 101 E San Fernando, Suite 402 San Jose, CA 95112 On Sun, 21 Mar 2004 15:25:44 +0530 [email protected] wrote: > Dear EMC Gurus, > I completed my pre-scans for an intentional > radiator (13.56 MHz) > in a 3 meters fully anechoic chamber and found that the > emission are well > within the Class B limits. The emission @ 108.48 MHz (8th > harmonic) was > around 35.8 db, @ 135.6 MHz (10th harmonic) was about 24 > db, @ 162.72 MHz > (12th harmonic) was about 29 db and @ 244.08 MHz (18th > harmonic) was about > 37.5 db. 3 samples were tested and the results were > similar for all the > 3. > But when another sample of the same model was > tested by my > counterpart in the 10 meters OATS (at USA) the results > are differing. > Emission @ 244.08 MHz is reported about 37.10 db, @ > 162.72 MHz is 31.1 db, > @ 135.6 MHz is 27.2 db and the device failed to meet > Class B limits. > The test setup is very simple and it is just the > 12 volts d.c. > output of a variable d.c power supply output connected > to the device. The > power supply used by me and by my counter part are not > same but similar. I > tried separately with a shielded cable and with an > unshielded cable and > the results were almost similar. My counterpart used only > a unshielded > cable. > Now it is great puzzle for us. What is wrong and > where? Apart from > the device being tested, the power supply and the cable > used there are no > other variables involved in the test set up. What other > factors we should > compare between both the testing? I request you all > provide your valuable > feedback. > > Sincerely > > K.Balasubramanian > Sr.Engineer, Hardware. This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. 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