All, Thanks for your all your input.
I don't think the listserve allows attachments, so I can't share setup photos. The setup was like that used for RTCA, MIL STD, or automotive(ISO 11452-2). 90 cm high, appeox 3.5 meters long copper bench bonded to the chamber wall. I have only done this limited range due to time constraints, and the probes having a limited range of overlap. I did use the calibration factors provided by the calibration lab or manufacturers. One concern is the cal factors are widely different for the two probes of identical model, but calibrated at two different places. One has a factor at 100 MHz of 0.44 dB, the other 2.27 dB. Should probes be calibrated in the far field? That might be difficult, as one of these probes is rated for 10 kHz to 1 GHz; another for 10 MHz to 40 GHz. If they're used only in the near field, a near field calibration makes sense. I suppose the same could be said for antennas - we get 1, 3, or 10 meter factors from a calibration, but I think all probes are calibrated at the same distance. I recall a cal house telling me once that all the probes they do are calibrated at 30 cm. Probe construction is different, and might have a major impact. That ties back in to my original concern - isn't the calibration supposed to ensure that each probe will return identical results in the presence of an identical field? I will plan to take measurements on a 90 cm non-conductive bench, and farther from the chamber walls. I'd like to see that data as well. Perhaps uniform field calibrations to IEC 61000-4-3 would be more consistent. Frankly, these results concern me. Do you tell Ford, Boeing, or the DOD that you don't really know what your field strength is? Yes, it's all calibrated equipment, but your actual field could be +/- 3dB due to just probe error. Thanks, David Schaefer From: Cortland Richmond [mailto:[email protected]] Sent: Sunday, March 04, 2018 7:02 AM To: [email protected] Subject: Re: [PSES] Field probe calibration I'm with Gert. Anything "antennas" is checked in the far-field -- especially if testing for accuracy. I'm a BIG fan of near-field probing for relative measurements and localizing emissions, but we use probes appropriate to what we are looking for; if I wanted to "calibrate" one there, I'd use a known current on a wire/trace or a known voltage on a small plate -- and not trust *that* much. Cortland Richmond On 3/4/18 5:35 AM, Gert Gremmen; ce-test wrote: IMHO all probes are calibrated under far field conditions. In general: Using probes in the proximity (< lambda) of anything conductive (including ground planes at 10 cm and including EUT) makes the measurement data useless. As James correctly states, the construction of the probe makes this effect different per type of probe, be it the construction, the size of battery or electronics on board or the lead (fiber or copper) , as long a other conductors are in proximity the read out has no relation to calibration data anymore. Using a probe near a ground plane, such as usual in automotive test set ups, indeed says not much about the test level of the EUT. Repeating this test under far field conditions, preferable on an antenna calibration facility, might give you much better results. (not that you are allowed to generate this much of power on air ;<) Gert Gremmen On 4-3-2018 11:06, James Pawson (U3C) wrote: Hi David, An interesting set of results! I'm going to ask some questions that I'm sure you've already considered so please bear with me being Captain B. Obvious. Do your field probes use frequency correction? I'm not familiar with a wide range of probes but my Narda PMM field probe has an internal calibration table; you tell it what the field frequency you are applying is and it makes the appropriate correction. However, looking at the typical correction data from the manual (see PDF page 12 of this doc: https://www.emctest.it/public/pages/strumentazione/elenco/Narda/EP%20600/Manuali/EP600-EP601EN-90302-2.02.pdf) it doesn't look like a large difference. Is there a difference in the probe construction between the probes used? Some probes like the Narda one above have two antenna per axis whereas ones like this Amplifier Research probe - https://www.arworld.us/html/18200.asp?id=636 only have one antenna per axis. Perhaps the proximity of copper plate makes a difference. On the subject of copper plate, what are the differences without this present? What are the dimensions of it and are they significant at the frequencies selected? Have you acquired just spot readings or a full frequency sweep? There may be some patterns in the frequency sweep data that give you more of a clue as to what's happening. An interesting puzzle and I look forward to hearing about your results further! All the best James From: Schaefer, David [mailto:[email protected]] Sent: 04 March 2018 05:22 To: [email protected]<mailto:[email protected]> Subject: [PSES] Field probe calibration I took data with 4 field probes, 3 different models. All calibrated. Two calibrations by the manufacturer, two by a reputable cal house. 200-1000 MHz data, 10 MHz step size, 60 V/m level. I recorded the forward power, and all equipment and software in the setup was the same, barring only the measuring field probe and associated probe factors. Composite values only. No 3-axis data as I don't have 3-axis calibration data for all probes. Probes were 10 cm above a copper bench, DRG antenna 90 from the bench. The results are not encouraging. The tables below show the results in watts of forward power for select frequencies. Antenna Horizontal - values in Watts Probe 1 Probe 2 Probe 3 Probe 4 Max-Min(Watts) 200 MHz 85.17 144.4 135.9 97.75 59.23 220 MHz 92.81 171.6 157.4 113.5 78.79 500 MHz 21.7 34.93 28.58 26.94 13.23 900 MHz 25.57 37.25 25.6 32.42 11.68 Antenna Vertical - values in Watts Probe 1 Probe 2 Probe 3 Probe 4 Max-Min(Watts) 200 MHz 18.94 25.12 22.55 18.82 6.3 330 MHz 34.1 40.69 46.29 39.41 12.19 780 MHz 35.52 53.03 29.87 32.83 23.16 930 MHz 56.63 47.01 64.26 107.7 60.69 There are trends in the data. Probe 1 was usually the lowest. Probe 2 was usually the highest, rarely the lowest. If you want to talk field strength effects this will mean, depending on the probe, you could have an E-field 40% higher between two 'identical' calibrations. The large variance between which probe was highest or lowest based on freq. is troubling, as is the clear difference between horizontal and vertical. I took additional data with two probes of the same model rotated around a center axis. I don't have that all compiled, but just comparing one probe against itself, laying on the left, right, and bottom sides, results in up 20% difference in required power. I have not read IEEE 519, but plan to soon. So my question to this group - do you think field probe calibrations are accurate? How can we have confidence in our results with such widely varying results? Thanks, David Schaefer - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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