Ken, This data was not taken with 61000-4-3 primarily in mind. We do -4-3, but also MIL, RTCA, and ISO testing. I should have had the probe at least 15 cm for ISO or 30 cm for MIL like you said, but 10cm is how I took the data.
Uniform field calibrations will be a concern eventually, but the variance is my problem. This was not four probes set up on a bench next to each other. This was data with one probe on the bench, centered in front of the antenna, then removed and replaced as precisely as possible with the next probe. So if I do a single point cal for ISO 11452-2, one probe might tell me 100 V/m and another 140 V/m. I'll get questioned by customers if they fail one day and pass another. This also runs into another issue - purchasing amplifiers. If I specify an amp to reach a desired field strength but when it shows up we can't hit levels due to using a different field probe, there will be hell to pay. Standards are silent on probe orientation as well. Do you position the probe to maximize field strength? If I can get an extra half a dB of power by having it an angle instead of straight on, why not do that? I can save that amplifier cost - at least until I get a new probe. The calibrations don't seem to mean that much based on my data, so with a composite reading whichever probe orientation gives me the highest field should be ok. Also, any replies I make may be delayed. It seems like I usually see a 4+ hour delay between when I email the listserve, and when it is delivered. Thanks, David Schaefer From: Ken Javor [mailto:[email protected]] Sent: Sunday, March 04, 2018 12:17 PM To: [email protected] Subject: Re: [PSES] Field probe calibration In turn: It is not surprising at all that it takes less power to generate the vertical field than the horizontal field. That's the effect of the conducting ground plane. The OP doesn't say what spec they are working to, but that is why MIL-STD-461 below 1 GHz has the probe 30 cm above the ground plane, to limit that effect. Comments, such as Gert Gremmen's, that measurements in the presence of a ground plane (or any conducting structure) are useless, are themselves useless. The comment reflects a difference in standards of value. If one is starving, food is the most important priority. If one is asphyxiating, oxygen is the primary need. It is logically incorrect for two people suffering these two conditions to point at each other and say the other one is wrong about his priorities: they are both correct within the scope of their individual circumstances. The only logical observation that can be made is that oxygen needs to be supplied sooner than food, if the standard of value is immediate survival. In the world of goods slated for use in home, office and factory, the coin of the realm is accuracy and minimum uncertainty, so that qualifications everywhere result in a level economic playing field. Required field intensities (1/3/10 V/m) are very low compared to the world of vehicle EMI testing (as high as 200 V/m, sometimes beyond), so that (again relative) low power amplifiers may be used with antennas separated from the test area by three meters instead of one, facilitating the calibration of a quiet zone in the complete and utter absence of any conducting surfaces, because the end-item use does not include installation on or near a conducting ground plane. This is all in sharp contrast to the qualification of equipment slated for use on a metal or partially metallic vehicle. The ground plane is of material use in reducing the intensity of the horizontal field near it, as noted in the OP and taken advantage of by the very standards that deal with such qualifications: the ground plane is our friend. Let us count the ways: As mentioned above, the ground plane reduces the intensity of a horizontally polarized field in its immediate vicinity. The presence of a ground plane causes cables in its vicinity to react to fields not as an antenna as in the 61000-4-3 and 61000-4-6 paradigm, but as a transmission line. Transmission lines radiate less per unit of current they carry, and couple less power from an incident field, than do antennas. Given the very stringent RE and RS requirements in vehicle standards, we need all the help we can get. And finally, metallic equipment enclosures bonded to a ground plane allows filters to efficiently shunt incoming noise to the ground plane and away from internal circuitry, and perform that same function for noise currents coupled to shields that terminate in a low impedance manner to the exterior of such metallic equipment enclosures. Now having dealt with Gert - his recurring comment about the futility of vehicle EMI testing re ground planes consistently eliciting the above response from me, ad infinitum and ad nauseum, lets look at the OP provided test data, especially in light of the 61000-4-3 required UFA (uniform field area) uniformity requirement of +6 dB, -0 dB for 75% of the required sixteen measurement points. What I see is that for a given polarization and frequency every single position measured is within 6 dB of the others. In the immediate presence of a ground plane, no less! The cup is not half full - the cornucopia is overflowing. This performance greatly exceeds the MIL-STD-461 requirement: there is no requirement for multiple measurement points, and if such are used, the only requirement is to use the average of the measured points as the leveling field intensity. In other such standards, such as RTCA/DO-160, there is a requirement to precalibrate the field in the absence of the test sample; I would say that the OP test data is evidence of an excellent chamber. If I wanted minimal variation between various positions down the length of a ground plane, I would not use an aperture antenna such as a DRG horn, but rather one with constant beamwidth vs. frequency, such as a log periodic. Assuming I could get the required field intensity with the amplifiers at hand. Finally, while all the standards of which I am aware allow leveling on the composite output of field probes looking in three orthogonal direction at once, it is unsurprising that this results in significant variations in required power level. Better testing results when the test equipment allows leveling on the polarization of interest. Although, as I said above, I consider the cited test data below to be exemplary. I would have been bragging to my colleagues, not complaining! Ken Javor Phone: (256) 650-5261 ________________________________ From: Cortland Richmond <[email protected]<mailto:[email protected]>> Reply-To: Cortland Richmond <[email protected]<mailto:[email protected]>> Date: Sun, 4 Mar 2018 08:01:31 -0500 To: <[email protected]<mailto:[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]] - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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