Trying again as the mail server disliked my text (or something)
I would say it might be difficult to see a difference in SWR to detect a problem. In some frequency ranges the SWR is very high anyway as we have antennas very close to the ground plane floor in shielded rooms. SWR will also be affected by having the EUT present, compared to not present during calibration. With the testing software of today there are capabilties to monitor all possible values. So why not do that and detect equipment failures or testing errors directly and aviod costly retesting. Petter From: Kunde, Brian [mailto:[email protected]] Sent: den 28 september 2011 17:49 To: [email protected] Subject: Re: [PSES] BS EN61000-4-3 and calibrated fields. I have to ask. If the power is going down the cable, where else would it be going if not radiated by the antenna? You can’t drive an open circuit and if the impedance is off by even a little it will throw the SWR all out of whack. The field probe will only tell you if it is radiating, it won’t tell you if it is radiating the correct field strength. A cable or connector going bad (but not open) or a loose radial on the antenna will still radiate but incorrectly. Having the field probe setup during testing seems to be an acceptable method for the required “sanity check”, however I feel measuring the forward and reverse power is an even better method and a more than acceptable alternative to the field probe. We upgraded to a dual channel power meter and probes about two years ago which can take power measurement extremely fast. Our current test method is to level on the forward power at each frequency then read the reverse power and calculate the Amp power and SWR on the fly. Then we turn on modulation for two seconds then off and go to the next frequency. By far I believe this to be the most accurate test method. Not only will it detect problems in the amp, cables, and antenna, but it also corrects for minor impedance changes caused by the EUT within the field. Since we have started using this test method, we have discover several cables going bad and loose “N” connectors by seeing an increase in the SWR. The field probe would not have detected these problems until they become very serious. Brian From: Sundstrom, Michael [mailto:[email protected]] Sent: Wednesday, September 28, 2011 10:36 AM To: Kunde, Brian; [email protected] Subject: RE: BS EN61000-4-3 and calibrated fields. Hello all, I.E. forward power: That will only tell you if you have power going down the coax. Not if it is actually being radiated. The probe in the field will tell you that all is working, the calibration file will set your levels. Michael Sundstrom OHD / TREQ Dallas Electronic Lab Analyst, EMC Lead 2170 French Settlement Rd, Suite B Dallas, Texas 75212 (214) 579 6312 (940) 390 3644c KB5UKT Albert Einstein once said, "The definition of insanity is doing the same thing over and over again and expecting different results". From: Kunde, Brian [mailto:[email protected]] Sent: Wednesday, September 28, 2011 7:43 AM To: [email protected] Subject: Re: [PSES] BS EN61000-4-3 and calibrated fields. Can’t you measure Forward Power via a power meter and bi-directional coupler and compare it to your calibration data as a sanity check? We actually do a quick check test prior to running an official test by measuring forward and reverse power with modulation turned OFF at 10% frequency points. We compare the data with our calibration data and calculate the amp wattage output and SWR. So if the antenna or cable was going bad this pre-test would find it without messing with the field probe. From: [email protected] [mailto:[email protected]] On Behalf Of Gärdin Petter Sent: Wednesday, September 28, 2011 5:55 AM To: Bryce Stammerjohan; [email protected] Subject: RE: BS EN61000-4-3 and calibrated fields. And anyone not having a field probe present during test should need a personal sanity check. How would you make sure you really have a field and not a broken antenna or something else. As per 17025 you need to make sure that you know what you are doing. Petter From: Bryce Stammerjohan [mailto:[email protected]] Sent: den 27 september 2011 22:19 To: [email protected] Subject: Re: [PSES] BS EN61000-4-3 and calibrated fields. I asked this just a few days ago when at a lab – this particular tech said it was a “sanity check”. The displayed 3-axis RMS value was close to the V/m setting I requested, but did vary with frequency and polarity, unsurprisingly. Bryce From: Kunde, Brian [mailto:[email protected]] Sent: Tuesday, September 27, 2011 1:19 PM To: [email protected] Subject: Re: [PSES] BS EN61000-4-3 and calibrated fields. Not to hijack this thread, but why do some test labs setup the Field Probe in the chamber next to the EUT during the test? The measurement with the EUT in the field should be all over the place and would have little value. Am I missing something or are some labs leveling off the Field Probe during testing? From: [email protected] [mailto:[email protected]] On Behalf Of Ian White (UK) Sent: Tuesday, September 27, 2011 6:00 AM To: IEEE Forum ([email protected]) Subject: BS EN61000-4-3 and calibrated fields. Could you please help: Could you explain what is the correct procedure when using an anechoic chamber which is calibrated and operating to EN61000-4-3. During calibration of the chamber, a calibration file is made and it is assessed according to the guidelines laid down under BS EN61000-4-3. During calibration the chamber is empty. During a normal test run a UUT is put into the chamber. The questions is : a) Do you use the calibration file as the starting point and adjust the field strength which may have changed due to the UUT being in the chamber. b) Just take the calibration file and run it and except the filed strengths would have changed due to the UUT being in the chamber. Thanks Ian White Compliance and Reliability. - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at http://product-compliance.oc.ieee.org/ http://product-compliance.oc.ieee.org/> Graphics (in well-used formats), large files, etc. can be posted to that URL. 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