You stated that the test was also done in a screened room. That is a room with all 6 surfaces made of metal. You can and will get a very large field in areas of the room just as you can get nulls in the room also. So basically you have a high gain antenna, a maxed power amp and a room that reflects rf energy in an unpredictable fashion. The RAM was probably in one of those spots of high field intensity caused by all the above factors. The result is poof, call the fire department.
Performing rf immunity is a shielded room is not the best of ideas as it is uncontrollable and unpredictable. Dennis Ward American TCB Certification Resource for the Wireless Industry www.atcb.com 703-847-4700 fax 703-847-6888 direct - 703-880-4841 From: emc-p...@ieee.org [mailto:emc-p...@ieee.org] On Behalf Of Jason Smith Sent: Monday, November 02, 2009 9:32 AM To: Price, Edward; EMC-PSTC@LISTSERV.IEEE.ORG Subject: RE: [PSES] 61000-4-3 and 61000-4-20 Field Uniformity Requirements Tim was not stating the amplifier was supplying 20 times its power but with the antenna gain included there was 20 times the power in the field. TWT amplifiers have high harmonics it is the nature of the beast when we test TWTA we filter out all harmonics to guarantee what the fundamental power is. Also a TWT does not draw more input power as it puts out more output power (or at least does not significantly change). So as the harmonics increase there is no more significant draw from the AC power. It’s like getting more power for free. I don’t think when using a waveguide antenna that there would be a good match outside of its band and the gain would not increase like this all the way up to the 7th harmonic but there no doubt would be significantly power in the field from the harmonics. And add the possibility of standing waves caused by the EUT (depending on how lager) which could have focused on a part of the RAM, putting it over the edge. Thank you, Jason H. Smith Manager Applications Engineer ar rf/microwave instrumentation ________________________________ From: emc-p...@ieee.org [mailto:emc-p...@ieee.org] On Behalf Of Price, Edward Sent: Monday, November 02, 2009 11:48 AM To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: RE: [PSES] 61000-4-3 and 61000-4-20 Field Uniformity Requirements Tim: I’m not doubting your story, but, I’m having some difficulty understanding where this power came from. If the sum of the incident power applied to the RAM was 20 times the level of the fundamental, then are you saying that the TWT tube was delivering 20 times its rated power, and that the amplifier was supplying 20 times its design level? And that the instrument was drawing 20 times its normal AC input power? Ed Price ed.pr...@cubic.com <blocked::mailto:ed.pr...@cubic.com> WB6WSN NARTE Certified EMC Engineer Lab Rat (day shift) Electromagnetic Compatibility Lab Cubic Defense Applications San Diego, CA USA 858-505-2780 Military & Avionics EMC Is Our Specialty From: emc-p...@ieee.org [mailto:emc-p...@ieee.org] On Behalf Of Haynes, Tim (SELEX GALILEO, UK) Sent: Monday, November 02, 2009 8:19 AM To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: RE: [PSES] 61000-4-3 and 61000-4-20 Field Uniformity Requirements Dear All, A cautionary tale. Some twenty years ago, a young engineer was doing some “immunity testing” of a circuit to high power uWave. The engineer “cranked” up the power into the TWT until the output was as high as he could get. The TWT was an experimental wide-band amplifier (many octave wide). The test set-up had a horn antenna radiating towards the circuit with a block of RAM behind – all in a screened room. He had done the safety calculations for the maximum power output of the TWT, the gain of the horn at the operating frequency and the distance between horn, and the RAM was within its ratings. Then the fire started – the RAM was smouldering, choking fumes everywhere. After the “fire” was out – I was called in to investigate. I am not going to describe how this was all investigated – but the findings were that the TWT was so overdriven that the harmonics were within fractions of a dB of the fundamental. The measuring equipment could only measure to the seventh harmonic and all the harmonics and the fundamental were present. The antenna was fed with waveguide which, although overmoded, presented little in the way of attenuation at the harmonics. The antenna itself also worked quite efficiently at the harmonics and the effective gain increased with the square of the harmonic number (approximately). The power at each harmonic was approximately 1/7th of the rated TWT output Instead of there being one unit of power at the RAM he had created 1/7+4/7+9/7+16/7+25/7+36/7+49/7 = 140/7=20 times the original incident power! It is the best example of harmonic induced problems that I have ever seen. Regards Tim ************************ Tim Haynes A1N10 Electromagnetic Engineering Specialist SELEX Sensors and Airborne Systems 300 Capability Green Luton LU1 3PG ( Tel : +44 (0)1582 886239 7 Fax : +44 (0)1582 795863 ) Mob : +44 (0)7703 559 310 * E-mail : tim.hay...@selexgalileo.com P Please consider the environment before printing this email. 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