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. There are 10 types of people in the world-those who understand binary and those who don't. J. Paxman - 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 <emc-p...@ieee.org> All emc-pstc postings are archived and searchable on the web at http://www.ieeecommunities.org/emc-pstc Graphics (in well-used formats), large files, etc. can be posted to that URL. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <emcp...@socal.rr.com> Mike Cantwell <mcantw...@ieee.org> For policy questions, send mail to: Jim Bacher <j.bac...@ieee.org> David Heald <dhe...@gmail.com>