Ed The scenario is not practical since there is no 1-8GH TWT amp or an amp with this many octave coverage.
The largest one I’m aware of is 4GHz to 18GHz which is just over 2 octaves. TWTAs are not rated for their linear power coverage. The 1dB point is some where between 25% to 70% of it rated CW power. TWTAs are used for transmitters where it must be linear, this is achieved by using a linearizer which corrects for the tube on the front end input. This can’t be used for EMC since it works only for very narrow band applications. In the linear range (25% power) of the TWT harmonics should be better but maybe 10dB down from the fundamental. 2nd and 3rd. But I doubt that many TWT users only are using 25% of their amps. So in most cases when TWTs are being used it is not in the linear range, there is ultimately going to be harmonics. Which many are rated -5dBc or worst. Each TWT is different and there are large variations even in production runs. I understand what you are getting at in your email so I will try to make some general statement. TWTs have bad harmonics and are not linear. This is why most other companies will not specify any specs like this. At 50% power the harmonics will be bad At full power the harmonics will be worse Out of band harmonics can be bad but usually fall off rapidly. Depending on the amp and how hard it is being driven usually 2nd and 3rd are contributing factors that I have seen. But if you drive it harder strange things can happen. If harmonics are not desired: don’t use a TWT, use harmonic filters, or use <1octive TWTs. 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 3:12 PM To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: RE: [PSES] 61000-4-3 and 61000-4-20 Field Uniformity Requirements Jason: Let me try to simplify this topic by talking about just the TWTA for now. We can get to waveguides and antennas later. Suppose we have a very broadband 100 Watt TWTA, possibly one with a 1 GHz to 8 GHz operational range. And let’s say we are driving this TWTA at 1 GHz, so there are the fundamental and 7 harmonics within the power band of this amplifier. And let’s also say that the amplifier is rated to deliver its full 100 Watts in the linear range of its operation. What power would you expect to see then at each of the harmonics? What power would you expect to see at harmonics beyond 8 GHz? Now, let’s take this 100 Watt TWTA and drive it a few dB harder, into full compression. Now what power do you expect to see at the fundamental and each of the harmonics? And now that we are really pushing the envelope on this TWTA, what is the total power output of this device? Ed Price ed.pr...@cubic.com <blocked::mailto:ed.pr...@cubic.com> WB6WSN NARTE Certified EMC Engineer Electromagnetic Compatibility Lab Cubic Defense Applications San Diego, CA USA 858-505-2780 Military & Avionics EMC Is Our Specialty From: Jason Smith [mailto:jsm...@amplifiers.com] 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 * 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> - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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