Bill:
I was responding with the best anecdotal experience that I had on the subject, but what I described was actually a big “science project.” I was testing, actually, investigating might be a better term, the shielding effectiveness of various compartments of what became the AGM-109 Tomahawk. That missile body had more seams and flaps that a biker’s jacket! We needed to expose the missile body to as high a field as possible to wring as much dynamic range as we could for measuring the SE. “Enough” field was never really enough, as the mechanical guys were sweating the designs to yield as much SE as possible. If we could prove better SE, then the theoretical guys could calculate that a mission could be flown closer to an emitter, thus influencing operational flight and attack profiles. If we could show that one technique gave 128 dB SE versus a different technique that gave 124 dB SE, rather than saying that both techniques were equally better than 121 dB SE, then at least a few mechanical engineers were happier at the end of the day. And the new SE figure could mean that a flight didn’t have to take the long way to a target, instead, it might be able to thread its way between two hostile emitters. In that instance, we were not considering the response time of a victim system, so all we cared was that the pattern shifted “enough” and that we identified how long it took for the pattern to repeat. I didn’t understand what you were saying about “…skew the reverb pattern…” All I know is that the stirring technique helped extend our SE dynamic range by a little at around 200 MHz, getting more helpful until we had, IIRC, around 12 dB betterment above 1 GHz. And 18 GHz was the high end of our investigation. Ed Price WB6WSN Chula Vista, CA USA From: Bill Owsley [mailto:[email protected]] Sent: Thursday, May 09, 2013 12:53 PM To: [email protected] Subject: Re: [PSES] Mode-stirred, Mode-tuned, Reverb - what's the difference? The large size you mention rings a faint bell of memory... There was some caution about using a chamber at frequencies above its first resonance which is based on size. The peaks and nulls of the multi-modal chamber then skew the reverb pattern along those peaks and nulls. Do I remember correctly? or even closely ?? _____ From: Ed Price <[email protected]> To: [email protected] Sent: Sunday, April 28, 2013 11:33 PM Subject: RE: [PSES] Mode-stirred, Mode-tuned, Reverb - what's the difference? I never studied the statistics of what you describe, but I recall some numbers that I had for a large (50’ long x 20’ wide x 12’ high) completely bare (no anechoic material and not even a test bench) shielded chamber. I had three “stirrers”; a large 4’ x 8’ flat panel rotating about 6 RPM, a medium stirrer that had several 24” square sheets tilted at odd angles turning about 30 RPM and a shaft with IIRC three 8” corner reflectors on a single shaft turning about 60 RPM. The rotational speed was continuous, not stepped, and the rates were not synchronized nor precisely controlled. At 100 MHz, shifts in the reverberant pattern were noticeable, but not enough change was seen to make a big impression. At 1 GHz, I could see greater than 20 dB of field variation, but I needed to wait about 3 minutes before I felt enough time had elapsed to allow for all combinations to have happened. I used the chamber from 1 GHz and up, with dwell times of 3 minutes at each frequency. Over the course of that three minute exposure, there were many dips and rises, but only a few combinations actually hit the peak exposure level. For monitoring, I used a spectrum analyzer set to zero sweep width to obtain a time domain view. The trace sweep was set to about 30 seconds per division. At the end of about 4 minutes, the analyzer could display the maximum and minimum signal strength. As I said, I didn’t think about the statistics, but it was tedious. I probably could have used several more stirring elements to shift the reverberations faster, which would have reduced the dwell time. Ed Price WB6WSN Chula Vista, CA USA - ---------------------------------------------------------------- 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://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. 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 <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]>
