------------------------ From: "Flinders, Randall" <[email protected]> Subject: RE: Indoor vs. Outdoor emissions testing Date: Wed, 28 Oct 1998 14:31:17 -0800 To: "[email protected]" <[email protected]>, 'Gordon Andrews' <[email protected]>
> Does anyone have any experience with mode sturring as a method to improve the > low frequency performance of test chambers? > > Regards, > > > Randy Flinders > EMC Engineer > Emulex Network Systems > (714) 513-8012 > [email protected] > Randy: About 10 years ago, I did a lot of emission and susceptibility work on the Tomahawk and Advanced Cruise Missiles, all in a totally reverberant (no absorber) 35' long by 26' wide by 12' high modular shielded enclosure. I used three paddles to "stir" the room. One was an 8' high by 4' wide open frame of firring strip, hung with aluminum foil and rotated at about 6 RPM. The second was a 12" by 12" sheet of aluminum, with each corner bent at a random angle, and rotated at about 60 RPM. The last was also a 12" by 12" sheet of aluminum, but I twisted and bent it till it had a lot of wrinkles and facets, and it was rotated at about 100 RPM. The frequency range of interest was 1MHz to 18GHz. Results were similar for both emission and susceptibility, so I'll describe the susceptibility example. The illuminating energy was brought into the room via a coax feedthrough mounted about 4 feet above the floor level at about 1/3 of the distance along the 35' side. An antenna, either a small commercial horn or a monopole or a long wire, was connected to the inside of the feedthrough. The monitor antenna was placed at several locations where the missile would later be located. The big stirrer was in the far corner, with just enough space to swing. The two little stirrers were placed, well, just sort of stuck, somewhere in the middle of the remaining room volume, at about two feet off the floor. I set a constant level, fixed frequency, CW drive into the illuminating antenna. A spectrum analyzer was tuned to the illuminating frequency and set for zero span. The analyzer was set for peak hold, and a fairly slow sweep speed of one division per second was used. Watching the spectrum analyzer, you could see the reflection pattern on the first scan. On subsequent scans, the trace may have exceeded or not exceeded the previous trace(s). When it did exceed the previous, the trace height increased. After a while, you had to wait a long time for any new trace to be written. Finally, the highest portion of the trace depicted the maximum field strength that could be three-dimensionally "walked" through the location of the monitor antenna. Moving the monitor antenna to a new location, you could compare the ability of the stirrers to achieve equally high amplitudes at any point in the room. (If you didn't use the peak hold, you could watch the reflection patterns rise and fall, with distinctive patterns related to each stirrers rotational rate.) So, what did I find? First, the min to max field strength at any observation point was about 35dB. Not too bad, I thought. That was a 50:1 shift in the field strength. Second, the same max field was achieved at every point throughout the center of the chamber. (I didn't monitor the far corners; I was only interested in the central volume of about +/- 5 feet off the centerline. Next, the rotational rates didn't matter too much; the faster the better, as long as the stirrers were not synchronized to each other. And the paddle shape (crooked, flat, corner reflector, bent) didn't matter either. (Obviously, a sphere is out of the question!) The big down side was that it took about 5 minutes to be certain that the maximum pattern would be seen at any given location. That's a real bummer for continuously swept frequency ranges. You just have to use a number of judiciously chosen discreet frequencies instead of a sweep. Lastly, frequencies. This method was great everywhere above about 400MHz. At 100Mhz, you could notice that the "dynamic stirring range" (uhhh, min to max) was not as good as at 400MHz. By the time you got to 30MHz, there was some "wiggle" to the trace, but maybe only around 10dB (Your milage may vary; data presented for amusement only). And, below 10MHz, the effect was underwhelming. I did get some slight effects (only a few dB) one day when I tried to look at 10KHz to 100KHz. Maybe I was seeing some effects of capacitive coupling of the stirrers to the room walls? (Just another chance for fame unfollowed.) Regards, Ed -------------------------- Ed Price [email protected] Electromagnetic Compatibility Lab Cubic Defense Systems San Diego, CA. USA 619-505-2780 List-Post: [email protected] Date: 10/28/1998 Time: 17:46:21 -------------------------- --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to [email protected] with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to [email protected], [email protected], [email protected], or [email protected] (the list administrators).
