25+ years ago, we had an audit facility in a semi-anechoic chamber at Wang Labs, in Tewksbury, MA. There were physical limits to height, so we never could get the full 4 meters of vertical travel out of the antenna mast, and the cones were a bit small -- pre-ferrite-tile you needed 8 foot cones down at 30 MHz. When we needed really accurate results we would go to Dash, Straus and Goodhue, in Boxborough. Glen Dash was adamant that we'd never get accurate result out of that chamber, but we convinced (paid) him to do an A vs. B test with a typical EUT so we could quantify the differences between an OATS and our chamber. I was there for a little over three tears and after we adjusted the limit lines we got pretty good results. At least, if we said something was going to be over, it WAS over.
But that was comparing apples to crabapples. Comparing the GTEM and the TEM cells to an OATS or a semi anechoic chamber is like comparing apples and pineapples. The TEM cell has resonances thatIMO make it less useful for a scan than a GTEM, and neither has what I'd call a usably large test volume. Dealing with peripherals and cables -- if any - is almost impossible in them. And to get an idea of elevation pattern (GTEM only, now) , you need to rotate the EUT on two axes, not just one. It has been done. But I can't imagine testing an 80 meg glass platter hard drive from the 80's, or a rackful of telecom gear that way. I was at AST Research in the mid 90's, and there, Jozef Baran got Dr. Andrew Podgorski to build one of his BGF chambers. It turns the whole chamber into field volume, or at least a much larger part than a GTEM cell, and shortly after I left in 1997 the FCC approved AST's for Part 15 measurements. Don't know what eventually happened to it when AST folded. It was a lot of work even so; the EUT was mounted on a dielectric table whose height had to be changed for horizontally and vertically polarized tests, and it may have been the only such facility built for non-military use. That's the impression I got when I saw Dr. Podgorski in Detroit last year. Andrew Podgorski and Jozef Baran presented a paper on it: http://ieeexplore.ieee.org/iel4/6068/16206/00750319.pdf?arnumber=750319 . Further papers followed later; I recall Mark Frankfurth doing at least one presentation. IMO, IF, the EUT is small enough, and IF it has no cables to speak of, one *might* correlate a TEM ort GTEM reading to an OATS. But that's just my opinion. Might be easier to use a mode-stirred chamber. Cortland Richmond KA5S GE Aviation My opinions, NOT my employer's > [Original Message] > From: <[email protected]> > To: <[email protected]> > Date: 7/3/2009 1:12:21 PM > > I am trying to understand how to properly use TEM and GTEM cells to take > radiated emissions data. Has anyone had any success in correlating far > field radiated emissions measurements with GTEM measurements? > > Thanks, > Cody - 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.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 <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]>
