Now we get to the heart of the matter that prompted the original post. That spike in the 137 cm tip-to-tip biconical used with a T1 dipole balun (28 200 MHz) behavior might have been objectionable, but you wouldn¹t miss it an analog sweep with even 1% frequency steps. You would only miss it if you calibrated every ten MHz as was done with the biconical antenna design in MIL-STD-461A (1968 when swept performance was difficult to do). There is no need to use frequency steps commensurate with the sorts of BWs used during EMI testing. A 1 - 5 MHz BW would more than suffice to sweep an antenna used above 30 MHz to ensure not missing anything.
The following quote is the rationale I have seen a few places with which I take issue: ³Though the >sweep< may be continuous, if you can read amplitude only by marking pixels, you need to insure .5 BW or less per pixel. That's straight sampling theory, right?² That is straight sampling theory applied to an unknown signal characteristic. If a continuous sweep using 1/2 BW steps as per sampling theory has been performed, then discrete points for an antenna factor file may be chosen by inspection so that in any particular frequency range either of the two following conditions are met: 1. If the data is noisy, with no obvious trend, then frequency points should be listed so that the antenna factor between any two listed points doesn¹t change by more than 1 dB (or whatever delta is deemed necessary for adequate accuracy). 2. If a linear or log-linear approximation can be made over some frequency interval, then only the two endpoints of that interval need be recorded, with the understanding that between those two points the appropriate interpolation will yield the desired accuracy. The interpolation has to be one which the automated software can be instructed to perform on its recorded antenna factor files. One of these two approaches should work when deriving a set of discrete points from a continuous sweep of any test transducer factor¹s performance. The question is whether this is ³cast in stone² anywhere or left up to the person doing the calibration, or the test engineer abstracting a subset of data points for inclusion in his files. Ken Javor Phone: (256) 650-5261 From: Cortland Richmond <[email protected]> Reply-To: <[email protected]> Date: Tue, 06 Dec 2011 20:43:38 -0500 To: <[email protected]> Subject: Re: [PSES] Calibration practice for EMI test transducers We might *like* measuring antennas to be low-Q, but remember the bicon shorting bars that were added some years ago? The skeleton bicons lacking them inserted a rather sharp notch that had to be accounted for in transducer tables. Baluns might go off from impact damage, too. And so on. I gave an ebay 50 MHz comb good past 2 GHz to a former employer who later gave all their own EMC lab gear to an NRTL. But you still have to check. For that matter, I've seen coax factors programmed with GAIN at a narrow range at microwave frequencies; that was caused by reflection from a crimped cable*. So we do need to be able to spot them. *This was due to the ignorance of operators collecting the data, IMO. I spoke to them and likely, they won't do it again. But they believed the SA/TG reading because they didn't know what they were measuring. That's another thread. Though the >sweep< may be continuous, if you can read amplitude only by marking pixels, you need to insure .5 BW or less per pixel. That's straight sampling theory, right? If your analyzer has a 1024 pixel wide screen you can't SEE less than about .1% of the scan width and can rely on less. Does it matter? Often it doesn't. Fun, isn't it? Cortland KA5S On 12/6/2011 5:11 PM, Ken Javor wrote: > Re: [PSES] Calibration practice for EMI test transducers This discussion has > gone far a field from the original post. I¹m assuming some sort of scan has > been run and a continuous sweep is available. The question is, how densely > does that sweep need to be digitized? Not as a practical matter, but as a > matter of compliance with standards or standard practice. > > What manner of antenna would have performance as cited below where at 100 MHz > the antenna factor is 10 dB and at 120 MHz it is 12 dB, but at 110 MHz it > could be 20 dB? Let¹s generalize the question to any closely spaced > frequencies. > > Let¹s look at the types of antennas available. > > 30-200 MHz: A half-wave tuned dipole is nowhere near that sharp. Neither is > a biconical. > > 200 1000 MHz: A half-wave tuned dipole is nowhere near that sharp. > Neither is a logperiodic, log-spiral, nor a Yagi. > > 1 GHz+: Logperiodic, log-spiral nor pyramidal horns act the way surmised. > > My conclusion, and the point here is to invite discussion, not close it out, > is that no test-type antenna is a high ³Q² device. Antennas can have > arbitrarily high gains, depending on construction, but the high gain is a > geometrical quality, not a high quality factor in the frequency domain. > > - ---------------------------------------------------------------- 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]> - ---------------------------------------------------------------- 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]>
