The example of the 41" rod is misleading, as detailed below.
The 41" rod antenna is specifically designed to pick up low-level electric fields. As such, it is designed to present a high load impedance to the rod, as described. But if such a signal (low-level, high impedance load) were actually transmitted down a cable between a test sample and auxiliary equipment, it would surely be shielded. As soon as that happens, the high impedance load on the cable end goes away, and the coupling is frequency dependent. But there is a more basic flaw in the argument. The 41" rod works against a ground plane. In the 1000-4-3 model (of which 1000-4-6 is but the low frequency portion), the ground plane is 80 cm below the test sample, and the only likely (not mandatory) connection of the test sample to that ground is the green (safety) wire of the power cord. And over most of the spectrum of this test, it is not a low impedance ground connection, so that for all practical purposes the test sample floats relative to ground. So the test sample, cable and any AE at the other end basically pick-up like a horizontally polarized dipole in space. The pick-up is all common mode relative to the individual wires within a cable. The issue of the impedance of a particular circuit between two wires within the cable is a moot point of at best secondary importance to the field-to-wire coupling phenomenon. > From: "Haynes, Tim (SELEX) (UK Capability Green)" <[email protected]> > Date: Tue, 27 Feb 2007 09:05:47 -0000 > To: <[email protected]>, "Ken Javor" <[email protected]> > Cc: <[email protected]> > Subject: RE: Conducted immunity lower frequency > > Hi, > > The length of the cable is one factor that determines the level of > coupled signal presented to the EUT. > EUT impedance is another factor. > > Take a look at the operation of an active broad-band antenna as used in > military EMC testing - I am thinking of the Singer - Ailtech design. > > It is a 39 inch rod, fed into a high impedance / low capacitance > amplifier. > Fields from 10kHz to 30MHz are received equally well - the response is > flat with an antenna factor of 8dB. > If the rod is extended to 41 inches and the capacitor hat is added (8 > inch disk on top of the rod) the antenna factor becomes 0dB. > > The sensitivity of the EUT is whatever it may be - that is what is being > tested. > > The level of pick-up at the EUT depends on the EUT loading of the wire, > the length of the wire, the effective screening of the wire (if any) the > field and the orientation of the wire to that field. > If the EUT looks like a high impedance low capacitance input and the > wire is orientated for maximum coupling - we could be talking about > levels of approx. 500mV/volt of field up to maybe 1V/ volt of field. > > If the EUT looks low impedance, then there will be a ratio'd voltage > presented of Zeut/(Zeut+Zwire) - whether this is significant in terms of > interference depends on the design of the EUT in withstanding the > applied voltage. > > No voltage, picked up on the cable and delivered to the EUT, is > insignificant if the EUT can't stand that level. > > I hope this makes sense. > > > Regards > Tim > 6239 > desk A1S77 > P Please consider the environment before printing this email. > > ******************************************************************** > This email and any attachments are confidential to the intended > recipient and may also be privileged. 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Website: http://www.ieee-pses.org/ > > To post a message to the list, send your e-mail to [email protected] > > 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] > > All emc-pstc postings are archived and searchable on the web at: > > http://www.ieeecommunities.org/emc-pstc > - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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