The way it is drawn in the cited figure is pretty much how it was done in the Air Force EMI lab where they worked out the technique. The issue that is resolved by the change that consists of floating the counterpoise from the ground plane connection, and using the coax cable with the bead to “ground” the counterpoise is that the old way generated a resonance between 20-30 MHz that disturbed the measurement accuracy. The -461F technique eliminates that resonance. I have seen other fixes for this problem pre-461F. One used either stacked pyramidal absorber, or floor absorber as a support for the counterpoise. Same idea – a resistive termination between counterpoise and ground, instead of a very conductive connection at one end, and an open circuit at the other.
Back to the precise position of the bead. From the cited figure, it is clear by inspection that the length of coax dropped vertically to the floor from the road antenna base cannot exceed 70 cm, and will in fact be closer to 50 cm, depending on the height of the rod antenna base, and where precisely along that vertical dimension the output rf connector is located. At 30 MHz, 70 cm is less than one-tenth wavelength, so that precise bead positioning cannot be critical. I would place it midway as shown, not right at the rod antenna end, and certainly not near the floor. I wouldn’t want the bead to be in intimate proximity with ether floor or coax connectors (metal). Ken Javor Phone: (256) 6 50-5261 ________________________________ From: "Price, Edward" <ed.pr...@cubic.com> List-Post: emc-pstc@listserv.ieee.org List-Post: emc-pstc@listserv.ieee.org List-Post: emc-pstc@listserv.ieee.org Date: Wed, 29 Jul 2009 08:44:20 -0700 To: <emc-p...@ieee.org> Conversation: Mil-Std-461F 41" rod Ferrite & Cable query? Subject: RE: Mil-Std-461F 41" rod Ferrite & Cable query? Ken: I don't see any reference in 461 to the height of the ferrite above the enclosure floor. Are you free to put the ferrite anywhere between zero and 120 cm above the floor, or is there some "sweet spot" height that is preferred? If so, then why? Ed Price ed.pr...@cubic.com <blocked::mailto:ed.pr...@cubic.com> WB6WSN NARTE Certified EMC Engineer Electromagnetic Compatibility Lab Cubic Defense Applications San Diego, CA USA 858-505-2780 Military & Avionics EMC Is Our Specialty ________________________________ From: emc-p...@ieee.org [mailto:emc-p...@ieee.org] On Behalf Of Ken Javor Sent: Wednesday, July 29, 2009 7:21 AM To: Untitled Subject: Re: Mil-Std-461F 41" rod Ferrite & Cable query? “In addition, Figure RE102-6 is revised to show the antenna lowered so that the center point of the 41” rod element is 120 cm above the test chamber floor. Further, this figure shows that the coaxial cable emanating from the rod antenna base is carried directly to the floor and grounded there, with a ferrite bead installed between the rod base and the floor ground point. The ferrite bead should have between 20 – 30 Ohms impedance at 20 MHz. A bead that works in this application is the Ferrishield B1642.” From an article in Conformity magazine that describe the –461F changes, January 2008. Ken Javor Phone: (256) 650-5261 - 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 <emc-p...@ieee.org> 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 <emcp...@socal.rr.com> Mike Cantwell <mcantw...@ieee.org> For policy questions, send mail to: Jim Bacher <j.bac...@ieee.org> David Heald <dhe...@gmail.com> - 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 <emc-p...@ieee.org> 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 <emcp...@socal.rr.com> Mike Cantwell <mcantw...@ieee.org> For policy questions, send mail to: Jim Bacher <j.bac...@ieee.org> David Heald <dhe...@gmail.com>
