On 12/22/2011 9:27 AM, John Woodgate wrote:
If the direct and reflected rays are equal amplitude and in phase, then it's +6 dB; if they are at 180 degrees, it's minus infinity dB. In between you can get any value, depending on relative phase and transmission loss.
Equal amplitude only occurs if the direct and reflected paths are the same distance. A 3 meter OATS can never meet that condition. If the source and antenna are at 1 meter height, the reflected path is 3.6 meters long and the lowest frequency where a null can occur is where the difference in path length is 180 degrees. That's at 250 MHz, but the reflected signal being subject to the inverse square law can't completely cancel the direct ray, only reduce it. and cna't increase it by the full 6 dB. I've played around in an old style TEMPEST test chamber with no cones or tiles and seen 30 dB+ cancellation between two Bicons, but that is from multiple reflections.
10 meter OATS are closer. Have used one for GR 0189. In Winter. In Collin County, Texas. Had to chip ice off the gate to get in. BRRRR!
Cortland KA5S - ---------------------------------------------------------------- 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]>
