On Thursday, December 13, 2012 12:44:00 Torsten Dreyer wrote: > Hi > - Performance > The most important limiting factor for radio propagation on VHF and up > is question "line of sight" or "obscured by terrain".
Hi again Torsten, Apologising for keeping this subject up, but I rather enjoy technical discussions, especially based on tested mathematical models and data gathered in the field. Here is what technical knowledge has to say about "line-of- sight" at VHF (or even higher frequencies): ### Terrain features, and buildings, usually attenuate signals. (NB in some circumstances knife edge diffraction can enhance propagation beyond the horizon)### (from http://goo.gl/1la6M ) You can read more about Longley Rice there, especially the part where "Longley Rice has been adopted as a standard by the FCC". Now for real data gathered by me in the field, which is consistent with ITM plots generated for the same locations. Case #1: Have one radio repeater station on 145 MHz, completely obstructed by a mountain edge. Distance between repeater site and mountain edge: ~40 km. As determined by me, acceptable and readable signal was present as low as 500 meters below the mountain edge: single horizon diffraction case. The mountain edge had no vegetation. Case #2: Have one radio repeater station on 440 MHz, completely obstructed by a mountain edge. Distance between repeater site and mountain edge: ~50 km. As determined by me, acceptable and readable signal was present as low as 300 meters below the mountain edge: single horizon diffraction case. The mountain edge had no vegetation. In both cases, no line of sight was ever present between the mobile station and the fixed station, and reflection was not possible. All signal was received from diffraction. Double horizon diffraction is possible too, depending on many factors, including how much power are you pumping into the transmitter. Sidney Shumate's improvements to the ITM, published in the latest IEEE Broadcast society newsletters, make this model the most reliable for irregular terrain. Unfortunately his modifications have a non commercial clause, but you can download, compile and test against the classic versions. Diffraction results will coincide with physical data gathered even more. I think we can close the "optical line-of-sight" debate now. Cheers, Adrian > > Regards, > Torsten > ------------------------------------------------------------------------------ LogMeIn Rescue: Anywhere, Anytime Remote support for IT. Free Trial Remotely access PCs and mobile devices and provide instant support Improve your efficiency, and focus on delivering more value-add services Discover what IT Professionals Know. Rescue delivers http://p.sf.net/sfu/logmein_12329d2d _______________________________________________ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
