Thanks for the questions. > If this is a multipath problem with the water, why does it occur at low > tide rather than high tide when the water is closer to the > line-of-sight?
My first astonishment was this very issue. My presumption is that however the link is behaving, low tide is the proper reflection offset distance for destructive interference to occur. I was even more astonished when the link went down at high tide. (more than 16 feet). And even more baffled when low-low tide brought the link back! The reflection angle between high tide and low tide is very small; Tide is changing +/- 2 meters and the path is 8000 meters. Dish beam is reportedly 8 degrees, diversion of direct and reflective paths is much less than one degree. No mud flats. Some searching has led me to see this as a variation of the two-slit optical interference experiment. (not the relativity one). (That the reason for the Lloyd's mirror links) The link would come and go as the tide falls and rises past the extremes of the local tide cycle, like the optical interference pattern shows. I live at 20 feet above the beach so I hope I never have the opportunity to test the pattern at 36 feet of tide. > If this was the case, would lowering the antennas by 10 feet or so have > the effect of artificially creating a permanent high tide. Thats the hard part; the cranky-up tower has the trick of suddenly telescoping in the crank-down direction, and it takes a crew to bring it to horizontal. Putting up a second diversity antenna 2 meters higher or lower would be the answer I think. > The web page says there is 27 dB of link margin. Who knows? a plug-in program I found on the net generated that info. > Intentionally pointing the antennas 5 or 10 degrees upward will chew up > some of the margin but might put the reflections off the main lobe and > actually improving the signal-to-noise ratio. Exactly my thoughts. But the angular difference between the direct and reflected paths are so small that I could just turn down the power in the radios and get the same effect. Higher power or better receiver sensitivity wouldn't do much good either; a null would still be a null, though the time of the outage might be a shorter duration. On Wed, 29 Oct 2003, Loren Zemenick wrote: > If this is a multipath problem with the water, why does it occur at low tide > rather than high tide when the water is closer to the line-of-sight? Is > there any type of reflecting surface that is being uncovered at low tide > like a sloping mud flat? Alternately, maybe the shallow angle of incidence > at high tide and surface ripples disperse the reflections. It's only at low > tide that the angle is steep relative to the size of surface ripples. > > If this was the case, would lowering the antennas by 10 feet or so have the > effect of artificially creating a permanent high tide. > > The web page says there is 27 dB of link margin. Intentionally pointing the > antennas 5 or 10 degrees upward will chew up some of the margin but might > put the reflections off the main lobe and actually improving the > signal-to-noise ratio. > > Loren Zemenick > ------------------------------------------------------------------------------- Andy Barlak On my desk I have a work station. Trains stop at train stations. -- general wireless list, a bawug thing <http://www.bawug.org/> [un]subscribe: http://lists.bawug.org/mailman/listinfo/wireless
