I need to do a reality check with those of you familiar with knife edge diffraction as a propagation medium. First, I should paint the scene:
I have a corporate farmer almost 16 miles away who is motivated. His options are satellite, dialup he currently uses, or us. A spectrum sweep of the property found absolutely no 2.4 signals. By in large, these rural areas are very quiet. There are no trees or obstructions in the near field or out quite a ways. However, there is a ridge almost half way between us. I am embedding an image of the path here created with alphimax.com path estimator. I have a test unit which is a 19 dB panel/radio with an AP fastened to the back. It lets me hand hold a test unit and see what it sees on a laptop. Standing on the ground on his property we got an ALMOST usable signal in a short test. He has a 35 foot TV tower next to the house on which we would install. Perhaps one would look at the path profile and common sense would dictate it won't work. However, I use knife edge diffraction successfully on a handful of installs. Besides, black magic sometimes trumps common sense. I have never used this technique where the ridge is close to mid point. On all others the ridge was closer to the user. All of them work except when tropospheric ducting enters into the equation, with one exception. I have told the users this is a 98% link and it WILL go down during those events. Earlier this winter we had a few days of ducting which caused a couple of them to fade. I saw a 15 dB fade on those. Statistically, ducting should only affect this area 20 some hours a year. The single exception was when the obstructing hill had soy beans growing on it. That particular one went down in late fall when the beans were ready for harvest. The previous 2 years the field had corn planted on it and had absolutely no issues. I think dry beans affect the signal because they are no longer "row" polarized and randomly scramble the signal beyond use. Once the beans were harvested, the signal came back like usual. On this path in question, I found the ridge. There are no trees, instead it is farmed. There is corn stubble on it right now. I am curious what others have found in these NLOS situations. Because the obstruction is mid path, will the signal still be there next fall as it is now? Are mid path obstructions on a long path better than obstructions closer to one end? Am I absolutely stupid for even considering this install? I went over all the physics involved and told him of my experiences. Like I said, he is motivated. I told him I wouldn't tie him into a contract, but we'd go month by month and if we found later in the year it wasn't working, we'd cut our losses. He was OK with that. Since I respect the viewpoints of many of you, bring it on! Friendly Regards, Mike Mike Gilchrist Disruptive Technologist Advanced Wireless Express P.O. Box 255 Toledo, IA 52342 Mike's <http://www.tamatoledonews.com/page/category.detail/nav/5001/Local-Columns.h tml> Weekly Column 239.770.6203 m...@aweiowa.com
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