900 MHz doesn't work well around here. The farmers have deployed GPS navigation systems using those frequencies.
Was the bean path you had NLOS? I'm curious what effects it had. I have seen a four foot change in elevation work like black magic. I think the beans get to blowing in the breeze and because there are hundreds of thousands of little hard points that randomly diffract the signal it fades. I saw this on a knife edge diffraction path as well as a distant, path where the CPE had to be mounted low to clear large branches of an oak tree 20 feet overhead. The next field over was a bean field one year and caused fits for a couple weeks, I believe because of the low angle. Is it that OFDM can survive in a multipath environment until the individually randomized signals number in the thousands? Mike -----Original Message----- From: wireless-boun...@wispa.org [mailto:wireless-boun...@wispa.org] On Behalf Of David Hulsebus Sent: Tuesday, February 23, 2010 3:04 PM To: WISPA General List Subject: Re: [WISPA] That black magic Mike, Interesting you mentioned soy beans. I have a customer (900-WaveRider) who was installed for 5 yrs next to a corn field. The crop was replaced with soy beans this past year and a month before harvest, as the beans dried out, we started having signal fluctuation issues. We raised the antenna 10 ft and the problem went away. It was their first issue in 5 yrs. Dave Mike wrote: > 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 > > > > > > > > ------------------------------------------------------------------------ > > > > ---------------------------------------------------------------------------- ---- > WISPA Wants You! Join today! > http://signup.wispa.org/ > ---------------------------------------------------------------------------- ---- > > WISPA Wireless List: wireless@wispa.org > > Subscribe/Unsubscribe: > http://lists.wispa.org/mailman/listinfo/wireless > > Archives: http://lists.wispa.org/pipermail/wireless/ ---------------------------------------------------------------------------- ---- WISPA Wants You! Join today! http://signup.wispa.org/ ---------------------------------------------------------------------------- ---- WISPA Wireless List: wireless@wispa.org Subscribe/Unsubscribe: http://lists.wispa.org/mailman/listinfo/wireless Archives: http://lists.wispa.org/pipermail/wireless/ -------------------------------------------------------------------------------- WISPA Wants You! Join today! http://signup.wispa.org/ -------------------------------------------------------------------------------- WISPA Wireless List: wireless@wispa.org Subscribe/Unsubscribe: http://lists.wispa.org/mailman/listinfo/wireless Archives: http://lists.wispa.org/pipermail/wireless/
