Actually, while Moire is relavent, this wiki page does a better job. http://en.wikipedia.org/wiki/Interference_%28wave_propagation%29 Notice just above the Examples heading, the 2 waves combine to produce a zero net result.
Scott Reed wrote: > The multipath you are describing is that in which the signal arrives > from two paths slightly out of phase. > If the path length of the reflected signal is just right, the signal > arrives 180 degrees out of phase, thus canceling the direct signal. > Google Moire Pattern for examples of how this works. > The fact you have LOS and yet only get -84 or so would seem to > indicate that multi-path is the issue or you have a hardware problem. > > John Vogel wrote: >> The terrain between the AP and the CPE is such that I would ordinarily >> consider it a slam-dunk. Standing there on the ground next to the house >> and looking at the (almost) clear view of the tower, with nothing in >> between that I would consider to be capable of creating multi-path >> reflections, my thought is that there is no way for this link to not >> work. heh... >> >> That being said, my experience with multi-path is that you may get wild >> fluctuations in RSSI, or you may get great signal, but lots of dropped >> packets. But you still get signal. In this case, at that particular >> elevation, nothing. Like something is completely blocking the signal, >> not that you are getting the signal from multiple directions. It's like >> there is a dead zone from 14.5 feet AGL to about 16 ft. AGL. And that >> tree isn't very big. >> >> The only thing comparable I have experienced is with a wireless security >> camera that was broadcasting enough signal that the CPE wasn't able to >> hear the AP, but I know of nothing within 1/2 mile of this house that >> could be generating any significant amount of signal. >> >> John >> >> Scott Reed wrote: >> >>> Or the tree is no longer blocking multi-path interference. >>> >>> Jayson Baker wrote: >>> >>> >>>> Multipath interference from the tree. >>>> >>>> On Mon, Oct 26, 2009 at 10:01 PM, John Vogel <jvo...@vogent.net> wrote: >>>> >>>> >>>> >>>> >>>>> So... I have a customer, been on for a couple of years now. The CPE on >>>>> their home quit working. I go to check it out, log into the CPE from >>>>> their computer, everything looks good, except that a scan for AP's shows >>>>> only the linksys router they have in the house. This is a MT411 with an >>>>> R52 card in it, with a 14dB panel enclosure. I assume the radio card >>>>> quit receiving or a bad pigtail, so I go retrieve the unit from the >>>>> mount, which is about 15 feet AGL (mounted on the facia at the peak of >>>>> the gabled end of the house). I have my bucket truck parked just below >>>>> where the unit was mounted, so I am standing just about straight under >>>>> where the CPE had been when I take it apart to check it out. >>>>> >>>>> First thing I notice when disassembling the unit is that the SMA >>>>> connector was pretty loose, so just for kicks, I tighten it down and >>>>> boot it up. I have my laptop right there at the back of the truck, so I >>>>> am powering it off of the truck and can log into it standing right >>>>> there. I can see the tower my AP's are on, just one lonely tree between >>>>> me and it, about a half mile away and it doesn't have any leaves on it >>>>> anymore (the tower is about 3 miles away). I can sometimes pick up the >>>>> tower directly from my laptop, so this link is a piece of cake, right? >>>>> So after tightening the SMA connector, and booting up the unit, I pick >>>>> it up and point it in the general direction of the tower. It links up. >>>>> -84 to -86 RSSI. So, even though I didn't really think the SMA >>>>> connector being loose had been the problem, it must have been. So I >>>>> re-mount the unit up where it had been. Log into it, and... nothing. >>>>> Scans for AP's show nothing except the linksys. While I am up there, I >>>>> can see about 80% of the water tower the AP's are on, that one lonely, >>>>> straggly, leafless little tree is technically denying me LOS, but, this >>>>> link worked all through the summer just fine, when that one tree had >>>>> leaves on it. But, no AP's showing in the scan. >>>>> >>>>> Maybe I knocked something loose, or there is a problem with the power >>>>> supply coming from the injector inside the house. So I grab my cable >>>>> from the truck, and plug it in (powering the unit off the truck now) and >>>>> try again. Still nothing. So I go back up, get the #...@! thing, and bring >>>>> it back down to take it apart again. While coming back down, about >>>>> halfway down I can see the laptop on the back of the truck, and winbox >>>>> says that the radio (which isn't even pointing at the tower now) is >>>>> associated to the tower. Go back up, and find that if I hold the radio >>>>> at the exact elevation I had it mounted at, a scan won't even SEE the >>>>> AP, much less associate to it. If I raise it about 18 inches, I get an >>>>> -84, same thing with lowering it 18 inches. I get -84 to -86. Moving >>>>> side to side, same thing. At the elevation the mount is at, nothing. >>>>> higher or lower, no problem. >>>>> >>>>> BTW, there are actually 3 120 degree sectors on the tower, and under >>>>> normal circumstances, I can pick up all three of them. Standing on the >>>>> ground, I get two of them. Where the radio had been mounted, nothing. >>>>> >>>>> This isn't a LOS issue, so I start looking for interference. MT reports >>>>> a noise floor of about -98, but I ask the customer if they have any >>>>> wireless stuff they might have added recently. Nothing. There is a >>>>> tractor and stock trailer parked across the yard, below LOS when the >>>>> radio is up on the mount, and impeding LOS if I am standing on the >>>>> ground. The stock trailer has a sheet-metal roof. Could that be >>>>> reflecting signal somewhere? Customer says the stock trailer has been >>>>> there all summer though. I think I remember it being there too. >>>>> >>>>> After spending an hour and a half pulling my hair out, it is starting to >>>>> get dark, so I decide to temporarily set the radio on a tripod on the >>>>> ground so they can have internet overnight and I'll come back tomorrow >>>>> and try to figure this out. Interestingly enough, the best signal I can >>>>> get with the radio on the tripod is when I move it to a point where the >>>>> tractor and stock trailer are completely blocking LOS. I get -83 there. >>>>> -85 when I move it to a point where I can actually see the tower. >>>>> >>>>> Anybody have a link to RF glasses? >>>>> >>>>> John >>>>> >>>>> >>>>> >>>>> -------------------------------------------------------------------------------- >>>>> 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! 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