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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