Would a point-to-point link be possible thru a city with a mountain in the way? When they say 900mhz is nlos, does that mean just a little bit ? What’s realistic? With a 3.5km distance what can you expect to be in the way and have it still work?
I’m guessing the EIRP is way illegal, but wondering if 900mhz is that good.
Sam
There’s no way that link will work. If it were a case of clipping the fresnel zone some for a short distance you’d be okay, but not that much obstruction. Trees are a little different than mountains when it comes to NLOS 
yea when i first read this i was kinda confused…ur trying to go through that mountain? right?
joe
A buddy of mine did a lot of radio stuff in the military.
He told me of a shot like you described, with a mountain right in the middle. I read about this technique in a book he gave me.
Take the first directional, point it almost directly at the top of the mountain, with a second on the other side pointing at the same point. The same rules apply here as with the fresnel zone, but in this case you try to take advantage of the obstical bending the radio waves.
I don’t know if it would work at all with 802.11, but it has been done in the past with analog for certain. I have a feeling it would change the wave too much, it would likely be inconsistant as hell if it worked at all.
Bending the radio waves is a trick but it only works for a few degrees, ie. light bends ever so slightly when hitting a sharp edge.
Bending radio waves around a large obstacle is really not possible.
However ‘sometimes’ and this is rare you can get lucky and bounce the signal off the atmosphere.
So 900mhz is nlos thru trees and smaller obstacles right, but nothing major ?
I assumed it wouldn’t work … but was curious what types of applications 900mhz helps with. If you look at the topo map above you can see its not that big of a mountain (just scrapes the pmountain1) but the radio link diagram shows it as a direct hit. Another few hundred feet in one direction and that hill is much smaller. Curious why radio mobile shows a green link ?
I’m working on getting those mountain tops setup (anyone dealt with the city on something like this?) so that i have a direct line and provide access to others.
Sam
I believe bending over objects works better with lower frequencies, dunno about bouncing off the atmosphere. Talked about it a bit, but haven’t read much.
it’s possible to make this link running very well, but 24dBi is quite high for 900MHz, 2meter (6 feet) yagi has about 16,5dBi gain…
Then try to correnct reciver sens.
Receive Sensitivity
Rate
1 Mbps
2 Mbps
5.5 Mbps
11 Mbps
6 Mbps
9 Mbps
11 Mbps
12 Mbps
18 Mbps
24 Mbps
36 Mbps
48 Mbps
54 Mbps Level
-93 dBm, +/- 2dB
-91 dBm, +/- 2dB
-88 dBm, +/- 2dB
-88 dBm, +/- 2dB
-90 dBm, +/- 2dB
-89 dBm, +/- 2dB
-88 dBm, +/- 2dB
-87 dBm, +/- 2dB
-86 dBm, +/- 2dB
-82 dBm, +/- 2dB
-79 dBm, +/- 2dB
-73 dBm, +/- 2dB
-70 dBm, +/- 2dB
so you’ll need about -80dBm to get this link running stable 365d/y
I was always taught that lower frequency radio waves do kinda roll over stuff like hills. With UHF You get a shadow effect behind large obsticals where the signal rolls over at a more gental curve so it depends on height of object and the distance behind it and of course TX power. I don’t have any actual data on 900mhz though. Still it look like he would kinda be skimming the side of the mountain rather than going strieght through.