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I use Cisco access points AIR-CAP1552E to connect buildings via WiFi, 802.11a/n (5 GHz). Normally, they are mounted upright, with three three omni directional antennas at the bottom. Now I consider a single AP on the roof of a very small building, connecting to a much higher one.
Would it be beneficial to turn the AP up some degrees, such as in the photo I made, or would it degrade the WiFi quality if the angle of the antennas doesn't match the other AP which is upright?
It definitely matters:
'Omni' refers to the symmetry in the plane perpendicular to the axis of the antenna; That is, the torus around the antenna axis. But 'omni' antennas are VERY sensitive to variation from the vertical.
If you're aiming down into an open area (to the left, as suggested by the photo,) then ok, tip them as is shown. But generally, you should have the antenna at about the same height at the other trans/receivers you're trying to cover.
If that photo is your actual installation: Be aware that a metal roof -- especially if it's grounded, which it almost surely is -- will be an RF reflector. So you will have an RF shadow from the roof below the visual line of sight. (If you can't see the antenna, the RF signal won't go through the roof either.)
Yes, it would be beneficial if you are angling the AP correctly (second AP would be up and to the right), even with the antennas on the other AP being vertical. If you angle this the wrong way (second AP is up and to the left), then you will be making things worse.
This type of antenna produces a signal pattern in a toroid or doughnut shape (with the antenna forming an "axel" through this pattern).
Angling the one would increase the energy that arrives at the higher AP and will allow for slightly better reception by the lower AP.
However, this may only be true if all you are concerned about was the signal between the two APs. If you are providing service to other devices from the lower AP, this could negatively impact the service to those other devices.
If it is only signal between the two APs, then I would question why you are using omnidirectional antennas over directional? Even if the higher AP is serving multiple lower APs, if the lower AP only connects to the higher one, it would be better served using directional antennas. If all the APs are lower than the higher one, then I would look for antennas with "down tilt" for the higher AP if possible or else something like a patch/panel pointed downward.
With any wireless, you should make sure to understand the signal patterns of the antennas you choose.
This will theoretically help. Your question seems to break down to: "Does it make sense to aim omnidirectional antennas?"
Since 'omnidirectional' dipoles are actually directional with a toroidal radiation pattern, the radiation from your angled antennas will be strongest along the plane perpendicular to the antenna.
The fact that the other antenna is perpendicular to the ground would not cause any issues, but if you wanted to optimize for this particular building to building link you could angle that one 'down'.
Whether or not this will help in any meaningful way in your particular situation is less clear: The only way to know would be to try it both ways and see if there is any difference in RSSI.
My hunch is that unless you're talking about extremely long distances or sharp angles, it won't matter much. If you are then you should probably be looking at more directional antennas.
