The AP is a 1300 Series with omnidirectional dipole antennas of 12 dBi. We are placing it like 4 meters from the floor and the signal strengh to a distance of aproximately 40 meters is about -70dB, (i think that this is too poor for this kind of antenna). This is located in a open space, (there is no buildings).

Well what are you using this for anyways... point to point? If this is for client access, then you have to look at the gain on your device to get back to the antenna. 12dbi can be heard further away, but your device might not be heard.

Is there any interference around the area, and what modulation are you seeing?

This is an omnidirecctional antenna with vertical linear polarization and really the issue is that a few meters (40 meters) from de antenna's base the signal strengh is too too low, even the clients can't to connect to it.

We already tried with another AP and the behavior is the same, there are others AP's around but our the all are in different channels.