The answer to this question is complicated.
You can place the tower (outside) antenna in your attic, but it may not be a good location.
There are two things you must consider before locating the antenna in your attic:
(1) Can the tower (outside) antenna meet the Required Minimum Separation Distance from the device (inside) antenna? The RMSD must be observed. Will there be enough distance between the antennas? Tech Tip: See the posts on antenna separation distance. You can combine vertical and horizontal separation distances to meet the RMSD.
(2) What's the signal strength in your attic? Many times we see signal strength readings in the attic that are just as weak as readings in other parts of the home. Locating the tower antenna in an area of weak signal can defeat the purpose of installing a cell phone signal booster! Bottom line: You will need a strong signal in the attic for this to be a beneficial location for the tower antenna.
If at all possible, you want that tower antenna located outside your home for optimal system performance.