Cell Tower Signal Patterns

Getting an idea of how cell towers radiate their signal and how your own antenna receives that signal can help you, if using a directional antenna, install and aim it in the most optimal position.
The above graphic is the typical pattern for a cloverleaf style tower most often found in suburban and rural areas. As you can see, the three sections that make up the pattern overlap each other and each of them have their own identifying number called a PN (pilot number). When you are online/connected that number will show up in your connection manager's debug screen as the "Active" PN:

In the debug screen you will see references to Active, Candidate, and Neighbor pilots. The below graphic helps explain where they are relative to you. A Candidate is a cell tower in a particular direction you would be handed off to if you were mobile. For this discussion we are only talking about Active pilots.

Your EVDO antenna also radiates a signal pattern and, if directional, that pattern is focused in one primary direction called a "lobe":

When the lobe of your antenna intersects the lobe of the cell tower you have a connection:
The above example will show up in your debug screen as being connected to one active PN. But what if you see two active PN's? That means that the pattern of your antenna is intersecting two lobes of the cell tower or a completely different tower:

This sometimes causes a performance degradation if you have both of those sector signals show up as active in your debug screen. What can you do? well you can try rotating your antenna to see if you can "lock" onto only one sector:
You won't be able to do this if you are using a omni style antenna because their signal pattern is 360 degrees and therefore cannot be directed. that is why I think a directional antenna is better if using your EVDO modem in a fixed position.

By using the same basic technique, if you are having problems with two towers at the same time you can try aligning the antenna so that one of the towers signal path is directed to the weakest section of your antenna:

In the above example a desired signal ("A") and a undesired signal ("B") are both arriving at the antenna. If we want to maximize "A" then we would point the main lobe of the antenna in a direction that puts undesired signal "B" into the weakest lobe, even if the main/strongest lobe is not optimally pointed at the desired signal. Remember the goal is to maximize the signal to noise ratio.

There you have it. A visual way to think about how you are getting the signal and a possible "tweak" to optimize it.