There are several styles that are popular with folks using a EVDO connection. Probably the most popular is the omni:
This antenna comes with six metal spikes that must be screwed in to provide a ground plane. The omni antennas provide a 360 degree horizontal radiation pattern. These are used when coverage is required in all directions (horizontally) from the antenna with varying degrees of vertical coverage. Its pattern looks like this:
This type of antenna design can deliver very long communications distances, but has one drawback, which is poor coverage below the antenna. For example your elevation is much higher than where the cell tower is located... you are at 1500' ...the tower is at 1000'. If you try to cover an area from a high point, you see a big hole below the antenna with no coverage.
This is why you can often get an ok signal with a omni even if you don't have it particularly high, as long as you are keeping that beam width in the cell tower radiation pattern.
They are popular because they are relatively cheap, work in most places, are easy to mount, and don't require any "tweaking". But, they don't work in all situations just as the example above shows.
Omni's are most often used in mobile applications where you are being handed off from tower to tower as you travel. They will work in a fixed location but why would you want to use one if the tower location is known?
You aren't moving. The tower isn't going anywhere. You should use a antenna that has better gain like one of the directional types.
