If you'd like to gauge the actual signal strength of your outside signal by Phone Jammers, then you can put your phone into field test mode, which will display the actual decibel reading of the current signal.

There are two different types of external antennas that you'll encounter when deciding on a cell phone signal booster, omni-directional and yagi (directional).

Omni-directional antennas are designed to send and receive signal in all directions, so you're able to reach multiple cell phone towers at the same time and boost cell phone signal for multiple providers. If you have a moderate to strong signal in the area that you're going to be placing your external antenna, then an omni antenna is good for you. Read more about omni antennas.

Jammer Antennas are designed to send and receive signal in a specific direction, so while you're not able to cover a broad area, you're able to reach significantly greater distance to cell phone towers that you would not be able to with an omni antenna. If you're going to be placing your external antenna in an area which has weak signal, and you only have one cell provider that you're looking to boost the signal for, then a yagi antenna is right for you. Read more about yagi antennas.