Cell phone antennas can do two things to boost your reception. The first is that they get the signal outside of the metal cage of your cars frame and the other, if they are large enough, is to amplify it.

Most small, magnetic mount antennas three inches or less do not amplify cell phone signals any more than the internal or external antenna of the cell phone but they do get the signal out of your car where it can be picked up by the cell tower easier.

To get the gain effect you will need a larger antenna that is rated in terms of decibel or DB gain. A decibel is a logarithmic measurement. Roughly, for every 3 DB gain you get a doubling in the strength of the signal.

A 3 db gain antenna may be twice as powerful as one that is neutral with double that gain for every extra 3db.

You may see a magnetic mount antenna with a gain up to 5 db or more and a fixed, yagi beam antenna that you aim in the direction of the tower with a gain of 12 db or more.

If you are in a fringe area where cell phone coverage is very poor then choose the highest db gain antenna possible.

The larger ones are prone to being knocked off by limbs so you may want to purchase on that has a choice of shorter screw on tips for when you may not need as much boost.

An omnidirectional cell phone antenna boosts your signal by sending it out in a narrower band toward the horizon instead of into all directions. The higher the db gain of an omnidirectional antenna, the narrower the band of radio energy that is beamed toward the horizon.