Only use external antenna approved to use with your GPS.
GPS units come with an internal antenna that detects your location to help lead you on your merry way without getting lost. However, internal antennas are not the only solution; you can also attach an external antenna to many models of GPS. Determining if an external antenna is necessary depends on how you use your GPS unit.
Types of External Antenna
There are two types of external antenna: passive and active. The passive antenna does not have a low-noise amplifier (LNA), which allows an antenna to amplify weak signals; thus, passive antennas must be used with a GPS that has LNA built-in. Passive antennas are best used very close to the unit, as signal is lost over the cable between the antenna and the unit. An active antenna has built-in LNA, which allows it to be further from the GPS unit and pick up more signals than a passive antenna.
Sensitivity
While you can purchase modern GPS units with high sensitivity internal antennas, generally speaking you're going to receive a better signal using an external antenna -- depending on the type of antenna. Not all antennas are created equal, and a low-end antenna with a GPS unit may not perform as well as a high-end GPS unit with a sensitive internal antenna. If you need to pick up a very weak signal, however, an active antenna is the best bet.
Convenience
Adding an external antenna to your GPS unit requires space to mount it. Where you mount it depends on how you use your GPS. Popular mount locations in cars include on the dashboard, on the roof or on the windshield. If you're hiking or using a handheld GPS, you can mount an antenna on your backpack; GPS accessory company Gilsson recommends hikers ground their external antenna with 2 inch-by-2 inch square of metal for better signal reception. In addition to requiring more space than the GPS reciever alone, active antennas also put an extra drain on your GPS unit's battery power, which can be inconvenient mid-trip.
When to Use an External Antenna
According to GPS manufacturer Garmin, you should only use an external antenna when your GPS receiver can't get a signal on its own. You might consider an external antenna on the roof of your car if your windshield has a heated windshield, which can interfere with your GPS signal inside the car. Before banking on an external antenna, test your GPS unit in the area you intend to mount it to see if it has any trouble receiving a signal.
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