70 Part I — Hardware Hacks

Antenna Placement

Where you place an external antenna is just as important as how you hold a GPS receiver. When using a fixed or semi-fixed antenna, you need to carefully consider where you place it in order to get the best coverage.

On a car, place the antenna as high as possible. The roof is the best place for it, while the hood and near the flat glass panels are worse. The area around the hood has high electromagnetic interference, while glass and flat metal surfaces cause signal reflections and signal loss (see Figure 4-10).

FIGURE 4-10: Good and bad placements for antenna on a car

Another possible area for placement is inside the front or rear plastic bumper, although bear in mind that small impacts can damage the antenna.

The problem with in-car use of GPS is that most people think that because they can get a satellite lock on their GPS when it is in their shirt pocket, in the car, they don’t need to worry about antenna placement. The truth is that poor antenna placement in a car, especially using a GPS signal from inside the vehicle that has traveled through the metal skin of the vehicle, is likely to result in a very inaccurate signal. If your vehicle has tinted metalized windows, this can severly degrade the GPS signal you receive, and an external antenna becomes a must.

Chapter 4 — Antenna Hacks 71

For trucks and vans, the best place for the antenna is going to be the cab or trailer roof. Both of these offer fantastic views of the sky.

Around buildings, keep the antenna away from walls. If possible, either put the antenna high up or at least well away from walls and obstructions. Keep the antenna away from trees (see Figure 4-11).

FIGURE 4-11: Good and bad antenna placement on buildings

In urban areas that have a large number of tall buildings, higher is better because it eliminates multipath errors caused by signal reflection (see Figure 4-12).

FIGURE 4-12: Where buildings are tall, higher is better.

Note one problem with the “higher is better” rule: lightning. A single lightning strike can completely destroy a GPS receiver. This is especially a problem with placing antennas on buildings.

If you are going to have a permanent antenna fixed on your building, then prevention is far better than the cure. You are safer if you attach lightning arrestors to the line.