Antenna Hacks
Making the most out of your GPS depends on getting a good signal from the satellites, which orbit the Earth at an altitude of around 20,000 kilometers (12,000 miles). This signal isn’t a strong one;
it’s roughly equivalent to detecting the light from a 25-watt lightbulb from 16,000 kilometers (10,000 miles) away.
In other words, without a good signal from the satellites (at least three of them, preferably more), you aren’t going to be able to use GPS at all, or the information it gives you will be erroneous. You can solve this problem by adding an external antenna to your GPS unit.
This chapter examines the GPS antenna and how you can maximize a weak signal when you are on the move.
The GPS Antenna
One part that all GPS receivers have in common is the antenna, whose job is to receive the signal from the satellites and pass that signal on for processing.
The signal the antenna picks up is a UHF signal with a frequency of 1575.2 MHz (this is the civilian, unencrypted frequency; the military signal uses 1227.6 MHz). This frequency offers all-weather navigation capability, but is blocked by walls, ceilings, and even trees.
GPS receivers usually come with one of the following antenna types attached:
Quad-helix
Patch
Receivers such as the Garmin III (shown in Figure 4-1), and most Magellan units, make use of the quad-helix style antenna, while units such as the Garmin eTrex (see Figure 4-2) use the patch antenna.
chapter
in this chapter
˛The GPS antenna
˛The helix antenna
˛The patch antenna
˛Antenna extras
˛Signal losses
˛Repeating antennas
˛Building your own antenna
62 Part I — Hardware Hacks
FIGURE 4-1: The Garmin III receiver
The debate as to which is best is a complex one, and there are pros and cons to each:
Patch antennas are smaller.
Patch antennas draw less power.
Quad-helix antennas are usually bulky and generally protrude from the unit.
Quad-helix antennas are less prone to having the signal from the satellites blocked (or masked, to use the technical term) by the receiver and the person holding the GPS.
The surface of the GPS over the patch antenna is prone to catching rain and losing the signal (simply because it is a flat surface).
A lot of myth and rumor surrounds antennas. Many people believe that quad-helix antennas offer greater performance under tree cover and are less prone to signal loss in valleys and in urban areas. However, based on personal experience, I’ve not found this to be the case. Usually, such phenomena can be attributed to the particular GPS unit, assuming that your course and speed are unchanged and retaining a false signal lock.
Chapter 4 — Antenna Hacks 63
FIGURE 4-2: The Garmin eTrex receiver
A good discussion on antenna sensitivity can be found at www.gpsinformation.net/ main/gpsant.htm. Generally, I don’t really think it matters much which type of antenna you use, but the one difference between the two types of antenna is how you hold them.
Quad-Helix Orientation
A quad-helix antenna works best when the antenna is placed in the vertical position (as shown in Figure 4-3). This gives the antenna the greatest sky coverage and offers the best signal reception.
Some GPS receivers allow you to place the antenna at odd angles, which enables you to get the antenna in the right orientation no matter how the GPS is being held or how it is mounted. However, make sure that the antenna is always vertical and not at odd angles (such as the one shown in Figure 4-4).
64 Part I — Hardware Hacks
FIGURE 4-3: A quad-helix antenna in the vertical position
When you are holding the GPS on the move, try to hold it with the antenna as vertical as possible and as far away from the body as comfortably possible to reduce the degree to which your body shields the signal.
The human body is very effective at shielding the microwave signal from GPS because your body is largely made up of water, and water is an effective absorber of microwave frequency electromagnetic radiation.
Patch Antenna Orientation
How you hold a patch antenna is completely different from how you hold a quad-helix antenna. To get the best out of these antennas, you want to hold them so that they are horizontal to the ground (see Figure 4-5).
This is by far the best orientation for patch antennas — the closer they are to being vertical, the less efficient they are. This may be a big part of the reason why some people think that the quad-helix antenna is better than the patch antenna. People have a natural tendency to hold a GPS receiver in a vertical position.