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GPS 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, and most
Magellan units, make use of the quad-helix style antenna, while units such as the Garmin eTrex use the patch antenna.
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.
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 F igure 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
When you are holding the GPS on the move, try to hold it with the antenna as vertical as pos-
sible 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 electro-
magnetic 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 .
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.
Best Performance Summary
Before moving on, let’s look at a quick “best performance summary" for built-in antennas for
GPS receivers:
1. Hold the antenna as far away from your body (and other human or animal bodies) as
possible and away from other electrical devices, as these can all degrade the accuracy of
your receiver.
2. If your GPS uses a patch antenna, keep it as close to horizontal as possible.
3. If your antenna is a quad-helix antenna, keep it as close to vertical as possible.
4. Whatever type of antenna you use, keep it as dry as possible — a coating of water
droplets will severely affect performance.
External Antennas
One problem with holding your GPS in a nearly vertical or nearly horizontal orientation is
that it can quickly become uncomfortable and cause you to start walking odd ly. If you are in a
vehicle, then the problem isn’t one of orientation but actually getting the signal to the GPS, as
most receivers find it hard to get a lock to the satellites when inside a car or boat. A far better
idea is to place the antenna away from the GPS receiver. Some GPS receivers come with either
a removable antenna or a socket that enables you to hook up an auxiliary external antenna to the device.
The connector is called a BNC connector, and you can get many types of
external antennas that connect to this port. F igure 4-7 shows a Lowe external antenna attached
to the Garmin GPS III.
This antenna has a magnetic base, and draws power from the GPS unit itself. It is a patch
antenna and offers better performance than the original antenna.
The antenna also has a strong magnet at the base that enables it to be firmly
attached to a metal surface (such as the roof of a vehicle).
The BNC-style connector isn’t the only style of external antenna connector available.
shows the connector on a Garmin 76 GPS receiver. This is called an MCX connector, and it is
much smaller than the BNC connector.
But they get even smaller!
GPS receiver. This is an MMCX connector (or Mini MCX or MicroMate). This connector is
much smaller than the MCX connector, but smaller also means that the threads on the connec-
tors are more delicate and prone to damage
These antennas either replace the existing antenna on the unit or they bypass the built-in
antenna after they’re attached.
External antennas enable you to place the antenna in a different location than the actual receiver.
This means that you (or whoever is holding the unit) will obscure the signal a lot less. It also
means you can take the unit into an area with poor signal coverage and put the antenna where
reception is better.
Some applications ideally suited to the use of an external antenna include the following:
Indoor use: If you want to bring a GPS signal indoors, an external antenna is a great
way to do it with little fuss.
Car use: If you want to use a GPS in a car, especially a car that has metalized glass
installed that blocks microwave radiation, an external antenna might be a possible
solution that will enable you to use a GPS.
Boat use: The antenna can be attached outside and the GPS kept inside, away from
exposure to the elements.
Mobile use: Place the external antenna in the top of your backpack while out and about.
This can really help to boost the signal! Usually, the higher the antenna, the better the signal.
As with most things, there are disadvantages to using external antennas:
Clumsiness: All the additional cables can be a pain, as they can easily get in a tangle.
Power: External antennas draw more power from the GPS unit. More power means
the batteries don’t last as long. Some external antennas come with their own power
source, but remember that this means you have something else to keep charged up
while on the move!
Antenna loss: If the GPS has a removable antenna, make sure that you don’t lose the original!
Cable length: The longer the cable you have attached to the antenna, the greater the sig-
nal loss. This isn’t a problem when you are routing the cable over a short distance, but if
you are routing the cable over a long distance, then this could be a problem. Keep cables
as short as possible; and above all, avoid keeping unnecessary loops of cable in the setup.
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
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.
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
In urban areas that have a large number of tall buildings, higher is better because it eliminates
multipath errors caused by signal reflection
Note one problem with the “higher is better" rule: lightning. A single lightning strike can com-
pletely 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.
There are several manufacturers of lightening arrestors. Here are a few to get you started:
Symmetricom:
www.symmetricom.com
PolyPhaser:
www.polyphaser.com
Radiall:
www.radiall.com
LightningMaster:
www.lightningmaster.com
This solution may seem pricey for something that may never be needed, but if you live in an
area where lightning is common, consider how much it will cost you if your antenna is hit and
you lose your GPS.
In addition, it’s not just the GPS that you can lose — if your GPS happens to be connected to a
PC at the time of the strike, you could also lose the PC and possibly even others connected on
the same network if you happen to use one. Other Things to Avoid
Here are a few other things that you should avoid when it comes to using external antennas:
Knots and kinks in cables damage the interior of the coax and can cause signal loss. Keep
the cables as straight as possible, and if the cable does have to travel around corners, don’t
make the loop too tight.
Be careful about bringing cables in through windows and door frames, as crushing can
severely damage the cable.
Another way to damage a cable is to stretch it, so take care to avoid this. If you install the
cable in warm weather, make sure you leave additional slack in the cable, as it will contract
when the temperature drops.
Keep the cable short and reduce the number of connectors: The greater the length of
cable and the more connectors and joints you have in it, the more signal loss you will experience.
Other information on antennas will be added soon... |
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