72 Part I — Hardware Hacks
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.
Reradiating Antennas
Another type of antenna you will come across is called the reradiating antenna. This is a combination GPS antenna and retransmitting unit.
It works as follows:
1.The main antenna picks up the GPS signal just like any other GPS antenna.
2.This signal is fed down a cable to a signal reradiator.
3.The signal reradiator takes the signal and reradiates it. This signal is then picked up by the antenna on the GPS.
Chapter 4 — Antenna Hacks 73
This sequence is summarized in Figure 4-13.
Signal reradiated to |
Signal received |
GPS receiver |
from satellites |
FIGURE 4-13: Reradiating antennas catch and retransmit the signals received from the GPS satellites.
There are two types of reradiating antenna:
Personal reradiating antenna
Communal reradiating antenna
Remember that a reradiating antenna can only pick up satellites that are above the horizon and not obscured by buildings or rock faces. Reradiating antennas can’t perform the impossible, they are just able to make better use of the existing signal.
Personal Reradiating Antenna
Personal reradiating antennas have a short-range reradiator that can re-transmit the GPS signal over a relatively short distance (between 40 and 60 centimeters). The reradiator is usually
74 Part I — Hardware Hacks
fixed to the GPS close to the antenna on the device. Usually, it is fastened on with Velcro fasteners and can be removed when needed.These antennas are great for in-car use or other applications for which you only want the signal to be transmitted over a short range.
Communal Reradiating Antenna
The communal reradiating antenna reradiator can transmit the GPS signal over a larger range than the personal reradiating antenna. Usually, these reradiators are capable of transmitting over a range of three to four meters.
Reradiating Antenna Considerations
The following sections describe some considerations for you to bear in mind when you are choosing and using reradiating antennas.
Power
Reradiating antennas require power. This power supply usually needs a 12-volt DC source, and a lead-acid battery is normally ideal. In a car or on a boat, you can tap into the existing electrical system. This will give you an almost unlimited power supply (given fuel and no mechanical failure). If you are on the move, you can use a portable 12-volt lead-acid battery. If your antenna uses AA or 9-volt batteries, then make sure that you have an ample supply of these.
You don’t have to use a lead-acid battery the whole time. You can use other batteries, such as AA or even D cells in series, which enables you to make a lighter and safer pack. If you use rechargeable batteries, it isn’t as wasteful and you don’t need to have a 12-volt charger.
Another power option is to use a solar charger to top up a battery pack. One such system is the SmartSolar solar panel and battery pack shown in Figure 4-14.
The 10 NiMH batteries provide a 12-volt power supply and can deliver 1 Ah of power. This is enough to keep a reradiating antenna working for several hours without recharge.
Range
If you want to use a reradiating antenna with more than one GPS unit, you will need the communal type that can serve more than one GPS. Technically, there is no limit to how many GPS receivers a single reradiating antenna can serve — as long as they all fall within the range of the re-transmitting antenna.
Remember that reradiating antennas can mask the proper GPS signal for those around you who may need to use it.
Tidiness
Reradiating antennas are made up of a lot of cables, including the cable taking the power to the reradiator and the coaxial cable to the antenna. Keep these cables tidy to prevent damage and tangles.