GPS is a nifty piece of technology. GPS hardware needs to have a clear view of the sky because it's satellite based, so using your handheld unit can be a problem when the roof of your car is blocking signal reception. The obvious answer is to add an external antenna. The catch-22 is that many GPS units lack external antenna jacks.
[Edit: Fixed the misspelling of NMEA in our diagram. (accursed nyquil) Thanks fabrizio]
If you crack open your GPS, you could extend the wire between the antenna and the internal antenna, but the loss from the extra wire would degrade the signal. But active antennas amplify the signal to get past the loss in the extra wiring; with a little bit of hacking, you can add an external active gps antenna to just about any GPS unit. Find out how in today's How-To.
To complete this project, you need:
A suitable GPS receiver
26 gauge wire (0.4mm thick)
100pF or 47pf ceramic capacitor
Active external GPS antenna
BNC panel mount connector (may vary, get one to match connector for the antenna)
1/8 inch mini jacks or similar for power and serial output (optional)
Tools: Soldering iron, pliers, wire strippers, drill
We wanted a low cost dedicated GPS receiver that could be mounted somewhere in the guts of our vehicle. We picked up a cheap GPS device made for the Palm III and used the instructions from Walter at radiohound.com to remove the 12 channel GPS board from the unit. It's just a matter of disassembling it and moving a few components around. The result is a stand alone GPS card. Power it up and it starts spitting out NMEA sentences via the serial port.
The GPS board isn't too shabby, but it does lack a backup battery (for faster acquisition) and support for an external antenna. By adding a power injector circuit, we can use a standard external active antenna. To make life easier, we used an antenna that accepts 5-12 volts DC. The GPS board we used accepts 4-16 volts DC. If your hardware needs a lower voltage like 3-5 volts DC, you'll need a power regulator to get power from your car. We scored a pair of 100pF capacitors, and a male BNC panel mount connector from RadioShack. The 3/32 inch drill bits (you only need one) came from Sears and the antenna from eBay.
How it works
Satellites orbiting the earth transmit radio signals down to the earth. The GPS antenna receives the signal (active antennas amplify the signal a bit) and sends it down the line to the receiver. The power injector does what it says, sends power up the antenna cable to the antenna. The signal is passed to the GPS receiver. Based on the time and the satellites the receiver can 'see' it calculates its position on the earth.
The Power Injector
The power injector is a simple circuit resembling the sound crossover you'd find in any decent set of speakers. The coil is a small inductor that acts as a low pass filter. It keeps the GPS frequencies from the power supply, and keeps interfering signals from the power supply out of the GPS while allowing the DC voltage to pass through. The capacitor acts as a high pass filter. It allows the GPS signals to pass while blocking the DC voltage from entering the GPS receiver
We'll make the coil from scratch, and the capacitor is either a 47pF or 100pF capacitor.
Building the coil
To make the coil, you'll need a piece of 26 gauge wire that's about .4mm thick. We pulled apart some 18 gauge stranded wire to get the right size. The digital caliper was oscillating between .39 and .40 mm, so we're good.
To make the coil, we wound our 26 gauge wire around the 3/32 inch drill bit six complete turns. It's easiest to wind it, squash it together and then evenly separate it evenly using your fingernails.
We decided to mount the hardware inside a penguin mint tin. To drill the large hole for the BNC connector, start with a small drill bit and work your way up in size. We used four or five bits to drill the hole.
Finishing up the job is pretty easy. A couple lines of hot glue provide a simple insulator to keep the board from shorting on the bottom of the tin. The coil is connected to the power pin on the GPS board. The capacitor runs to the signal line of the coax to the board. The shielding is soldered to the outer tab of the BNC connector. We couldn't get the mini-jacks in time for the photo shoot, so just imagine them being wired up. There's plenty of room for them next to the BNC connector.
The mod is so simple, it's almost a let-down. Fortunately, the results aren't. The lock speed of the GPS card decreased to a consistent 60 seconds, down from over two minutes. The extra shielding helped along with the now externally mounted antenna. If you don't want to crack open your shiny GPS, you an build a re-radiating antenna using the same circuit with a passive antenna in place of the GPS unit.