GPS is a nifty piece of technology that needs a clear view of the sky being 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, however many GPS units lack external antenna jacks.

If you crack open any GPS unit, you can extend the wire between the antenna and the internal antenna, but with a little bit of hacking. Find out how in this 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



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 lacks 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. 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 the GPS antenna receives (active antennas amplify the signal a bit) and sends it down the line to the receiver. The power injector sends power up the antenna cable to the antenna.

The Power Injector


The power injector is a simple circuit resembling a sound-crossover you'd find in a 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. Make the coil from scratch, with a capacitor that is either a 47pF or 100pF capacitor.

Building the coil


To make the coil, take a piece of 26 gauge wire that's about .4mm thick. The digital caliper was oscillating between .39 and .40 mm.



Then wind the 26 gauge wire around a 3/32 inch drill bit six complete turns. It's easiest to wind it, squash it together and then evenly separate it using your fingernails.

Case modding


Mount the hardware inside a tin can. First to drill the large hole for the BNC connector, start with a small drill bit and work your way up in size. Use four or five bits to drill the hole.

Building


Finishing up the job use 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..

Performance

The mods results are the lock speed of the GPS card decreased to a consistent 60 seconds, down from over two minutes. And the extra shielding helped along with the now externally mounted antenna.