Filed under: Features, GPS

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: Yup, we
misspelled 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.
Case modding
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.
Building
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.
Performance
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.
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