How-To: Add your own external GPS antenna

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.

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.

[Mike @ Mar 28th 2006 10:20PM]

theres no way in hell that i will void my warranty for an expensive item by ripping it open - but this is a nice tut for extreme modders/hardware hackers.

[Sam @ Mar 28th 2006 10:46PM]

"GPS hardware needs to have a clear view of the sky..."

Not anymore. My BT-338 uses a SiRF III chip which can function fine without a direct view. I can leave it in my pocket when I drive, and use TomTom on my Treo without any problems.

[billybob @ Mar 29th 2006 12:13AM]

In addition to the obvious dangerous of cracking your small electronics device (like the possibility of never getting it to work again), you can't just stick any old gps antenna on a receiver. It is very easy to mismatch the receiver & antenna and either provide too little or swamp the input. Either will cause the unit to never or poorly perform in acquiring satellite. Tell the truth - you posted this story because it involved a soldering iron, right?

[BronzeG3 @ Mar 29th 2006 1:55AM]

I find it amusing that some people have trouble messing with hardware that can be found under $100 on ebay, while others have no trouble at all installing a system that pumps water around in a $3,000 computer or increasing voltages on $500 processors to get a bit more speed. I guess some people's "safety zones" are just different.

I think this mod is great, and would love to perform it if I can only find where I put my old GPS receiver.

[onii @ Mar 29th 2006 8:03AM]

I have a holux, and it works fine sitting near the window. I have a 350z, so it just sits in the trunk area near the hatch window. signal works fine..I don't even suction cup it to the window.

[Shade @ Mar 29th 2006 8:07AM]

I also have the BT-338, its great bluetooth gps device.. don't really need this mod as u can tuck it away in your glove box or pocket :)

[Joseph Heller @ Mar 29th 2006 8:25AM]

That's not a catch-22; it's just a problem.

[jds @ Mar 29th 2006 8:26AM]

Or... you could just spend $40 on a re-radiating antenna and spend the time tucking the wire away.

[fabrizio @ Mar 29th 2006 8:35AM]

Isn't it spelled NMEA?

[RichC @ Mar 29th 2006 10:44AM]

What's the big deal? The Garmin GPS 18 puck (2.5" dia) velcros nicely to the dash and is powered by one USB plug into a notebook computer. The included software maps and talks through turns just like many of the 'tiny screen' models all for a list retail price of $129.00. I assume a shopper can save a few buck on top of that. I use mine daily is my business sales trips to find client addresses and routes through cities and its a no brainer since I already travel with my notebook and can tuck the USB powered receiver in my computer bag. (I'll give it 4 out of 5 stars -- maps could be better)

[Jon Lind @ Mar 29th 2006 3:58PM]

What do you do with the signal from the GPS unit?

[Martin H. @ Mar 29th 2006 4:34PM]

There were two problems with the Streetfinder GPS:
The current time is lost and some Palmsoftware sent a wrong initstring, causing the Streetfinder to send only $GPRMC and $GPGSV. Both problems can be solved by correctly initalising the GPS.

[Kevin @ Mar 30th 2006 8:05AM]

This is a good mod for those of us that have an older GPS. I use a Garmin GPS III+, so I've got a BNC mount for my external antenna already provided.

Lots of naysayers here that have newer models that don't realize how crappy the receive used to be on GPS units. My III+ would hardly work driving around in downtown Seattle until I got an external antenna, with which it now has no problems.

Again, great mod, and wow! A soldering iron? -New concept!- :D

[Sutherix @ Apr 3rd 2006 1:03PM]

Honestly, antenna impedance matching is not as simple as this. But hey, a much easier, cheaper hack is an RF reradiator. Are you listening Garmin, TomTom R&D departments? How about in marketing? Great parallel sales (accessory) idea. Put one on every bus!!!

If you use this idea you are financially obligated to support Engadget.

[GBrown @ Feb 10th 2007 12:55PM]

Unfortunately, devices which re-radiate GPS signals can not legally be marketed in the United States because they operate in restricted frequency bands allocated for authorized safety-of-life operations only, so you won't see Garmin, Gillson, or any other company subject to US laws selling these. See FCC regs for more or simply do a web search for FCC GPS re-radiator for plenty of background. Makes this mod a bit more reasonable alternative.

[www.espow.info @ Sep 15th 2006 8:02AM]

I think it is a great mod . Some of my friends love to perform it .

[[e-mail] @ Dec 1st 2007 4:20PM]

[e-mail]

[amin qamar @ Apr 24th 2008 1:44AM]

I want to attch an antenna chip directly to a holex gps receiver module.
In this way , i want to calculate the resistance of equivelent wire of antenna . because when i connect an anteena module to a gps module
without wire it never connect to a valid data mode .
please guide me any body in this way.
thanx