TripMate external antenna

Foreword from the author

Hi folks,
since these Delorme Tripmate boxes are so extremely cheap ($50 refurbished at delorme.com) and so extremely useful to make a poor man's APRS solution, here's a write-up on a mod I did that I haven't seen documented before. Feel free to forward this email to other related email lists, or publish it at relevant websites. If you do put it on the web, please let me know so I can brag about it to others :-)

Thanks,
Ramon, W1/PA3EUG, [FN42KI], Boston, MA, USA, mail:
legalalien30@yahoo.com

Modification guide

The current problem is that the Tripmate does not allow you to have an external antenna, and therefore will not function from the trunk of your car (or wherever out-of-sight you want to hide it). After some shopping, I came across a relatively cheap solution for an active external antenna.

The Antenna

I bought the "ANT-GPS-SE-SMA- ND" from Digikey for $37.54 + $3.50 s&h.
This is one of the cheapest I've seen around. For the specs, see http://www.antennafactor.com/m_gpsselect.html (it's the SE series on this page). At DigiKey's website, this antenna can be found by searching for the part (use the search string I wrote above) from their main page at http://www.digikey.com. (DigiKey ships internationally if needed.) But anyways, I have no commercial connection to digikey, and the remainder of this description would be valid for almost any active GPS antenna that can run on 5 Volts fed via its coax cable. Two more components are needed: a small 1 nF capacitor and a small 100 microHenry coil. The latter should preferable be a pre-fab coil in a resistor-like packing.

Construction Details

power consideration

The antenna uses 5-11 mA at 2.5-5.5 Volts, so it's not wise to install this antenna if you haven't already made the power modification that allows you to run your Tripmate from external power. Even if the Tripmate is "asleep" (nothing connected to the 9-pin sub-D connector), the antenna will consume power. So beware if you rely on battery operation. You may want to consider an add-on switch or something to power-down your antenna. Further, if you use any external power source, just make sure you don't feed your Tripmate with anything more than +5 Volts. Use a LM7805 or similar if necessary.

Warning

Installing this will void your antenna warranty as well as the warranty of your Tripmate. You will need some experience soldering, a steady hand, good lighting, and good eyes. Although you are not directly touching the SMD components on the Tripmate, you will be soldering components that carry 1.5 GHz signals. Make a mess of it, and I guarantee you that it won't work. The first of these modifications I did, took me about 45 minutes to an hour to complete. Be patient and take your time...

preparing the construction

You need a "regular" screwdriver and a small philips screwdriver for this part.
1. Open the Tripmate (two screws) and disconnect the battery cable
2. Remove the metal plate that is fixed using the 3 small screws.
ATTENTION! This *will* void your warranty, so if you mess things up, Lelorme won't fix the thing for free!
3. Now the real work starts. Take the complete GPS module out of the plastic box, and put away the plastic box so it won't get damaged. If you can't do this (e.g., because you already installed cabling, etc. to your builtin TinyTrak), continue, but just be careful what you do.
removing the existing antenna You need a 15-25 Watts soldering iron with pencil-style tip for this, preferably with a wet sponge on the side to keep this tip clear. Keeping the tip clean is paramount to your effort here!. You also need a rasor-knive to cut through some doublesided tape. A solder-pump (to remove solder) or similar desoldering tools can be handy and will save time. I also used
some "Goof-Off" (you can probably use any solvent) to clean the metal box after removing the antenna. A small, flat, screwdriver can be handy to "pry" between the PCB and the box.
1. Study the object. First turn the box around, and look at the antenna-side. The antenna consists of a square piece of circuit board that is fed using a piece of RG-178. The center connector of the coax is connected through a hole in this PCB-antenna, the shielding is soldered on the part that you cannot see, between the metal enclosure of the electronics
and the antenna. Then, the antenna is connected to the metal box using doublesided tape. You cannot (yet) see this, but the other side of the coax cable is connected directly to the bottom-side of the PCB. We will get back to that. Turn the box around again so you see the exposed part of the PCB, with all the integrated circuits. Note that the PCB has been soldered to the box on 4 or 5 point all around. Note also, that at these soldering points, there are two little nodges in the metal that "clamp in" the PCB.
2. Start by cutting through the doublesided tape that holds the antenna it its place.
3. Then, remove the antenna by desoldering the coax cable where it connects to the antenna. The antenna is no longer needed, but you could keep it for any future GPS projects.
4. Turn to the component side. Remove the soldering where the PCB is attached to the boxes at all these points. The way I did it was by roughly removing the solder from all points, then heat up one point and put a small screwdriver or rasor knive between the box and the PCB. Be careful not to damage anything. When the solder cools down again (and the PCB is disconnected from the box at this point), move to the next point on the same
side. After doing this for all point, you can remove the PCB from the metal box.
5. Turn over the PCB to the side where the coax is connected. Study this connection carefully. CAREFULLY desolder the coax cable from the PCB. Don't touch or damage any components with your fingers, the iron or any other object.
installation of the active antenna 1. Take your active antenna. Cut off the SMA connector (or any other connector that is attached to it. Carefully pass the coax through the rubber ring, where your serial cable also comes in. Then, pass the coax (from the outside to the inside) through the hole in the metal box that was used for the original coax. Give yourself some working space here, you can pull back the coax later. Just be careful: when I did this the first time, I broke the rubber loop where the coax/serial cable come into the Tripmate yellow plastic box.
2. Clean the end of the coax cable. From the outside to the inside, you want to see: 1 mm center conductor, 1/2 mm center insulator, 1 mm shield conductor, and from there the rest of the coax. For the Americans among you, take 1/16" for 1 mm, that will do (it's not that critical). Then, carefully solder the center and shield conductors. Use the coax cable of the original antenna as an example on how to cut this.
3. Where the center connector was originally attached to the PCB, connect one side of the 1 nF capacitor. KEEP THE LEADS AS SHORT AS YOU CAN, and work clean. This is important!
4. On the other side of the capacitor, attach the center lead of the coax and one side of the 100 uH coil. Again, KEEP THE LEADS AS SHORT AS YOU CAN, and work clean. This is important!
5. I put some electrical tape around all of this. Just make sure you stay away from the coax shield connector.
6. Solder the coax shield connector to the PCB, right where the original connector was.
7. Connect the other side of the coil to the 5+ Volt, using a small piece of extra cable. This cable is not critical as there is no HF going through it. You can find the +5 Volt on the bottom of the small modular connector that is used to power the Tripmate.
8. Put power on the Tripmate, and test it. Connect the Tripmate to your computer and look on the Serial port. Make sure your antenna looks out on the sky (putting it in your window sill should be enough). You can use a serial program at 4800 bps. If you didn't do the "autostart" modification (see http://wes.johnston.net/aprs/tripmate.htm), send the word "ASTRAL" to the Tripmate as soon as it starts sending this word itself. When you see strings appear that start with $GPxxx, you are halfway. Now watch the $GPRMC string. The 3rd field can contain an "A" or a "V". When your antenna works well, it will eventually show an "A". If you see a "V", don't despair! It can take up to 20-30 minutes for your receiver to lock, depending on how long ago you used the Tripmate for the last time, where you were when it locked on the last time, etc. As soon as you see the "A"s roll in, you're in business! If not, use your multimeter to figure out if you're feeding +5 Volts onto the coax center lead, and check that you DON'T see this +5 Volts
on the PCB-side of the capacitor that you installed. Check for short circuits. Make sure you didn't leave any solder clutter on the PCB, or any strands of the coax outer conductor. If that all didn't work... well...eh.... then PANIC! :-)
re-assembly the TripMate 1. Re-install the PCB in the metal box, and solder it fixed.
2. Carefully pull back the coax cable through the rubber ring so you can install the box in its original position. Put the metal box on top of the 3 plastic nodges, and re-install the metal plate on top.
3. Reconnect the battery connector (if applicable), re-set the rubber gasket, close the yellow box with the 2 screws, and you are all set!
Afterword from the author
If you used the antenna I described on top, this one is magnetic. Just dump it on the roof of your car, and it will stay there. I tested it up to speeds well over the speed limit :-). If you want to see it working, take a look at http://map.findu.com/pa3eug-9.
(If you see me parked at a lake... that's my QRL. Unfortunately, I'm not out fishing ;-((( ).

Good luck, if you have any questions, feel free to drop me a line!

--Ramon W1/PA3EUG, Boston, MA [FN42KI]
Comment from DK7XE
I received this modification info via the TinyTrack mailinglist. This modification is not tested by me so please send any remarks/comments directly to Ramon legalalien30@yahoo.com!!

Owner: DK7XE

external antenna for Tripmate GPS - DK7XE/OE6DLD

Update: 15.05.2006