Making APRS Trackers

My good and bad experiences

Ralph Fowler N4NEQ

I doubt that there is anyone on this SIG more mobile than I am! Yes, Peter drives all over the place, but always has his setup in his truck. He doesn't reinstall it twice a week.

My tracker is moved from my personal van, to carry-on airplane luggage, to rental car, and back about once a week. It even got left on a mtn top in California as a test digipeater for a weekend (and a permanent one is going up there next week, I hope). It has flown to the Cuban border in a Cessna 337 with the Brothers To The Rescue and has also been a part of numerous special events. Components of it were even used during the 1996 Olympics.

I wanted to share a few lessons I have learned.

Radios I have used

Kenwood TH-something AT (the shirt pocket wimpster). Poor choice power too low. Weird 7v power requirement.

GE Century II mobile. Worked pretty well, but poor RX sensitivity. Rockbound. Not useful when traveling.

Icom 2AT- Worked pretty well. Gave about 5 watts on 12 volts, power still too low for reliable tracking in and out of city.

Azden PCS-4000- Old technology. LED readout a power hog. Ran van battery down all the time.

Kenwood 241- What a dog! Besides the display problem that plagues EVERY 241, and the replacement control board needed at age 3 months, it refused to transmit data (sounded more like an obscene noise) if the voltage wasn't right up at 14 volts. I sometimes got down to 10 volts on TX with a weak battery. Interestingly enough, It did fine on voice. Several folks here are familiar with my problems with that rig!

Yaesu Vertex 2011- This is my current rig. It is small, light, delivers about 30 watts and runs great. Have you seen the Mil spec Yaesu 2400? This is the commercial equivalent. It has a plain front without display, is PC programmable, and mine has 4 channels. I have it programmed for 145.79 145.01 144.39 and 146.655 (my voice repeater for use with Mic-E).

GPS units

I started off (like most others ) with the Garmin 45. It was inadequate. When I added a Motorola active antenna, it became better at receive, but still did not perform well in the service I subjected it to. It would never recover satellite lock after being left on while out of sky view overnight. Of course the power button was always a problem as well. Now that one is my spare and used for fox hunting, hiking, etc.

Next, I got one of the 6 channel Motorola Oncore units. This was a nice GPS, but rather fickle about getting into a strange mode and losing satellites. Then it was back to the GPS-90 program for a test and reset. Before Bob made the HSP mod to the MOTOROLA.BAS program, It was a real pain to disconnect the GPS from the HSP adapter and wire it direct to the PC to change programming. I had a couple that just died. I have no idea what happened- they just lost satellites and no amount of programming or resetting would help. I'll bet they got the bugs out with the latest generation of them. As far as receiving goes- the 6 channel parallel tracking receiver beat the old single channel scanning Garmin by a country mile!

Now I use a couple of Garmin 35 units. They look like a computer mouse and have 12 parallel tracking channels. They always lock and stay locked. I have the PC version . This great if you want to plug it into a PC and use it with Street Atlas. Or if you have a 2nd port on your APRS machine. Otherwise, the separate 9 pin plug and cigarette lighter power plug make it cumbersome to deal with. The PC version has not got a DGPS input either. Next time, I will buy the pigtail-ended GPS 35. The power/data cable going to the GPS gets very brittle and hard to handle in sub 35 degree weather.

Note that the 35 is the 12 channel parallel tracking unit. The guts are the same as the GPS 25 and as the 12XL and 2 Plus. The 3 Plus probably has this engine too. The 30 is the same engine as the 20 and the 45 and 38. It has one channel.

GPS antennas

I've used two: The Motorola and the Trimble/Lowe unit. (I think they are the same)

The Motorola is built like a tank. The Lowe/Trimble is flimsy (especially the cable). If you use the Motorola, folks will be pointing and trying to tell you that you left your gas cap on the car roof or trunk. If you use the Trimble/Lowe, you risk chopping the cable in half easily.

But for the relative size and unobtrusiveness, I guess I could stand it.

Try to mount it in the rear deck of your car if you can. I've never noticed any degradation of signal.

HSP Adapter

I've used two, the Paccomm and the AEA.

The Paccomm one was an early unit. It was looking for circuit power on pin 6 of the laptop. My Compaq 386-25 LTE did not power this pin, so I rewired it to steal power from the TNC. Not Paccomm's units get power from either the TNC or the Computer. It had all 9 pin connectors, so I had to change the TNC plug. I was using a KPC-3.

The AEA unit looked like a gender changer of the 25 pin variety. It had a small wire coming off the side with a mini phone plug on it for the GPS. I think it was designed for that old Icom GPS. I can't remember where it got power, but I rewired it to get power from pin 25 of the KPC-3. There is a KPC-3 mod to supply power to or from this pin. I had to call AEA and their tech support folks had no idea what an HSP adapter was or what it did. I wired a switch in mine (per Bob's diagram) to jump the GPS data out over to the TNC data in for stand alone tracker use. I was going to put a relay that would sense the unplugging of the laptop, but gave up when I couldn't ever remember to type INTFACE GPS and RESET each time I got out of the car.

The HSP in the Pico Packet is by far the best option.


I've used three: The MFJ, the KPC-3 and the Pico-Packet (dual port).

The MFJ without GPS support was scary to use. The TNC was left in CONV mode and the GPS set to output sentences every 180 seconds. I was always afraid something would reset the GPS and I would stream posits out every second. I think this is the way Bob got started and he may have still done it this way during the Football Run of '97 MFJ may offer GPS support now, but Paccomm has an EPROM that will work in it too.

The KPC-3 made a nice tracker. It also makes a nice TNC for use with a laptop. Too bad it is so cumbersome to switch between both uses. It has 4 possible GPS beacon paths and can use different GPS sentences with them. I utilized them in several Helicopters and Motorcycles during the 1996 Olympics for APRS tracking of events. There was one on top of the Olympic Stadium in a Tupperware box used as a digi.

The TNC I settled on is the Dual-Port Pico Packet. This one of a kind device allows out of the way, set and forget operation in a tracker OR mobile APRS setup The GPS goes in one port and the Laptop goes in the other. When the APRS program is not running, the GPS feeds the tracker and the beacons go out to keep you on the map. When the TNC detects a running APRS program, the automatic beacons stop and the program handles all TNC activity. The Pico has been used in numerous balloon tracking packages. Early units had a reset problem, but that has been cured and a remote programming operation is available too. My Pico has the callsign-substituting EPROM used in digis. It is great to be able to see whether my tracker unit is helping other users make APRS contacts.


I used to use a military surplus ammo box. I mounted BNC feed thru connectors to bring out the GPS and radio antennas. Boy did that ammo box bring on the stares when I carried it on to the airplanes! I never had it searched, though- even at the X-Ray checkpoint.

Now I use a small carrying case that originally held a portable Telxon data terminal. It is a bit larger than a Weller soldering gun case. I have the feedthrus on it for the antennas and just stuff the GPS and all the cables inside when I pack up. It works great and looks unobtrusive when carrying on to the plane.

There you have it: a lot of musings detailing my good and bad experiences making trackers.

I hope it provides a bit of information to those contemplating building their own.

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This page was written 12/8/1997 by Ralph Fowler N4NEQ
It was formatted into HTML by Steve Dimse K4HG