Mobile APRS rig

What I have here is my mobile APRS rig. I originally put this together sometime around October 2011, but didn't fully refine it until late December 2011.

For those unfamiliar, APRS, or Automatic Position Reporting System, typically combines a GPS receiver and an amateur radio of some kind, with either a standalone tracker, computer, or other device in between the two which takes telemetry data from the GPS and transmits it over the radio using the typical AX.25 packet radio signalling, but in a more or less unidirectional mode. Each packet will usually contain a callsign and optional ssid, a short text message, and your position (or weather telemetry, but I won't go into that here).

Receiving stations can pick up these packets and display the position of every station that has been heard on a map, and also track stations that are moving by interpolating their path over multiple position reports. At the same time, receiving stations called I-Gates can "gate" your packets to the internet, or APRS-IS, enabling your position to be tracked over the internet. Other receiving stations called digipeaters can repeat your packets according to certain algorithms involving hop counts and station type, enabling your position to escape otherwise dead zones over long distances or high mountains before finally reaching an I-Gate, making the system more reliable than cellular signals in some respects.

An APRS station that only transmits is called a tracker, and is most often used in personal or mobile configurations. What I have in my mobile tracker is an old RadioShack HTX-202, a Byonics TT-3+, and an old DeLorme Tripmate serial GPS receiver (not shown).

The package is housed in a watertight storage box with feed through connectors for Anderson PowerPole +12VDC input, BNC for 2M RF, and DB-9 for serial and power out to the GPS.

I've also included a low-pass filter to deal with alternator noise that I was having issues with, which consists of about 20-30 turns of wire wound on a closed stainless steal quick link and a sizeable electrolytic capacitor.

Internal view showing low-pass filter and feed through connectors

The TT-3+ is configured using a null-modem serial cable and the included software, and is configured to pass through +12V to the GPS. The GPS itself has been modified internally with a simple LM7805 regulator and some caps to take the +12V from the tracker unit and regulate it down to +5VDC in order to replace the stock battery pack. The receiver was then spray painted flat black (originally they're bright yellow), and fitted with a hard drive magnet so it'll stick to the car roof.

BNC bulkhead connector and DB-9 male

This rig is for 2M, transmitting on 144.390 MHz, the standard APRS operating frequency. For antenna I'm using an MFJ-1721 1/4 wave mag mount, which is about as inconspicuous as you can get on 2M.

Anderson PowerPole panel mount adapter plates.

Power is supplied through Anderson PowerPoles and a standard cigarette lighter outlet (for now). At this point I'm rather undecided regarding PowerPoles as a legitimate power connector. They're rather pricey, can be difficult and expensive to crimp and assemble right, feel cheap, and can sometimes fail if the contacts are bent even the slightest. I'm very much considering switching over/back to the venerable "T" connector. <rant>Just look at the prices of some of the "Rig Runner" distribution blocks for the PowerPole. What a load of overpriced junk! That's not to say that the larger size powerpoles as used in large DC power systems like fork lifts and UPSes, but the low current sizes are really only good on paper.</rant>

I highly recommend Byonic's products, particularly in kit form. The instructions for their kits are reminiscent of the old Heathkit or Ramsey Electronics kits, and are rather pleasant to follow for even a seasoned electronics enthusiast like myself.

I'm now gathering components to make a self contained personal tracker based around an Icom micro-2AT. More on that when I start the build.

Anyway, you can track me via aprs.fi/KB3EWY-12 or any of the other online APRS systems using that callsign and ssid.