Fall 2006 Issue

Building an APRS Tracker
Part 2 – The “Powerhouse” Tracker

In the Summer 2006 issue of CQ VHF, in part one of this article, W3DOE described how
he built a simple, what he calls “nimble,” Automatic Position Reporting System tracker.
This time he tells us about the first tracker he built, a high-power unit that turned
into a challenging construction project.

By Carlton Doe, W3DOE

Photo 1. The author chose a toolbox that can be purchased from most any hardware store as the enclosure for the “powerhouse” APRS tracker. (Photos and figures by the author)

Welcome to the second and final part of this series on building APRS trackers. In the first part (Summer 2006 CQ VHF, p. 15), I introduced the Automatic Position Reporting System (APRS), described what a tracker is, related the design criteria to be evaluated, and then detailed the construction of a low-power, portable APRS tracker—what I call a “nimble” tracker. This time I will cover the first tracker I built, a high-power unit that turned into an over-the-top experience during construction.

Recap of Part 1

If you missed part one, let me briefly recap. APRS was developed by Bob Bruninga, WB4APR, as an enhancement to regular packet-mode transmission oriented specifically for publishing location-based information via RF. There are two parts to an APRS system—one part transmits where the tracking object is (the job of a tracker), and the second part is the display software, such as UI-View (http://www.ui-view.org), which receives APRS transmissions and plots the position information on maps. An APRS tracker has four basic components:
• A Global Positioning Satellite (GPS) receiver to generate the location-based information.

• A Terminal Node Controller (TNC) to interface with the GPS receiver and translate its data stream into a signal that can be transmitted by a radio. The TNC also connects to and controls the radio, determining how often or under what conditions the tracker will “beacon” or send out its location information.

• A 2-meter radio able to transmit at 144.390 MHz, the APRS frequency for almost all of the United States.
• An enclosure.

Since a tracker can be built into almost any type of enclosure, building one requires balancing a number of factors such as radio power, battery capacity, size, weight, and so on. The relative importance of one factor versus another will determine the size, capability, power, and portability of the tracker. There isn’t one set of “correct” answers to any of these issues. You as the builder have to decide what you want to do with the tracker and that will guide the rest of your decisions. As I mentioned at the end of part one, the most important components of a project such as this are the design goals and criteria. With these in place, you can build to your own specifications and enjoy a high chance of success.

As I started to build this, my first tracker, I didn’t have a clue what I was doing nor did I have anything in the way of a design specification other than I wanted to use a 50-watt radio. I thought I needed the RF power here in the Dallas, Texas area to get my signal out. It turns out I was wrong, but at the time I didn’t know about APRS digi-peaters.

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