A Lego Enclosure for an APRS Tracker with a Built-In Antenna

My APRS tracker seems to work quite well, but it was difficult to use with various pieces of equipment sloshing about on the dashboard. Initially, I’ve been using the short vertical antenna that came with the UV-3R radio. Both the radio/antenna and the GPS antenna were placed on the dashboard. This antenna did not work well inside the car, especially when it was lying horizontally with the radio on the dashboard. Then I switched to the small PCB magnetic loop antenna I built a while ago. This worked much better, especially when the antenna was standing up. But now I had three pieces of equipment (the radio, the loop antenna, and the GPS antenna) sloshing on the dashboard. This was only good enough for a bit of testing.

I was considering how to mount the antenna and the radio more securely on the dashboard. I didn’t feel like spending a lot of effort on building an enclosure. Then the idea of building the enclosure, or at least a prototype, from Lego. The box you see in the picture above is the result. It actually works very well. It’s a bit heavy, so it doesn’t move at all on the dashboard. It contains the antenna, the radio, and the GPS antenna. There are holes in the box for the audio cable to the microcontroller, for the GPS antenna cable, and for the display and buttons of the radio. Being able to turn the radio on and off is important, since it runs on its internal battery. The hole on the side is also large enough for the radio’s charging cable. The antenna and radio slide into specially-built compartments that hold them securely. It’s also easy to slide them out. Both antennas work well inside the plastic box on the dashboard (Lego bricks are usually made of ABS plastic). The box has enough space for a small microcontroller board and for a GPS module, but my current microcontroller board is a large development board, so for now it and the GPS module still lives in a cardboard box that I stick in the glove compartment.

As you can see in the snapshot from aprs.fi below, this Lego tracker actually works, even though it’s inside the car and even tough the UV-3R puts out only 2 Watts. Still, I plan to deploy a 1/4-wave vertical antenna on the car’s roof for even better coverage, and I’ll probably replace the Lego with some other box, mostly to avoid having the Lego bricks ruined by exposure to sunlight. But for now, I’m using the Lego tracker. It’s also very good to know that the 10cm magnetic loop antenna works well in APRS trackers; such antennas can be used in small trackers that are completely self contained.

Two more comments are in order. First, driving with a heavy tracker on the dashboard is possibly unsafe, since in an accident it might hit the driver or a passenger. I’m mostly using it around the neighborhood, but once I switch to the vertical antenna on the roof, I’ll move the whole thing off the dashboard. The other thing I wanted to mention is that I performed the two almost-necessary modifications of the UV-3R radio. One adds a capacitor to the VHF low-pass filter, to attenuate the second harmonic, and another adds a decoupling capacitor to the external transmit-receive control signal; without it, the radio often locks in transmit mode when used with a headset or an external modem.

2 Responses to A Lego Enclosure for an APRS Tracker with a Built-In Antenna

  1. va3paw says:

    Very cool use of Lego for an enclosure!
    Could you please add a bit more details about modifications to the UV-3R radio? May be as a separate post? I ran into the same accidental transmit-lock issue with my Yaesu FT-60 when interfacing it to a PC’s soundcard. I’m guessing that a decoupling capacitor in series with the audio is probably all I need?
    Thank you.

    • Sivan Toledo says:

      Both mods are described in detail in the files section of the Yahoo UV-3R group, http://groups.yahoo.com/group/UV-3R and in various other places on the net. People found different ways to improve the VHF low-pass filter. I think I implemented the version from http://wa5znu.org/2011/06/uv3r-lpf/, but I don’t remember for sure.

      As for the PTT mod, there only one mod, and it is to add a capacitor of about 100pF from the PTT line on the speaker-mic connector to ground. The value is not very critical; I didn’t have 100pF so I used 220pF and it still works great. Again, the yahoo group has documents with pictures showing where to add the capacitor.Before the modification, the radio would get stuck in transmit mod when used with the headset or with the tracker. Three turns of the cable on a snap-on ferrite bead cured the problem, but the ferrite bead was annoying.

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