Category: Machine Shop

APRS coverage of this part of the Mighty Wappinger Creek Valley isn’t very good, particularly for our bicycle radios (low power, crappy antennas, lousy positions), so I finally got around to setting up a receive-only APRS iGate in the attic.

The whole setup had that lashed-together look:

It’s sitting on the bottom attic stair, at the lower end of a 10 °F/ft gradient, where the Pi 3’s onboard WiFi connects to the router in the basement without any trouble at all.

After about a week of having it work just fine, I printed a case from Thingiverse:

Minus the case, however, you can see a TNC-Pi2 kit atop a Raspberry Pi 3, running APRX on a full-up Raspbian Jessie installation:

You must solder the TNC-Pi2 a millimeter or two above the feedthrough header to keep the component leads off the USB jacks. The kit includes a single, slightly too short, aluminum standoff that would be perfectly adequate, but I’m that guy: those are four 18 mm lengths of heatshrink tubing to stabilize the TNC, with the obligatory decorative Kapton tape.

The only misadventure during kit assembly came from a somewhat misshapen 100 nF ceramic cap:

Oddly, it measured pretty close to the others in the kit package. I swapped in a 100 nF ceramic cap from my heap and continued the mission.

The threaded brass inserts stand in for tiny 4-40 nuts that I don’t have. The case has standoffs with small holes; I drilled-and-tapped 4-40 threads and it’ll be all good.

The radio, a craptastic Baofeng UV-5R, has a SMA-RP to UHF adapter screwed to the cable from a mobile 2 meter antenna on a random slab of sheet metal on the attic floor. It has Kenwood jack spacing, but, rather than conjure a custom plug, I got a clue and bought a pair of craptastic Baofeng speaker-mics for seven bucks delivered:

For reference, the connections:

Unsoldering the speaker-mic head and replacing it with a DE-9 connector didn’t take long.

The radio sits in the charging cradle, which probably isn’t a good idea for the long term. The available Baofeng “battery eliminators” appear to be even more dangerously craptastic than the radios and speaker-mics; I should just gut the cheapest one and use the shell with a better power supply.

I initially installed Xastir on the Pi, but it’s really too heavyweight for a simple receive-only iGate. APRX omits the fancy map displays and runs perfectly well in a headless installation with a trivial setup configuration.

There are many descriptions of the fiddling required to convert the Pi 3’s serial port device names back to the Pi / Pi 2 “standard”. I did some of that, but in point of fact none’s required for the TNC-Pi2; use the device name /dev/serial0 and it’s all good:

<interface>
serial-device /dev/serial0 19200 8n1 KISS
callsign $mycall # callsign defaults to $mycall
tx-ok false # transmitter enable defaults to false
telem-to-is false # set to 'false' to disable
</interface>

Because the radio looks out over an RF desert, digipeating won’t be productive and I’ve disabled the PTT. All the received packets go to the Great APRS Database in the Cloud:

server   noam.aprs2.net

An APRS reception heat map for the last few days in August:

The hot red square to the upper left reveals a peephole through the valley walls toward Mary’s Vassar Farms garden plot, where her bike spends a few hours every day. The other hotspots show where roads overlap the creek valley; the skinny purple region between the red endcaps covers the vacant land around the Dutchess County Airport. The scattered purple blocks come from those weird propagation effects that Just Happen; one of the local APRS gurus suggests reflections from airplane traffic far overhead.

An RPi 3 is way too much computer for an iGate: all four cores run at 0.00 load all day long. On the other paw, it’s $35 and It Just Works.