APRS Standalone Unit

VA2XJM Standalone APRS Unit

Its been a while since I had/took the time to write over here. But with the COVID-19 forced break, I decided to get back here and show some projects I’ve been working on.

So… Here is a standalone APRS unit I built for fast and easy field deployment. Please be kind, There are many steps where I do not have pictures because the project slept a bit and then I needed it to work for a demo. Eventually I plan to make some changes and I will take pictures and update the article.

Take the time to read the full text twice if you are looking to achieve something similar, some information needed before the assembly are located by the end of the article.

Objectives

What I wanted:

  • Be able to transport the whole kit easily.
  • Be able to view and send APRS positions.
  • Be able to send and receive APRS messages.
  • Be able to connect to AREDN mesh network when available.
  • Have a small power reserve for transitions.
  • Be able to repair with only my Leatherman multi-tool.

Equipments

For this built, I used the following components:

  • Nanuk 925 cases (Nanuk are simply the bests!)
  • Raspberry Pi 3b+ (Pre-configured)
  • 12v powered screen (from amazon)
  • Wireless keyboard/touchpad
  • USB GPS Antenna
  • TNC-X Pi-TNC (Now sold by MFJ)
  • Alinco DR-135T (Because of the DB9 connector)
  • PL-239 to SMA pigtail (Check for long SMA bulkhead)
  • WMR PWR Gate
  • A battery (I used AGM)
  • Anderson PowerPole connectors (30 & 45 Amps)
  • DC-DC Converter 12v to 5v
  • 12awg cable
  • Digital voltage meter
  • On/Off switch
  • Push button switch
  • OTC fuse holder and fuses
  • DB9 connectors
  • Some Velcro strips with glued back
  • Many small component that you will pick according to your needs…
  • Some 3D printed components, see below…

If you do not own a 3D printer, there is multiple services on Internet that will rent you one on the cheap.

The case

For few years now I buy Nanuk cases for 3 reasons:

  1. The cases are made in Quebec.
  2. The latching system is just the best on the market
  3. To have abused some at work, I trust them more than the other well-known brand when it comes to harsh environments.

So I went with the 925 for its perfect size, not too big, not too small. I designed modular panels that will fit inside the case using the mounting holes to have a way to customize them to my needs. You can find the files to do your own right here: https://www.thingiverse.com/thing:4280661. Don’t forget to make the modifications (read here holes) for the switch, push buttons, meters and other things you might want to mount.

So you will need to print the 4 top sections, 4 bottom section and 2 joiner. I also installed a «T» inside the joiners to hold the center of the panel as it is used as work surface. To fix the bottom panels, I used Velcro strips so the panels can be removed later. The use of those panels allows to fix the components without drilling a hole in the case.

Depending on what kind of connections you will want for antenna, power and data, you may need panel mountable connectors. For power, I use Anderson PowerPole, so I 3D printed a panel mount found on Thingiverse.

For the antenne I went with a SMA connector (find the extra long ones, the case is thick). Even if the Raspberry Pi 3B+ have onboard Wi-Fi, I also installed an Ethernet jack on the side of the case to connect to AREDN mesh when no Wi-Fi access point is available. More on AREDN eventually.

To mount the Raspberry Pi and the TNC, I use a VESA mounted case found on Thingiverse for the Raspberry Pi and I also remixed it to fit the TNC into a second second. Then I used screws to hold both case one above the other. The 2 wires on the left of the ribbon cable as been separated and wired to a push button. A script on the Raspberry Pi monitor the GPIO pins and shutdown the Raspberry Pi if the button is pressed for more than 5seconds.

Raspberry Pi 3B+ on top of its TNC

Prepare everything

So now you need to get everything ready. First, I installed the biggest components as you can see below. Everything is fixed to the bottom panels. This allowed me to drill holes for power, antenna and network bulkhead. Before installing the PWRGate remember to make proper preps (jumper for AGM battery, set fuses correctly for the charge speed you wish to achieve).

Sadly, I have no pictures of the wiring, but I will try to descibe it as best as I can. The space beside the radio has been reserved for the 15Ah AGM battery. I’ve set the battery terminals toward the back/bottom of the case to ensure there is no short circuit if the battery move. The battery is kept in place by the 3D printed panel that squeeze it and I added zip ties with zip ties mounts sticked to the side of the case, just-in-case.

The space remaining on the right is used for cable management. I 3D printed a 5 ports PowerPole breakout box for power distribution. 3 ports are used at the moment for the radio, the screen and the Raspberry Pi’s DC converter. For the radio, the original fuses has been used. For the screen and the Raspberry Pi, automotive type fuses and in-line fuse holders has been used.

The PWRGate input is directly connected to PowerPole bulkhead that will be installed next. The PWRGate battery input is connected to the battery terminals with a 40Amps fuse at the battery positive. The PWRGate output goes to the volt meter circuit and the breakout box.

The volt meter circuit consist of the digital volt meter in series with a push button so it is not always ON.

Just before the breakout box, a 50Amps switch is installed on the positive side as an electrical main. Because the volt meter circuit is connected on the PWRGate output, the volt meter is working even if the main is OFF.

Radio, Raspberry Pi 3B+ and the TNC below, WMR PwrGate

Then I drilled two holes side-by-side that I squared using a coarse file to be able to fit the 3D printed Anderson PowerPole bulkhead. I added silicone to the bulkhead flange and at the back of the connector where the cable is located before inserting it so it is as sealed as it can be.

After that I drilled and installed the Ethernet and SMA bulkhead. Both Ethernet and SMA bulk head are waterproof if covers are ON.

Panel installation

So I made the panels with holes already where I wanted them for the components I used. If you are not willing to redesign the panels, you can simply drill the holes. So to start, I bolted the joiners to the sections using carriage bolts and butterfly nuts (#14 IIRC). This way I now have only 2 panels to install (Back and Front).

Place the back panel and insert components (switch, push buttons, volt meter, cables for the screen, USB GPS…). Once you are satisfied, screw everything in place.

Unless you have something special for the front panel, make final check and test, then screw it.

Almost done!

So the worst part is done. Now, the screen. I ordered the largest touchscreen I was able to find on Amazon that requires a 12Volts power input, have an HDMI input and fits into the case cover. Since I built it, the screen I have is not available anymore, but you will be able to find many models to fit your needs. Touchscreen is not mandatory so you may cut the bill.

Another option, could be to use a tablet with a VNC client to connect to the Raspberry Pi. It all depends on you, but I wanted a screen integrated into the kit, so if I lend the kit to another operator, everything is inside.

I attached the screen using Velcro to the cover as shown below.

Accessories

To provide proper location and time to the Raspberry Pi, use an external USB GPS antenna. The longer the cable, the better. The antenna can be installed at a better position if needed depending of where you operate.

For user input, I found out the nice logitech wireless keyboard/track pad combo. It fits the case perfectly and works like a charm.

I also keep inside the case some cables and spare parts.

I keep a SMA-to-SO-239 adaptor for antenna connection, a 12V cigarette lighter plug to Anderson PowerPole and a 8 feet long PowerPole to PowerPole extension.

The spare parts are AA lithium batteries, fuses, PowerPole connectors and the wrench that came with the radio to remove the radio from its bracket is needed.

The Raspberry Pi

Prior to even built this kit, the Raspberry Pi was already configured for APRS. Raspbian is the host I prefer. For APRS operation I use Xastir as it is quite easy to work with and reconfigure on the fly when needed. It is also possible to cache maps and use them while offline.

Xastir can also be used as a digipeater and a igate. It is possible to connect to multiple APRS-IS servers at the same time, so it can be used without a radio just over Internet or network. This feature also get interesting as we have our own APRS-IS server inside the club AREDN mesh. If the Raspberry Pi is connected to the mesh, via Ethernet or Wi-Fio Access Point, it connects to the APRS-IS server of the club that relay the packets to other APRS devices on the mesh or over Internet if available.

You may also consider to install a push button to shutdown the Raspberry Pi. To achieve so, consider one of the available tutorial on Internet using GPIOZero. Just remember to select some pins that are not used by the TNC. For those of you saying you have a keyboard to shut it down, I once got stuck with dead batteries… The button has been helpful.

Conclusion

This is the first time I take the time to explain one of my project like this. I hope you are not lost. I can only promise you that I will do better next time, especially for the step-by-step pictures.

This project is not done yet. It is fully operational but I have some idea here to improve it eventually:

  • Install a fan to cool down the inside with some sort of wind tunnel to direct the airflow where needed.
  • Find a 1/4 wave antenna that fit in the case and design a bracket to mount it on the lid.
  • Consider to change the AGM battery and PWRGate. LifePO4 is lighter and offers more capacity.

73 de VA2XJM

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