Mobile APRS Digipeater ("MAD")
A self-contained portable APRS digipeater
APRS is the Automated Packet Reporting System. It was created by Bob Bruninga, WB4APR, and subsequently grew to global proportions. APRS can be used to transmit and receive virtually any type of data, such as GPS position reports, telemetry data, weather data, and text messages. What makes APRS different from using a mobile phone is that APRS uses amateur (ham) frequencies instead of cellular or Internet services. A large network of APRS repeaters (aka digipeaters) operate similarly to the ham voice repeater network, with the important distinction that there is only a single frequency (no split). (Note that the use of APRS requires an FCC amateur radio license).
Currently, the Yavapai Amateur Radio Club(YARC) provides event communications for over a dozen public events each year in Yavapai County. A large contingent of YARC members staff multiple stations along the course providing race status information, transportation requests for participants who are unable to continue, and safety-related information such as vehicles trespassing on the closed course or participants in need of medical attention. Many of these events are quite extensive, covering a large geographic area and encompassing well over one thousand participants. These larger events, in particular, are outside of urban centers and cover such a widespread area that amateur radio is the only consistent means of communication for the event. A key component to providing event communications is managing the course vehicles, which are typically provided by the event organizers and staffed with a YARC member as a passenger to provide communications. These vehicles handle logistics such as participant pickup, supplies and equipment (SAG), and “sweeping” the event to track the end of the race and ensure that all participants get back safely. This is a very dynamic situation with assignments frequently changing with the flow of the event. As such, keeping track of the current location of these vehicles is critical to ensure the right vehicle is assigned to a given task.
Our club is using mobile APRS transmitters in these vehicles to track their location. APRS packets are received over a VHF frequency and decoded. The decoded GPS information is used to plot the position on a map. Internet services, such as https://aprs.fi can be used for tracking, but in our situation the Internet isn't available, so we use UI-View software instead.
While the aforementioned network of digipeaters vastly increases the coverage area for APRS, Yavapai County terrain can vary significantly, resulting in coverage gaps especially on the trails used for mountain bike and marathon events. The portable APRS digipeater was designed and built to address this problem. We use terrain mapping software (ubiquiti or radio mobile) to determine the best location to cover the event area and we preposition the MAD before the start of the event. Properly placed, the MAD will relay the packets from our tracking units to one of the 3 area mountaintop APRS digipeaters, enabling position reports to be received in our communications van. To simplify deployment, everything is pre-programmed so all one needs to do is to attach an antenna and power it on.
We also plan to use this digipeater within our local Yavapai County ARES/RACES emergency response group.
A Mobile APRS Digipeater (MAD)
The MAD consists of a 12V power supply, APRS transceiver, an LCD display, a voltage regulator for the HT battery backpack, an 8W handitalkie (HT), and an SMA-F antenna mount. I used the SMA-F mount to match our APRS mobile transceivers and enable antenna interchangeability (we use 1/4 wave and magmount antennas). It is encased in a Pelican-like case with foam cutout spaces for the SLA battery, HT, and a 3D-printed control unit case.
The control unit housing contains the Byonics TinyTracker4 (TT4) transceiver, HT adapter cable, DC buck converter, and a 4-line LCD to display decoded APRS messages.
The MAD is housed in an Apache 2800 12" IP65 weatherproof case. There is a panel-mount PowerPole connection on the side that provides a direct connection to the 12V SLA battery, either for 12V out or to charge the battery. On the top is an SMA-F antenna mount.
I chose a Baofeng UV82HP HT for the radio. This HT is 8W, slightly higher than typical HTs. I chose this unit since we have extensive experience with Baofeng and a lot of accessories on hand. Also, Byonics (see below) makes a Baofeng-specific interface cable. I did not program the radio, other than to set the 144.390 MHz APRS frequency and change/confirm a few settings:
- 0: SQL 5
- 2: power HIGH
- 3: SAVE OFF
- 4: VOX OFF
- 5: Wideband
- 6: 5 sec backlight timeout
- 7: TDR OFF
- 29: standby color OFF
- 30: RX color blue
- 32: TX color orange (RX & TX colors are useful when remotely troubleshooting with inexperienced operators)
- 35: STE OFF
- 39: Roger beep OFF
We keep the HT keyboard locked when in use.
The control unit houses the Byonics TT4 APRS transceiver, Byonics HTK2P interface cable, the Byonics TT4 LCD display, and a DC-DC buck converter. A mount is provided for the TT4 PCB, with adequate space to attach the interface cable inside the enclosure. There is an opening for the DB9 serial connection required for programming. You will need either a DB9 serial with null modem or a USB to serial adapter, e.g. FTDI (null modem), to configure the TT4. Supposedly, a PS2 keyboard can be connected to the LCD adapter board and used to configure the TT4, but I did not verify that this approach works. There are also 2 panel-mount PowerPole connections: one on the left side for power in, and one on the right to power the HT. I used the Baofeng 12V battery adapter, but the voltage regulation electronics are in the 12V lighter socket assembly, and thus useless in this application. Instead, the 12V battery power is routed through a DC-DC buck converter to maintain the HT voltage at 7.8V. A Powerwerx PowerPole distribution hub is included to manage the power connections inside the enclosure.
The top has the power switch and the LCD display. The switch controls the power from the external PowerPole connection to the distribution hub; switching it on turns on the power for the TT4, HT, and display.
I chose to use the assembled TT4 and the LCD display kit. The rationale for this is that the assembled SMT board, unlike the through-hole (PTH) kit, has a header for the display unit. If you use the TT4 PCH kit, you'll need to disassemble and strip the ends of the ribbon cable leads and solder the wires to the underside of the board.
Printing the Enclosure
There are no special requirements for printing the enclosure. I used 0.4mm layers, with 2 solid bottom layers and 3 solid top layers (makes for better screw connections in the lid supports) on a Taz6. I used high temperature PLA to avoid any issues with high temperatures that might cause standard PLA to deform (yes, it does happen). You can either print the washer to secure the TT4 PCB to the case or just use a standard nylon or other non-conductive washer.
The battery is a standard 12V sealed lead acid (SLA) battery, with a 5A blade fuse on the positive lead. There are no special requirements for the battery, but I prefer the Keyko AGM gel batteries.
Helpful hints for assembly:
- The Bill of Materials (BOM) file in the repo has links to purchase the required components.
- The diameter of the PowerPole panel mount is 22mm.
- The external PowerPole connector goes directly to the battery, with a parallel connection going to the control unit (left side connector). This branch has a 5A inline blade fuse.
- The battery is mounted upside down to simplify the wiring and maintain a "clean" look.
- Note the spare fuse taped to the bottom of the battery.
- The control unit is a tight fit; it needs to fit inside the Apache case, and many small printers have a limit of 150mm. Therefore, to provide some flexibility in stuffing everything in, the PowerPole hub is not attached to the case.
- 4 #2 1/4" pan head screws for the LCD
- 2 #2 1/4" pan head screws for the buck converter
- 1 #2 1/4" pan head screw for the TT4
- 6 #2 3/8" flat head screws to secure the lid to the case
- When printing the PowerPole panel mounts, I used 0.1mm layers, with the first layer at 300%. The face (bottom) did not print well with the first layer at 0.1mm. However, the screw (and nut) threads need a very fine layer setting to move properly, so all but the first layer is printed at 0.1mm.
- When assembling the unit, manage your wire gauge and length carefully due to the tight fit. I used 14 AWG wire, which is overkill given the low power draw, but that's what I had on hand and what would fit the crimped PowerPole connectors. 16 AWG would probably be a better choice, with the smaller gauge PowerPole crimp connectors, of course.
- On the Baofeng HT 12V adapter, I cut the cord where it starts to coil and soldered the very thin wires to 2 14 AWG stubs that were crimped to the PowerPole connectors. Be sure your wire gauge is thick enough to get a solid crimp. (And use PowerPole crimpers, not pliers, please!).
- The enclosure design was done with a 0.5mm printer nozzle in mind. You could possibly have issues with the case print if your nozzle is significantly different.
- Be sure to use supports when printing the base, especially for the 2 rectangular openings.
- The battery goes through both foam inserts. Everything else rests on top of the first insert.
- I use a BatteryTender to charge the battery. I built a cable with the standard "split" connector on one side and PowerPole connectors on the other to allow the battery to be charged using the side connector without removing it.
- The TT4 config settings can be a bit touchy. You can use the TT4 settings file in this repo - just change "MYCALL" to your callsign and SSID (e.g. W7YRC-15 in our case). (This file is just a screen capture of the "display" command output.) Unfortunately, there is no way to upload a settings file to the TT4, so you'll need to manually enter each setting. Most of them are default settings, though, so you don't need to do them all!
- Be certain your BTEXT string begins with ">" and the TSTAT string begins with "/" or things will not work.
- TOSV must be false to broadcast the location without a GPS attached. Set this to true if you don't want to broadcast the digipeater's location.
- The TXLEVEL and RXAMP settings were tuned for my HT. If you use the same radio these will probably work, but I suggest you verify the calibration using the process explained in the TT4 Quick Start Guide.
- You may wish to adjust the settings that determine how often things are transmitted. If your local APRS net is busy, try not to overwhelm it with unnecessary broadcasts. Remember, in APRS every packet is broadcast multiple times, all on the same frequency.
The MAD was designed to be as simple as possible to operate. Prior to deploying the unit, we select the best location as previously described, and configure the GPS location (degrees and decimal minutes followed by the heading, e.g. DDMM.mmmmH). The ham operator who deploys the unit (and acts as the control operator) simply attaches the best available antenna and switches the unit on. The club can provide operators an SMA whip antenna or an SMA magmount antenna. In addition, we keep an SMA to SO-239 adapter cable between the inside of the lid and the top foam insert to allow use of an antenna with a UHF connector, e.g. a mast or other vertical.
To confirm the unit is operating properly, 2 things can be observed:
- The LCD display will display the last decoded packet. We ask operators to periodically check that this is being updated.
- The adapter cable silences the HT's speaker, but the HT LCD will turn blue while receiving and orange when transmitting (see settings, above).
I have tested the run time of the MAD up to 22 hours. The battery level drops below 12V, but both the TT4 and the UV82HP operate at much lower voltages, so it is safe to assume the unit can operate much longer than 22 hours.
Most APRS packets are set to be rebroadcast 2-3 times. With the config settings in the repo the MAD is configured as a fill-in digipeater, so it will "consume" one of these broadcast slots, including the WIDE1 slot. Since this includes your event APRS packets along with everyone else's, when placing the unit be sure it has a path to the next hop, typically a mountaintop digipeater. This is only one approach to operating - you can alter this behavior by setting the TT4 PATH and ALIAS variables appropriately. While explicit paths are an option, we chose not to take that approach because the MAD will not be in range of all APRS transmitters and we do not always have the ability to configure all of the APRS devices in an event. A very complete description of how APRS paths work can be found at http://wa8lmf.net/DigiPaths.
Copyright 2018 Rob Redford. This work is licensed under the Creative Commons Attribution-ShareAlike 4.0 International License. To view a copy of this license, visit BY-SA.