LoRa port for TARPN node

A complete TARPN 1-port node using LoRa. The smaller device is the LoRa KISS TNC Transceiver, and the larger Raspberry Pi is the TARPN node. It requires about 1.5 watts to run, just plug it into a USB power source.

KV4P has been experimenting with adding LoRa ports to his TARPN node. LoRa is very desirable for TARPN because LoRa transceivers are very inexpensive yet have good range, are very small and low-power, and reduce the overall setup cost of a new link.

Materials:

• Raspberry Pi Zero 2 W (~$15 when in stock), WITH gpio header. You can use any Rapsberry Pi model for this, but this is what I used.
• Adafruit LoRa Radio Bonnet with OLED - RFM95W @ 915MHz - RadioFruit ($32.50)
• 78mm of 16AWG magnet wire (33cm band 1/4 wave antenna), soldered to the radio bonnet antenna center conductor via (right next to the connector we won't use) (~$12 for a spool)

This assumes you already have a TARPN node. With this experimental guide, you'll be building a LoRa radio that shows up on your network as a TCP-based KISS TNC, which we'll wrap in a virtual serial port so TARPN ports 11 or 12 can be configured to access it like a normal TNC.

I'm going to assume you already have the Raspberry Pi configured with Raspberry Pi OS, and connected to your local network (the same network as your TARPN node). You'll want to assign a static IP to it (rather than DHCP), so you can ensure your TARPN node will be able to find it on the network after restarts.

A few notes:

• This has only been tested with the "Lite" Raspberry Pi OS image, not either of the "with desktop" images. They might work, but have not been tested.
• This definitely does not work on a Pi that has TARPN installed. Something in the TARPN setup script is incompatible with the LoRa bonnet. We may be able to fix this in the future, but it would require significant debugging to figure out what TARPN setup step breaks the LoRa bonnet functionality.

Get LoRa bonnet working

You can read more about these steps on this Adafruit page.

1. sudo apt install python3-pip
2. sudo pip3 install --upgrade setuptools
3. sudo pip3 install --upgrade adafruit-python-shell
4. wget https://github.com/adafruit/Adafruit_CircuitPython_framebuf/raw/main/examples/font5x8.bin
5. wget https://raw.githubusercontent.com/adafruit/Raspberry-Pi-Installer-Scripts/master/raspi-blinka.py
6. sudo python3 raspi-blinka.py (this will require reboot at the end)
7. sudo pip3 install adafruit-circuitpython-ssd1306
8. sudo pip3 install adafruit-circuitpython-framebuf
9. sudo pip3 install adafruit-circuitpython-rfm9x
10. create this test script rfm9x_check.py, which you should run ("python3 rfm9x_check.py") to prove your bonnet is properly connected and working:

# SPDX-FileCopyrightText: 2018 Brent Rubell for Adafruit Industries
#
# SPDX-License-Identifier: MIT

"""
Wiring Check, Pi Radio w/RFM9x

Learn Guide: https://learn.adafruit.com/lora-and-lorawan-for-raspberry-pi
Author: Brent Rubell for Adafruit Industries
"""
import time
import busio
from digitalio import DigitalInOut, Direction, Pull
import board
# Import the SSD1306 module.
import adafruit_ssd1306
# Import the RFM9x radio module.
import adafruit_rfm9x

# Button A
btnA = DigitalInOut(board.D5)
btnA.direction = Direction.INPUT
btnA.pull = Pull.UP

# Button B
btnB = DigitalInOut(board.D6)
btnB.direction = Direction.INPUT
btnB.pull = Pull.UP

# Button C
btnC = DigitalInOut(board.D12)
btnC.direction = Direction.INPUT
btnC.pull = Pull.UP

# Create the I2C interface.
i2c = busio.I2C(board.SCL, board.SDA)

# 128x32 OLED Display
reset_pin = DigitalInOut(board.D4)
display = adafruit_ssd1306.SSD1306_I2C(128, 32, i2c, reset=reset_pin)
# Clear the display.
display.fill(0)
display.show()
width = display.width
height = display.height

# Configure RFM9x LoRa Radio
CS = DigitalInOut(board.CE1)
RESET = DigitalInOut(board.D25)
spi = busio.SPI(board.SCK, MOSI=board.MOSI, MISO=board.MISO)

while True:
    # Clear the image
    display.fill(0)

    # Attempt to set up the RFM9x Module
    try:
        rfm9x = adafruit_rfm9x.RFM9x(spi, CS, RESET, 915.0)
        display.text('RFM9x: Detected', 0, 0, 1)
    except RuntimeError as error:
        # Thrown on version mismatch
        display.text('RFM9x: ERROR', 0, 0, 1)
        print('RFM9x Error: ', error)

    # Check buttons
    if not btnA.value:
        # Button A Pressed
        display.text('Ada', width-85, height-7, 1)
        display.show()
        time.sleep(0.1)
    if not btnB.value:
        # Button B Pressed
        display.text('Fruit', width-75, height-7, 1)
        display.show()
        time.sleep(0.1)
    if not btnC.value:
        # Button C Pressed
        display.text('Radio', width-65, height-7, 1)
        display.show()
        time.sleep(0.1)

    display.show()
    time.sleep(0.1)

Turn it into a TCP KISS TNC

See the original github project this is forked from for more info.

1. sudo apt install git aprx python3-rpi.gpio python3-spidev python3-pil python3-smbus
2. git clone https://github.com/VanceVagell/RPi-LoRa-KISS-TNC-2ndgen.git
3. Edit config.py and update the frequency (search for "frequency"). You can/should customize the frequency to anything in the 33cm band (but at least 500kHz from the band edge).
4. Optional: The default config.py settings are for balanced speed and range. If your connection is strong, you can increase bandwidth up to 500000 (500kHz) which will significantly improve speed. 500kHz is roughly equivalent to 9600 baud, and 62500 is roughly equivalent to 1200 baud. If you need even more distance, you can also increase spreadingFactor above 7 (to say 8 or 9), which will make it transmit more slowly but be received much better. There are other settings in config.py that can affect range and speed as well, but this is a good starting point.

Note that if you're using a ham band other than 33cm you may need to use lower bandwidth settings to adhere to FCC requirements for spread spectrum transmissions (<100kHz bandwidth below 33cm). You can use the full 500kHz 33cm and above.

Create log placeholders that the library expects to exist:

1. sudo mkdir /var/log/lora
2. sudo touch /var/log/lora/lora.log

Start/test your TCP-based KISS TNC with this command (you should see it print out the config values and say it's listening for connections, with no errors listed):

• sudo python3 Start_lora-tnc.py

This is what you should see:

########################
#LORA KISS TNC STARTING#
########################
#Lora parameters:
frequency= 910300000
preamble= 8
spreadingFactor= 7
bandwidth= 500000
codingrate= 5
APPEND_SIGNAL_REPORT= False
outputPower= 15
TX_OE_Style= False
sync_word= 0x12
crc= True
########################
2023/08/15 00:27:18 - KISS-Server: Started. Listening on IP 0.0.0.0 Port: 10001

2023/08/15 00:27:18 - LoRa radio initialized. Waiting for LoRa Spots...

You'll want this to start every time the LoRa pi reboots, so follow these steps (TODO replace this crontab approach with systemd):

1. sudo crontab -e (If you've never used crontab it will ask you to select you preferred text editor, I use nano)
2. Paste this line at the end of the crontab file, and save:

@reboot /usr/bin/python3 /home/pi/RPi-LoRa-KISS-TNC-2ndgen/Start_lora-tnc.py &

TARPN configuration

TARPN only supports serial TNCs, so first we need to "fake" a serial TNC that actually connects to our TCP-bsaed LoRa KISS TNC using the "socat" command.

• sudo apt install socat
• sudo nano lora-port-up.sh
• Use these contents but replace the IP address to your LoRa KISS TNC's address:

#!/bin/bash
printf "Bringing up LoRa port as /dev/ttyS0\n"
while true
    do
      sudo socat -T 600 pty,link=/dev/ttyS0,b115200,raw,nonblock,echo=0,mode=777,wait-slave tcp:192.168.1.90:10001
      printf "LoRa port /dev/ttyS0 disconnected, waiting 1 second and bringing it up again.\n"
      sleep 1
    done

• chmod +x lora-port-up.sh
• Start the fake serial device with "./lora-port-up.sh &" and it will show up as /dev/ttyS0 like a serial TNC would.

You'll want this LoRa virtual device to appear on device reboot, you can do this with crontab (TODO replace this crontab approach with systemd):

1. crontab -e (If you've never used crontab it will ask you to select you preferred text editor, I use nano)
2. Paste this line at the end of the crontab file, and save:

@reboot nohup /home/pi/lora-port-up.sh &

Then you can edit your TARPN node.ini to configure port 11 as follows:

usb-port11:ENABLE
portdev11:/dev/ttyS0
speed11:115200
txdelay11:1
frack11:2000
kissoptions11:disable
neighbor11:KV4P-3

Of course, assign "neighbor11" to the callsign/ssid of the node you plan to connect to via LoRa on your new port 11.

Performance

915MHz LoRa, bench test

With 500kHz bandwidth and spreadingFactor of 7 in config.py, a throughput test on my own TARPN node with a "bench setup" (2 LoRa transceivers right next to each other), I achieved 135bytes/sec. That's about as good as a very solid 9600 baud link with a NinoTNC and high quality mobile radios.

Both LoRa nodes used a 1/4 wave vertical antenna made of magnet wire (no counterpoise or ground plane).

915MHz LoRa, quarter mile

With 62.5kHz bandwidth, spreadingFactor of 8, and coding of 6, the module worked very well at a distance of about 1/4 mile. I live in a valley, and put one LoRa transceiver in my attic (3rd floor) and I tested it from a nearby hill. That worked well. I also walked down a flat road that's level with my house (no intervening hills) and that also worked well. I did not do a throughput test, but at these settings it's roughly equivalent to a 1200 baud typical TNC link.

Both LoRa nodes used a 1/4 wave vertical antenna made of magnet wire (no counterpoise or ground plane).

915MHz LoRa, transceiver 60' up a tree

LoRa "port" enclosed in a waterproof project box with a homebrew groundplane antenna.

For the next test, I put the LoRa + pi "port" about 60 feet up in a tree, with a homebrew groundplane antenna instead of the quarter wave vertical (i.e. piece of wire). The idea was to give the test a better chance at success. I also added a 5dBi rubber duck antenna to my portable TARPN node's LoRa port, so it was a little better than the piece of wire as well.

Observations:

• I ran my first set of tests at 64.5kHz bandwidth and 8 spreadingFactor.
• My house is surrounded by hills on most sides (I live in a valley), and I could not get any signal to the south, southwest, or southeast. I tried many different spots, from about 2 miles away, all the way down to about a half mile down the road. None worked, no packets received.
• On a lark, I also tried 500kHz bandwidth and 7 spreadingFactor. This did not help, which was not surprising since the same power was more spread out.
• I was FINALLY able to get packets to arrive when I was 1/3 mile north of my house. There are no hills to the north of my house.

Overall, 33cm is clearly heavily blocked by hills (all VHF and UHF is to some extent, but 33cm is clearly far more sensitive than either 2m or 70cm to hills, I have other TARPN links on those bands that work OK for several miles).

915MHz LoRa, transceiver 60' up a tree, avoiding hills

In the next test, I drove north / northeast from my house, where there are no hills higher than my house's elevation. I drove to one location at 1.5 miles, and another at 4 miles.

Unfortunately, neither of these locations could receive the 33cm LoRa signal (8 spreadingFactor, 62.5kHz bandwidth, 6 coding rate). In fact, the only location I've been able to receive the signal (other than literally at my home QTH) is 1/3 mile straight north. This tells me two things:

1. Trees / forest likely attenuate 33cm signals fairly strongly, since other locations within ~1/3 mile of my home QTH did not work
2. Hills are a complete no-go for 33cm signals

So a LoRa link for TARPN could be an excellent inexpensive option if you live in a flat area and/or have a clear line-of-site to your nearest TARPN neighbor. For example, I'd expect it to work pretty well in a suburban housing district without hills, letting neighbors link up. Or if you're in a part of the US that just isn't very hilly (e.g. farmland area). Where I live is hills everywhere, and I'm in a valley!