Details

Overview

My goal is to design a simple, cheap and easily replicable hardware to build a 144MHz APRS iGate and Digipeater.

Sources

I collected two initial hardware resources using ESP32 and SA818:
https://oshwlab.com/APRSTH/esp32sa818#P6
https://github.com/nakhonthai/ESP32IGate/blob/master/doc/ESP32DR_Simple_Schematic.pdf

and two software:
https://github.com/erstec/APRS-ESP
https://github.com/nakhonthai/ESP32IGate

but, after trying to receive some RF APRS messages on 144.8 MHz to test rx capabilities, I realized that the SA818v receiver hadn't enough sensitivity. Read more about this in the Log pages.

So I ended up to a different solution, using:

SA818v for transmitting
RTL2832 for receiving
Direwolf running on a Raspberry PI zero 2 W to decode received / encode tranmitted APRS messages
A MAX98357 board to output audio waveform (AFSK1200 modulated APRS messages)

Schematic

Hardware

The Raspberry PI, by the I2S interface, send the modulated AFSK 1200 (Bell 202) APRS messages to the microphone input of the SA818v 144.8MHz transmitter.

The RTL8232 board (tuned at 144.8MHz) is connected via USB to the Raspberry PI, which decodes the APRS messages.

RPI outputs

GPIO 23 is the TX signal (PTT) which:
- switches the antenna relay to the SA818v transmitter, short circuiting the RTL2832 RF input to the ground (for RX protection).
- commands the SA818v to turn on its transmitter
- turns on the blue led
GPIO 24 is the RX signal (DCD) which is turned on by Direwolf when a valid APRS message is received
GPIO 25 is the On signal which is turned on after RPI booting.

GPIO 23 and 24 are configured in the file rtlfm_sdr.conf (see the software section on GitHub)

Software

The rtl_fm program configures and tunes (at 144.8MHz) the RTL2832 and demodulates narrow band FM.

The Direwolf program, running on the Raspberry Pi, demodulates, decode, encode, modulates the APRS messages and manages everything.

See https://github.com/guido57/APRS-raspi-rtl2832

Project Logs

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Second solution (RTL2832)

Guido • 06/24/2024 at 19:48 • 0 comments

At the same time, as a comparison, I tried to receive using an RTL2832 SDR receiver (NooElec R820T2) which appeared only a little worse than the RSP1, probably because of the reduced number of bits (8 bits vs 12 bits samples).

For this test I received the 144.8 MHz with a multi band discone antenna, and the GQRX software on Ubuntu 22.04.

First steps (SA818v Transceiver)

Guido • 06/24/2024 at 19:44 • 0 comments

I started testing the SA818v 144MHz FM transceiver module, which appeared very cheap (around 15 Euro) and simple to configure. I haven't tested the TX side yet but, after some struggling, I understood that the RX sensitivity (declared -124 dBm) was insufficient in my situation. In fact two APRS iGates that I received at around -40dBm with my Sdrplay RSP1, were barely listenable and not decodable using the SA818v receiver.

For this test I sent the audio output from the SA818V or from thr Sdrplay RSP1 to the GQRX SDR receiver (which has built in AFSK 1200 decoder) on Ubuntu 22.04 or to the Direwolf software

The resources I found at the beginning

Guido • 06/24/2024 at 19:37 • 0 comments

Sources

I collected two initial hardware resources using ESP32 and SA818: https://oshwlab.com/APRSTH/esp32sa818#P6 https://github.com/nakhonthai/ESP32IGate/blob/master/doc/ESP32DR_Simple_Schematic.pdf

and two software: https://github.com/erstec/APRS-ESP https://github.com/nakhonthai/ESP32IGate

and I'm now trying to receive some RF APRS messages on 144.8 MHz to test rx capabilities.

Discussions

144MHz APRS iGate with RPI Zero+RTL2832+SA818V

Description

Details

Overview

Sources

Schematic

Hardware

RPI outputs

Software