Table of Contents
- What is the SA818?
- Two Paths: Ready-Made Kit or DIY
- The Easy Route: SHARI & Co.
- The Maker Route: SA818 + Raspberry Pi + USB Sound Card
- Wiring (DIY Build)
- Programming the SA818
- CTCSS Tones: Standard Values
- Configuring SVXLink
- The Hotspot Dashboard
- Optional: EchoLink on the Hotspot
- Frequency and Regulations
- Further Resources
- Transparency Notice
In our overview of SVXLink in Austria we showed two ways to get on the network: via a participating repeater – or via your own hotspot. This article covers the second path. An FM hotspot generates a weak 2 m or 70 cm signal at home, bringing the SVXLink reflector directly into the shack without needing a repeater in range. The heart of almost every such build is the same small component: the SA818.
What is the SA818?
The SA818 (manufacturer NiceRF) is a postage-stamp-sized FM transceiver module for the 2 m or 70 cm band, with around one watt of output power. It combines transmitter, receiver and tone signalling (CTCSS/DCS) in a single chip and is programmed via a serial interface (UART, 9600 Baud, 8N1, 3.3 V). Because it is cheap, compact and power-efficient, it sits inside many ready-made hotspots – and is equally well suited to homebrew builds.
Two Paths: Ready-Made Kit or DIY
The Easy Route: SHARI & Co.
Those who prefer not to solder reach for the ready-made SHARI (SA818 Ham Allstar Radio Interface, a design project by N8AR). SHARI combines an SA818S module with a CM119 USB sound card on a small PCB, plugs directly onto the USB ports of a Raspberry Pi and delivers power, audio and programming over USB. Status LEDs show power, connection, COS (receive) and PTT (transmit); output power is 0.5 to 1 watt depending on the model, in either 2 m or 70 cm variants. Besides the original (available from kits4hams.com) there are numerous clones such as the AURSINC Shari PiHat or Jumbospot SR110U. Originally designed for AllStarLink, the same hardware works equally well for SVXLink or EchoLink.
The Maker Route: SA818 + Raspberry Pi + USB Sound Card
Those who build from scratch need three things: an SA818 module, a Raspberry Pi (Zero 2 W, 3, 4 or 5 – any model is more than sufficient) and a simple USB sound card with a CM108/CM119 chip that provides PTT and COS lines via its GPIOs in addition to audio. Add a small antenna and a stable 5 V power supply. The SA818 draws a noticeable amount of current when transmitting – so the power supply should not be under-dimensioned.
Wiring (DIY Build)
The key connections of the SA818 and where they go:
| SA818 Pin | Function | Connection |
|---|---|---|
| VBAT | Supply (~3.7–5 V) | stable power rail |
| GND | Ground | common ground |
| PTT | Transmit key (active low) | GPIO of sound card / Pi |
| MIC | TX audio | audio output of sound card |
| AF / RF | RX audio | audio input of sound card |
| SQ | Squelch/COS (carrier detect) | GPIO input |
| H/L | Power level switch | to ground = low power |
| PD | Power-Down / enable | to VBAT = module active |
On the ready-made SHARI this wiring is already done; on the RF.Guru hotspot, for example, PTT runs via GPIO16 (pin 36) and COS via GPIO12 (pin 32) directly to the Pi. Those building their own follow exactly these same lines: key the carrier (PTT), detect the carrier (SQ/COS), route audio in and out.
Programming the SA818
Before first use, the module receives its frequency and – if desired – a CTCSS tone. The easiest way is with the open-source Python tool sa818 (by Fred, W6BSD). For programming, the module is accessed via a 3.3 V USB-serial adapter (FTDI) or directly on the Pi:
pip install sa818
# Read firmware version (communication test)
sa818 version
# Set frequency and CTCSS (70 cm example)
sa818 --port /dev/ttyUSB0 radio --frequency 434.925 --ctcss 88.5 --squelch 4A practical tip from experience: if you let SVXLink handle the pre-emphasis/de-emphasis and filtering itself, disable the module's built-in filters with AT+SETFILTER=1,1,1. This results in a cleaner, more even audio response.
CTCSS Tones: Standard Values
When programming, you specify the CTCSS tone as a frequency in Hertz. There are 38 standardised tones — which one is actually needed depends on the target repeater or reflector (if in doubt, ask the sysop). The standardised values:
| 67.0 | 71.9 | 74.4 | 77.0 | 79.7 | 82.5 | 85.4 |
| 88.5 | 91.5 | 94.8 | 97.4 | 100.0 | 103.5 | 107.2 |
| 110.9 | 114.8 | 118.8 | 123.0 | 127.3 | 131.8 | 136.5 |
| 141.3 | 146.2 | 151.4 | 156.7 | 162.2 | 167.9 | 173.8 |
| 179.9 | 186.2 | 192.8 | 203.5 | 210.7 | 218.1 | 225.7 |
| 233.6 | 241.8 | 250.3 |
In the sa818 tool the tone is passed as a number (e. g. --ctcss 88.5); transmit and receive tones can be set independently.
Configuring SVXLink
A hotspot runs in SVXLink as a SimplexLogic (not a repeater, but a simplex channel). A few values in svxlink.conf are critical for the SA818 – above all the transmit lead-in delay:
[SimplexLogic]
TYPE=Simplex
...
[Rx1]
AUDIO_DEV=alsa:plughw:1 # USB sound card is card 1, Pi internal is 0
SQL_DET=CTCSS # or HIDRAW (COS via GPIO)
[Tx1]
AUDIO_DEV=alsa:plughw:1
PTT_TYPE=Hidraw # or GPIO
TX_DELAY=800 # IMPORTANT: SA818 starts up slowly
PTT_HANGTIME=200
TIMEOUT=300 # Emergency cut-off against permanent carrier
PREEMPHASIS=1
SQL_TAIL_ELIM=300The two most common stumbling blocks are already in these lines: TX_DELAY=800 gives the SA818 around 800 milliseconds until the transmitter is stable – without this lead-in, the first syllables are clipped. And TIMEOUT=300 is the safety switch: should the module ever get stuck in a permanent carrier (a known SA818 phenomenon), SVXLink shuts down after 300 seconds.
Those who want to skip the manual setup will find the RF.Guru Analog-HotSPOT – a ready-made Raspberry Pi image (flash with Balena Etcher, then run hotspot-config) – complete with a pre-configured SVXLink setup and the correct GPIO assignments.
The Hotspot Dashboard
A fully configured hotspot usually comes with a small web dashboard, accessible via the local IP address of the Raspberry Pi on your home network (e.g. http://192.168.x.x/). There you can see at a glance which repeaters and hotspots have recently transmitted and which talk groups are active — and connect a group or send a DTMF command with a single click. That is more convenient than typing everything on the radio.
Optional: EchoLink on the Hotspot
SVXLink can also run as an EchoLink node. To do this, register your callsign once at echolink.org and enter the credentials in the SVXLink configuration. The EchoLink ports must be open in your router: 5198 and 5199 (UDP) and 5200 (TCP).
Frequency and Regulations
A hotspot transmits – so: a valid amateur radio licence is required. Choose a frequency designated for simplex or hotspot use according to your national band plan, never a repeater input, and operate at minimum power – a few milliwatts are enough to cover your own home. This keeps the hotspot a pure shack tool and causes no interference. Which talk groups await you (e.g. TG 232 for the Austria-wide net) is covered in the SVXLink overview.
Further Resources
- Video: "SHARI Pi HAT – Allstar SA818 Radio Module for Raspberry Pi" – YouTube
- Video: "Setting up a SHARI Allstar Node with Kyle, AA0Z" – YouTube
- Video: "Building a new SHARI hotspot for Echolink and Allstar" – YouTube
- Programming tool and documentation: github.com/0x9900/SA818
- Ready-made SVXLink hotspot image: RF.Guru Analog-HotSPOT-SVXLink
- Kits: kits4hams.com/shari
Related articles: SVXLink in Austria · EchoLink, AllStarLink and SVXLink · VHF Repeaters in Austria.
This page is part of our digital voice overview — compare all modes there.
Transparency Notice
This article was researched and written with the support of AI (Claude, Anthropic) based on public sources – including the ÖVSV Wiki, the official SVXLink documentation, the SA818 project pages by 0x9900/W6BSD and RF.Guru, and freely accessible video tutorials. Always verify specific configuration values against your own hardware. All content has been editorially reviewed. Questions, corrections or additions? Write to us at [email protected].
