HF digital modes
Mercury HF Modem
Mercury is a fully open-source HF data modem from Rhizomatica, built as an alternative to closed-source options like VARA. WG0A runs a Winlink RMS gateway mode over Mercury, giving operators an open protocol path for HF Winlink traffic.
What is Mercury?
Mercury is an open-source software modem for digital data communications over HF radio, developed by Rhizomatica. It was created to provide a free, open alternative to proprietary HF modems, with a focus on robust performance over noisy, low-bandwidth HF channels.
Because Mercury is open source, its protocol and code can be inspected, modified, and improved by the community, unlike closed HF modem products.
Mercury at WG0A
WG0A runs a Mercury Winlink RMS gateway as one of several HF modem options offered alongside Dire Wolf (AX.25/FX.25/IL2P) and ARDOP. Current station configuration:
| Port | Frequency | Mode | Notes |
|---|---|---|---|
| 6 | 7.1013 / 14.1023 MHz USB | Mercury HF Modem | BW2750 |
This port is served through the WG0A-10 / MSPRMS Winlink Gateway service,
with band selection following the station's normal HF auto-switching schedule
(20m 12:00-00:00 UTC, 40m 00:00-12:00 UTC). See the full
WG0A node status page for live port and service details.
VARA compatibility
Mercury exposes a VARA-compatible TCP TNC interface, so Winlink programs that already know how to drive a VARA modem can drive Mercury the same way, with no protocol changes on the Winlink side. For Winlink Express or Pat Winlink clients, you would point the VARA configuration at Mercury's address and ports, just like you would for a VARA modem.
| Port | Purpose |
|---|---|
| 8300 (base port) | Control port — VARA-style text commands (MYCALL, LISTEN,
CONNECT, BW2750, etc.) and status lines. |
| 8301 (base port + 1) | Data port — raw application payload for a connected session. |
In practice, this means Winlink Express (or another Winlink client) can be configured
with a VARA HF session pointed at Mercury's control/data ports (typically
127.0.0.1:8300 / 8301) exactly as it would for the real VARA
modem software, and Mercury handles the RF side transparently.
Mercury also emits VARA-style asynchronous status lines such as BUSY ON/BUSY OFF,
PENDING, and CONNECTED, which is what lets existing VARA-aware
hosts like Winlink Express and BPQ32 hold off transmitting and react to link state
without any special-casing for Mercury.
Mercury TNC command reference — the full VARA-compatible command/response set and port layout.
Setup guides and videos
How to install Mercury open source HF modem on Linux & Windows (YouTube)
VARA Winlink Email Speeds With MERCURY HF! (YouTube)
Winklink P2P over Mercury HF - R&D Demo for Preppers (YouTube)
Key resources
Mercury source code (GitHub) — the modem implementation and build instructions.
Rhizomatica: Mercury release announcement — background on the project and its goals.
ARRL: Mercury overview — ARRL's coverage of the modem for amateur radio operators.
Amateur Radio Newsline: open-source modem called a VARA replacement — news coverage framing Mercury as an open alternative to VARA.
Related WG0A notes
Mercury is one of several HF modems in use at this station; see also Dire Wolf installation notes and the main page for the full modem lineup.