VARA Modem for APRS: What It Is and Why It Matters

VARA is a Windows-based soundcard software modem developed by EA5HVK in Spain. It comes in two versions: VARA HF for SSB transceivers, and VARA FM for VHF/UHF FM transceivers. Both versions use up to 52 simultaneous QAM audio subcarriers and multiple layers of forward error correction (FEC).

Performance

On HF, VARA can exceed the data rate of a 1200-baud VHF packet TNC while operating within a 500 Hz CW filter bandwidth. Using the full SSB voice bandwidth of around 2300 Hz, VARA HF can exceed 7,000 bits per second. On VHF/UHF, VARA FM can exceed 12,000 bps via mic/speaker connections, and over 25,000 bps when connected to the radio’s rear-panel data port in 9600-baud packet mode.

More importantly for APRS, VARA can produce reliable copy on signals too weak to hear in the radio speaker. Classic 300-baud AX.25 packet on HF has no FEC. When a packet is lost to noise or fading, the receiver simply waits for the next beacon. VARA’s FEC layers repair corrupted packets at the receiving end without retransmission. For one-way APRS beaconing, where there is no ARQ or NAK mechanism, this is a significant practical advantage.

KISS Interface and APRS Compatibility

VARA exposes a KISS-over-IP interface, making it a drop-in TNC replacement for any APRS client that supports KISS. Applications including YAAC, PinPoint APRS, and APRS-is32 can connect directly to VARA’s KISS IP port. UIview32 and other programs that only support KISS over a serial port require a bridging utility such as VSPE (Virtual Serial Ports Emulator) or the open-source Com0com/Com2TCP combination.

HF Frequencies

Standard APRS-over-VARA frequencies in North America:

• 40 metres: 7083.500 kHz USB
• 30 metres: 10148.200 kHz USB

Note that 40 metres uses USB, not the usual LSB for that band. Programming a memory channel with USB or DIG mode selected is the recommended workaround on radios that bury this setting in menus.

Licensing

The free version of VARA is speed-limited. Full speeds on both HF and FM versions are unlocked by purchasing a single license for approximately USD $69 from the author’s website. One license covers unlimited copies as long as they use the same callsign. For HF APRS use, the free version is sufficient.

The VARAtrack

WA8LMF built a mobile APRS-over-VARA tracker using a Higole stick PC: a compact Windows 11 device built around a quad-core 64-bit Intel Celeron J4115, 4 GB RAM, 64 GB eMMC storage, built-in sound, Wi-Fi, Bluetooth 5.0, Ethernet, HDMI, and USB-C. The device is repurposed from commercial digital signage applications and is available from around USD $130.

The stick runs a stripped-down Windows 10 installation with VARA, UIview32, VSPE, and Visual GPS, all launched automatically via Windows Scheduler at boot. VARA uses the device’s built-in 3.5 mm TRRS audio jack, with the connected radio using VOX to key the transmitter. A cable adapts the TRRS plug to the radio’s 6-pin mini-DIN data port. For hard PTT keying, a DigiRig or SignaLink can be added via USB.

The system can run headless as a transmit-only mobile tracker, with a monitor as a receive-and-map station, or as a full home igate when connected to the internet. Total power draw with monitor is approximately 6 watts (0.5 A at 12 VDC).

Download

VARA modem: https://rosmodem.wordpress.com/ VarAC chat: https://www.varac-hamradio.com/

Full documentation and experiment results comparing VARA against 300-baud AX.25 packet on 60 metres: http://wa8lmf.net/VARA/