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/

The post VARA Modem for APRS: What It Is and Why It Matters appeared on Hamradio.my - Amateur Radio, Tech Insights and Product Reviews by 9M2PJU.