Field Guide · protocol

Also known as: VARA, VARA HF, VARA FM

VARA is a software modem for amateur radio that moves error-free data over ordinary HF and VHF/FM voice channels using OFDM — dividing the audio passband into many closely spaced subcarriers, each carrying a slice of the data.1 Running entirely as a PC sound-card application (no special hardware), it pairs that OFDM waveform with an adaptive ARQ protocol, and has become the dominant data mode for Winlink radio email, largely displacing the hardware-modem Pactor it was designed to rival.

many OFDM subcarriers across ~2.3 kHz → each subcarrier: PSK/QAM, level set by SNR
VARA packs dozens of narrow OFDM subcarriers into the voice channel; each is modulated with PSK or QAM at a density the modem raises or lowers to track conditions.

Overview

VARA comes in two forms. VARA HF occupies about a 2.3 kHz SSB channel and is tuned for the fading, multipath conditions of shortwave; VARA FM runs through a standard VHF/UHF FM voice radio for high-speed local links. Both use OFDM: spreading the data across many subcarriers makes each symbol long in time, so a multipath echo or a narrow interferer damages only part of the signal rather than the whole. On top of that, VARA runs an adaptive ARQ — it measures the channel, chooses a modulation/coding level for the subcarriers, and retransmits any block that fails, so throughput scales smoothly from a few hundred bits per second on a bad HF path up to tens of kilobits on a clean FM link.

Technical characteristics

Property Value
Waveform OFDM, adaptive PSK/QAM per subcarrier
Access ARQ, half-duplex, connected sessions
VARA HF ~2.3 kHz SSB; ~few hundred bps to ~7 kbps
VARA FM Through an FM radio; up to ~25 kbps
Error control FEC + CRC + ARQ retransmission
Implementation Windows sound-card software (proprietary)

History

VARA was written by José Alberto Nieto Ros (EA5HVK) and released around 2017, with VARA FM following soon after. By offering Pactor-like (and often better) throughput using only a sound card and a licence fee to unlock full speed, it rapidly became the default HF and VHF data mode for Winlink, especially in emergency-communications circles.1

Deployment

VARA is amateur-focused, used above all for Winlink email over HF and for high-speed local VARA FM gateways. Its low cost of entry — a computer, a sound-card interface, and any SSB or FM radio — is the main reason it has spread so widely for served-agency and disaster communications.

Decoding it with GopherTrunk

GopherTrunk does not decode VARA. It is a closed, proprietary OFDM modem for amateur HF/FM data, entirely outside GopherTrunk’s digital land-mobile trunking scope, and only the author’s software can transmit or receive it. GopherTrunk does handle the general primitives VARA is built from — OFDM concepts, PSK/QAM subcarriers — but not this specific waveform or its ARQ protocol.

Sources

  1. VARA (software) — Wikipedia, for VARA’s OFDM waveform, HF and FM variants, adaptive ARQ, throughput, and origin. ↩ ↩2

See also