Table of Contents
- SCS and the History of PACTOR
- The PXdragon DR-9400 in Detail
- Hardware Specifications
- Supported Modes
- PACTOR-4: The Technology Behind the Protocol
- Adaptive Modulation
- OFDM in a 2.4 kHz Channel
- ARQ and Error Correction
- Data Compression
- Winlink: The Primary Use Case
- Emergency Communications: PACTOR as a Lifeline
- PACTOR vs. VARA HF: The Big Comparison
- Maritime Use: SailMail and Sea Radio
- Setup: Connecting the DR-9400
- Cost and Availability
- Who Benefits from a PACTOR Modem?
- The Future: PACTOR-5 and Beyond
- Conclusion
- Transparency Notice
Sending emails over shortwave while the internet around you goes dark — with a PACTOR modem, this becomes routine. Among HF data modems, PACTOR has been the gold standard for reliable data transmission on shortwave for over three decades. With the PXdragon DR-9400, German manufacturer SCS GmbH from Hanau introduced the next generation in 2024: a powerful single-board computer running Linux that combines PACTOR-4 and numerous other operating modes in a single compact device.
SCS and the History of PACTOR
The PACTOR story begins in 1987 when two German radio amateurs, Hans-Peter Helfert (DL6MAA) and Ulrich Strate (DF4KV), started a project that would revolutionise HF data transmission. Their goal: develop a protocol combining the advantages of Packet Radio (error-free transmission through ARQ) with the robustness of AMTOR (narrowband FSK). By 1991, PACTOR-I was born — an adaptive protocol with automatic error correction that reliably worked at 200 baud on shortwave.
From this project emerged SCS GmbH (Special Communications Systems) in Hanau near Frankfurt. The company has been exclusively developing and manufacturing all PACTOR hardware in Germany ever since — a niche product with global significance. The evolution of PACTOR reads like a chronicle of digital shortwave communication:
- PACTOR-I (1991): 200 baud, FSK, ARQ protocol — the first step beyond AMTOR
- PACTOR-II (1995): Up to 1,200 bps using DPSK modulation and Huffman data compression — a quantum leap in speed
- PACTOR-III (2002): Up to 5,200 bps thanks to OFDM (18 parallel carriers in a 2.4 kHz channel) — the first practical data rates on HF
- PACTOR-IV (2010): Up to 10,500 bps gross, effectively higher with compression — ten speed levels, adaptive modulation, works down to -20 dB SNR
Each new PACTOR version remained backwards-compatible: a PACTOR-4 modem can connect to any PACTOR-1 station and automatically negotiates the highest possible speed.
The PXdragon DR-9400 in Detail
The PXdragon DR-9400 succeeds the well-established DR-7800 and DR-7400 modems, which set the standard in the PACTOR world for years. Both predecessors have since been discontinued. The DR-9400 represents a fundamental architectural shift: instead of a specialised DSP chip, it employs a full ARM v8 single-board computer running Linux.
Hardware Specifications
- Processor: ARM v8 quad-core, 1.8 GHz — significantly more processing power than DSP-based predecessors
- Operating system: Embedded Linux — enables future firmware updates and new modes via software
- Connectivity: USB-C (host and device), Ethernet/LAN, optional Wi-Fi and Bluetooth
- Storage: SD card slot for configuration, log files, and firmware updates
- GPS input: For automatic position reporting (e.g. in Winlink forms)
- Radio connection: Standardised audio and PTT interface for common HF transceivers
- Encryption: Optional AES-256 encryption for governmental and security applications
- Warranty: 2-year manufacturer's warranty
Supported Modes
The DR-9400 is far more than a pure PACTOR modem. It supports an impressive range of operating modes:
- PACTOR 1, 2, 3, 4: Full support for all four generations
- Packet Radio: 300 and 1,200 baud AX.25 — the classic packet mode
- Robust Packet: A hardened variant of Packet Radio for challenging HF conditions
- APRS: Automatic Packet Reporting System — on HF and VHF
- Weather-FAX: Reception of weather charts as broadcast by coastal stations — especially useful for maritime operation
- RTTY: Radioteletype — the oldest digital mode in amateur radio
- 2G ALE: Automatic Link Establishment — automatic connection setup on the best frequency
This versatility makes the DR-9400 a universal HF data platform: a single device covering virtually all common HF data modes.
PACTOR-4: The Technology Behind the Protocol
What makes PACTOR-4 special? At its core, it is a sophisticated interplay of several techniques:
Adaptive Modulation
PACTOR-4 uses ten different speed levels. Depending on channel quality, the protocol automatically selects the optimum level — from a robust 200 baud at poor SNR up to 10,500 bps under good conditions. This switching happens seamlessly during an active connection: if propagation deteriorates, PACTOR shifts down within seconds. If conditions improve, speed increases automatically.
OFDM in a 2.4 kHz Channel
PACTOR-3 and PACTOR-4 use OFDM (Orthogonal Frequency Division Multiplexing) — the same fundamental technique employed in modern Wi-Fi and 4G/5G mobile networks. In PACTOR's case, multiple parallel carriers are used within the standard 2.4 kHz SSB channel. Compared to FT8, which also uses narrow channels, PACTOR offers a crucial difference: bidirectional connection with error correction rather than one-way signalling.
ARQ and Error Correction
PACTOR uses ARQ (Automatic Repeat reQuest): every received data packet is acknowledged. Corrupted packets are automatically retransmitted. Combined with Forward Error Correction (FEC), this delivers error-free data transfer — even under conditions where the human ear hears nothing but noise. PACTOR-4 operates reliably down to -20 dB SNR.
Data Compression
Built-in data compression significantly increases the effective data rate beyond the raw bitrate. Text-based messages — such as emails — benefit particularly. In practice, PACTOR-4 achieves effective rates for text transmissions well above the 10,500 bps gross rate.
Winlink: The Primary Use Case
By far the most important application for PACTOR modems is Winlink — the worldwide email system for radio amateurs. Winlink enables sending and receiving emails via shortwave when no internet connection is available. The combination of PACTOR modem and Winlink provides:
- Reliable email delivery: ARQ-based transmission guarantees error-free transfer
- Worldwide reach: Over 1,000 RMS gateways (Radio Message Servers) worldwide, operated by volunteers
- Internet gateway: Messages are seamlessly routed between radio and internet — a radio email can reach any regular email address
- Form system: Standardised forms for emergency communications (ICS-213, Welfare Messages, situation reports)
- Position reports: Automatic GPS position in Winlink messages (when using the DR-9400's GPS input)
In Austria, OE3XEC near Amstetten is one of the most important Winlink RMS gateways. It provides PACTOR access on multiple shortwave frequencies and serves as a central node for Austrian emergency communications. Vienna's historical connection to the Winlink system is also noteworthy: one of the early Common Message Servers (CMS) ran in Vienna — all CMS have since migrated to Amazon AWS.
Emergency Communications: PACTOR as a Lifeline
When infrastructure fails, PACTOR becomes a critical communication tool. In Austria, A.R.E.N.A. (Amateur Radio Emergency Network Austria), coordinates the use of amateur radio in emergencies. PACTOR and Winlink form part of the core infrastructure:
- Independence: PACTOR requires no internet connection, no mobile phone mast, no mains power — just a radio, a modem, and a battery
- Reliability: Error-free data transfer even under catastrophic conditions (storms, ionospheric disturbances, weak signals)
- Standardisation: Winlink forms are internationally standardised — emergency responders worldwide can read the data
- Range: On shortwave, connections spanning hundreds to thousands of kilometres are possible — a single emergency go-kit with a PACTOR modem can serve entire regions
Find more details in our articles on emergency communications in Austria and self-sufficient radio operation during blackouts.
PACTOR vs. VARA HF: The Big Comparison
For several years now, VARA HF has offered a software-based alternative to PACTOR — and the debate in the amateur radio community is lively. Here are the key differences:
- Hardware vs. software: PACTOR runs on dedicated hardware (DR-9400), VARA HF is a pure Windows application working through the sound card
- Price: The DR-9400 costs approximately EUR 1,599 (at WiMo), VARA HF costs USD 69 for the full version
- Maximum speed: PACTOR-4 achieves approximately 10,500 bps gross, VARA HF up to approximately 8,490 bps — similar order of magnitude
- Robustness at poor SNR: This is PACTOR's greatest strength. At -15 dB SNR and below, PACTOR-4 reliably maintains the connection, while VARA HF often drops the link or becomes extremely slow under such conditions
- Latency: PACTOR's hardware DSP responds more quickly to channel changes than a software-based solution on a Windows PC
- Operating system: VARA HF runs only on Windows; PACTOR modems work independently of the OS since they are accessed as serial devices — including Linux and Raspberry Pi
- Reliability: A dedicated modem has no driver issues, no untimely OS updates, and no latency fluctuations from other processes
In practice, this means: VARA HF is an excellent and affordable entry point into Winlink and HF data. For emergency communications, maritime use, and situations where absolute reliability matters, PACTOR remains the gold standard.
Maritime Use: SailMail and Sea Radio
Beyond amateur radio, PACTOR has a second major domain: the maritime sector. SailMail is an email service specifically for sailors and boaters that uses PACTOR as its primary transmission protocol. For approximately USD 250 per year, sailors gain access to a worldwide network of coastal stations and can receive and send emails, weather reports, and GRIB files (weather data for route planning) over shortwave.
For ocean-going sailors, a PACTOR modem is often the only reliable means of communication beyond satellite phones. Compared to Iridium or Inmarsat, PACTOR is significantly cheaper to run — the hardware costs more upfront, but SailMail's annual fees are far below the per-minute charges of satellite providers.
Many bluewater sailors use the DR-9400 or its predecessor together with an Icom IC-7300 or Yaesu FT-991A as the HF transceiver. Some sailboats even have dedicated PACTOR stations with automatic antenna tuners and long-wire backstay antennas permanently installed.
Setup: Connecting the DR-9400
Setting up a PACTOR modem is less complicated than one might expect:
- Prepare the transceiver: Set the HF transceiver to SSB (USB), disable compressor and equaliser, set audio output level to medium
- Connect the DR-9400: Run audio cables between modem and transceiver (microphone input and speaker output or DATA jack). Connect the PTT line.
- Connect to computer: USB-C cable or LAN cable from DR-9400 to computer. The modem appears as a serial device.
- Configure Winlink Express: Select PACTOR as session type, specify the modem's COM port, enter your callsign
- Select an RMS gateway: Enter the frequency of an RMS gateway (e.g. OE3XEC on 3,607.5 kHz). Establish the connection and send a test email.
The DR-9400 additionally offers a web-based configuration interface via LAN or Wi-Fi — a significant improvement over the purely serial configuration of its predecessors.
Cost and Availability
The PXdragon DR-9400 is a premium product — in both performance and price:
- SCS PXdragon DR-9400: Approximately EUR 1,599 (as of 2026, from WiMo and other dealers)
- Additionally required: HF transceiver (e.g. Icom IC-7300 from approximately EUR 1,100), antenna, cable set
- Second-hand market: Older SCS modems such as the PTC-IIusb or DR-7800 regularly appear on the used market, offering PACTOR-3 for significantly less
Yes, EUR 1,599 is a lot of money. But for that investment, you receive a device that works reliably for years, requires no software licences, is operating-system independent, and functions under conditions where every other solution fails. For maritime use, emergency communications, and expeditions, there remains no more reliable HF data modem.
Who Benefits from a PACTOR Modem?
- Emergency communicators: Those seriously involved in emergency communications and part of A.R.E.N.A. or similar organisations benefit from a PACTOR modem's reliability — especially combined with an emergency go-kit
- Sailors and ocean-going vessels: For maritime use with SailMail, PACTOR is effectively the standard
- Expedition operators: On DXpeditions where no internet exists, PACTOR enables contact with the outside world
- Gateway operators: Anyone wanting to run a Winlink RMS gateway needs a reliable modem for continuous operation — PACTOR modems are designed for exactly this
- Technically curious: Those who appreciate the technical elegance of a mature HF data protocol and have the budget
For the occasional Winlink user who primarily operates under good conditions, VARA HF with a Digirig interface for under EUR 120 is the more economically sensible choice. Getting started in the world of HF data need not be expensive.
The Future: PACTOR-5 and Beyond
With the Linux-based DR-9400, SCS has created a platform that can be extended through software updates. The ARM architecture provides enough processing power for future protocols — a PACTOR-5 could be realised purely through software on the existing hardware. Whether and when a fifth PACTOR generation will arrive has not been announced, but the hardware is ready.
In parallel, the amateur radio community is working on open alternatives such as ARDOP (Amateur Radio Digital Open Protocol) and the aforementioned VARA HF. The coexistence of proprietary PACTOR and more affordable software alternatives energises the market — and advances the development of reliable HF data transmission overall.
Conclusion
The SCS PXdragon DR-9400 is a specialist tool for a specific purpose — and at that purpose, it is the best there is. Those who need reliable data transfer on shortwave, whether for Winlink, emergency communications, maritime use, or expeditions, will find in the DR-9400 a device that keeps working when everything else fails. The ARM/Linux architecture makes it future-proof, and the broad range of supported modes — from Packet Radio through APRS to Weather-FAX — makes it versatile.
PACTOR has been the most reliable way to transport data over shortwave for over 30 years. With the DR-9400, SCS has carried this legacy into the future.
73 – your oeradio.at editorial team
Transparency Notice
This article was researched and written with the assistance of AI (Claude, Anthropic). The editorial team has reviewed and edited all content. Despite careful review, occasional inaccuracies may occur — we welcome corrections via email to [email protected].
