Zero Retries Guide to Amateur Radio HF Data Communications

The following is the Zero Retries perspective of this subject, primarily the knowledge and perspective of Steve Stroh N8GNJ.

Index:

Introduction to this Zero Retries Guide

Amateur Radio data communications over HF has come a long way since Radio Teletype (RTTY) developed in WW2 and 300 bps Audio Frequency Shift Keying (AFSK) AX.25 Packet Radio developed in the 1980s.

“300 baud HF packet” was never a good, fast, or reliable system.
It only kind-of worked, some of the time.
It had poor ability to deal with the noise and other vagaries of the HF bands.
The only redeeming features of “300 baud HF packet” were:
1) It was better than RTTY for data communications,
2) It was the best we could do at one point (using TNCs),
3) It was included in every legacy Terminal Node Controller (TNC) after the TAPR TNC-1.

Whither TNCs for Use on HF

The following information is intended for those considering doing data communications on Amateur Radio High Frequency (HF) bands, and don’t have the background to understand the differences in Amateur Radio catalogs between TNCs and “sound card” interfaces.

The primary use case for using a TNC for HF data communications is “I already have one” / “I’ve been using it for years” / “It works fine for me and what I want to do on HF”. Those are all fair points, and this information is probably not applicable for you.,

While there are many implementations / variations of “TNCs” (including some developed in the 21st century), there is no significant advantage to using a “TNC” where protocol functions are performed in a microcontroller in the TNC, such as implementing a “KISS” interface. The reason to prefer modems versus TNCs is that for HF use, most “TNCs” implement legacy HF packet radio - 300 bps, or perhaps a bit faster.

Some TNCs add Forward Error Correction for 300 bps AFSK packet for HF:

NinoTNC hardware TNC (implements IL2P FEC).
OpenTNC hardware TNC (implements FX.25 FEC).
Dire Wolf software TNC (implements FX.25 and IL2P FEC).

An article by Billy Penley KN4NKB makes the case, in the 2020s for modems versus TNCs - The Hard Truth about Hardware TNCs in Packet Radio. KN4NKB also has an excellent YouTube channel - ModernHam.

Amateur Radio can now use advanced modulation techniques such as Orthogonal Frequency Division Multiplexing OFDM within the audio bandwidth of the transmission, and other techniques such as Forward Error Correction (FEC) to achieve reasonable data rates and reasonable reliability for data communications on HF.

In the US, HF Maximum Bandwidth is 2800 Hz

A significant improvement in HF data communications for US Amateur Radio took effect in 2024-01 - “symbol rates” were removed, and…

Maximum bandwidth of 2800 Hz for most HF bands

was implemented by the US FCC for US Amateur Radio Operations.

A bandwidth of 2800 Hz allows many modern data communications systems, that achieve reasonable speeds (10 kbps), to be used on the US Amateur Radio HF bands.

Use Modems, Not TNCs

Terminology - Modems used in Amateur Radio data communications are often referred to as “Sound Card” Interfaces, or “Audio Interfaces”. The former term is from the era when audio circuitry in a personal computer was an add-in card - a sound card. The latter term Audio Interface is a reasonable description of the function under discussion, but Modem (Modulator / Demodulator) is pretty universally understood to be an interface between audio signals and data communications. Thus, the term modem is used in this document.

The best implementation of “advanced” Amateur Radio HF Data Communications is to use a modem (USB connection, with no intelligence) between a computer and an HF radio.

Computer <—> Modem <—> HF Radio

The modems listed below also provide the ability for the computer to send a “hardwired” Push To Talk (PTT) control signal to the HF radio. (Using a “hardwired” PTT is strongly preferred, and more reliable and efficient, than the use of Voice Operated Transmit - VOX.)

Commercial Amateur Radio Modems for HF

Some well-supported commercial audio adapters for Amateur Radio HF data communications:

Systems for “Advanced” Data Communications on Amateur Radio HF

Pactor 4

Data rates up to 10500 bps using 2400 Hz bandwidth.
Requires a proprietary (and expensive) hardware modem by SCS
Usable for network communications - just a bit pipe, no integrated applications.

VARA HF

Adaptive data rates - see chart; (bandwidth unspecified):

Proprietary, Windows only, requires $69 license for full functionality
Download and information at https://rosmodem.wordpress.com
Usable for network communications - just a bit pipe, no integrated applications.

FreeDATA

Data rates up to 9800 bps using 1700 Hz bandwidth.
Open source project, uses the OFDM modem technology developed for FreeDV Amateur Radio Digital Voice mode for HF, adapted to transport data instead of digital voice.
Download and information at https://freedata.app.
Usable for network communications, includes a messaging system.

Rhizomatica Mercury

Data rates up to 5735 bps (bandwidth unspecified).
Open source project (not specifically for use in Amateur Radio).
Download and information at https://github.com/Rhizomatica/mercury
Usable for network communications - just a bit pipe, no integrated applications.

Amateur Radio Digital Open Protocol (ARDOP)

ARDOP was originally a project by Rick Muething KN6KB to develop a software modem for HF use by Winlink. That (original) development work (and use of the ARDOP software developed) was largely abandoned as VARA HF was developed and subsequently widely adopted by WInlink HF users. “Largely” - some usage of ARDOP has continued. There was some subsequent “forked” development of ARDOP:

ARDOPOFDM

Data rates up to 4600 bps (bandwidth unspecified).
Open source project (status unknown).
Download - https://www.cantab.net/users/john.wiseman/Downloads/Beta/.
Information - https://www.cantab.net/users/john.wiseman/Downloads/Beta/ARDOP1OFDM.txt

ardopcf

Data rates and bandwidth unspecified.
Open source project.
Download and information - https://github.com/pflarue/ardop/tree/master.

Feedback Requested

Readers of this page, please advise if there are other “advanced” data communications systems for Amateur Radio HF data communications that are not listed above.

Please email editor@zeroretries.net with any feedback or comments.

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