A Look at HF Digital Modes in Ham Radio

Voice communication methods such as SSB, AM, and FM are fundamental to the hobby. Digital modes present a captivating and often more efficient way to communicate, particularly when signals are weak or there is interference. These digital modes convert your text or voice into digital signals that can be transmitted over the airwaves, enabling reliable communication even under challenging conditions. Let's look at some of the most popular digital modes used on HF.

Radio Settings

Select the appropriate operational mode by choosing "USB-D" or "Data" instead of the standard USB setting. These specialized modes enhance signal processing for digital communication. Set your radio's filter bandwidth to 2.5 to 3 kHz to achieve optimal signal clarity for digital modes.

It's also important to turn off any signal processing features that could degrade digital transmissions. Turn off speech compression to avoid data distortion, deactivate noise blankers that may cut off critical signal components, and switch off automatic notch filters, which could remove essential elements of the digital signal.

These settings will ensure clean and accurate digital signal transmission by preserving the integrity of your signal.

Overdriving Ruins Your Signal

Overdriving the audio input to a transmitter, based on the misconception that "more power is better," leads to signal distortion. Clipping¹ occurs when the audio stages exceed their limits, resulting in unwanted harmonic artifacts.

Distortion may also arise from the receiver. An overloaded receiver can create artifacts in the waterfall display that mimic those from an overdriven transmitter.

Excessive audio drive does not improve communication in digital modes; it often increases decoder errors. A properly driven signal at lower power is always more effective than a distorted, overdriven one.

The most popular modes

FT8

This mode, part of the WSJT-X suite, has revolutionized weak-signal communication. FT8 (and its variants like FT4) is designed for contacts where signals are extremely weak. It uses short transmissions and automated protocols, allowing for contacts even with low power over vast distances. It's particularly popular for DXing (long-distance contacts).

FT8 uses 8-FSK modulation with forward error correction (LDPC codes). Messages are 77 bits encoded into 79 symbols. FT8 uses 15-second cycles, and FT4 uses 7.5-second cycles. FT8 works best for weak signal communication, DXing, and quick contact under marginal conditions. FT8 and FT4 send short, structured messages containing minimal information to make a valid QSO, and they are not suited for conversation.

PSK31

One of the most popular digital modes, PSK31 (Phase Shift Keying, 31 baud), is known for its narrow bandwidth and resistance to interference. It's excellent for weak signal work and ragchewing (casual conversation). Its text-based nature allows for keyboard-to-keyboard communication.

PSK Uses binary phase shift keying (BPSK) or quadrature PSK (QPSK) with varicode² encoding. Each character has a variable length, with common letters using shorter bit patterns. The narrow bandwidth achieves excellent spectral efficiency. PSK is good for low-power operation and crowded band conditions. This mode is used primarily for keyboard-to-keyboard conversation. Unfortunately, it has no native error correction, making it unreliable in poor propagation conditions. PSK is also susceptible to multi-path distortion.

RTTY (Radio Teletype)

A classic mode dating back to the early days of radio, RTTY uses Frequency Shift Keying (FSK) to transmit characters. While older than many other modes, it's still used and has a certain nostalgic appeal.

RTTY Uses 5-bit Baudot code with 170 Hz frequency shift keying (FSK). The standard speed is 45.45 baud, with mark and space frequencies separated by 170 Hz. Used for contests, DX operation⁴. This mode is good for keyboard-to-keyboard and point-to-point communication and is robust in good condition. Widely supported by a myriad of software. RTTY's lack of error corrections makes it unsuitable for long conversations in poor conditions.

Beyond the Basics

While the above modes are common, many other digital modes cater to specific needs and preferences. Here are the modes that utilize FEC (Forward Error Correction) that are worth exploring.

MFSK⁵

MFSK modes use multiple tones sent sequentially to represent the characters. Common variants include MFSK16 (16 tones) and MFSK32 (32 tones). Includes interleaving to combat fading and forward error correction, offering increased robustness in the face of interference. There are various MFSK implementations, each with its own characteristics. They often find use in situations where other modes struggle.

MFSK-16 allows you to send images in black and white, as well as 24-bit color.

Thor

Designed for challenging conditions, Thor is known for its ability to dig signals out of the noise. Thor is a robust mode with a slower transmission speed than PSK, making it ideal for operators interested in long, casual conversations. It uses multiple frequencies with sophisticated symbol interleaving and forward error correction. Available in multiple speeds (THOR8, THOR16, etc.). Thor is reliable in poor conditions, especially with multi-path.

Domino

Domino, another mode designed for weak signals, performs well under difficult conditions. It was often used for contesting and DXing, now replaced by FT8. It uses MFSK with strong forward error correction and interleaving. Various sub-modes (like DOMINO-EX) offer different speed/reliability trade-offs. Thanks to its strong error correction and multi-path handling, Domino works great in challenging band conditions and reliably delivers messages.

Olivia

Olivia is a robust mode that's particularly effective in the presence of selective fading, a common problem in HF. It's often used for long-distance communication and situations where reliability is paramount. Uses 32 or 64 tones with forward error correction. Tones and bandwidth are configurable (common formats: 8/250, 8/500, 16/500, where the first number is tone, and the second is bandwidth).

It is the best mode for extremely poor band conditions and provides reliable long-distance communication on weak signals, very reliable. Unfortunately, this comes at the price of slow speed and broad bandwidth requirements compared to other modes.

Contestia

As the name suggests, Contestia is a mode optimized for radio contesting. Contestia is similar to Olivia but focuses on performance rather than reliability. Its speed and efficiency make it well-suited for rapid information exchanges during contests. Uses multiple tones, 4 to 256, with forward error correction. Bandwidth and number of tones can be adjusted (common formats: 8/250, 8/500, 16/500). Provide reliable communication in poor band conditions and fading signals.

Contestia speed lies between Olivia and simpler modes of communication. It delivers a high level of reliability, ensuring that messages are transmitted effectively. However, this enhanced performance comes with a trade-off, requiring a wider bandwidth to function optimally.

Feld-Hell (Feldhellschreiber)

Feld-Hell is a digital mode that transmits text as a series of graphical images, similar to early mechanical Hellschreiber machines developed in the 1920s. Feld-Hell sends each character as a series of ON/OFF keyed pulses displayed on the receiving end as a scrolling visual representation of text. This method allows operators to decode messages by eye, even under weak signal conditions where noise might corrupt traditional digital transmissions.

Feld-Hell uses on-off keying (OOK) modulation, where a single-frequency carrier is rapidly switched on and off to create a visual pattern of each letter. The receiving station reconstructs the characters on a screen in real-time, where they appear as a continuous scrolling text stream. This mode is particularly effective in noisy HF conditions since the human eye can often recognize distorted characters better than a computer can decode garbled digital text. Because of its resilience and simplicity, Feld-Hell remains popular among amateur radio operators who enjoy historical and visually distinctive digital modes.

JS8Call

JS8Call is a digital mode for ham radio that extends FT8 with keyboard-to-keyboard messaging, relays, network-style communication, and automated messages. It uses the same 8-FSK modulation and error correction found in FT8. This mode has good weak-signal performance, but it is the slowest of the keyboard-to-keyboard modes of communication and has a complex setup.

Choosing the Right Mode

The best digital mode for your needs will depend on your requirements and operating conditions. FT8 is an excellent choice for exchanging call signs, and signal reports in harsh conditions or with weak signals. PSK31 is quite popular for more casual conversations. For challenging conditions or long ragchew, consider using modes like Thor, Domino, Olivia, or MFSK, which may be ideal for such situations.

Conclusion

Exploring the world of HF digital modes can greatly enhance your ham radio experience. Ham radio is a fascinating hobby, and digital communications provide opportunities for experimentation, weak-signal communication, and making contacts around the globe. So, power up your radio, install the necessary software, and start decoding the airwaves!