G8C: Digital Emissions – Ham Radio General License Study Guide

G8C covers the digital emission modes used in amateur radio, from classic RTTY and packet to modern weak-signal modes and digital voice systems. The fifteen active questions in this group cover a broad range of digital technology including propagation beacons, data encoding standards, error correction protocols, mesh networking, signal display tools, and digital voice.

Topics include what digital mode is used as a low-power HF propagation beacon, which part of a packet radio frame carries routing information, how Baudot code is structured, what a NAK response means in ARQ, what happens when ARQ retransmissions fail, which digital mode works with very low SNRs, a key characteristic of PSK31, how mesh network nodes behave when one fails, how FEC allows error correction, how FSK frequencies are identified, what code PSK31 uses, what vertical lines on a waterfall display indicate, how a waterfall display is oriented, what an FT8 signal report of +3 means, and which modes are digital voice.

Weak-Signal Modes: WSPR and FT8

WSPR (Weak Signal Propagation Reporter) is a narrow-band digital mode used as a low-power beacon for assessing HF propagation. Stations transmit short, highly structured WSPR signals, which automated receiving stations decode worldwide and upload to the WSPRnet database. Operators can monitor where their signals are being received using just a few watts or even milliwatts. WSPR is not a conversational mode — it is strictly for propagation assessment.

FT8 is a narrow-band weak-signal digital mode that can receive signals with very low signal-to-noise ratios — well below the audibility threshold for human ears or for CW. FT8 signals are structured into 15-second transmission intervals, decoded by software, and use a precise time reference. Signal reports in FT8 are expressed in dB relative to a reference noise floor in a 2.5 kHz bandwidth. A report of +3 means the signal-to-noise ratio is equivalent to +3 dB in a 2.5 kHz bandwidth.

PSK31, Packet Radio, and Baudot

PSK31

PSK31 is a narrow-band phase-shift keying mode popular for HF keyboard-to-keyboard contacts. It uses Varicode — a variable-length encoding where common letters (e, t, a) use short bit sequences and less common characters, particularly upper case letters, use longer Varicode bit sequences. This means typing in upper case actually slows the effective transmission speed. PSK31 does not include built-in error correction, but its narrow bandwidth and slow baud rate give it good performance in noisy conditions.

Packet Radio

Packet radio organizes data into frames — self-contained units containing the data and all the information needed to route and verify it. A packet radio frame has distinct sections:

• Header — contains the routing and handling information: source address, destination address, and control flags
• Data payload — the actual information being transmitted
• Trailer — typically contains a cyclic redundancy check (CRC) for error detection

The exam specifically asks about the part carrying routing and handling information — that is the header.

Baudot Code

Baudot code is the character encoding used in RTTY (radioteletype). It is a 5-bit code with additional start and stop bits. Because 5 bits provide only 32 combinations, Baudot uses two shift states (letters and figures) to represent both the alphabet and numbers/punctuation. Baudot code predates ASCII and is still used in amateur HF RTTY operation.

Error Correction: ARQ and FEC

ARQ (Automatic Repeat reQuest)

ARQ is a two-way error-correction protocol in which the receiver acknowledges each packet received. If a packet arrives with errors, the receiver sends a NAK (Negative Acknowledgment), which tells the transmitter to retransmit that packet. If the packet arrives correctly, the receiver sends an ACK (positive acknowledgment).

ARQ requires a reliable two-way link to work. When the link becomes too poor to successfully exchange packets — after excessive retransmission attempts — the connection is dropped. ARQ modes include PACTOR, WINMOR, and VARA, used in systems like Winlink.

Forward Error Correction (FEC)

FEC (Forward Error Correction) takes a different approach. Rather than requesting retransmission, FEC allows the receiver to correct errors without any feedback path. It does this by transmitting redundant information along with the data. The extra bits let the receiver reconstruct the original data even when some bits are corrupted by noise. FEC is used in one-way transmissions (broadcast), weak-signal modes, and situations where a return path is unavailable. FT8, for example, includes FEC in its encoding.

FSK, Mesh Networks, and Waterfall Displays

FSK Mark and Space

In Frequency Shift Keying (FSK), the transmitter shifts between two frequencies to represent binary data. These two frequencies are called mark and space. Mark traditionally corresponds to a binary 1 (or key-down in RTTY); space corresponds to a binary 0 (key-up). The separation between mark and space is the FSK shift, typically 170 Hz for amateur HF RTTY.

Mesh Network Microwave Nodes

A mesh network is a topology in which each node can route traffic through multiple paths. The key property for the exam: if one node fails, a packet can still reach its target station via an alternate node. The network automatically finds an alternate path, making mesh networks resilient to single-node failures. Amateur mesh networking uses modified commercial Wi-Fi hardware operating on Part 97-eligible frequencies.

Waterfall Display

A waterfall display presents radio spectrum information in three dimensions simultaneously:

• Frequency — horizontal axis (left to right)
• Signal strength — color intensity (brighter or different color = stronger)
• Time — vertical axis (top = most recent; scrolls downward as time passes)

A signal being transmitted appears as a vertical stripe running downward in real time. When a data mode or RTTY signal is overmodulated, one or more vertical lines appear on either side of the main signal stripe — these sidebands are spurious emissions caused by the overmodulation.

Digital Voice Modes

Digital voice encodes speech as compressed digital data rather than analog FM or AM. The three principal digital voice systems in amateur radio are:

These are the digital voice modes. WSPR, FT8, FT4, and similar modes are weak-signal data modes, not voice. Winlink, PACTOR, and VARA are data/messaging modes. The exam asks specifically which modes provide digital voice — the answer is DMR, D-STAR, and SystemFusion.

G8C Practice Questions