E2E: HF Digital Modes

E2E covers digital mode operating on HF: the modulation types, timing, bandwidth, and technical characteristics of FT8, FT4, FST4, Q65, PACTOR, PSK31, WSPR, RTTY, MFSK16, ALE (Automatic Link Establishment), and the distinction between direct FSK and audio FSK.

The Extra exam tests precise knowledge of what each mode does and how it works — not just the name, but the specific technical details that distinguish one mode from another.

HF Data Emissions: FSK Below 30 MHz

Data emissions below 30 MHz use FSK (Frequency Shift Keying) as the modulation type. FSK shifts the transmitter frequency between two or more defined frequencies to encode data. This is used by RTTY, PACTOR, and various other HF data modes. DTMF tones on FM, pulse modulation, and spread spectrum are not used for HF data emissions under standard amateur radio practice.

WSJT-X Timing Synchronization

WSJT-X digital modes (including FT8, FT4, WSPR, JT65, and others) synchronize transmit/receive timing by synchronization of computer clocks. Both stations must have their computer clocks accurate to within about 1 second for FT8 (and tighter for some modes). This is typically accomplished using internet time servers (NTP) or GPS-disciplined clocks. The synchronization is not achieved by aligning frequency shifts, sync-field transmission, or sync-pulse timing — the precision clock synchronization is what enables the fixed, predictable TX/RX switching windows that WSJT-X modes depend on.

FT8: 15-Second Cycles

The length of an FT8 transmission cycle is 15 seconds. Each FT8 transmission occupies exactly 12.64 seconds within a 15-second window, leaving time for switching and decoding. The 15-second period does not vary with the amount of data — every FT8 message is encoded at a fixed length. The 8-second choice would describe an 8-second cycle (which is not FT8), and 30 seconds applies to JT65 periods, not FT8.

FT4: Four-Tone CPFSK

The "4" in FT4 refers to four-tone continuous-phase frequency shift keying (CPFSK). FT4 uses four tones with continuous phase transitions between them, which maintains spectral efficiency. This is not a reference to bits of user information, not to four TX/RX cycles per minute, and not to all of those simultaneously. The "4" specifically identifies the four-tone modulation scheme.

FST4 Characteristics

FST4 is a WSJT-X mode with several distinctive characteristics — all of which are correct:

Q65 vs JT65

Q65 differs from JT65 in that Q65 averages multiple receive cycles before attempting to decode. This averaging technique improves decode probability under very weak signal conditions — conditions where a single receive cycle would not provide enough signal-to-noise ratio for successful decoding. Q65 does not support keyboard-to-keyboard operation (neither does JT65), and Q65 does not use quadrature modulation as its defining difference from JT65.

WSPR: No Keyboard Operation

WSPR (Weak Signal Propagation Reporter) does not support keyboard-to-keyboard operation. WSPR is a beacon mode — it transmits a fixed, structured message containing the operator's call sign, grid square, and power level. Stations receive and decode WSPR beacons and automatically upload spots to the WSPRnet database. There is no provision for typing messages in real time. RTTY, PSK31, and MFSK16 all support keyboard-to-keyboard (conversational) operation.

PACTOR: Binary Transfer and Throughput

PACTOR is the HF digital mode that can transfer binary files. PACTOR's protocol includes error checking and retransmission at the link layer, enabling reliable transfer of arbitrary binary data — not just printable ASCII text. PSK31, RTTY, and AMTOR are text-oriented modes that do not support binary file transfer.

PACTOR IV has the highest data throughput of the modes listed in the exam question set under clear communication conditions. PACTOR IV uses multi-tone waveforms and adaptive modulation to achieve rates far above what MFSK16, 45-baud RTTY, or FT8 can achieve. FT8 is optimized for extreme weak-signal operation, not throughput.

PSK31: Variable-Length Encoding

PSK31 uses variable-length character coding. Specifically, PSK31 uses Varicode — a variable-length binary code where common characters (like 'e', 't', 'a') are assigned shorter codes and uncommon characters are assigned longer codes. This makes PSK31 efficient for English text transmission because the most frequently used characters take less time to transmit.

RTTY uses fixed 5-bit Baudot codes. PACTOR uses its own encoding scheme. MT63 uses a fixed parallel tone structure. Only PSK31 uses variable-length character coding (Varicode) among these options.

Narrowest Bandwidth

Among the modes compared on the exam, FT8 has the narrowest bandwidth. An FT8 signal occupies approximately 50 Hz of spectrum. For comparison, MFSK16 uses about 316 Hz, 170 Hz shift 45-baud RTTY uses approximately 250–300 Hz total, and PACTOR IV can use up to 2.4 kHz. FT8's extreme spectral efficiency is one reason it has become the dominant weak-signal HF digital mode.

Direct FSK vs Audio FSK

The key difference between direct FSK and audio FSK is that direct FSK modulates the transmitter's VFO (voltage-controlled oscillator) directly. The data signal shifts the actual transmit frequency of the radio. In audio FSK, the data is encoded as audio tones fed into the microphone or audio input of an SSB transmitter, which then transmits those audio tones as sideband signals — the result is frequency-shifted output, but achieved through the audio path rather than by directly controlling the carrier frequency.

Direct FSK does not inherently occupy less bandwidth, cannot transmit higher baud rates simply by being direct, and the correct answer is the single distinguishing fact: direct FSK modulates the transmitter VFO.

ALE: Automatic Link Establishment

ALE (Automatic Link Establishment) stations establish contact by constantly scanning a list of frequencies and activating the radio when the designated call sign is received. ALE systems monitor a set of predefined HF frequencies in rotation, listening for their own call sign to be transmitted by another ALE station. When a match is detected, the ALE system completes link setup automatically — selecting the best frequency, synchronizing, and connecting — without requiring the operator to manually tune the radio.

ALE does not monitor internet sites, does not send constant tone codes to reserve frequencies, and does not use backscatter echo for activation. The defining characteristic is automatic frequency scanning with call-sign-triggered activation.

E2E Practice Questions