THOR is a family of offset incremental multi-frequency shift keyed modes with low symbol rate, closely related to DominoEX. A single carrier of constant amplitude is stepped between 18 tone frequencies in a constant phase manner. As a result, no unwanted sidebands are generated, and no special amplifier linearity requirements are necessary. The tones change according to an offset algorithm which ensures that no sequential tones are the same or adjacent in frequency, considerably enhancing the inter-symbol interference resistance to multi-path and Doppler effects.
The mode has full-time Forward Error Correction, so is extremely robust. The default speed (11 baud) was designed for NVIS conditions (80m at night), and other speeds suit weak signal LF, and high speed HF use. The use of incremental keying gives the mode complete immunity to transmitter-receiver frequency offset, drift and excellent rejection of propagation induced Doppler.
These are unconnected, manually controlled message asynchronous simplex chat modes, using binary convolutional Forward Error Correction. The default calling mode is THOR11.
A binary varicode with ASCII-256 user interface (same as MFSK16) is used. Lower case characters are sent faster. An ASCII-128 secondary character set extension allows a fixed (typically ID) message to be sent whenever the transmitter is idle. Modulation uses two dibit pairs, symbol synchronous, differential.
The FEC system uses binary convolution to generate two dibits per varicode bit, and halves the corrected data rate compared to the equivalent DominoEX mode. Rate R=1/2, Constraint length K=7, Interleaver L=10 (40 bits).
| Mode | Symbol Rate | Typing Speed1 | Duty Cycle2 | Bandwidth3 | ITU Designation4 |
| THOR45 | 3.90625 baud | ~ 1.25 cps (12 wpm) | 100% | 173 Hz | 173HF1B |
| THOR55 | 5.3833 baud | ~ 1.6 cps (16 wpm) | 100% | 244 Hz | 244HF1B |
| THOR85 | 7.8125 baud | ~ 2.5 cps (25 wpm) | 100% | 346 Hz | 346HF1B |
| THOR116 | 10.766 baud | ~ 3.5 cps (35 wpm) | 100% | 262 Hz | 262HF1B |
| THOR16 | 15.625 baud | ~ 5.0 cps (50 wpm) | 100% | 355 Hz | 355HF1B |
| THOR22 | 21.533 baud | ~ 7.0 cps (70 wpm) | 100% | 524 Hz | 524HF1B |
1. WPM is based on an average 5 characters per word, plus word space. Values are approximate because a variable length code is used.
2. Transmitter average power output relative to a constant carrier of the same PEP value.
3. This is the "Necessary Bandwidth" as defined by the ITU.
4. A summary of the ITU Designation system can be found at