Page 29 January February 2018 TCA

The type of FEC is denoted by LDPC for and one synchronizing bit. Thus, 50% of FT8 – Designed especially for
low‑density parity check, C for the transmitted energy is devoted to propagation conditions, such as
convolutional, RS for Reed‑Solomon, and synchronization. Submodes JT4A through multi‑hop sporadic E at 50 MHz –
QRA for Q‑ary repeat‑accumulate. JT4G have tone spacings at increasing circumstances where signals are weak
Chosen keying rates make the length of a multiples 1, 2, 4, 9, 18, 36, and 72 times and fading, openings short, and quick
transmission approximately 13 seconds the keying rate of 4.375 baud. The wider completion of reliable, confirmable
for FT8 and 48 seconds for the other submodes are useful on propagation contacts is particularly desirable. Timed
modes. For JT4, JT9, JT65, and QRA64, paths with large Doppler spread. For sequences for transmission and reception
this leaves plenty of time at the end of a example, JT4F is frequently used for EME are 15 seconds rather than 1 minute.
transmission for a message to be communication on the 10 GHz band. Shorter transmissions mean that FT8 is
decoded and the receiving operator to about 6 dB less sensitive (for steady
decide how to reply, before the start of WSPR – Designed as a propagation signals) than it would be if its duration
the next minute. With 15‑second T/R probe rather than for making two‑way was commensurate with the other
sequences, FT8 is much more tightly contacts, it differs from other WSJT-X slow modes. Message packets include
constrained. An optional auto‑sequencing slow modes by using message lengths 75 information bits and a 12‑bit cyclic
feature allows the software to generate k = 50 bits and 2‑minute T/R sequences. redundancy check (CRC) that helps to
suitable messages in response to Message packets normally include a ensure a very low false decode rate.
received information. Exact values for 28‑bit call sign, a 15‑bit grid locator, and Modulation uses eight‑tone FSK at
keying rates were chosen so that at seven bits to convey transmitter power in 6.25 baud, and synchronization uses
12,000 samples per second the number dBm. Alternative formats can convey a three 7 × 7 Costas arrays. As suggested
of digital samples per channel symbol is compound call sign and/or a six‑digit grid by a snapshot from the PSK Reporter
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an integer with no prime factor greater locator, using a two‑transmission website (see Figure 3 on the next page),
than 7. This advantageous choice makes sequence. Typical WSPR usage was FT8 has become popular very quickly
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some of the digital signal processing described in QST. after its introduction in early summer
algorithms more efficient. JT9 – Eight tone frequencies are used to of 2017.
convey message information, and one
In the following, modes are described in Protocol Details for Fast Modes
chronological order of their development: additional tone is used for synchronization.
JT65 was first introduced in 2003, JT4 in The slow submodes JT9A‑H have tone The WSJT-X fast modes take advantage
2007, WSPR in 2008, JT9 in 2012, spacings at multiples 1, 2, 4, 8, 16, 32, of brief propagation enhancements that
QRA64 in 2016, and FT8 in 2017. and 64 times the 1.736‑baud keying rate. bring a signal up to useful levels for a
JT9A (often called simply JT9) uses less very short time. Keying rates and
JT65 – A detailed description of the than 10% of the bandwidth of JT65, and occupied bandwidths are much larger
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JT65 protocol was published in QEX. for steady, undistorted signals is about than for the slow modes, because we
Half of its channel symbols are used for 2 dB more sensitive than JT65. For these want full messages to be conveyed in a
synchronization, using a pseudo‑random reasons, JT9 is popular for low‑power very short time. Table 2 lists the essential
pattern at the lowest tone frequency. DXing on crowded HF bands. parameter values for these modes. The
The other symbols carry encoded last column gives the time required to
information using 2 = 64 different tones. QRA64 – An experimental mode intended transmit a message once. In these
m
Special features (used only for Earth‑ for EME and other extreme weak‑signal modes, the transmitted information is
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Moon‑Earth – EME) can convey the paths. Its internal code was designed by repeated for the full duration of a T/R
EME‑style “OOO” signal report and Nico Palermo, IV3NWV. Synchronization is sequence.
short messages interpreted as RO, accomplished by using three 7 × 7 Costas
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RRR and 73. EME submodes JT65B arrays. Submodes QRA64A‑E use tone ISCAT – Messages are free‑form, up to
and JT65C use tone spacings two and spacings 1, 2, 4, 8, and 16 times the 28 characters in length. The protocol
four times larger than JT65A. JT65 has 1.736‑baud keying rate. Early tests have uses no FEC other than repetition. ISCAT
become very popular for low‑power shown QRA64A to be very effective for has proven especially useful for making
DXing at MF and HF, as well as for weak‑signal work at MF and HF, and for aircraft‑scatter contacts on the microwave
EME on VHF and higher bands. EME on the VHF and UHF bands. The bands.
wider submodes QRA64C‑E work
JT4 – Each channel symbol carries one extremely well for EME on microwave JT9 Fast – Submodes JT9E‑H differ from
information bit (the most significant bit) bands up to 24 GHz. their slow counterparts by using much
faster keying rates. Otherwise the coding,
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