Quick-Start Guide to Q65
Joe Taylor, K1JT; Bill Somerville, G4WJS; Steve Franke, K9AN; and Nico Palermo, IV3NWV
April 3, 2021

WSJT-X 2.4.0 introduces Q65, a digital protocol designed for minimal two-way QSOs over especiallydifficult propagation paths. On paths with Doppler spread more than a few Hz, the weak-signal
performance of Q65 is the best among all WSJT-X modes. Q65 is particularly effective for troposphericscatter, rain scatter, ionospheric scatter, TEP, and EME on VHF and higher bands, as well as other typesof fast-fading signals.
Q65 uses 65-tone frequency-shift keying and builds on the demonstrated weak-signal strengths ofQRA64, a mode introduced to WSJT-X in 2016. Q65 differs from QRA64 in the following importantways:•

A new low-rate Q-ary Repeat Accumulate code for forward error correction•

User messages and sequencing identical to those in FST4, FT4, FT8, and MSK144•

A unique tone for time and frequency synchronization. As with JT65, this “sync tone” is readily
visible on the waterfall spectral display. In addition, Q65 provides a sensitive “sync curve” near
the bottom of the waterfall window. Unlike JT65, synchronization and decoding are effective
even when meteor pings or other short signal enhancements are present.

•Optional submodes with T/R sequence lengths 15, 30, 60, 120, and 300 s and different tonespacings.

•

A new, highly reliable list-decoding technique for messages that contain previously copied
message fragments. No use is made of a callsign database.

•Highly effective message averaging for situations where single transmissions are too weak orsignal enhancements too sparse for a signal to be decoded.

•A “multi-decode” option that attempts to decode all Q65 signals in the received passband.

Basic parameters of Q65 for each of the five T/R sequence lengths and their minimum tone spacings(“A” submodes) are summarized in the table below. Threshold sensitivities (SNR in 2500 Hz
bandwidth yielding 50% probability of decode) were measured for each submode using simulations
over the additive white Gaussian noise (AWGN) channel. As with other recently developed modes inWSJT-X, a feature called a priori (AP) decoding improves sensitivity by several additional dB as
information is accumulated during a standard minimal QSO.
T/R
Period(s)SymbolLength(s)“A”
Tone
Spacing(Hz)“A”
Occupied
Bandwidth(Hz)Transmission
Duration
(s)

SNR
(dB)

Max AP
SNR(dB)

15

0.150

6.667

433

12.8

-22.2

-23.7

30

0.300

3.333

217

25.5

-24.8

-26.6

60

0.600

1.667

108

51.0

-27.6

-30.2

120

1.333

0.750

49

113.3

-30.8

-32.5

300

3.456

0.289

19293.8

-33.8

-37.4

Forward error correction (FEC) in Q65 uses a specially designed (65,15) block code with six-bitsymbols. Two symbols are “punctured” from the code, yielding an effective (63,13) code with apayload of k = 13 information symbols conveyed by n = 63 channel symbols. The punctured symbols
consist of a 12-bit CRC computed from the 13 information symbols. The CRC is used to reduce thefalse-decode rate to a very low value. A 22-symbol pseudo-random sequence spread throughout atransmission is sent as “tone 0” and used for synchronization. The total number of channel symbols ina Q65 transmission is thus 63 + 22 = 85.
For each T/R sequence length, submodes A - E have tone spacings and occupied bandwidths 1, 2, 4, 8,and 16 times those specified in the above table. Full submode designations include a number forsequence length and a letter for tone spacing, as in Q65-15A, Q65-120C, etc. Tone spacings andoccupied bandwidths for the wider submodes are summarized in the table below. Additional submodes
120F, 300F, and 300G might be implemented in future if there is a perceived need.
T/RABCDEPeriod Spacing Width Spacing Width Spacing Width Spacing Width Spacing Width
(s)
(Hz)
(Hz)
(Hz)
(Hz)
(Hz)
15

6.67

433

13.33

867

26.67

1733

N/A

N/A

30

3.33

217

6.67

433

13.33

867

26.67

1733

60

1.67

108

3.33

217

6.67

433

13.33

867

26.67

1733

120

0.75

49

1.50

98

3.00

195

6.00

390

12.00

780

300

0.29

19

0.58

38

1.16

75

2.31

150

4.63

301

N/A

On-the-air experience during the first six months of testing has shown that Q65 is more sensitive thanany other WSJT-X mode when path Doppler spread is more than a few Hz. An excellent example oftargeted uses of Q65 is ionospheric scatter on the 6 m band. Extensive tests on the 1150 km path
between K1JT and K9AN have shown that with 300 W power output, nearly every Q65-30A
transmission is copied correctly by the other station. Q65 will enable stations with a modest Yagi and100 W or more and to work one another on 6 m at distances up to ~2000 km on most days of the year,
in dead band conditions. Ionospheric scatter is best near mid-day and in summer months, but is present
at all times.
Many tests of Q65 on EME, troposcatter, rain scatter, optical scatter, and other potentially interesting
paths have been conducted over the past six months. We find that suitable Q65 submodes perform wellin a wide variety of conditions. Decoding is effective for signals with Doppler spread up to ten times
the tone spacing, and even beyond.
Recommended submodes known to be effective for specific applications include the following.
Obviously, you should feel free to experiment with other combinations. Please report your results on asuitable forum!
•••••••Trans-Equatorial Propagation (TEP) on 50 MHz: 15C, 30C
Ionospheric scatter on 50 MHz:30A
QRP ionospheric scatter on 50 MHz:
120E
Ionospheric scatter on 144 MHz:
60C
Troposcatter and rainscatter at 10 GHz:
60D
Small-dish EME, 10 and 24 GHz:
120E
Other EME: 50, 144 MHz 60A; 432 MHz 60B; 1296 MHz: 60C; 10 GHz: 60D

Some representative sensitivity curves from simulation measurements are presented in Figures 1 and 2.
-21
Q65-60D No AP

SNR for 50% decode probability (dB)-22-23

Q65-120E No AP-24-25

Q65-60D Max AP-26-27

Q65-120E Max AP-28-29-30
020406080

100

Doppler Spread (Hz)Figure 1. – Threshold sensitivities for Q65-60D and Q65-120E as a function of frequency spread.

100Percent Decodes

8060
0510fSpread = 20 Hz

40200-30-28-26-24-22-20

SNR2500 (dB)Figure 2. – Decoding probability of Q65-60A (no AP) as a function of SNR and frequency spread.

In most ways operating with Q65 is like that for other popular WSJT-X modes, but you should know
about some differences. Many of these are introduced in the following short tutorial, which I
recommend following before you proceed to make QSOs with Q65.
1. Download a collection of Q65 sample files fromhttps://physics.princeton.edu/pulsar/k1jt/Q65_Samples.zip and unzip it into a convenient
directory such as C:\WSJT-X\Q65_Samples
2. Start WSJT-X 2.4.0, select Q65 mode, and configure other parameters as shown in the screen
shot below. On the File → Settings → General tab, be sure to check Enable VHF andsubmode features and Decode after EME delay, and uncheck Single decode. On the Decodemenu select Fast, Enable averaging, and Auto Clear Avg after decode.
3. Go to File → Open, navigate to where you saved the sample files, and open the file...\Q65_Samples\60A_EME_6m\210106_1621.wav. You should see something like thisscreen shot, which shows decodes of four 6-meter EME signals received at W7GJ on January 6,
2021. Note that decoding takes place first at the selected Rx Freq, then at other frequencies.

4. Notice that Q65 Sync has been selected at the bottom of the Wide Graph window. This choiceenables the orange “sync curve,” a special Q65 feature that provides sensitive indications ofwhere Q65 sync tones may be present. Here you can see significant spikes at the frequencies
697, 943, 1420, and 1620 Hz, and the Single-period Decodes panel shows decodes at each ofthese frequencies. Strong Q65 signals show additional “grass” on the orange curve to the rightof the sync tone. The decoder generally ignores these.
5. Q65 decodes are always tagged with the lower-case letter “q” followed by one or two decimal
digits. The first digit indicates the type of a priori (AP) information used, if any. When present,a second digit shows the number of Rx sequences that have been averaged to produce thedecode.
6. To get a feeling for some of these features, switch to submode Q65-30A, set RxFreq to 1000
Hz, and open the first sample file in directory ...\Q65_Samples\30A_Ionoscatter_6m.
This file will not decode, but if you select File → Open next in directory (or hit F6) twice toread the next two files, you should see022900 -19 0.4 1010 : K1JT K9AN R-16

q03

in the Average Decodes panel. (Be sure to wait until the blue Decode indicator has cleared,before hitting F6.) None of the files at 022700, 022800, or 022900 UTC will provide a decodeby itself, but the average of all three does.
7. Go to File → Settings → General (or hit F2) and set MyCall temporarily to K1JT. Doubleclick the Erase button to clear both panels of decoded text and then repeat the sequence ofopening the first two sample files in the 30A_Ionoscatter_6m directory. You should nowsee022900 -20 0.4 1010 : K1JT K9AN R-16

q22

in the Average Decodes panel. The “q22” flag shows that with the first callsign alreadyavailable as AP information, a valid decode is obtained from the signals in just two Rxsequences.
8. Hit F6 two more times to read and process the third and fourth files in this directory. The file at022900 UTC will not decode, but the one at 024000 produces the result
024000 -21 0.3 1010 : K1JT K9AN R-16

q2

in the Single-Period Decodes panel. Again, AP information for the first callsign has made thedecode possible.
9. Double-click on the line of decoded text so that K9AN will be copied into the DX Call entry
field. You should now find that two of the four files in the 30A_Ionoscatter_6m directory
will decode immediately with “q3” flags. This happens because both callsigns are now
available as AP information. You may also notice that a red sync curve now appears along withthe orange curve. For “q3” decodes the red curve is the most sensitive indicator of propersynchronization.
10. By now you should know enough to examine and decode signals in all the remaining
subdirectories of sample files. Signals in these files are weak and may not be readily seen onthe waterfall. Set the submodes as indicated by the subdirectory names and Rx Freq to thevalues shown in the table on the next page. All of the sample files are real over-the-air signals,
recorded by users of WSJT-X 2.4.0 who were part of the Q65 test group. They provide good
examples of signals on a wide variety of propagation paths.

Subdirectory

Rx Freq

Message60B_1296_Troposcatter

1000

VK7MO VK7PD QE38

60D_EME_10GHz

1000

VK7MO K6QPV DM1212D_Rainscatter_10GHz

1000

VK3WE VK7MO QE37

120E_Ionoscatter_6m

1800

KB7IJ N0AN 73

300A_OpticalScatter

1000

VK7MO VK7PD QE38

11. While exercising the sample files you should experiment with turning on and off various
program features such as Single decode, Enable averaging, and Auto Clear Avg after decode.
Try double-clicking on the waterfall to decode a particular signal, using for example signals inthe 60A_EME_6m subdirectory. Do not pay too much attention to displayed values of SNR for
received signals. We are working toward a more reliable algorithm for estimating thesenumbers.
12. To get an even better feeling for how Q65 works for EME on lower VHF bands, you candownload many additional files here:
https://physics.princeton.edu/pulsar/k1jt/60A_EME_6m_2.zip Each of these files recorded on 6
meters by W7GJ includes at least two decodable EME signals. For reasons you should now
understand, full sensitivity will require decoding them with MyCall set to W7GJ.
13. Finally, here’s a summary of meanings of the “q#” flags:
◦ q0?

?

?

◦ q1 CQ?

?

◦ q2 MyCall?

?

◦ q3 MyCall DxCall?

◦ q4 MyCall DxCall [<blank> | RRR | RR73 | 73]