A software package
for VHF DXers
WSJT: What is it?
• WSJT = Weak Signal by K1JT
• Developed by Joe Taylor, K1JT
• Supports two digital modes:
FSK441 for meteor-scatter
JT44 for extremely weak signals
• Uses computer soundcard
• Requires no fancy equipment
Meteor scatter anytime and
Meteor-scatter basics
Science fiction?
• Pieces of space dust enter the earth’s
atmosphere at relatively high speeds
• Mostly “dustballs” –light, porous particles
composed of light metals.
• About the size of dust-specks to grains of
• Friction rips electrons away from their
parent atoms ( = ionization)
Time is of the essence!
• Very short communication windows
• SSB and fast CW (>30 WPM) are the
traditional mediums
• (Good) operators employ special operating
practices to be most efficient
• SSB/CW QSO’s on 144 MHz or higher only
possible during major showers
“Underdense” vs. “Overdense”
• Overdense bursts are caused by bigger and
faster meteors. They ionize a thick column
of air that cannot neutralize quickly.
• Bursts can last many seconds, often long
enough for a complete QSO.
• Useful for SSB and CW
• Associated with major showers, and very
uncommon at other times.
Typical overdense burst
(70WPM CW)
Here’s a weird one…..
“Underdense” vs. “Overdense”
• Underdense trails are caused by smaller and
slower meteors. These meteors create just
enough ionization to scatter a radio wave.
• These meteors case very short bursts of
signal, usually < ½ second at the longest.
• Not very useful for conventional SSB or
CW—often referred to as “pings”
• Very common, even outside showers
Underdense “ping”
Using underdense trails—HSCW
• Send short messages in a loop
• Operators use several short bursts of signal
to piece together a QSO over time, rather
than “all-at-once.”
• Tape recorders slow down the 100 WPM or
faster CW. (Computer software has
increased speed to ~2000 WPM.)
• Popular in Europe since the 1960’s
Problems with HSCW
• Lots of energy wasted in keying sidebands.
• S/N gets worse as speed increases.
• Key-clicks can be a problem for other
users, especially if transmitter is
• Requires a full-duplex soundcard, or two
A digital solution: FSK441
• Like HSCW, one uses a high-speed loop to
complete a QSO over several short bursts of
• 100% duty cycle, so no extra energy wasted
by OOK.
• More user-friendly interface--like RTTY or
PSK31 modes.
• Much better S/N than HSCW at same speed
FSK441: What’s in a name?
• “FSK…” –it uses frequency shift keying.
You can think of it as fancy RTTY,
however, FSK441 switches among four
tones rather than alternating between two.
• Tones: 882Hz, 1323Hz, 1764Hz, 2205Hz.
• “…441” –Each character takes about 2.3ms
to send. Each character is composed of
three tones. That’s 441 baud.
Spectral display: FSK441 “ping”
Spectral display in WSJT
How it works….
• Operators take turns transmitting in 30-sec
• This ensures that only one station is sending
and only one station is listening at a given
• After each receive period, the program
decodes any signals that it detected and
displays the text on the screen.
How it works….
• The program calculates the average
amplitude for the recorded audio.
• The program looks for “spikes” in the
amplitude—these could be meteor pings
above the noise floor…could be QRN!
• If the detected spike satisfies certain other
parameters, the program will decode it as
text and display it on the screen.
WSJT in FSK441 mode
The FSK441 code
• Code only supports characters we are
interested in sending: callsigns, signal
reports, and very short messages.
• Uses the PUA43 alphabet: A-Z, 0-9, space,
period, comma, ?, /, #, and $. No formatting
characters, such as <CR> or <LF>.
• No stop bits: synchronization achieved with
no overhead!
RTTY and FSK441
RTTY (5-bit)
<SP> 00100
FSK441 (3-bit)
<SP> 033 *
FSK411 synchronization
• Space is encoded as “033”.
• No character starts with a “3”.
• All messages contain at least one space. If
the user does not enter one, the program
will add one to the end of the message.
• When WSJT finds a signal, it “looks” for
the sequence “033”. This is the point of
FSK411 synchronization
• A burst of signal contains the following:
• WSJT finds the “space” character:
• WSJT can now find the message:
….123 001 122 210 033 123 223 203 131…
K 1
K 0 S M
Single-tone messages
• Each character in the FSK441 code contains
at least two different frequencies—no “000”
“111” “222” or “333”.
• These characters are reserved for
“shorthand” messages: “R26” “R27”
“RRR” “73”.
• If one sends one of these messages in a
loop, the result is a pure single-frequency
carrier. (Hence the name!)
Single-tone messages
• These messages are shorthand for the most
common messages in an FSK441 sked.
• WSJT can use a separate algorithm to look
for single-tone messages, which means
better S/N than with the multi-tone
• It can occasionally result in false signals.
• You have to use your ham skills: Listen!
FSK441 operating procedures
• Very similar to SSB meteor-scatter
• Operators send information based upon
what they have copied from the other
• QSO is complete when both stations have
received complete callsigns, a piece of
information (usually report), and a
confirmation that it was received (“roger”).
FSK441 operating procedures
• 30-second sequences are standard.
• Western-most station transmits first. This is
in the Western hemisphere.
• DXpeditions usually run all schedules and
CQ’s on the same frequency and period,
regardless of direction.
• “Regular” CQ’s can be either first or second
period. This eases QRM (contests/showers)
FSK441 reporting system
First number (1-5)
Second number (6-9)
1: no info (not sent)
2: up to 5 seconds
3: 5 to 15 seconds
4: 15 to 60 seconds (!)
5: more than 60 s (!!!)
6: up to S3
7: up to S5
8: up to S7
9: S7 or stronger
Making a QSO
If you have copied….
• Nothing……………..
• Partial callsigns……..
• Both callsigns……….
• Both calls and report...
• “R” + report…………
• “RRR”……………….
then send….
Callsigns only
Callsigns only
Calls + report (or grid)
“R” + report
QSO is complete, send
“73” or (or QRZ, CQ)
….other meaningful messages…
Sometimes you need specific information:
• MMM………. “I need my callsign”
• YYY………… “I need your callsign”
• SSS………….. “I need your report”
• UUU………... “Your keying is unreadable”
These messages could be very useful when
pings are very short—222 and 432 MHz.
What equipment do I need?
• Most popular bands are 144 and 50MHz. There is
growing activity on 222MHz, and a few 432 MHz
QSO’s have been made.
• More is better, but “brick and yagi” are sufficient
on 144 and 222 any time of year.
• Brick and yagi has worked on 432, but more
operation is needed to draw conclusions.
• Preamp—you don’t know what you’re missing!
What equipment do I need?
• A computer and a soundcard-- 60Mhz
Pentium with 24Mb of RAM will work, but
you’ll be happier with more!
• The (free!) software
• A way to interface the soundcard with the
radio. You can use a commercial “PSK31”
interface (Rigblaster, MFJ, etc) or make
your own.
WSJT Station
What can I expect to work?
• On 144MHz, the average “brick and yagi”
station should be able to work a similar
station in the 600-1000mi range fairly
consistently, any time of year.
• Geometric limit of ~1400mi (based on the
height at which meteors ionize sufficiently)
• Limits, schlimits! Records are meant to be
144 MHz QSO’s from EN10rt
When should I operate?
• The daily “random” meteor rate peaks
around sunrise local time, but QSO’s are
possible anytime of day or night—it just
might take longer to complete it.
• Minor showers can “enhance” the
background meteor rate (June-December)
• Before and after major shower peaks
How about portable operation?
• No need to haul large antennas and big
amplifiers to the top of a mountain for
• One doesn’t need to plan DXpedtion around
major showers
The road to DN90wp
K0SM/P DN90wp
CY9DH—FN97we July 1-7
144 MHz FSK441 (37 QSOs)
FSK441 in contests
• Rovers can stray to distant grids and still
make contacts—that means new grids and
more multipliers.
• “Big guns” should be able to work anyone
within 1200mi on 144Mhz.
• Fills the “dead time” in the small hours of
the morning with valuable mulipliers.
A really weak signal mode
• Uses long term signal averaging to recover
a signal that is below the noise floor.
• Humans have “short ears” limited by their
sensory memory—they can only analyze a
signal in a small timeframe.
• Computers can analyze a signal in relatively
large timeframes.
Time for a demonstration!
“Long Ears” and “Short Ears”
The JT44 code
• Inspired by the PUA43 mode
• Uses 44 tones, one for each character in the
PUA43 alphabet (same as FSK441), plus a
synchronization tone. Each character is
assigned a unique frequency.
• Slow transmission speed: 5.38 baud.
• Highly redundant (FEC)
JT44 Tones
1755.0 Hz
A 1485.8 Hz
$ 1475.0 Hz
1399.7 Hz
1302.8 Hz
1270.5 Hz
• Bandwidth of 485Hz
• Tones spaced at
• Sync tone 32.3Hz
below data.
• Allows for frequency
error and EME
Doppler shift with and
2.7 KHz passband.
The JT44 code
• Transmission lasts about 25 s, with a gap at
the beginning and end to allow for timing
error and EME delay. (Like FSK441,
operators use alternating 30-second periods)
• 135 intervals (bits)
• 69 are devoted to sending the sync tone
• Other 66 intervals are used to send the
22-char message three times.
Why have a sync tone?
• Tones spaced at 10.3-Hz intervals.
• Most VHF radios are not that accurate—the
digital readout is lying to you!
• Most radios drift a little over long periods
of time.
• Sync tone provides a frequency reference
for the data.
How WSJT decodes JT44:
Finding the frequency
• WSJT does a frequency-analysis the whole
30-sec. Because the sync tone is sent over
half of the time, it should outweigh any
other frequencies (characters).
• This gives WSJT a frequency reference to
find the data.
• The frequency discrepancy is displayed in
the “DF” column on the screen.
Spectral analysis of a strong
(audible) JT44 signal
The pseudo-random pattern of
the sync tone
1-20: 11101000011100110000
21-40: 10010001010111010111
41-60: 10010010111001110000
61-80: 00111011101001111010
81-100: 10010100000010101010
101-120: 11111010110100000110
121-135: 111011011010110
How WSJT decodes JT44:
• WSJT looks for the “best match” to this
known pattern. Quality of sync from 0-9 is
displayed in the “sync” column.
• The time discrepancy is displayed in the DT
column on the screen Clocks must be
synchronized within ~1 sec of each other.*
Decoded JT44 signal
JT44 Message Averaging
• Single letters will appear 6.8dB below the sync
tone because less time is spent sending these
frequencies. (Sync tone sent 69/135 of the time,
individual character sent 3/135 of the time)
• Every doubling of the number of receive periods
adds 1.5dB of in S/N.
• Four periods get you 3dB improvement, 16
periods get you 6dB, etc.
• This assumes JT44 can synchronize each time
(i.e., signal is stronger than ~-29dB).
Message “folding”
• Some messages have symmetrical content:
• 22 characters is often enough room to send
two pairs of callsigns:
• Message can be “broken in half” and
averaged to achieve a better copy:
Line averages
• Some messages are even more redundant:
• And finally:
• Averages of even, odd, and last four
characters given after each reception.
Line averages
JT44 and CW comparison
Type of signal
(50Hz BW) (2500Hz BW)*
Minimum intelligible CW
JT44 random message
JT44 message after 4 min
JT44 limit of synchronization -12.9dB
*WSJT reports S/N in a 2500Hz bandwidth (dB column)
A demonstration…
• Copy this CW signal:
• Here’s a JT44 signal at an even lower S/N:
JT44 signal decoded in WSJT
JT44 operating procedures
30-second sequencing
Same as MS for terrestrial operation
EME operation almost always by schedules
Some “big guns” call CQ, but will often
announce beforehand (e-mail, web, packet).
JT44 operating procedures
• Terrestrial operation is similar to FSK441,
though grid squares are usually sent instead
of a report.
• Westernmost goes first (Region 2)
• EME operation usually uses the “TMOR”
system—same as CW.
• Always check with your sked partner!
Operator responsibilities
• Lock onto the other station’s signal and
adjust for frequency drift
• Identify and reject “bad syncs” so they are
not counted in the average message
• Adjust controls for QRN and birdies
JT44—terrestrial operation
• JT44 works well on troposcatter paths that
are too short for MS enhancement.
• Several operators have used it on 6m for
borderline TE, and marginal Es openings.
• Microwave non-line-of-sight paths
Other terrestrial applications
• Perhaps useful on extremely long IOS paths
between big stations.
• Could be a good way of overcoming
atmospheric absorption above 10GHz.
JT44: EME applications
• JT44’s ability to recover extremely weak
signals makes it ideal for EME
• Most activity by arranged schedule.
• 144MHz is the most popular band.
• Also operation on 50 MHz, 432MHz, and
1296 MHz so far….
What do I need to make EME
• A pair of single-yagi stations should be able
to work each other, with QRO power.
• “Brick and yagi” stations can work larger
stations at moonrise/moonset.
• Single-yagi and a few hundred watts on 6m
to work big guns (W7GJ, ON4ANT)
• W5UN has been copied with a “RingoRanger” vertical on 2m!
EME possibilities
• EME is possible with medium-power, solidstate amplifiers (bricks)
• Portable EME operation can be a reality.
• DXpedtions can afford to bring gear
CY9DH—FN97we JT44
• Worked JT44 on 50MHz “marginal Es”
• VE1ALQ and VE9AA (FN65) 2m JT44
• W7GJ on 144Mhz and 50MHz EME
• Partial with W7MEM 144MHz EME
• EME limited by terrain—no elevation
control, hill to east.
Clock synchronization
• Dimesion 4 Clock utility:
• Keeps computer clock synchronized with
WWV by internet
• You can also use GPS synchronization.
• Setting by hand is difficult.
Where to get WSJT:
• WSJT homepage:
• 5.6Mb download
• Self-extracting .EXE file— “click-click!”
• You get a color, 38-page manual in .PDF
format, complete with pictures and index of
buttons and other controls!