EME with JT65
Joe Taylor, K1JT
Most readers of this column already know about WSJT, a free computer program 1,2 that cangreatly enhance your station’s capabilities for making distant VHF/UHF contacts. Theprogram was designed with meteor scatter and Earth-Moon-Earth (EME) communication
especially in mind. Anyone who has attempted such QSOs knows they are not easy atamateur power levels — which of course is one reason why they can be so much fun.
Successful EME stations, in particular, require the very best of amateur engineering. It’salways a stretch: we strive for every possible dB of antenna gain and transmitted power, while
minimizing our feedline losses and receiver noise figures. The necessary hardware can belarge and complex, and some of it is expensive. Optimizing the various parts of our stations
while minding the budget is a game that all EME enthusiasts must learn to play.
These days, not all good engineering involves hardware — and that’s where WSJT and its
operating mode JT65 come in. The JT65 transmission protocol uses state-of-the-art message
encoding with powerful error-correcting features, coupled with highly efficient modulation
based on a constant-envelope waveform and 65-tone frequency-shift keying. The
combination is much more effective for EME than Morse code and on-off keying. Together
with well optimized decoding algorithms, it enables JT65-equipped stations to make QSOs atsignal levels some 10 to 15 dB below the minimum required for “ear and brain” CWcommunication. This huge advantage brings EME capability within reach of a much wider
range of stations. Perhaps best of all, WSJT is designed to facilitate contacts between humanoperators, not between computers. The internet does the computer link so easily that we don’teven pause to think about it; JT65 EME contacts, on the other hand, are thrilling and fun.
Over the past year, JT65 has been used to make at least several thousand EME QSOs onbands from 50 to 1296 MHz. On 2 meters, the most popular EME band worldwide, 300
Watts and a good yagi pointed at the Moon can enable you to work dozens of other EME
stations, anywhere in the world. Somewhat larger stations will find hundreds of workable
QSO partners, and the number of active stations is increasing all the time.
JT65 was designed to work efficiently with the existing standards and conventions for EMEcontacts. It starts by ensuring that its transmitted messages are compact and efficient. On
users’ computer displays, the exchanges in a JT65 EME QSO typically look something likethis:
1.
2.
3.
4.
5.
6.
7.

CQ VK7MO QE37
VK7MO K1JT FN20
K1JT VK7MO QE37 OOORORRR
TNX REX 73 GL
FB SIG JOE 73

1J. Taylor, K1JT, “WSJT: New Software for VHF Meteor-Scatter Communication,” QST December 2001, pp.
36-41.
2J. Taylor, K1JT, “JT44: New Digital Mode for Weak Signals,” QST June 2002, pp. 81-82

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Transmissions 1–3 use the basic JT65 message format of two callsigns, a grid locator, and an
optional “OOO” signal report. (CQ or QRZ can be substituted for the first callsign.) After
callsigns have been exchanged, transmissions 4 and 5 complete the QSO by sending a return
signal report and acknowledgments that all necessary information has been received.
Transmission 6 lets VK7MO know that his “RRR” transmission was received, and furtherconversational chit-chat may follow. Aside from the embellishment of exchanging gridlocators, this model QSO closely follows standard EME practice for CW QSOs. JT65
transmissions start at the top of each UTC minute, so if copy is good a minimal QSO can becompleted in 5 or 6 minutes.
Detailed operating instructions are provided in the WSJT User’s Guide 3 . You can learn theessential operating skills by reading the manual and making a few practice QSOs, perhaps
using extremely low power or a difficult tropospheric path to ensure that signals are barelyaudible, or even sub-audible. As you accumulate experience you will see your JT65 skills
continue to improve, and your rate of successful contacts should rapidly climb.
Unlike Morse code, JT65 does not transmit messages character by character. The forward
error correction (FEC) built into every encoded message makes it highly probable that JT65
messages will be received in their entirety, or not at all. The decoder does occasionally makemistakes, most commonly because a birdie or other interfering signal is present. In such cases
errors in the falsely decoded messages are large, easy to recognize, and readily rejected by theoperator. You will never see broken callsigns with a few missing or incorrect characters, ormessage fragments of any sort. There is no chance for the letter O or R in a callsign to beconfused with a signal report or an acknowledgment, or for a fragment of a callsign likeN8CQ or a grid locator like EM73 to be misinterpreted.
Just how does JT65 work, and where does its mysterious 10 – 15 dB of “gain” over CW
originate? The answer lies in the use of coding and modulation explicitly designed tooptimize exchanges of short messages at a low but acceptable throughput, around 0.3
characters per second. Standard JT65 messages are compressed or “source encoded” using aclever scheme proposed some years ago 4 by W3IWI and KA9Q, enabling two callsigns and agrid locator to be transmitted with just 71 bits. In JT65, a 72nd bit serves as a flag to indicatethat the message consists of arbitrary text (up to 13 characters) instead of callsigns and a gridlocator. Special formats allow other information such as callsign prefixes (e.g.,ZA/PA2CHR) or numerical signal reports (in dB) to be substituted for the grid locator. The
aim of source encoding is to compress the common messages used for EME QSOs into aminimum fixed number of bits.

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See http://pulsar.princeton.edu/~joe/K1JT/Documentation.htm to download the WSJT User’s Guide. Thanks toa number of WSJT users, the manual is available in French, Italian, Japanese, Russian, and Serbian, in additionto English.
4Clark, T. W3IWI, and Karn, P., KA9Q, “EME 2000: Applying Modern Communications Technologies toWeak Signal Amateur Operations,” Proc. Central States VHF Society, 1996.

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After compression, each 72-bit message is augmented with 306 error-correction bits. This
process is defined by a mathematical code known as RS(63,12) — a so-called “Reed Solomoncode” — which converts 72-bit user messages into sequences of 63 six-bit “channel symbols”
for transmission. Every sequence used in the code differs from every other one in at least 52of the 63 symbols — which, in a nutshell, is why the code is so powerful. By actual
measurement, the full and exact transmitted message has a high probability of being received,
even if the average key-down SNR is as low as 2 to 6 dB in 2.7 Hz bandwidth (roughly –28 to–24 dB in 2500 Hz, the conventional reference bandwidth used in WSJT).
Like the traditional CW procedures for EME, JT65 uses special signal formats to convey afew frequently used messages in a robust and efficient way. Three such messages arepresently defined, conveying the messages “RO”, “RRR”, and “73”. Instead of keying asingle-frequency carrier on and off according to a pattern like di-dah-dit, dah-dah-dah, …,
JT65 sends “RO” by transmitting two alternating tones with specified frequencies andswitching rate. Such waveforms are easy to recognize and to distinguish from one another, as
well as from “normal” JT65 messages. Indeed, as many users have discovered, the shorthandmessages of JT65 are readily decodable by human operators using sight and sound, as well as
by a computer.
JT65 uses one-minute T/R sequences and requires tight synchronization of time andfrequency between transmitter and receiver. Typical amateur equipment cannot accomplish
this task with enough accuracy in open-loop fashion, so JT65 signals carry their own
synchronizing information. A pseudo-random “sync vector” is interleaved with the encoded
information symbols. It allows calibration of relative time and frequency errors withaccuracies of about 0.03 s and 1.5 Hz, respectively, thereby establishing a rigorous framework
within which the decoder can work. In addition, it enables the averaging of successive
transmissions so that decoding can be accomplished even when signals are too weak forsuccess in a single minute. The synchronizing signal is so important that (except in shorthandmessages) half of every transmission is devoted to it.
A JT65 transmission is divided into 126 contiguous time intervals, each of length 0.372 s(4096 digitized samples at 11025 samples per second). Within each interval the waveform isa constant-amplitude sinusoid at one of 65 pre-defined frequencies, and frequency changes
between intervals are accomplished in a phase-continuous manner. A transmission nominallybegins at t = 1 s after the start of a UTC minute and finishes at t = 47.8 s. The synchronizing
tone is at 1270.5 Hz, and is normally sent in each interval having a “1” in a pre-defined
pseudo-random sequence of 0s and 1s known to both transmitter and receiver. This sequence
has the desirable mathematical property that its normalized autocorrelation function falls from
1 to nearly 0, for all non-zero lags. As a consequence, it makes an excellent synchronizing
vector.
Encoded user information is transmitted during the 63 intervals not used for the sync tone.
Each channel symbol generates a tone at frequency 1275.8 + 2.6917 Nm Hz, where N is thevalue of the six-bit symbol, 0 ≤ N ≤ 63, and m is 1, 2, or 4 for JT65 sub-modes A, B, or C (seethe WSJT User’s Guide for further details on the sub-modes). The signal report “OOO” isconveyed by reversing sync and data positions in the transmitted sequence. Shorthandmessages dispense with the sync vector and use intervals of 1.486 s (16,384 samples) for thealternating tones. The lower frequency is always 1270.5 Hz, the same as that of the sync tone,and the frequency separation is 26.92 nm Hz with n = 2, 3, 4 for the messages RO, RRR, and73.
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As a teaser, let me offer two examples of what can be done with JT65 on the 144 MHz EMEpath. Joop Mutter, PA0JMV, has made a special game of working the smallest stations he cancontact by EME. His list now includes 39 stations at ERP levels (transmitted power times
antenna gain) between 1.5 and 8 kW. More than half of the stations on his list run less than200 Watts and use one or two yagis. Joop has an excellent EME setup, but it is hardly in thesuper-station category: he runs 1.5 KW to a pair of 16 element yagis mounted in his garden,and positioned by hand.
Bill Davis Jr., K0AWU, joined the JT65 EME fun in February 2005, just two weeks before I
write these words. It was mid-winter in Minnesota (see picture), so Bill erected a simple mastin his driveway, guyed it to some handy anchoring points, and put a 13 element yagi on top.
He also uses the “armstrong” method to point it at the moon. With this temporary lash-up andrunning 700 Watts, Bill has worked 44 different EME stations, including 22 DXCC entities,
in two weeks.
Working stations that you can’t hear (or can hardly hear) takes some getting used to. New
operating procedures need to be developed that will make it easier for stations to find eachother and start a QSO. At present, a majority of JT65 EME QSOs originate when someone
posts a CQ notice on the JT65 EME Link, http://www.chris.org/cgi-bin/jt65eme, or bymaking a real-time schedule there. Schedules are a good way to start, and if you needcoaching the internet chat pages are excellent places to get it. However, to ensure that yourQSOs are legitimate, be sure to suspend any use of the reflector (and any other non-EME
means of communication) for any contact-related information while the QSO is in progress.
It is considered very bad form, and invalidates your QSO as a true EME contact, if you
convey something like “I have copied both calls, I’m now sending my report” via some othermeans while your QSO is in progress.
It will be helpful for the community of EME users to give collective thought to operatingprocedures and band-plan issues relating to JT65. On 2 meters, most JT65 activity is foundbetween 144.100 and 144.160. Agreement on a small range of calling frequencies for CQs,
such as 144.120 – 144.130, could provide a significant boost to random (as opposed toscheduled) operation. Japanese users generally transmit JT65 below 144.100, and othernational or regional issues may be important. EME activity on 6 meters seems to centeraround 50.190, and I believe there is some concentration toward 432.044 and 1296.044 on theUHF bands. I encourage wide discussion of such questions, and hope that a consensus can beachieved.
Finally, I should briefly mention FSK441, the popular WSJT mode designed for making
meteor scatter contacts. Unlike JT65, which transmits slowly and works with extremely weaksignals, FSK441 transmits at the high rate of 147 characters per second to take advantage offraction-of-a-second reflections from the ionized trails of meteors. Meteor scatter QSOs in
the 800 – 2000 km range can made be on the VHF bands at any time, using FSK441. Suchcontacts are a excellent way to get your feet wet, before trying EME with JT65.
WSJT can be freely downloaded from http://pulsar.princeton.edu/~joe/K1JT, and from theEuropean mirror site http://www.dk5ya.de/.

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