Being the geek that I am, I oftenenvision what the invisible worldaround us might look like if we

could see beyond the visible light spec-trum; cosmic rays and neutrinos streak-ing through the sky like a meteor storm,electric and magnetic fields surround-ing antennas and power lines, uninsu-lated walls and windows leaking heatvia infrared. Likewise, it’s fascinating tothink that at any given moment, thou-sands if not millions of distinct radio, TV,and cellular phone signals surround us,unseen and unfelt, their existencerevealed by the familiar magic of a radioreceiver’s ability to convert microvoltsof RF energy into audible sounds. Yetthis leads me to wonder: How many sig-nals are there that we can’t hear, andhow might we detect them? When wehear a rush of static from our rig’sspeaker, is there something under-neath the noise unheard and undetect-ed? What if there was a way to receivethose messages, to add greater sensi-tivity to your HF station, using the equip-ment you likely already have? As it turnsout there is, and it’s called JT65A.

The JT65A communications protocolwas conceived and first implementedby Joe Taylor, K1JT. Joe, a ProfessorEmeritus of physics at PrincetonUniversity, shares a Nobel Prize withRussell Alan Hulse (ex-WB2LAV) forthe discovery of the first pulsar in a bina-ry system as well as the first confirma-tion of the existence of gravitationalradiation in the amount and with theproperties first predicted by AlbertEinstein. Joe has contributed to theamateur radio community in much thesame way, changing the playing field forweak-signal operation.

Joe was first licensed as an amateurradio operator while he was still ateenager. His ham radio interest led himinto astronomy (see CQ interview,October 2009 issue—ed.). When heapplied his mind to the idea of develop-ing a communications protocol thatwould work well under very low signal-to-noise ratio conditions on a communi-cations signal path between, say, themoon and Earth-bound amateur radiostations, he formulated a number of pro-tocols that have revolutionized the worldof amateur radio weak-signal DXing.

In 2001, Joe wrote the WSJT (for“Weak Signal/Joe Taylor) software

(http://physics.princeton.edu/pulsar/K1JT/wsjt.html) that implemented thesenew weak-signal communications pro-tocols. WSJT offers several modes(including FSK441, the JT65 family, andJT6M) intended to support meteor-scat-ter, troposcatter, and Earth-Moon-Earth(EME, or “moonbounce”) communica-tions. JT65A is a specific protocol de-signed for weak-signal conditions on theshortwave (HF) frequencies (see figs. 1and 2), taking into account the specificways in which an HF radio signal prop-agates via the ionosphere and “suffers”under changing conditions.

JT65A is actually a “sub-mode” of

Ever wonder how much of the noise you hear on the HF bands is actually comprised of signals too weak to be copied? W6DTW takes us exploringwith a weak-signal mode that can pull great DX out from under the noise!

Communicating Under the NoiseJT65A on HF – Part I

BY DAVID T. WITKOWSKI,* W6DTW, with TOMAS HOOD,† NW7US

*1525 Altamont Ave., San Jose, CA 95125e-mail: <w6dtw@arrl.net>†CQ Propagation Editor, P.O. Box 1980,Hamilton, Montana 59840-1980e-mail: <nw7us@cq-amateur-radio.com>

Fig. 1– A screen capture showing the WSJT software by Joe Taylor, K1JT, in theJT65A digital communications protocol mode on the 20-meter JT65A frequency,14.076 USB. NW7US just made a successful two-way digital exchange with HB9ARI.

(Source: NW7US, using WSJT)

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Joe’s original JT65 protocol, which hedesigned to optimize EME contacts onthe HF and VHF bands. JT65 includeserror-correcting features that make itvery robust, even with signals much tooweak to be heard. It was later realizedthat this protocol, with some adaptation,would also be very usable for terrestri-al HF communications.

A Bit of BackgroundBefore we can talk about the benefits ofa mode such as JT65A, we need todelve into a bit of background on com-munications and information theory. Inthe earliest days of wireless, the con-version mechanism between receivedsignals and language was via thehuman ear, the difference betweenbackground static and the static of aspark-gap transmitter interpreted asMorse code and written down by anoperator at the receiving end. Tech-nology advancements would later giverise to continuous-amplitude wave(CW) and voice (phone) transmitters,the difference between the two being atradeoff between better detection ofweak signals for CW and faster through-put for phone. Figs. 3 and 4 illustratethis concept by revealing the “footprint”of a “usable” CW and voice (using sin-gle sideband), respectively. These fig-ures reveal that, using the same anten-na and power level, the useful range ofthe CW signal is much greater than thatof an SSB signal. This is why CW hasbeen noted as a great mode for weak-er-signal operation, and why low-power(QRP) operation is typically a CW-mode endeavor.

Speaking strictly in terms of de-

tectable signal-to-noise ratios (SNR), aCW signal that is “encoded” at twelvewords per minute (12 wpm) is generallyheld to be copyable at an SNR of –15dB, whereas a phone transmission thatsends information at 250 wpm requiresan SNR of +6 dB. (These ratios are typ-ically calculated based on a 2.5-kHzchannel bandwidth.) If we normalizethese to a 1 character-per-second (cps)rate—e.g., 12 wpm CW versus speak-ing one letter per second phonetically inphone—the detectable SNR for phone

becomes –8 dB. Therefore, on a trulylevel playing field, CW yields an im-provement of 7 dB over phone.

The adoption of machine-to-machinecommunication (for instance, RTTY,Hellschreiber, etc.) in the early to mid-20th century provided faster throughputand a marginal increase in SNR perfor-mance, but at the expense of channelbandwidth. The normalized SNR ofthese early machine-to-machine modesworks out to be only about 2 dB, hardlyan improvement worth getting excitedabout. (Although to be fair, the value ofRTTY was not so much from SNR im-provements, but rather that it printeddirectly to paper, freeing the radio oper-ator to do other tasks.)

Even the development of PSK31 in thelate 1990s by Peter Martinez, G3PLX,did not yield an improvement in normal-ized SNR, although it did reduce thebandwidth requirements through the useof Varicode, a form of data compression.

If the application of data compressioncan reduce bandwidth requirements, arethere other techniques that can beapplied to improving SNR performance?And how much room for additionalimprovement might there be? In the1940s, Claude Shannon and RalphHartley, both of whom were researchersat Bell Labs, developed the Shannon-Hartley Theorem. This theorem providesan equation (proved by Shannon in1948) for calculating the maximumamount of digital information that can be

Fig. 2– The “waterfall” display, a feature of the WSJT software, showing several JT65Adigital signals. One of these, the “trace” on the left, is the signal from HB9ARI as

received at NW7US. (Source: NW7US, using WSJT)

Fig. 3-– The “footprint” of a 100-watt CW signal at 0500 on the 20-meter band. Comparethis with the footprint of a 100-watt SSB signal at the same time, as seen in fig. 4.

(Source: ACE-HF Pro [http://hfradio.org/ace-hf], as used by NW7US)

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reliably decoded over a communicationschannel with a specified bandwidth in thepresence of noise (see Equation 1).Shannon-Hartley doesn’t tell us how toreach the theoretical limit, it just tells uswhat that limit is.

As it turns out, for real-time datastreams we can’t get to the theoreticallimit. Each modulation technique (forexample, RTTY uses “frequency-shiftkeying,” CW and Hellschreiber use “on-off keying,” and PSK31 uses “phase-shift keying”) has an inherent limitationin the ability of the receiver system,whether machine-human or puremachine, to discriminate betweenstates. Improving SNR beyond a certainpoint becomes impossible.

However, all is not lost. An alternatetechnique for improving SNR is toimplement redundancy in the data. Weuse redundancy all the time in amateurradio—repeating callsigns, signal re-ports, locator grids, etc. Of course, thiseffectively reduces the channel capac-ity, or throughput, which appears inShannon-Hartley as bits/second—i.e.,a function of time. If PSK31 has athroughput of 30 wpm, and we repeatour callsign six times to overcome aweak path, then clearly our throughputis less than 30 wpm. What we’ve effec-tively done by using redundancy is toreduce the SNR required for detectionof our callsign. Of course, in this exam-ple, we still rely on the operator to lookat the decoded text and, using thehuman mind’s awesome ability to dopattern recognition, extract the callsignfrom the garbled text.

Thus, if we’re willing to accept lowerthroughput and use redundancy, wecan improve SNR for a given modula-tion method. Further improvement canbe achieved by using an error-correct-ing code, leveraging the power of acomputer to encode the data in aprocess known as Forward ErrorCorrection (FEC). We can then use acomputer on the receiver to invert the

FEC encoding and correlate the redun-dant data blocks into a single error-freeblock of data. Combining redundantsending and error-correcting codesallows us to reach a throughput close tothe limit predicted by Shannon-Hartley.JT65A’s performance tracks well withtheory and has been shown to yield anadditional 7 dB of detectable SNR(nearly approaching the theoreticallimit), which equates to a 5× improve-ment in system performance. Thismean that reliable decoding of a signalat –24 dB SNR is now possible, andeffectively turns your 20-watt portablestation into a 100-watt boomer!

JT65AIn late 2006 Victor, UAØLGY, andTetsu, JE5FLM, completed the firstJT65 QSO on HF. Interest grew quick-ly in 2007 as several members of the“digitalradio” Yahoo Group beganexperimenting with applying JT65A toweak-signal DX. People such as Andy,K3UK, created helpful guides for newoperators looking to get involved. Dialfrequencies range from 80m (3.576MHz) to 10m (28.076 MHz), but 90% ofthe activity happens at 14.076 MHz. (Allare USB; more on this in Part II.)

Benefits of JT65AJT65A on HF offers several benefits. Itrequires minimal transmit power, mak-

Fig. 4- The “footprint” of a 100-watt SSB signal at 0500 on the 20-meter band. Notetha using the same power level and antenna, the footprint of a “usable” CW signal isgreater than that of a SSB signal. (Source: ACE-HF Pro [http://hfradio.org/ace-hf], asused by NW7US)

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Equation 1– Considering all possible multi-level and multi-phase encoding tech-niques, the Shannon-Hartley theorem states that the channel capacity C, meaningthe theoretical tightest upper bound1 on the information rate (excluding error cor-recting codes) of clean (or arbitrarily low bit error rate) data that can be sent with agiven average signal power S through an analog communication channel subject toadditive white Gaussian noise of power N, is:

where:C is the channel capacity in bits per second;B is the bandwidth of the channel in Hertz (passband bandwidth in case of a modulated signal);S is the total received signal power over the bandwidth (in case of a modulated signal, often denoted C—i.e., modulated carrier), measured in watts or volts2;N is the total noise or interference power over the bandwidth, measured in watts orvolts2; andS/N is the signal-to-noise ratio (SNR) or the carrier-to-noise ratio (CNR) of the com-munication signal to the Gaussian noise interference, expressed as a linear powerratio (not as logarithmic decibels).

Note:1. For an explanation of this terminology, see <http://oakroadsystems.com/math/polysol.htm#Bounds>

C = Blog2 (1 + S/N)

ing it suitable for highly portable sta-tions, DXpeditions, and situationswhere running QRO (high power) mightcreate interference and draw theunwanted attention and ire of neighborsor, worse yet, spouses. The vast major-ity of JT65A QSOs on HF are complet-ed using less than 50 watts ERP.

Perhaps best of all, JT65A allowspeople whose living situations requirethe use of stealthy antennas to worksome real DX! My first-ever confirmedcontact with South Africa was madeusing JT65A from my home in northernCalifornia using 50 watts into a jury-rigged doublet made from TV twin-leadand 16AWG speaker wire, and hung outa second-story window. An apartment-dwelling friend of mine got started onJT65A by attaching an auto-tuner to arain gutter on his building, a setup thatallowed him to work a VK4 station fromwhat normally would be a much-com-promised location.

JT65A also allows operation in high-noise and high-QSB environments,because the redundancy provided bythe forward-error correction allowsnearly 80% of the transmission to be lostand still be decoded.

HardwareHardware requirements for JT65A arestraightforward and no different frommost other digimodes. JT65A uses anAFSK interface between your PC andrig. If you’re already set up to run PSK31

via Ham Radio Deluxe’s DM780 pack-age, MixW, MultiPSK, etc., then you’regood to go with one exception; you’llneed a method for accurately syncingyour PC’s clock. PTT can be accom-plished through either serial-port trig-gering or VOX. The only twist is that theaccuracy of your PC clock will have adirect effect on your ability to decodeand be decoded, and if you’re more thana second or so off-sync, nobody willdecode you and you won’t decode any-one else. More about this in Part II.

SoftwareOperators wishing to try out the JT65Amode have several different softwarepackages to choose from. The originalWSJT package, originally coded byK1JT, is now an open-source project(under GPL license) that is maintainedand enhanced by a small group ofdevelopers. To use JT65A you will wantto obtain WSJT7, because as of June2010, WSJT8 contains some newexperimental modes such as JT64Aand JT8 but does not contain JT65A.WSJT7 is available as a binary down-load for Windows® and a package forDebian-based Linux distros. If you’recomfortable with compiling your ownsource code, it is fairly straightforwardto get WSJT7 running on FreeBSD,Macintosh OS/X, and most other UNIX-like operating systems.

In 2008 Joe Large, W6CQZ (nowW4CQZ), branched off from the main

WSJT codebase and developed theJT65-HF software package (fig. 5).JT65-HF (information on downloadingand operation can be found at<http://tinyurl.com/JT65-HF>) is intend-ed, as the name implies, to be used onHF and as such only offers JT65A (itdoes not offer wider-bandwidth sub-modes JT65B for VHF or JT65C forUHF; FSK441 for Meteor, WSPR, andso on, the modes that are part ofWSJT7). However, JT65-HF offers sev-eral enhancements such as simultane-ous decode of all signals in a full 2-kHzpassband, a real-time client that reportsthe decoded “captures” from the water-fall to a DX cluster and/or a receptionlogging system, automatic soundcardrate calibration, and the ability to query your rig’s dial frequency viaHam Radio Deluxe, OmniRig, or DXCommander.

Patrick, F6CTE’s MultiPSK applica-tion also offers JT65A. However,MultiPSK is commercial (versus WSJTand JT65-HF, which are free) and assuch I have no experience with it. Youare encouraged to experiment with thevarious software packages and deter-mine which is best suited for your oper-ating style.

Regarding the requirement to keepyour PC clock synchronized: If your sta-tion is at home, and you have internetaccess, then you should use a time syncclient such as Dimension4 orSymmtime. Both are free and readilyavailable online. The reason you wantthis is that the built-in time-sync featurein Windows XP/Vista/7 is not accurateenough to allow proper JT65A opera-tion; you should disable it and use a ded-icated sync client. If you don’t have inter-net access at home, or are workingrover/portable, then you might considerusing a GPS dongle together with a soft-ware package that locks the PC’s clockwith the time signals received via GPS.(Many GPS vendors provide a smallsoftware utility with the GPS which willdo just that, but I’ve also used the UI-View32 APRS software package whichcan link up with many GPS dongles andadjust your PC’s clock.) If you’re in apinch, on a tight budget, and still wantto work JT65A, you can try syncing tothe WWV tones from NIST in Boulder,CO or other shortwave sources.F6CTE’s MultiPSK package comes witha WWV clock receiver application(clock.exe), but bear in mind that PCclocks tend to drift a lot even during ashort period of time, so you’ll have totune back to WWV and readjust yourclock about every 30 minutes. For best

Fig. 5– The window of the JT65-HF software showing the waterfall capture of the JT65Adigital signal of VK3XQ and the “captures” of the QSO between VK3XQ and NW7US.

(Source: NW7US, using the JT65-HF software)

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performance you’ll want a GPS dongle; these can be pur-chased online for about US$30.

Reverse Beacons and Propagation MapsOne of the enhancements offered by JT65-HF is the report-ing in real-time of decoded messages to both a DX clusterand a reception reporting system, often referred to in the JT65community as a “reverse beacon.” Those familiar with theautomated DX cluster reporting in Alex, VE3NEA’s CWSkimmer, or the ability of DM780 (part of Ham Radio Deluxe)to upload PSK31 decodes to Phil, N1DQ’s excellentPSKReporter, will quickly grasp the value of this feature;every person running JT65-HF can effortlessly become partof a worldwide network of monitoring stations that report theirdecoded messages to a web-based server for use by theamateur community (fig. 6). The PSKReporter website is at<http://pskreporter.info>.

The data provided by automatic collection/aggregation ofreverse beacons from JT65A users around the world, com-bined with the ability of JT65A to decode signals approach-ing the Shannon-Hartley limit, has been very valuable inshowing that propagation often exists where common sensesays it shouldn’t—such as 40-meter and even 80-meteropenings that occurred nightly for almost a week last winteraround 0700Z between South Africa and the western USA.It also provides a method for visualizing worldwide propaga-

tion of JT65A messages via maps such as those provided byPSKReporter. Call CQ and within a minute or two you cancheck the map to see just how far away you were heard! Inaddition to monitoring my propagation, I’ve used the world-wide reverse beacon network to do things such as comparethe relative performance of antennas.

If you’re looking for less visual and more detailed propa-gation data in a DX cluster style interface, then JT65A recep-tion reports are also available via Laurie VK3AMA’sHamSpots system, and via W4CQZ’s website. W4CQZ’swebsite also hosts a JT65A “chat-room” which features a live-updating list of reception reports displayed right on the page.

Using JT65A is not only interesting from the perspective ofstudying propagation on HF, but is useful for communicationwith DX stations around the globe which might not be possi-ble using other protocols and modes. Often, JT65A users canwork DX on bands where no other mode, including CW, isworking at that time.

Coming Up in Part IINext month, we’ll dive into how to use JT65A in real-worldcommunications. We will use the JT65-HF software as ourexample because of its features and because of the ongo-ing improvements being made by its author. Until next month,if you are adventurous and jump into this exciting area ofweak-signal DXing, enjoy all that JT65A offers.

Fig. 6– The PSKReporter map showing the reception of the JT65A signal as sent by NW7US. The stations “hearing” NW7US (asa “capture” by the remote JT65A software) are shown with the time since NW7US was last heard. (Source: PSKReporter at

<http://pskreporter.info>)

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