A, B, C's of Dx Fundamentals of the Art of DXing XI
W5FKX, Don Boudreau
Operating Modes
Is there a "Best" mode for DXing?
From the single mode of keying a spark-gap signal, to more than a dozen different ways of communicating over the radio waves, ham radio has undergone a phenomenal technological evolution over the relatively brief period of its existence. Despite arguments to the contrary (" ... CW always gets through"; "... PSK is the most efficient ..."; " ...SSB is the most robust method ..."; " ... you can't beat the fidelity of AM ..."), there is no single mode that is best for all people, purposes, or operating conditions. Although most DXers enjoy mixing up their use of operating modes, there are some who prefer to operate only on a single mode. Others find this too limiting since, depending upon circumstances, it often happens that one mode is more productive in making a DX contact than others. For example, if a DX station only operates with a single mode, you may not be able to make a contact if that particular mode is not available to you. For that reason, it is useful to be proficient in the use of - and have the ability to operate - as many of the modes that are available. For the purpose of chasing DX, all of the available modes are useful at one time or another and, the fact is, that most of the truly serious DXers use them all. Indeed, many have had great success with all three of the modes for which the DXCC award is now offered (CW, Phone, RTTY/Digital), achieving DXCC Honor Roll awards for all three. But the over-riding consideration is, as with any hobby, you should choose to do that which you enjoy the most. Since it should be the enjoyment that counts, you should use whatever mode or modes that you like.
Let's take look at the currently most popular modes used in DXing. In decreasing order of current users among DXers, the modes are:
SSB - Single Side-Band telephony is today, by far, the most frequently used mode on the bands, and may well be considered the universal mode. Its popularity undoubtedly derives from the fact that its use is intuitive, as voice communication is learned from birth, so telephony is easy to use, requiring no additional effort at learning a new skill - you just talk! Unlike the older telephony modes of AM and FM that require up as much as 6 KHz of bandwidth, SSB uses 3KHz or less. Those who remember the days before SSB can tell you how much of a significant improvement in communications capability it provided over AM phone. The lack of a base carrier and elimination of one of the sidebands meant that more of the output power was usable for signal information output as compared to AM. SSB signals were much more reliable, especially at lower power output, than the former telephony modes. The combination of narrow bandwidth and power efficiency means that under severe conditions, SSB can be intelligible within as narrow a bandwith as 1.2 KHz, and this means everything to a DXer! Also, since SSB output is proportional to modulation (see Equipment chapter), speech compression is an extremely useful DXing aide. Most SSB transceivers have selectable, adjustable speech compressors built in. DXers should take full advantage of this useful signal enhancement technique by following the manufacturers' recommended settings for speech compression. Always solicit over-the-air critiques of your compressed audio signal from others, especially DXers, as it will not necessarily sound "normal" to someone who is interested only in audio quality. As long as you're not distorted, or splattering your signal beyond the normal 2-3 KHz bandwidth, don't be disturbed by comments from non-DXers that " ... you don't sound normal ...". Of course, for casual contacts, you may want to switch off the speech compressor.
CW - After SSB, the next most popular mode on the bands is Continuous Wave (CW) as we hams call it, but more widely known by the encoding schema used: International Morse code. CW is a term that is a throwback to the early days in which this newer method of generating a constant (continuous) RF signal at a single frequency was distinguished from the spark-gap method that was essentially a ragged sequence of rising and decaying broadband RF noise. Interestingly (and unknown to most), CW was actually the first digital mode used by hams, since it a binary (ON/OFF) encoding scheme for information exchange. Compared to SSB, CW uses even less of the spectrum, needing at most a few hundred Hertz. In fact, CW signals are copiable in a filter bandwidth of as little as 50Hz, allowing one enjoy a greatly improved signal-to-noise ratio (Ever since I was able to afford one of the transceivers that employed DSP within the IF chain, I have used a 50 Hz bandwidth for all of my CW DXing - quite a change from the 4 KHz bandwidth of my S-38C!).
With rare exceptions, almost all DX stations will, at some time or another, operate CW. This is good news for DXers, especially those with modest stations, because a good operator can often copy a weaker station on CW than on SSB, especially under adverse interference conditions. DX station operators are usually excellent CW operators and when using split, they are quite capable of picking up even a very weak station that is calling from a clear frequency. That is why most successful DXers operate CW, knowing that there is often a better chance of working a DX station on CW than on SSB, especially when competing with the Big Guns. For this reason, it is definitely worth the effort to learn how to use CW. Since all transceivers today offer CW output along with SSB, all that is needed to operate this mode is a key and a bit of skill (you DID pass the code test, didn't you? ). The fact that DX station operators, like contest operators, generally send at a high rate of speed may scare off some DXers who don't feel that their CW ability is up to the task. Should you feel this way, my advice is not to give up because all you really need to be able to do is reliably copy your callsign, and most can recognize their own callsign even at very high speed. Since a DX contact only requires the exchange of a callsign and signal report, and since all pile-up reports are "599", if you can copy your callsign you can work the DX on CW. Your sending speed is not an issue - send at whatever speed is comfortable for you. Just get in there and do it! Which brings up another option for CW operation: with the appropriate software, one can interface the transceiver to the soundcard of a PC and use the PC keyboard to send CW at any rate of speed desired (see References). Some of the programs will also decode received CW; however, the software generally lacks the ability to copy well under adverse conditions. Note that software packages that provide a transceiver-PC interface for using the soundcard to code/decode digital modes can generally do more than one mode, if not all (CW, RTTY, PSK, etc).
The fact that CW is less popular than SSB among the ham population at-large should not be misinterpreted to mean that competition will be less among DXers. Anyone who believes that has never been in a CW pileup! Be assured that in a pileup for a moderately rare DX station, it will seem as if the whole world is there - and it is worse for the very rare ones!
RTTY - Next on the list of popularity among DXers is Radio TeleType, or RTTY. It is a relatively narrow bandwidth (300 Hz) digital mode that transmits keyboard-typed text encoded in the Baudot code, a 5-bit code devised in the late 1800s and used to develop wireline teletype communications. RTTY offers the advantages of both SSB (intuitive) and CW (narrow bandwidth). At the same time, since its user base is the least among the modes, it does offer a somewhat less competitive field of play for DXing. That alone makes it worthy of prime consideration! The only disadvantage of RTTY is that it usually requires additional equipment. In the old days before computers, large, heavy, pedestal-mounted teletypewriter devices (more akin to machinery than electronics!) were used to generate and receive (print on paper) RTTY signals. With the advent of the early computer chips, teletypewriters were replaced by a relatively small device called a Terminal Node Controller (TNC) for encoding/decoding, a terminal (monitor) for print display, and interface connections between the devices and the radio. In recent years, as computers with sound-processing capability became widely available, software was developed to allow the computer to replace the TNC, interfacing directly to the transceiver. While there may be extra costs and effort needed to implement RTTY, it is certainly not a foreboding obstacle. In fact, many stations today already have a computer. Free RTTY software is available for download from the Internet (see References). Rig-to-PC interfaces that isolate any stray RF from the PC are available commercially from $50 - $300), or can be readily built with low cost parts.
One popular and freely downloadable software program that is widely used is called MMTTY, developed by Mako, JE3HHT and available on his website at mmhamsoft.amateur-radio.ca/. With this program (and many others), one can set up "macros", activated at the click of a button, that will automatically send information. For example, a typical macro may send the following information:
%c DE %m
OK TNX %n UR %r %r
NAME IS DON ... DON
QTH IS RIVER RIDGE, LA ... RIVER RIDGE, LA
BTU %n
%c DE %m KK
This is a macro that I use in MMTTY, where "%c", "%n", and "%r" are the respective values for the called station's callsign, the called operator's name, and the signal report. The variable values are typed into appropriate windows on the screen, as may be seen in the examples below. The figure shows a screen-shot of a RTTY session "printing" of Ed, D2PFN from Luanda, Angola calling "CQ NA". (Ed developed his RTTY interest while operating as P5/4L4FN from Pyongyang, DPRK). Note the error in the first line where "D" was decoded instead of "DE", and in the third line where the callsign was decoded as "D2PKN". These are typical of decode errors due to fading or interference when printing DX signals.
Some seem to believe that RTTY requires high-power for reliable contacts, but this is really not true. RTTY reliability compares well with that of CW. For example, in the above screen-shot, print copy of D2PFN was better than 98%, despite the fact that Ed was operating "barefoot" and using (I believe) a Butternut multiband vertical antenna. Even under extremely poor conditions, RTTY operators can usually complete exchanges of contact information (callsign and RST) with as little as 40-50% error-free printing. An example of this can be seen in the figure below of 3C0M working RTTY on 20m, with plenty of print errors but still enough information content to complete a contact (Clicking on the image requires Shockwave Flash player for video display).
The MMTTY software provides highly automated RTTY operation. Clicking on the callsign in the Rx window automatically causes it to appear in the "call box", ready for making a call using a macro button, and the same can be done for the operator's name if it appears on the screen. If desired, upon completion of the QSO, a button-click will log the contact.
Notice in the RTTY screen-shots that all the letters are capitalized. Baudot code provides encoding for capital letters only. In general, binary coding schema can be used to represent up to 2-nth power characters. In this case, 2 raised to the 5th power is 32, so coding of the 26-character alphabet is possible, and upper-case was chosen (probably because telegraphers traditionally wrote telegrams in caps). Since the null-code (00000) is unused. the remaining 5 binary 5-bit codes are used for control characters (carriage-return, line-feed, space, and two "shift" character controls). By preceding a character code with a "shift" code (as on a standard keyboard), it is then possible to "re-use" the 26 alpha-codes to also encode the numbers 0-9, along with the commonly used symbols that appear on standard keyboards ( period, comma, etc).
There are two ways in which RTTY can be transmitted: either by using Frequency Shift Keying (FSK), or by using Audio Frequency Shift Keying (AFSK). The difference between these two is very simple to understand. FSK mode may be thought of as a form of CW in which the key is not only closed and opened, generating a "tone" at the desired HF, but may also be done at second frequency spaced 170 KHz higher. The first (lower) frequency is the "Mark" tone and the second (higher) frequency is the "space" tone. Because it is essentially a form of CW, it is possible to improve selectivity for RTTY reception by making use of the narrow filters typically available for CW. On the other hand, in AFSK mode, the Mark/Space tones are generated at audio frequencies and then transmitted as a SSB signal (by convention, using LSB). While the Mark/Space separation of the audio tones is the same (170 Hz), the AFSK signal is received in SSB mode where usually only wide filters are available, in which case selectivity is not as good as for FSK. Another important difference between FSK and AFSK is that on the receiver end, the reception tuning bandwidth for FSK is centered on the actual transmit frequency, while in AFSK, the transmit frequency is centered on the LSB signal that is 2.4 KHz below the transmit (zero-beat) frequency. To illustrate, if one reports ("spots") the frequency of an AFSK signal on a DX cluster as 14085.0 MHz, then in order for someone tuning in FSK mode to be able to hear it, they will have to tune their receiver to 14083.6, since the LSB signal tones are 2.4 KHz below the actual transmit zero-beat frequency. In other words, FSK (MHz) = AFSK (MHz) - 2.4 KHz. Which is best? Most serious RTTY DXers prefer FSK because of the selectivity issues; however, with the great improvements in DSP filtering now widely available for any mode of operation, selectivity may be less of an issue. One note about AFSK and FSK: RTTY spots posted on the DXCluster should ALWAYS indicate whether the spotter is using AFSK or FSK in order to allow others to tune to the correct frequency. It is frustrating to look for a DX station spotted at 14085.0 and then have to tune around to finally find them 2.4 KHZ lower at 14082.6 or higher at 14087.4!
One last thing to know about RTTY is that it can happen (and frequently does!) that the Mark/Space tones may be received in reverse. This can be duse to an error on the transmitter end (the tones are transmitted in reverse due to an incorrect connection or menu setting at the transmitter), or on the receiver end (incorrect receiver-TNC or PC sound card connection/setting). This becomes apparent when, despite careful tuning, only garbled print is received. All RTTY software, TNC or PC based, allows selection of "Reverse mode" reception for this purpose, and reversing the tone-decode process at the received end will allow proper printing. If it should happen that you are told that you are transmitting RTTY "in reverse" (or "upside down"), you should immediately look into correcting the problem. For FSK, first check your transmitter options for RTTY and, if available, reverse the Mark/Space polarity. If the option is not available or you're using AFSK, then swap the signal wires connecting your radio to the TNC or PC sound card.
See some of the links below for more information on getting started with this great mode. An especially good website is the one provided by Don, AA5AU, for all that you need to know about RTTY.
PSK31 - More properly, BPSK31, Binary Phase Shift Keying at 31.25 Hz is one of the most efficient HF modes currently available and has grown in popularity to the point where it has greatly surpassed RTTY in prevalence on the bands and may become the digital mode of choice for DXers. More detailed information sources can readily be found on the Internet and some are listed below. Using a variable length binary digital encoding scheme called Varicode, it offers the use of a fully complete character set with both upper- and lower-case. Further, the code is optimized to provide the least amount of code-level changes for the most frequently used characters, thereby minimizing dropped bits without the use of cumbersome error correction methods An alternate form, QPSK , provides error correction, but at the expense of significant reduction (3db) in signal-to-noise ratio, thereby rendering it less useful than PSK31 for DXing. Because of its extremely narrow bandwidth (31 Hz), PSK31 offers excellent signal-to-noise ratio to the extent that many have reported solid printing for signals that were almost imperceptible to the ear. The usual output power level is less than 50 watts (more power can cause obnoxiously severe splattering). A screen image of a PSK31 QSO between Serge, UA0FO and the author, using one of the free PSK software packages, DigiPan ("Digital Panoramic Tuning "by KH6TY, UT2DZ, & UU9IDR; www.digipan.net/download/) is shown in the figure below (Clicking on the image requires Shockwave Flash player for video display).
A few things to note in the figure (showing DigiPan v. 1.6d):
In addition to the macro buttons, there are three windows: Rx#1 (top), Rx#2 (below #1), Tx (small, below #2), and Waterfall (bottom).
Bothe upper- and lower-case characters are seen; in fact the Varicode encoding schema supports the entire set of ASCII characters & symbols.
PSK31 band segments are as wide as a receiver SSB passband (3 KHz wide; shown above is 14.070 - 14.073)
A PSK31 signal is a 31Hz-wide set of audio tones generated somewhere within the 3 KHz passband and seen as a bright line "flowing" down the waterfall display.
Tx frequency selection is done by placing the mouse curser at some spot in the waterfall display and left-clicking (red diamond w/green flag in figure).
On the Rx end, one can look at the waterfall and see all signals currently being received within the passband; placing the cursor on a signal and left-clicking will allow print to begin.
While printing a signal in Rx#1, a second signal may be printed simultaneously in Rx#2 by placing the cursor on it and right-clicking the mouse (blue triangle in figure).
Because of its superior qualities (low power, narrow bandwidth, excellent signal-to-noise ratio), many stations all over the world now operate in PSK31 mode, offering many opportunities to "work a new one" with modest equipment and antennas.
Other Modes
Of course, there are a number of other modes that may also be used, such as PSK63, FM, SSTV, and MFSK to name a few. However, these are all less frequently used for DXing due to (1) the lower number of overall users; and (2) the fact that they are somewhat less efficient.
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