On The Art of NDB DXing
by Sheldon Remington
©
1987-2000 All Rights Reserved
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CHAPTER NINE: THE FREQUENCY DOMAIN
Part Three - Frequency Measurement and Defects The two preceding chapters have dealt with the layout of the maze we call the NDB band, first in the general terms of channelization and modulation pitch, then by breaking down individual clusters. Discovering and assembling the pieces of the puzzle may have been challenging and satisfying, but its lasting value will be as little more than esoteric theory, unless each DXer takes one more step. We must have a method of determining the frequency of the beacon idents we hear; i.e., what frequency our receiver is tuned to at a given moment. Otherwise, we will be severely limited in our ability to benefit from, and contribute to, the exchange if DX tips. The measurement of frequency demands not so much in the way of expensive hardware as it does of our approach to the particular hardware we're using. An adept DXer who has no receiver dial whatsoever can still zero in on an ident and measure it with greater accuracy than a DXer who owns an $800 digital receiver but who doesn't know how to use it properly on the NDB band. Let's divide the frequency measurement capability or receiving setups into three general groups. Each will require a particular set of techniques. Coarsely Calibrated Receivers Many receivers, even those in navigational service, have frequency markings so broad and/or inaccurate that there is simply no way we can read out which cluster we are receiving. For this, we need readouts no coarser than 1 kHz. Frequency calibration spacing of 3-10 kHz is common on lower-priced or older receivers, and it is fine for most broadcast and hamming situations. But on the NDB band, its only benefit is to get us close to the frequency of interest; then we are in the same boat as those who have no dial at all. This requires the blindfold technique, which in essence depends on solid knowledge of commonly heard signals on the band. Just as a navigator learns to find his way around by familiarity with landmarks, so we DXers can steer our receivers around the band by utilizing known reference signals. We're lucky that the 1-kHz channelization is so widespread, because this gives us fairly precise markers 1 kHz apart across the band. Some of these markers will consist of idents only, some will consist of carriers only, but most of them will have a mixture of both, as discussed in Chapter Eight. So, the strategy is to tune in a known beacon component (carrier or ident), and then tune away carefully and slowly until we come upon the next cluster. We can assume we have now moved more or less exactly 1 kHz upward or downward. We can stop here and work on the idents in this cluster, or we can just use it as a signpost on the way to the next cluster. We can practice and test ourselves by counting clusters as we tune the space between two loud local signals, and seeing if the calculated frequency fits the known frequency. In this way we can always determine the frequency of any signal, or we can park the receiver on a frequency reportedly occupied by an interesting DX target. By rough interpolation, we can even take a guess at split off-channel signals with an accuracy of a few hundred Hertz. Interpolation can also guide us through the few 1 kHz multiples that may not have any audible signals at a given moment. And, of course, if we're hearing the 400-Hertz clusters, they then become even closer calibrators. Another technique for receivers with coarse analog dials is to modify the dial itself. Most amenable to this are "slide-rule" dials, particularly that generation of 1960's receivers which have an additional slide-rule pointer for bandspreading the ham bands. This writer used to NDB DX with an Allied A-2515 whose dial was modified by taping a cardboard holder to the front of the bandspread scale. Into this was inserted any one of a set of hand-calibrated cardboard scales, each of which covered a small slice of the band. Then the main tuning was adjusted until a reference beacon was correctly shown on the proper cardboard scale. This allowed easy measurements to much better than 1 kHz with fine mobility within a given scale. But changing the scales was a nuisance when slewing around the band looking for particular targets during a hot opening.
Closely Calibrated Receivers Increasingly, modern receivers come equipped with a digital display of frequency, with increments of either 1 kHz, 100 Hertz, or 10 Hertz, depending somewhat on price level. A few of the 1 kHz types simply have conventional variable oscillators coupled with a frequency counter. This can prove even less useful than a home-calibrated analog dial, because any displayed frequency represents a possible range of frequencies over 1 kHz in width, and interpolation is impossible. The only solutions are to add more digital circuitry to the counter to get more digits, (and the stability to match), or to resort to the "blindfold" channel-counting technique. The remainder of the current digital receivers are actually using digitally-controlled phase-lock synthesizers. These tune in more-or-less repeatable steps, so that each time we dial up a particular setting, we can trust that the receiver is on exactly the same frequency. Such receivers generally have memories as well, which also allow us to bring the receiver up on a precisely repeatable frequency. Unfortunately, most of these receivers are designed so that the digital display reads correctly when (in CW mode) a monitored signal is tuned for a pitch around 700-1000 Hertz. Since, as explained in Chapter Seven, we must use lower pitches for successful NDB DXing, we end up with a displayed frequency that is offset by several hundred Hertz. On receivers with a RIT (Receive Incremental Tuning) control, this problem is easily solved by offsetting the RIT by the same amount. The procedure is to pick a known-frequency signal (say, ZBB-396's upper sideband ident on 397.02), tune the receiver until the display reads correctly, then adjust the RIT until we hear that signal at our chosen pitch. So anyone shopping for a digital receiver for NDB work should try to get one having RIT, in addition to previously-mentioned features like AGC defeat, switchable narrow IF filtering, etc. Lacking RIT, another choice would be to adjust the frequency of the receiver's BFO. This internal control can be found by referring to the operating or service manual-set it by the same procedure as discussed above for a RIT control. Finally, if all the above fails to correct the display, we can just accept the offset error, and mentally add or subtract a standard correction factor from every reading. So, if ZBB's USB ident reads 396.6 when tuned for our chosen pitch, we know we'll have to add 0.4 kHz to all readings. External Frequency-Measurement Apparatus No matter what receiver we use, it probably doesn't give frequencies more precisely the 10 Hertz. Since it is useful to be able to measure NDB idents to 5 Hertz or better, all receiver users should seriously consider assembling this capability with external equipment. The time honored method is to use a tunable RF oscillator coupled to a frequency counter, which then becomes the receiver's "tuning dial." To measure an ident, we simply adjust the oscillator until its signal, as heard in the receiver, exactly matches the pitch of the ident. The counter then directly shows the exact frequency of the ident. The counter can be home-brewed (see recent issues of the ARRL Handbook for such a project), but it is a lot easier to purchase a commercial unit. These can be found selling in the neighborhood of $100, via ads in Radio-Electronics, Monitoring Times, QST, and Popular Communications. Radio hobbyists can find many other uses for such a counter as well. The oscillator is a simple homebrew project. Mitchell Lee has written up a surplus oscillator modification for the Feb. 1988 Lowdown; see also his article on Page 27 of the July Lowdown. For our purposes, this oscillator need not have great stability, nor even be calibrated, since the counter provides those attributes to the system. An alternative to the counter and oscillator would be a precision frequency synthesizer. This is, in effect, an oscillator whose frequency can be set so accurately that a counter is not needed. Max Carter published a simple synthesizer scheme on Pages 17-20 of the July 1987 and pages 16-17 of the Sept. 1987 Lowdown. A deluxe synthesizer project has appeared in the ARRL Handbook. For those who really want to pursue precision frequency measurements, the NRC Reprint Service offers some detailed papers on this topic. With techniques described therein, it might be possible to keep track of local temperatures of NDB's by measuring their tiny hour-to-hour drift! Frequency-Domain Summation This and the preceding two chapters have presented a set of techniques which enable us to thoroughly dissect the NDB band in the course of our DXing. What appears, upon first hearing with a broad AM-mode receiver, to be a band with a few score effective channels now is revealed to contain literally thousands of effective channels, each quite separable, measurable, and stable. By specifying 212.86 as the place to find a Vietnam ident, or 278.79 as the frequency of Easter Island's ident, we can give another DXer a major key to logging those places, provided he knows how to combine selectivity and frequency readout. This writer has been collecting such data for several years and publishing it in ever-increasing detail, formerly via the West Coast Checklists and the NBD DXers Newsletter-hopefully it will eventually become a part of the Lowdown. It is a literal map of the NDB band as would be seen on the ultimate spectrum analyzer, and now contains details for the idents of over 2,500 NDB on all continents except Europe. So if you have developed narrowband capability to at least the extent that you know what cluster you're listening to, your input would be very much welcomed, either to the Newsletter or to the Lowdown's "DX Downstairs" column, or both. To eliminate confusion, specify "±261.0" rather than "upper sideband of 260" to identify the cluster. If it's a 400-Hertz beacon, specify it as "±260.4." If you can supply more precision, by all means do so, and drop the "±" symbol. Let's take full advantage of the frequency domain, one of the great attributes of NDB DXing! Beacon Defects In The Frequency Domain A significant portion of the NDBs have one or more defects in their spectral emissions. This occurs, of course, because of aging parts and other maintenance needs which may not be met due to remote locations and low priority. There are five types of spectral defects to be heard on the NDB band: carrier harmonics, ident harmonics, spurious emissions, instability, and negative keying. Carrier Harmonics normally fall outside the longwave spectrum. These are just like the BCB and SWBC harmonics popularized by Glenn Hauser in that they are found at multiples of the carrier frequency. All beacons have some trace of these, just like any kind of transmitter, which can be heard (especially the second and third harmonics) when within a mile of an NDB. But a few rare cases are known of these harmonics being radiated as DX heard via skywaves. One notorious widely-heard harmonic is the seventh of CQI-274 in Council, Idaho, found on 1918 kHz as a negatively-keyed carrier harmonic. Ident Harmonics occur when the beacon's ident modulation contains harmonic distortion. For example, HBT-390 in Alaska has DXable idents on 387.96, 388.98, 391.02, 392.04, and 400-Hertz NY-350 in British Columbia has weak ones on 350.82 and 351.23 kHz as well as the proper 350.41 kHz. As with carrier harmonics, these are found in trace amounts on all A2 beacons. But they can't be confidently recognized without a narrowband receiving system, since they're usually so much weaker than the proper ident. Spurious Emissions include any signals radiated besides the carrier, the idents, and their harmonics. Basically, this is the "garbage" category. Sometimes these signals are steady, and sometimes they are keyed along with the ident. COR-205 in California emits a nest of carriers a couple of kHz wide. And ZBB-396 occasionally can be heard with a raspy-sounding identifier which resembles a 6-meter (50 MHz) auroral scatter CW note. Instability refers to frequency drift, particularly that which is fast enough to be audible while monitoring. This is quite rare in the U.S., Canada, and Down Under, but several examples have been noted among Russian Far East beacons. These actually are doing what hams know as "chirping,"i.e., drifting at the start of each Morse element, and bouncing back to the "rest" frequency after each element. Cases are known where the chirp moves the signal out of a narrow passband, although these beacons key so slowly that the receiver can be tuned along with the drift. Negative Keying is a defect occurring when the carrier of an NDB drops somewhat in strength during each transmitted ident element. It can be useful in identifying a beacon whose carrier is in the clear but whose ident is buried under QRM. In some cases, it may propagate farther than the genuine ident, since more RF power is radiated in an NDB carrier than in the ident. A few beacons appear to be only negatively-keyed, with no normal ident being audible (from a distance). One example is the aforementioned CQI-274, which has caused numerous reports of an unidentified IAEIN or IAEIK over the years in "DX Downstairs." One giveaway to negative keying is that it tends to have an excess of dots compared to normal Morse Code idents. Be aware, however, that a wide-selectivity receiver with AGC on can easily cause the carrier to falsely sound negatively-keyed. Poor transmitter power-supply regulation probably accounts for real negative keying. In order to decipher the negative keying, we must invert the elements. So a beacon whose format is dash-after-ident will not have DAID on its negative keying, and vice versa. A dot will become an interelement space, and vice versa; a dash will become an intercharacter space, and vice versa. Be careful with the lengths of the beginning and ending elements, where it is easy to mistake dots for dashes. The translation process is straightforward if we do it on paper, preferably graph paper, using IAEIN (w/DAID) as an example, first we draw the negative-keying characters being heard:
Next, we add vertical lines attached to the ends of each dot and dash:
Finally, we connect the verticals with horizontal lines where the negative keying has spaces:
These top lines can now be read out in plain Morse code, and we can look up CQI and put a new one in the log. Of course, a variety of NDB defects are also found in the time domain,
which will be covered later. Next time, in hopes of influencing DXers'
vacation plans, we'll discuss DXpeditions and visiting NDB sites.
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