Listening for NDBs:

One of the sub categories of DXing (that is, listening for distant radio stations) involves listening for NDBs - Non Directional Beacons (see sidebar.)  One of the objects is to try to pull as many of these NDBs out of the noise as you possibly can - hoping to hear new and ever more distant ones.

The big question:  Why?

That is a good question.  Typically, these NDBs have nothing in particular to say - except their callsign over and over again.  However, there is a certain fascination to be had with trying to hear an ever more distant signal - not to mention the vague mysteries of low-frequency propagation - especially during the winter months when the atmospheric noise is at its seasonal low.

Listening for NDBs has some more practical aspects as well:  Working to pull a signal out of the "muck" hones ones skills in being able to copy signals under the worst conditions - a skill that can come in handy when trying to communicate on the amateur radio bands when conditions are poor, when there is interference, or when the signals are just weak such as during EME operations.
 

What are NDBs? NDB is an acronym that stands for Non Directional Beacon.  What does that mean? NDBs are intended to be used by pilots as an aid in flying toward a point or as an aid to mariners in navigation.  The Non Directional part comes about because these beacons are not intended to "beam" their signal in any particular direction unlike several aero-navigational systems.  NDBs typically operate in the 190-535 KHz region - a frequency range that gives them good groundwave coverage during the daytime - but it also allows for often excellent propagation during the night via skywave.  NDBs identify themselves with a series of 1-3 letters (usually) that they send in "Morse" Code at a low enough rate that they may be identified by someone who does not even know the code.  The pilot (or captain) using an NDB will tune to the frequency of the beacon near their intended point (either their destination, or a point along their planned course) and using an RDF (Radio Direction Finder) know what their heading is with respect to the bearing of the beacon.

Techniques for listening for NDBs:

No doubt that every NDB DXer has his/her own listening techniques, but it all boils down to one main thing:  Patience.  If you expect to take a quick spin across the dial and hear rare and distant NDBs, you will likely be very disappointed.

One of the very best sources for techniques on listening for these beacons is the series of articles that appeared on the LWCA Lowdown beginning in the late 1980's entitled On the Art of NDB DXing by Sheldon Remington.  Fortunately, this series of articles has been made available online here on the LWCA website.  Although some of the specific information is somewhat outdated, the general techniques are more enduring.

It can be safely assumed that almost everyone doing NDB DXing these days uses a synthesized receiver - or at least one with a digital readout with resolution of at least 100Hz.  One of the more common techniques when doing a "survey" is to start at one end of the band (or even in the middle) and move up or down in systematic 1 KHz steps, pausing to see what can be heard at each step, doing some fine-tuning as necessary.

There are several problems that arise in NDB DXing making it more difficult to pull out some of the weaker beacons.  These include:

• Too much noise.  This could be man-made (powerline, etc.) noise, or just the atmospheric noise that plagues the LF bands - especially in the summer months.  Numerous techniques are available to reduce man-made noise and they are covered elsewhere.
• Receive system deficiencies.  Often the LF DXer notices that there seem to be quite a few AM stations in the low-frequency band:  This is almost always a result of intermodulation distortion - usually in the receiver or in an active antenna that might be used.  This sort of problem may usually be reduced (or eliminated) with the use of a tuned active antenna (or one with a good lowpass filter)  or a preselector of some sort.
• The use of directional antennas (such as a loop) can help reduce manmade noise sources as well as null out a beacon that may be covering over a more desirable catch.
• Assuming that your receive system is working perfectly (i.e. no intermod and no manmade noise) and you are listening in the dead of winter (minimal atmospheric noise) that doesn't mean that you will be able to hear everything that is out there to be heard:  Propagation is the next thing to contend with.  This is where patience comes in.  You may have to "sit" on frequency for a while and hope that QSB will work in your favor - or even come back at a later date.
• You can do everything right, but still not hear that beacon you are searching for.

A Screen Capture of the Spectran program with the receiver tuned to 311 KHz USB.  There are four beacons that are easily visible here.  Around 400 Hz, the lower and upper beacons are 9Y (Pincher Creek, AB) and HY (Hay River, NT) respectively. Around 1 KHz, the MVI (Monte Vista, CO) and BFE (Brownfield, TX) are the top-most and the next one down respectively.
(Click on the image for a larger version with labels)

Again, NDB DXing takes patience, concentration, and skill.  As technology improves, however, it is certainly within our rights to use it to pull out those beacons that you may have previously missed.  The most helpful accessory to a computer in NDB DXing (besides a program to log reception of beacons) may be a sound card:  Programs are available to not only provide good audio filtering, but to visually represent what is being received as well.

One of the best-known of these is Spectran.  Although this program is undergoing continuous evolution, it already not only provides a "waterfall" display that graphically shows the audio spectrum in three dimensions (frequency, time, and amplitude) but it can do real-time audio filtering as well in Versions 4 and above.  While this program is suited for extremely slow-speed copy of CW (such as QRSS) it can provide an aid to NDB DXing.

The image shows the result of listening on 311 KHz using USB:  I prefer to use USB as the RF frequency of a signal displayed is simply the addition of the dial frequency on the receiver and the audio frequency on the screen.  To understand this display requires a little bit of knowledge on the nature of NDBs.  Typically, Canadian NDBs have their idents modulated at 400 Hz and U.S. NDBs typically modulate their idents at about 1020 Hz (there are exceptions, of course.)  NDBs are modulated using AM (but often with just a single sideband) which means that they will also have a continuous carrier.

When a beacon is zero-beat one will see the modulation sideband received (in this case, the upper sideband) and modulation of signals with the displayed passband - at 400 Hz and 1020 Hz, as in the image.  Fortunately (for us NDB DXers) not all beacons end up precisely on their designated frequencies.  This error arrives from two sources:  The carrier frequency itself may be off causing its sidebands to be off-frequency as well and/or the modulating tone may be off-frequency, causing it to appear at a distance different from the usual 400 or 1020 Hz away from the carrier.  (Note that some NDBs actually consist of two transmitters:  One providing a continuous carrier, and another with a CW-keyed offset frequency providing the "sideband" carrying the identifier.)

The settings for Spectran for this image were a 5.5 KHz sampling rate and a 2.7 Hz resolution:  This provided a display bandwidth from about 100 Hz to 1.8 KHz, showing everything over a range greater than a 1 KHz "slice" of spectrum.  While this resolution is far too narrow to show the individual dits and dahs of the beacons, one can easily ascertain the existence of beacons.  What does this display tell us?

• Right away, we know that there at least four beacons that we may be able to hear.  We would have quickly known if there were any beacons worth spending time trying to hear at that moment:  Keep in mind that the conditions may change, bring in a new crop of beacons to hear.
• The signals down near 400 Hz are most likely Canadian.  Most of these operate with a 400 Hz tone, and in the DAID (Dash After ID) format.  This also means that we are most likely zero beat with the carrier associated with those beacons and therefore, our dial frequency is the beacon's frequency.
• The signals near 1000 Hz are probably U.S beacons and we note that these are not DAID types because of the "bursts" of signal correlating with each ID.
• We cannot be sure that we are at zero beat with the beacons around 1000 Hz.  When a carrier is Amplitude Modulated with, say, 1020 Hz, you would end up with a signal that was 1020 both above and below the carrier frequency.  What we may be seeing is not the beacon whose carrier is 1020 Hz below the tone (i.e. at 0 Hz, or zero beat and therefore on the frequency indicated by the receiver) but rather the lower sideband of a beacon whose carrier is 2 KHz above the frequency indicated on our receiver.
• At just above 1000 Hz, we see a  solid line - this is most likely a carrier from an NDB that is 1 KHz above the receiver's frequency.  Just above this solid line we see another weaker line that may be solid - or maybe not?  It's hard to tell - it could be another NDB's carrier, or it could be another beacon using the DAID format, or it could be a combination of several carriers, with our without some idents.  From this snapshot it is impossible to tell - but if you were to wait for a while, QSB could change things, or a yet another beacon may appear.

Using Spectran's audio filters to "get" NDBs:

There is really no substitute for good IF filtering.  A narrow filter can help prevent a much stronger NDB on an adjacent frequency from throttling back the AGC, for example.  However, you aren't likely to find an IF filter much narrower than 250 Hz or so - with 500 Hz being more common.  Unless you are a truly avid NDB chaser (or an amateur radio operator that does a lot of CW operation) you aren't likely out to invest the $60 or more (sometimes much more) that it may take to get another filter.  An audio filter can certainly fill a niche - especially one that is narrower than available IF filters.

There are very specific limitations to audio filters, however:  If a nearby signal is causing the AGC to "throttle back" the desired, signal, an audio filter won't necessarily help you.  Turning off the AGC may help, but whether you have an AGC or not you are still faced with the same ratio between the weak and the strong signal.  Switching to a narrower filter and/or adjusting the radio's PBT (PassBand Tuning) or IF Shift and appropriate selection of the receiver's sideband selection (to prevent audio "images" from appearing in the audio) may allow you to put that offending signal outside your existing filters.

Looking at the above image of the four NDBs, listening with the "naked ear" may allow one to pick out one, maybe even two of the signals.  As you can see, there is really nothing in the audio other than the signals near 400 Hz and 1000 Hz.  The trained ear is perfectly capable of picking out the stronger of each of the two signals at each of these two different frequencies - assuming that there are different signal strengths.  However, what if each "pair" of beacons were equal strengths, what would you do?  Several things come to mind:

• Wait for a while.  Conditions change and one of those (currently) equal-strength beacons may become stronger or weaker than the other for long enough to pull out an ID.
• Waiting also works for trying to hear the ID.  If the two beacon's intervals aren't identical, they will eventually occur at different times as they "slide" past each other, allowing the listener to pick out different elements of each ID.  Sometimes simply writing down the letters of the two beacons as they send together will yield a pattern, allowing you to accurately deduce the beacons you are hearing.
• If you have a notch filter, you can try to notch out the beacon that is slightly different in frequency.
• If you have a directional antenna, you can try to null out the one that you are trying to avoid.
• Use a bandpass filter and allow only the one you are targeting to pass through.

To be able to receive the signal from BFE I placed the filter's limits just below the line for MVI and just above that line just below BFE's signal (keep in mind that I didn't know which beacon I was "seeing" until after I successfully copied its ident.)  It is also worth pointing out that at the precise moment that this image was captured, BFE's signal was not copiable - I had to wait for a few minutes before it slowly faded back in before I could copy it.

Setting up audio filters in Spectran:

Setting up the audio filter in Spectran is fairly easy - if you know how:

• Along the left side, note the BandPass "button" - click on this so that it is red.  Also present is a "BandRej" button - this cannot be active at the same time as the BandPass button - see below.
  
• At this point, you may have to bring up the "mixer panel" for your sound card.  The easiest way to do this is to double-click on the "speaker" icon in your task bar (on the far right) if it is there  Once this is brought up, make sure to "un-mute" the Wave Volume output and make sure that the volume is turned up slightly.
• Hit one of the SHIFT keys on your keyboard.  This should cause the filter's adjustment slider to appear just above the frequency scale.  Moving your mouse cursor into this region should now cause the cursor to change from its usual "crosshair" symbol to that of an up-pointing arrow.  You should now see a blue "bracket" indicating the highest and lowest bandpass filter frequencies.  If you don't see this "bracket" you may need to "slide" the frequency scale up or down to find it (i.e. it may be off-screen.)
• Now, you can use the left mouse button to set the lower frequency of the bandpass filter and the right mouse button to set the upper frequency.  Of course, you cannot set the higher frequency to be lower that that of the lower frequency - and the program won't let you.
• If you want better display resolution in order to set the filters more accurately, select a narrower bandwidth for display resolution.  This will not affect the audio bandwidth, but it allows you to "zoom in" on the spectrum display, and the filter bandwidth markers are "magnified" appropriately.
• Note also the BandRej button:  As the name implies, it is used to "reject" a specific band of frequencies, selected in exactly the same way as the bandpass response.  Note, however, that it cannot be active at the same time as the BandPass function - but it can be used in conjunction with the CW Peak filter mentioned below.
  

When setting filters there are several things to remember:

• If you set the filters to be too narrow they will cause the dits and dahs to run together.  As a general rule, the following formula should be kept in mind:
• CW Speed (in WPM) * 2.5 = Minimum filter bandwidth in Hz
• Most NDBs identify at between 5 and 7 words per minute.  This means that CW received through a bandpass filter that is narrower than 15 Hz or so may start to sound excessively "soft."  Nevertheless, it is possible to copy a typical NDB's ident through a filter that is as narrow as 8-9 Hz without too much difficulty.
• Avoid the temptation to turn up the audio gain too high when setting a narrow filter.  Pay attention to the "audio level meter" located immediately above the frequency scale so that it doesn't turn red - even on peaks:  If it does, than reduce the input level to the computer's sound card via the sound card's mixer panel and/or the radio's audio level control.  The Volume control on Spectran adjusts only the output to the speaker and has no effect on the input level.

Other filter modes:

I would be remiss if I failed to mention that Spectran has three another filter modes available:  The Denoiser, and the Humid filter, and the CW Peak filter.

The Denoiser:

This filter is a typical noise-reduction algorithm found in many audio DSP implementations.  It works by mathematically eliminating those spectral components that are random, revealing (hopefully) those components that aren't just randon noise.

HUMID:

The HUMID algorithm (short for HUM Instant Destroyer) is a comb filter.that is designed specifically to remove noise that is a result of AC powerline interference - specifically AC hum.  Note that this is primarily intended to remove the AC hum that would be received by a VLF antenna system - that is, one that is designed to directly intercept RF in the frequency range capability of the computer's sound card (that is:  Plugging your VLF antenna directly into the sound card's input!)  Note that direct VLF reception is best done with a specialized antenna system and that simply plugging your HF antenna into your sound card's input is not recommended.  If you want to try your hand at receiving VLF/ULF signals,

The CW Peak Filter:

• Along the left side, note the CW Peak "button" - click on this so that it is red.
• As with the BdPass filter, at this point, you may have to bring up the "mixer panel" for your sound card.  The easiest way to do this is to double-click on the "speaker" icon in your task bar (on the far right) if it is there  Once this is brought up, make sure to "un-mute" the Wave Volume output and make sure that the volume is turned up slightly.
• Hit one of the SHIFT keys on your keyboard.  This should cause the filter's adjustment slider to appear just above the frequency scale.  Moving your mouse cursor into this region should now cause the cursor to change from its usual "crosshair" symbol to that of an up-pointing arrow.  Note that the "bracket" for the bandpass filter may appear whether or not you have the BdPass mode active.
• Holding down the Control key, click the left mouse button.  A red "arrow" (visible on the image above) will signify where the CW bandpass is centered.
• The CW Peak filter has a fixed width and somewhat less of a "brick wall" response than the BdPass filter.
• It is important to note that both the CW Peak filter and either the BandPass or BandRej filter may be used simultaneously (remember that you can't use "BandPass" and BandRej at the same time.)  With the effect of using both is as if the filters were placed in series.  Because the BdPass filter has a "brick wall" response, it can occasionally introduce audible artifacts.  These artifacts may sometimes be reduced somewhat if the CW Peak filter is turned on as well and this benefit is most apparent if the BdPass filter is set slightly wider than the CW Peak filter.

Links:

Return the KA7OEI main page, or look at these other pages:

The Radio Waves below 22 KHz site. - This site has a lot of articles about VLF/ULF topics - a must-read!

Online NDB (and related) Databases:

AirNav's Navaid Information - This provides a database that is searchable by frequency, identifier, or name.  This database encompasses North American and at least some Central-American beacons.

Canadian Nondirectional Beacons - A searchable database offered by Pierre, KA2QPG

William Hepburn's TV & Radio DX Information Centre - This has information on some LF transmissions as well as HF, VHF, TV and other radio.
 

Do not miss Sheldon Remington's series of articles: On The Art of NDB DXing from the LWCA website.
 

DSP Software for digging out the weak/buried signals (These are definitely worth getting!):


Spectran Beta 4 and ARGO - Spectran is the successor to the well-known Hamview program.  This windows version can use any standard (full-duplex) sound card. Like "Spectrogram" this program produces a graphical "waterfall" display and has a real-time audio bandpass filter.  This program is more specifically suited for amateur/radio use as it's display algorithm can more distinctly show "buried" signals than Spectrogram.  The ARGO program is specifically tailored for QRSS - extremely slow speed CW.  It's frequency range is limited to audio, however. (By I2PHD and IK2CZL. Freeware - non-commercial use.)  You can download both of these from the Weak Signals web site.

ON7YD Slow-CW Software - This software (called QRS) is designed to send both CW and DFCW (Dual Frequency CW.)  For more information about this program, go to the description at the LWCA web site.  (Freeware for non-commercial use.)

ONEasygram - This software is uses the "core" .DLL of Spectrogram (above) and provides a user-interface that is better-suited for QRSS use.   (By OK1FIG.  Freeware for non-commercial use.)

Bill DeCarle's Page (VE2IQ) - This is where you can get programs such as CRUNCH, AFRICA, and COHERENT.  Please take a look at the "Readme" files included with the programs before you run them.

KK7KA's WOLF Page - This is where you can get the WOLF program as well as some tips for using it.  Also, look at  Lyle Koehler's Wolf For Dummies page.

Spectrogram - This is a great program that may be used for analyzing audio and VLF/ELF/ULF signals.  (For radio-related applications, refer to the Radio Waves below 22 KHz site.)  This program produces a graphical "waterfall" display showing time, frequency, and amplitude at frequencies from DC through 22 KHz.  (By Richard Horne.  Older versions were shareware, but the current "full" version allows for a 10-day evaluation before requiring a purchase.)

Any comments or questions?  Send an email!