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By Don Moore

This article was originally published in The Journal of the North American Shortwave Association.


If you've been reading NASWA very long, you've noticed by now that shortwave broadcasters aren't the only broadcast stations using the shortwave bands. On occasion loggings of mediumwave (AM) "harmonics" creep in. But, although some mediumwave stations broadcast on shortwave at times, usually when they do they don't even know it! And that's just part of what makes DXing mediumwave harmonics such a challenge.


All radio transmitters naturally put out harmonics, which are multiples of the fundamental frequency. For example, a station on 1400 kHz will transmit harmonics on 2800, 4200, 5600, etc. Twice the fundamental frequency, e.g. 2800 khz, is called the second harmonic; three times it, e.g. 4200 khz, is called the third harmonic, etc. The first harmonic, or one times the fundamental frequency, is the fundamental itself. Harmonics should not be confused with images, which are generated internally in a receiver, and are usually received plus or minus 910 kHz or 1000 kHz of a frequency, depending on the receiver, or receiver mixing products that cause exceptionally strong stations to appear where they don't belong. Harmonics are produced at the transmitter, not the receiver.

Harmonics may seem like a free way for a small AM station to become an international broadcaster, but AM stations don't want to intentionally transmit harmonics, as any power that goes into a harmonic frequency is not being used for their fundamental frequency, which means a less powerful signal for their main audience. In addition, harmonic transmissions are prohibited by law as these unintentional broadcasts on higher frequencies may interfere with other stations which are licensed to use those frequencies. To prevent these problems, station engineers suppress harmonics below a maximum level allowed by the FCC or their nation's equivalent agency, although enforcement may vary (especially in Third World countries). But, not all stations have competent technical help watching over the equipment, or even if they do, mistakes can happen. Sometimes something gets maladjusted or whatever and suddenly there is maybe fifty or a hundred watts going into a harmonic instead of half a watt or a watt. Usually the problem is so small that it won't be noticed until the engineer does a regular check-up on the transmitter, or someone hears the harmonic and tells the station about it. In the meantime, it's happy harmonic hunting for DXers!


The question as to whether or not harmonic receptions are a technical problem in the receiver is logical considering how many receivers have image and overloading problems. But remember, to have an image or overloading problem putting the station where it doesn't belong, the signal first has to be coming in very strong on the frequency where it belongs. That is, your receiver can't screw up and put a signal someplace other than where it belongs if the receiver isn't receiving the signal in the first place!

As to whether receiving a harmonic is a receiver problem, there is a very easy way to check for that. If you have a harmonic of a 1400 kHz station on 2800 kHz, just tune down to 1400 kHz and see if you hear the same signal. If you don't, there's no way the harmonic is being generated by your receiver - it must be coming from the transmitter. Usually that's the case as the harmonic is likely coming from several hundred miles away and there are probably other AM stations on the fundamental that are closer to you, and those are the ones you're going to hear on the fundamental frequency. Even if you do hear the same station on 1400 kHz, that still doesn't mean the harmonic is a receiver problem. If on 2800 you hear the station with a fair and interference-free signal, but on 1400 it is weak and mixing with three other stations, you again have a transmitter-generated harmonic. Receivers can't take a weak signal and produce a stronger image out of it, nor can they filter out co-channel interference in the process (although sometimes we might wish they could!).

Receiver-generated harmonics are always a possibility, however, and you should cross check possible fundamental frequencies for the same programming before spending any time trying to ID a harmonic. In reality though, only very-strong local AM stations are going to cause receiver-generated harmonics, unless you have a cheap single-conversion receiver. Even then, if you hear the second harmonic of a 5 kW or less AM station a couple of hundred miles away you can be sure it is not the fault of your receiver. The signal from such a station just won't be powerful enough to overload any receiver.


First of all, it is possible to hear many stations that you simply can't hear otherwise. That's true of almost all the 130+ plus AM harmonics I've heard over the past 20 years. After all, what's your chance of hearing a 1 kW Guatemalan AM station in the central US? Secondly, DXing MW harmonics is one of the most challenging types of DXing and can be a lot more exciting than hearing a station on its fundamental. Most harmonics are second harmonics and fall at the bottom of the SW spectrum - the 2 MHz area, which is the most difficult part of the SW bands to hear any kind of distant stations on. Increasing the challenge, harmonic signals are extremely weak. As the signals are unintentional, powers are on the order of a hundred watts or less - real DX.

Another challenge is that because the signals are weak, it is hard to understand enough to narrow down who the station is, and there are so many more choices. For example, if you have a US harmonic on 2800 kHz, just take a look at how many stations are on that frequency. Was that static-covered identification WBDA or WEBH or ...? Or, maybe a harmonic has lots of news of Colombia, but there are six Colombians on the probable fundamental. Finally, the signals are very intermittent. You can tune through the 2000 - 3200 range every evening and morning for a month and not here a peep of a harmonic. Then in a week's time, there might be three or four. Some last a day or two, few more than a couple of weeks. Whether or not something is there is a combination of chance and good propagation.


DXing harmonics requires just three things. The first is a sensitive receiver as the weak signals don't propagate well over long distances. The more sensitive the receiver, the more audio will be recovered from the weak noise-covered signals. Of course your receiver also must cover the 2MHz band; many of the smaller shortwave portables do not.

The next important piece of equipment is a good antenna. The best 2MHz antenna is the probably a lengthy beverage or mini-beverage antenna, but there is good news for DXers who don't have that much real estate - the second best antenna will even fit inside an apartment. Loop antennas, which are commonly used on medium wave (AM), will also work very well on 2MHz. Building loop antennas is beyond the scope of this article, but the National Radio Club's two antenna reference manuals have lots of plans and information. Send $1 to National Radio Club Publications Center, PO Box 164, Mannsville, NY 13661 for an NRC catalog. To make a box or other wire loop work usable on 2 MHz, it may be necessary to use a little less wire than is needed for MW. Build the loop according to their plans, try it out, then gradually shorten it, if necessary. Alternately, Kiwa is reportedly soon to come out with a commercially-built shortwave loop for the tropical frequencies. If it's anything like their medium wave loop, it should be a great harmonic-snatcher!

The budget conscious DXer with some backyard space might try a Dipole, Inverted V, or a Delta Loop cut to 2.5MHz. However, at 120 meters, even these common antennas take up space! Some DXers have reported good 2 MHz reception by winding 120 meters of wire around a 10 foot section of pvc pipe. Finally, if none of these specialized antennas is suitable, there's the good old random wire with an antenna tuner. I now use a Drake R-8 and two 400 foot mini- beverages (which, in length, are roughly resonant at one wavelength on 2 MHz). But years ago I had success DXing harmonics with a Barlow-Wadley XCR-30 portable with a 3 foot whip antenna and after that with a FRG-7 and a 30 foot wire on the roof. Many of the harmonics I hear today with the R-5000 and mini- beverages are also audible, if not as strong, on my Sony ICF-2010 with the whip antenna.

The third necessity for DXing harmonics can't be bought from any radio supplies vendor. It's called patience. Because signals here are weak and very susceptible to the vagaries of propagation, it's common to go one or two weeks without hearing a thing. The DXer has to consistently tune through the 2 - 3.2 MHz band at least two or three times during each nighttime or morning DX session. Weeks may pass without a thing being heard and when something is heard, it may be too weak to identify. Many DXers soon give up and say it's impossible to hear anything down there. But through the patience of daily or near daily checks, some great catches can be made. The best way to look for harmonics is to tune from the bottom of the band to the top in USB mode, stopping and checking out any suspicious carriers, especially those on frequencies ending in zero.

Actually, two more things are needed to DX harmonics. You need to have a good list of AM stations, such as the WRTH for foreign medium wave and the National Radio Club's "Domestic Log" for domestic stations. And, unless you have a good head for mathematics, a calculator will be very useful. When you hear a probable harmonic, you need to be able to divide the frequency you're hearing and find possible fundamental frequencies to look up in the AM station guide to help identify it. For example, 2920 divided by 2 is 1460, by 3 is 973, and by 4 is 730. Therefore you are probably hearing a second or fourth harmonic. A third is only possible if the station's fundamental frequency has drifted off center.


Mediumwave harmonics can be divided into two categories, domestic stations (US & Canadian) and international harmonics. Let's look at the easier, domestic, ones first. Domestic harmonics are easy to spot because they sound just like everyday AM stations, which is exactly what they are. The first harmonics you hear will probably be your local AM stations, although be careful - if the signal is loud and clear, it is probably a product of overloading in your receiver. But, even if your local stations' harmonics have been properly suppressed within the legal limits, it still may be possible to receive the second and maybe third harmonic a few miles away with a good antenna and a sensitive receiver.

Of course hearing harmonics from your local stations isn't much of a challenge. The challenge is to pick up harmonics from stations several hundred miles away. Because harmonic DXing is essentially DXing someone's mistake before he catches it, it's truly a matter of "here today, gone tomorrow". A harmonic strongly received today may not be there at all the next week, or even the next day. Usually, however, domestic harmonics will be around for a few days before the problem is caught and fixed. If you hear a harmonic and can't identify it, check the next day about the same time. A good time to go hunting for domestic harmonics is around your local sunset and sunrise (especially sunrise), when "grayline" near-darkness conditions exist. Because the grayline runs roughly north-to-south, harmonics from a southerly direction are much more common than those from the east or west.

Most harmonics you hear will be second harmonics; third and higher are very uncommon. Some harmonics actually fall inside the AM band, such as those of stations on 800 kHz and lower (e.g. 2x800 = 1600 kHz). These will probably be covered-up by stronger signals from stations legally on the frequency, unless you live very close to the offending transmitter. While some harmonics can be heard in the "no-man's land" of 1605 - 2000 khz, between where AM ends and SW begins, and in the lower part of the 2 MHz band, the best frequencies for domestic harmonics seem to be between 2460 - 3200 kHz. These frequencies are the second harmonics of 1230 - 1600 kHz, which is where most of the smallest AM stations are - the ones more likely not to be technically up to standards.


Domestic harmonics are tough, but international ones are even tougher. But, eventually you'll come across a Spanish station on a frequency where there isn't supposed to be one. Engineering requirements are less enforced in Latin America than in the US and Canada, and low-powered harmonics are common among the less well-run rural AM stations. Rarely, though, are these harmonics strong enough to be heard more than a few hundred miles away. But, under really good reception conditions, some of these may be heard in North America for a night or two. On occasion, a station will put off transmitter maintainence long enough, that the harmonic will become strong enough to be heard in North America on a semi-regular basis, albeit weakly. For example, Radiotelevision Dominicana put out a harmonic on 2540v (2x1270) for over two years and in January, 1993, Ecuador's Radio Runacunapac Yachana put out a harmonic on 2968 (2x1484) strong enough to rival signals in the sixty meter band.

Because they are so weak, long-distance Latin harmonics are best received when static levels are lowest - in the winter and as late at night as possible. The best reception is usually in the early morning before dawn, right after the stations have just signed on for the day. As with domestic harmonics, just divide the frequency you are hearing by 2, 3, and 4 (and maybe 5, 6, and 7), and check the WRTH lists for any "even" channels (frequencies ending in zero) that you get. Occasionally a Latin American harmonic will be heard on a non-even channel, such as Colombian Una Voz en la Frontera, heard on 3041, or the second harmonic of the off-frequency fundamental of 1560.5 kHz.

Latin American harmonics come from the south, so what about harmonics from the east and west. From either direction harmonics have a huge ocean to cross, which is difficult for such weak signals. I know of only two cases of trans-oceanic reception of mediumwave harmonics. For several weeks in 1979, Radio Tirana, Albania put out a second harmonic of its 1458 kHz broadcast on 2912 kHz, which was heard by several DXers in Eastern North America. The second case was in July, 1989 when DXers John Bryant and Dave Clarke, DXing on vacation in the Juan Fernandez islands off the Washington coast, picked up an unidentified Indonesian on 3144 kHz. John later played the tape to some of his Indonesian students and with their help was able to identify it as the second harmonic of Radio Veronica Sonata on Sulawesi Island. This must surely be the best harmonic reception of all time - and John even later received a verification for it!


In the case of domestic harmonics, if you want a QSL, it probably isn't worth the effort to report your harmonic reception to the station. Verifications are all but unheard of because it's highly unusual for a chief engineer or station manager to be willing to admit on paper that his station violated FCC regulations and the harmonic was heard several hundred miles away! Besides the high probability of not getting a reply, another drawback of sending a reception report on a harmonic is the possibility that the harmonic may disappear before other interested DXers get a chance to hear it. The best remedy for this is to report the reception to a rapid DX news source, such as the DX Party Line, before sending your report, so that other DXers will have at least a few days to look for it before the report arrives. There is, however, a way to verify a harmonic reception in a round-about fashion. Send a report to the station, but neglect to even mention the frequency heard in the report. The station then naturally assumes that you heard the fundamental and sometimes sends a QSL. It's a bit devious, and I don't know if it really counts as a QSL. But, if you really want a reply from a domestic harmonic reception, it works.

But, regardless of whether one is interested in a QSL or not, should we inform the station of the problem? Some DXers feel we should do a good deed and call or write them about it. Others point out that it is the station's job to adequately maintain their equipment, so let the harmonic on the air so that other DXers can hear it.

One common argument for telling stations that they are producing harmonics is that the harmonics may be interfering with vital communications. In reality, that's rarely true. Tune through the area between 1600 - 3200 kHz a few times and see what you hear. There's not much there. A few frequencies are used for maritime communications, but the US Coast Guard & similar agencies elsewhere monitor those frequencies constantly, so no AM station is going to get away with a harmonic on a vital frequency very long. If you are familiar with medium wave DXing, think of how quickly that Turks & Caicos AM station on 530 got moved when it began interfering with aeronautical beacons a few months back. Or, for another example, when WRMI first began testing in April, their transmitter was putting out a twelth harmonic which was interfering with the VHF airband and were told about it in just a few days. In short, the organizations that are using those vital frequencies keep a close eye on them. Usually, harmonics are very low-powered by-products, and anything legitimately on a frequency is bound to be much more powerful and would blow the harmonic signal away. The only reason harmonics can be heard at distances is because they have clear channels with no other station using the frequency.

Getting back to QSLing, Latin American station engineers and managers, on the other hand, don't seem to be as reluctant to admit their mistake in the form of a QSL. I have received several QSLs for Latin American harmonics, and have seen several more in DX bulletins over the years. QSLing Latin harmonics doesn't seem to be any more difficult than QSLing Latin American stations in general. Curiously, few of the Latin American harmonics that I have reported to, whether or not an answer was forthcoming, fixed the harmonic very quickly after the probably arrival of my report. In fact, three were still heard over a year later!


In over 20 years of off-and-on harmonic DXing, I've heard around 130 medium-wave harmonics about equally divided between US and foreign stations in more than 20 different states and over 15 countries (all from Latin America except for Albania). Most were second harmonics, with a couple of third and one fourth thrown in. The exception to that are the Cuban stations, especially those on 590, 600, and 720 kHz, which have been heard on harmonics as high as the twelth. Harmonics are, of course, unpredictable. A harmonic heard last year, or even last week, is probably not going to be there today. But, to give you an idea of what can be heard via harmonics, here's a list of some that have been reported over the last year or so. Just don't expect to hear these same stations - they've probably fixed the transmitters by now! The fundamental frequency is given in ().

(This list is from about 1995)
1720 Radio Clarin, Dominican Rep (860)
1760 Radio Progreso, Cuba (880)
2020 Radio Lobo, Chihuahua, Mexico (1010)
2040 Caribbean Christian Radio, Turks & Caicos (1020)
2060 Radio Francisco Orellana, Ecuador (1030)
2100 LV Cuchumatanes, Guatemala (1050)
2120 La Voz de Centroamerica, Honduras (1060)
2200 LV de las Islas, San Andres Island (1050)
2300 WCEN, Mount Pleasant, MI (1150)
2560 Radio Alajuela, Costa Rica (1280)
2580 Radio Enciclopedia, Cuba (1290)
2640 XERJ, Mazatlan, Mexico (1320)
2720 WBLC, Lenoir City, TN (1360)
2840 WJOB, Plymouth, WI (1420)
2800 XELH, Acaponeta, Mexico (1400)
3000 R Color, Dominican Republic (1500)
3060 Emisora Claridad, Colombia (1530)
3100 Radio Fantasia, Colombia (1550)
3120 WAGC, Centre, AL (1560)
3510 Radio Central, Ecuador (1170)
3600 Radio Rebelde, Cuba (600)
4530 WLAC, Nashville, TN (1510)
4800 Radio Rebelde, Cuba (600)

AM harmonic DXing is an interesting, if obscure, branch of the the listening hobby, but I hope this little article helps convince a few others out there to try it. If so, be sure to report your results here so the rest of us can go after the stations, too!


This article is copyright 1995 by Don Moore. Not to be reproduced without permission.

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Association of North American Radio Clubs
DXer of the Year for 1995

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