|by Dr. Bruce F. Elving|
|FM radio is situated in a most ideal portion of the electromagnetic spectrum
since it benefits from all types of propagation. Within its 88 to 108 MHz
confines, the listener can experience reception by means of skip, extended
groundwave, tropo and line-of-sight reception of local stations --- all with
beautiful high fidelity monophonic sound, or in full stereo. Adding to the
excitement of FM DX'ing is reception of weak signals by meteor scatter, auroral
DX, or by bouncing off lightning flashes and airplanes. Sometimes these types of
reception cause interference to nearby stations and annoyance when a non-DX'er
is trying to listen to a favorite program! To the DX enthusiast, however, the
presence of unusual reception conditions means that many stations not usually
heard are coming in, with the attendant opportunity to add new stations to his
or her log. The propagation section of this guide will fill you in on the
Not all DX reception is perfect or even "good", as anybody who is serious about DX'ing soon discovers. Some of the signals --- even on "static-free FM" --- will be fadey, weak, or suffer from interference, but it is this element of unpredictability and willingness to listen to FM stations under less than perfect conditions that helps to make FM DX'ing an exciting pastime. But, from this writer's experience in hearing over 1500 FM stations from one location (Duluth, Minnesota) over a number of years, I can attest to the fact that a high percentage of what you do receive as DX will come in with dazzling fidelity and stereo --- more so than the stuff that takes a trained ear and a practiced hand to get.
In the United States and Canada the FM band contains 100 DX-ready very high frequency channels, comparable in their reception characteristics to television channels 2 through 6, with the FM band lying immediately above channel 6 in the spectrum. [In other parts of the world, the FM band is similarly situated, except for eastern Europe & Russia, where it was formerly in the 66-72 MHz region. In these countries, both frequency ranges are used for FM broadcasting although the most popular stations have moved to the 88-108 range. In Japan the band is 72-76MHz.] With thousands of commercial and educational FM radio stations presently on the air in North America, there is a lot of potential DX for the serious listener. In addition, thousands of low-power translator stations are on the air rebroadcasting existing stations on new frequences. These translators are interesting DX targets on their own.
Many DX enthusiasts, including members of WTFDA, DX all the VHF and UHF bands, including FM. Others specialize; still others engage in AM radio (broadcast band) and shortwave DX listening, as well as FM DX'ing. The latter involve little or no conflict with the DX'er's time, since AM tends to be best during the winter months, while FM is at its seasonal best during the summertime, daytime as well as night. Furthermore, students and teachers who are away during the school year find the opportunity to add to their FM logs when at their homes during the productive May through September period.
Equipment for FM DX listening need not be elaborate, especially for skip, which occurs primarily during the early and mid-summer, mainly during the day and early evening. To test your FM DX receptionability, tune in a weak FM station and devise experiments to improve the reception of this station. If you have access to more than one FM receiver, use the set that gives the best reception of that weak station. If your set uses a line-cord antenna (the old Zenith tube-type radio with this kind of antenna were popular), place the radio with its power line extended straight broadside to the station. This type of antenna will not be suitable for much DX. An upstairs room in a wooden frame house is better than the basement when using an indoor antenna. If you live in an apartment or a high-rise building, your best reception will be from the direction your windows face. If you have a rabbit ears antenna, extend the rods in a flat position, put the antenna on the window sill, connect the lead-in wire to the antenna terminals of your radio (forget about using the radio for DX'ing if it lacks provisions for an outside antenna), and see what weak or distant stations come in. With such indoor equipment, you should be able to receive DX when conditions are favorable.
Some people connect their FM radios on TV antennas, and this can be tried. The best method is by means of an antenna switch, which diverts all of the antenna's energy to the TV or to the FM set. FM from a TV or even from a combination FM-TV antenna is, however, a compromise. There is a compromise in time (FM DX may be good when the rest of the family wants to watch a favorite TV program), and the FM signal is diminished by virtue of its being received through a system designed to favor TV reception. A true FM antenna is best.
Just about everyone who enjoys FM for its distance-carrying abilities (and FM, despite what its detractors have said, is not merely a local broadcast service) eventually acquires better equipment than that described above. The first item should be an external FM antenna --- either an all-direction "turnstile" of "S-type", which can clamp to your existing TV tower or weathervane, or ideally, a directional "yagi" FM antenna with rotator. These antennas are best bought either through a radio parts jobber or through a mail order parts company rather than through a local TV dealer. They can be installed either by the DX'er or by a professional. Even DX'ers living in apartment buildings are often surprised to learn how easy it is to obtain a landlord's permission to install rooftop antennas, despite wording to the contrary in leases. Advice on antenna selection and erection is available from WTFDA's technical editors and DX'ers who may live near you, as well as from frequently-appearing technical columns in the VUD. Ordinarily, it's best to mount your antenna as high as possible. Selection of a low loss lead-in is also important.
When buying a new receiver, try to select component high fidelity equipment, such as stereo receiver or tuner with good cross-modulation (90 db or better) rejection and selectivity. Avoid used equipment that may employ transistors in the FM front-end that are not field-effect (so-called "bi-polar transistors"), as well as newer "cheap stereos" that give mediocre performance on FM, especially if you live in an area close to one or more FM or TV transmitters. FET's do not overload in the presence of strong local signals as easily as the bi-polar transistors found in early receivers (and found to this date in almost all portable radios). Older tube-type FM sets work well too, though lack the sensitivity of newer receivers. Stereo receivers with crystal or ceramic IF filters or older monophonic tuners generally have the best selectivity (ability to tune a weak station clse on the dial to a very strong local signal). Most modern receivers have more than adequate sensitivity (ability to receive very weak signals). With increased crowding on the FM dial, it is more necessary than ever to pay primary attention to selectivity when purchasing a receiver, in order to build log totals and to enjoy the maximum of FM DX.
Tuning and signal meters on a receiver, as well as digital frequency readouts, are helpful, especially when using an antenna rotator to correctly point the antenna at the station. Pushbuttons, on sets so equipped, can be a valuable DX'ing aid. On the other hand, automatic tuning, muting, and unswitchable automatic frequency control could be a hindrance to DX'ing. Look for sets with a sensitivity rating of less than 2 uv, an alternate channel selectivity rating of at least 70 db, and a capture ratio of less than 2.
Naturally, an FM DX installation permits listening to many distant stations for your normal music, informational, or hi-fi enjoyment, therefore it is good to include the FM DX receiver as the heart of a component hi-fi or stereo system, incorporating speakers in more than one room in the house controlled by a switch at the receiver.
Many FM stations send out a subcarrier (SCA) in addition to what they are broadcasting to the public. This SCA signal is of interest to many DX'ers, as it hs programs that are unique and something the FCC has not encouraged people to own. On SCA, depending upon where you live, you can hear background music, reading to blind persons, proram to physicians, lawyers and farmers, and network relay. SCA DX is a challenge because it takes stronger signals to pick up and a certain expertise to connect an SCA adapter to an FM receiver.
Good equipment and knowledge of the stations is essential to receiving the most FM DX within a given period of time. However, FM DX'ing is much like fishing. You have to be around when conditions are favorable. Generally, the best antenna and receiver will not work as well on the poorest day as the poorest set on the best day. Like the fisherman who caught his limit, but who is experiencing success, you should stay with your receiver as much as possible when conditions are extraordinarily good, lest you miss some never-to-be-forgotten DX opening. If, like the fisherman, you develop experience, have good equipment, and are on hand when conditions are favorable, who knows what limit you may exceed.
This article was written for WTFDA by Dr. Bruce F. Elving of FM Atlas Publishing, and is used with permission. Copyright reserved.