By Scott Fybush and Earl Higgins
One of the easiest parts of the radio spectrum to explore are the broadcast bands. This page will attempt to offer some hints to make your exploration of the mediumwave and VHF-FM bands more enjoyable.
The mediumwave (commonly referred to as AM) broadcast band currently extends from about 525 to 1605 kilohertz. In the Western Hemisphere, channels are spaced in even 10 kilohertz increments; i.e.: 530, 540, ..., 1600 kilohertz. Elsewhere, channels are spaced in 9 kilohertz increments, i.e.: 531, 540, 549, etc.
In the United States, the band is currently being expanded to 1700 kilohertz. Stations which are currently experiencing high levels of interference have begun appearing on the 1610-1700 kilohertz frequencies. They will simulcast the new frequencies with the old for a period of a few years, eventually dropping the `old' frequency. This opening up of new channels presents some once-in a lifetime opportunities for the alert mediumwave DXer. The Federal Communications Commission's current list of stations moving to the expanded band is at http://www.fcc.gov/mmb/asd/exband.html.
The VHF-FM broadcast band in the United States, Canada and Mexico extends from 88 to 108 Megahertz. Channels are assigned at 200 kilohertz increments; i.e.: 88.1, 88.3, 88.5, ... , 107.9. In the United States only, the channels from 88.1 to 91.9 are reserved for noncommercial educational stations. Outside the United States, Canada and Mexico, the boundaries and channel spacing vary. In Japan, the band starts at 76 Megahertz. In Western Europe, the band generally runs from 88-108 Megahertz, but channels can be irregularly spaced, i.e.: 101.25 Megahertz.
The distant stations you are able to receive will depend largely upon signal propagation. This varies depending upon the time of day, the season, and other factors. For mediumwave, the single most important factor for good DX is the time of day. Mediumwave signals almost always get absorbed by the D Layer of the ionosphere during daylight hours. As a result, all mediumwave signals received during midday hours will arrive by ground wave, making reception of signals over a few hundred km/miles away unusual in daylight. At night, however, the ionosphere reflects mediumwave signals, making it possible for signals to be heard at much greater distances, up to a few thousand km/miles, via `skywave'. To a lesser extent, the period up to two hours after local sunrise, and two hours before local sunset, called "Critical Hours", have varying levels of skywave, and also can provide some very unusual reception opportunities for the mediumwave DXer. Reception also tends to be better in winter than in summer, due to lower levels of atmospheric noise and longer hours of darkness. In the United States, due to the large number of stations, many smaller mediumwave stations are required to sign off or reduce power sharply at sunset so as to reduce interference to distant stations.
Whereas the mediumwave band can be counted on to provide distant reception with much dependability, this is not the case at all on the VHF-FM band. Under `normal' conditions, VHF-FM signals generally carry no more than 150-250 km (100-150 miles), or `line of sight', since the ionosphere generally does not reflect VHF-FM signals. VHF-FM transmitting antennas are thus usually located as high as possible. Tall towers, high buildings, and mountaintops are common VHF-FM transmitter sites.
However, under certain rare conditions, the atmosphere will even reflect VHF-FM signals, thus making it possible to receive these stations at quite long distances.
During the warmer (especially Summer) months, two types of long-distance VHF-FM reception can occur. The first is tropospheric ducting, or "tropo" for short. This happens mainly during temperature inversions. During the day, the ground absorbs the heating rays of the sun. At night, radiational cooling occurs where that absorbed heat rises from the Earth and goes about 2-5 km (1-3 miles) up to the higher areas of the troposphere. If the conditions are right, mainly during times of high humidity, the troposphere can act like a sort of mirror or waveguide. Tropo is more prevalent during the late night and morning hours. Since the ducts are 5-7 km (3-4 miles) above the Earth, the line of sight can be roughly 170-800 km (100-500 miles). If a stalled weather front occurs, that ducting can bring stations over 1000 miles away. A Massachusetts DXer recently got local reception of WHTK/99.7 in Port Royal, SC, over 1300 km/800 miles away. UHF-TV also can get into the act during tropo. A viewer in Tulsa, Oklahoma got a perfect picture of Boston's WSBK-TV, Channel 38 in 1979 and got it verified!
The other relatively widespread form of VHF-FM DX is called Sporadic E, or E-skip, because it is the E Layer of the ionosphere which reflects the signals. The E layer is located roughly 80-170 km (50-100 miles) above the Earth. Like the name implies, this form of propogation is very sporadic, yet very intense. When it's in, it is VERY strong. Stations from a relatively limited geographic area 1300-2000 km (800 to 1200 miles) away will suddenly boom in, strong, often in stereo but quite fadey, even overpowering semilocals in many cases. This happens primarily during late May through late July (in the Northern Hemisphere), but it can happen at any time of the year on a limited scale. There is no known reliable way to predict E-Skip, but you can get a hint that E-skip might occur if you have a police scanner, capable of receiving the VHF low band (30-50 MHz). Set one of your memories to a channel not being utilized in your area. If you are beginning to get some activity on that channel, chances are the bands are going to "open up". After that, check any low band TV channel that is not active (Channels 2-6) in your area. If you are beginning to see some video or if are experiencing some co-channel interference, you may have Sporadic E! Sporadic E conditions move up in frequency depending upon the strength of the opening. The highest frequency at which signals are reflected by the ionosphere is called the Maximum Usable Frequency (MUF), just as it is in shortwave, and it can occassionally surpass the top of the VHF-FM dial in an unusually good opening.
It is also possible to get ID's of distant VHF-FM radio stations, via the momentary reception of "meteor scatter". During times of meteor showers, the quick bursts of meteors or "shooting stars" with their subsequent burning up in the atmosphere can provide a few precious seconds of long- distance reception, 300-2500 km (200-1500 miles) away. These meteor trails pass through the ionosphere providing a brief electrical charge strong enough to act like a mirror for those few seconds. The trick is to find a nice open frequency, and be very patient. Sitting on the open frequency for about five minutes during a meteor shower, you should at least hear a few "pings" to learn what meteor scatter reception sounds like. If you're lucky, you could hear a cluster of "pings" that can last 20-30 seconds. It will sound like someone is changing the station but in fact, the meteor trail is moving so fast that it is picking up stations like a fast moving mirror located many miles above the Earth. Whereas the mediumwave band can be counted on to provide distant reception with much dependability, this is not the case at all on the VHF-FM band. Under `normal' conditions, VHF-FM signals generally carry no more than 150-250 km (100-150 miles), or `line of sight', since the ionosphere generally does not reflect VHF-FM signals. VHF-FM transmitting antennas are thus usually located as high as possible. Tall towers, high buildings, and mountaintops are common VHF-FM transmitter sites.
Almost any radio is capable of some broadcast-band DXing, especially long-distance mediumwave reception. However, most recent radios, even those designed for quality shortwave reception, do not have outstanding broadcast band reception. Some important things to seek out in a receiver are:
One exception is the General Electric Superadio III (Model 7-2887.) The SR III is designed for optimum AM/FM broadcast performance, incorporating:
The SR III is a bulky portable radio which can be run off 120V AC or 6 "D" batteries, providing over 400 hours of battery life. This radio has become popular among the DX community for its exceptional performance.
It costs between thirty and sixty dollars in the US, and may be found at many discount outlets. It can be obtained from Bennett Brothers (Order #R3116) at 1-800-621-2626 or 1-800-631-3838, or from Best Products (Order # 140457) at 1-800-950-2398.
For mediumwave reception, most receivers have a short internal ferrite rod. This will provide acceptable signals for high-powered distant stations. Ferrite rods are quite directional, and the radio can thus be turned to null out strong interfering signals, or to improve reception of the desired signal.
For more advanced DXing, external antennas offer certain advantages. The most common external antenna is a simple random wire, 15m (50 feet) or more run out the window and then as high as possible (up in a tree, for example). The wire can be connected to the external antenna terminal. If none exists, you can open up the radio and wrap the wire a few turns around the ferrite rod inside. It is also possible, although less desirable, to simply wrap the wire around the entire radio. If the radio has a terminal marked "ground" or "GND," another wire can be run from this terminal to a copper rod driven a meter/a few feet into the earth.
One problem with a random wire antenna for mediumwave work is it's inability to reject strong local signals. Most receivers today lack the dynamic range to effectively deal with the extremely strong signals from a local mediumwave broadcaster as picked up by a random wire antenna. Thus, some sort of tuned antenna is best for all but the most isolated, rural locations.
The most popular antenna for mediumwave DX today is called a `loop' antenna, and can be either of two types: ferrite rod or air-core wound wire loop. These antennas are small, 25-100 cm (1-3 feet) in diameter, and sit on the DXers desk or shack table where they can be easily turned by hand for optimum peak or null of a signal. Each design works with a tuned circuit before feeding the signal into your receiver, and usually this circuit includes a small powered amplifier. Generally speaking, the longer the ferrite rod, or the larger the diameter of the aircore loop, (to a point), the sharper the null of the antenna. 45 to 55 cm (18 to 22 inches) would be optimum for a ferrite rod antenna.
Air-core loops need to be made by hand, as there are none on the market. Ferrite loops, however, are available commercially from at least two manufacturers; Kiwa, Palomar Engineers and Radio West. Plans for building all sorts of mediumwave loop antennas are available through National Radio Club publications. The address is found later in this FAQ.
A more advanced antenna is the "beverage" antenna. This is a length of wire 300 m (1000 feet) or more, with extremely high gain and narrow-beam directional characteristics. It is usually, but not always, terminated at the far end with a 450 ohm resistor connected to a metal stake driven into the ground. It should be pointed in the direction of the desired station. The beverage antenna can, under good conditions, be used for transatlantic and transpacific DX.
For VHF-FM, the important factor is height. The higher one can place an antenna, the better reception will be. A multielement Yagi antenna, which can be found in Radio Shack or similar stores, will often produce excellent reception. Since a yagi is quite directional, the use of a rotor is essential for reception of stations in different directions.
There are over 10,000 radio stations in the United States alone. It's important to have some idea of what to expect to hear. A good directory is important (see Station listings below), but it's essential to know what the station information means.
For mediumwave, North American frequencies fall into three basic classes:
On VHF-FM, the US FCC has reduced its restrictions on power and antenna height considerably. Today, VHF-FM stations are allowed up to 50,000 watts from a 150 meter antenna in the Northeast and California; 100,000 watts from a 610 meter antenna elsewhere. Many stations, however, serve much smaller areas. These "Class A" stations use only 6000 watts or less. They were formerly restricted to just the following frequencies: 92.1, 92.7, 93.5, 94.3, 95.3, 95.9, 96.7, 97.7, 98.3, 99.3, 100.1, 100.9, 101.7, 102.3, 103.1, 103.9, 104.9, 105.5, 106.3, and 107.1 Megahertz. While the FCC no longer restricts class A stations to those frequencies, most are still found there. Likewise, only a few of the high-power stations are found on the old class A channels. For stations with extemely high antenna, the FCC mandates that transmitter power be reduced proportionately; thus a station with only 430 watts from a 220 meter antenna will reach about as far as a 3000 watt signal from the usual 91 meter antenna will.
The most important element for beginning DXers is to get to know the dial. Spend a few hours scanning up and down the dial both during the day and at night. At night, many of the clear-channel 50kw stations, particularly the non-directional ones, should be easy to hear. Learn all the local stations. Find out which ones sign off at sunset, which ones go off late at night, and which stay on all night. This will affect which distant stations can be received. If you have a local station on all night, you shouldn't expect to hear another station on that frequency without some tedious nulling. Experiment with radio placement. As you slowly turn the radio, observe its directional characteristics. The Superadio III will receive signals coming from the front and back of the set the best, and attenuate the signals coming in `off the side' of the set. You can use this to your advantage and log several stations on a single frequency.
By all means, keep a log of all new stations you hear; someday you'll be glad you did! Better yet, keep it on the computer; that way you can share it with others on the net. After a few months of listening; you should have a list of 200-300 stations you've heard. Most of these will be your `regulars', audible almost daily. Beyond that, you can hunt for the rarer stuff. Try for all 50 US states (very, very difficult), or 15 countries (fairly easy in most locations). Within a few years, you'll likely have heard a thousand or so stations and have a very interesting logbook. Here's a `beginner's target list' of widely heard clear channel AM stations to start you out. All of the following stations either operate with at least 50,000 watts on a I-A clear channel, or operate with at least 50,000 watts nondirectional at night. From most locations in North America you should be able to hear all but six or seven of these fairly easily.
Once you've mastered the basics, here are some suggestions for areas of specialty DX:
Radio stations in the United States are required to identify with their full call letters and city of license once an hour, between 10 minutes before and 10 minutes after the top of the hour. Canadian stations are never required to identify, and many never use any identifier other than "Q107" or "Toronto's 590 AM." It's therefore important to use other clues:
An essential tool for any DXer is a good station listing. This will help identify what's being received, as well as provide a way to contact the station being heard.
For North American AM stations, one essential tool is The NRC AM Radio Logbook. It includes day and night antenna and power information, format, hours of operation, address, verie signers, network affiliations, etc. Price is $19.95 for US, $20.95 for Canadian, three dollars cheaper for NRC Members. To order write NRC Publications, Box 164, Mannsville NY 13661-0164 USA. NRC also publishes FM and TV Logs. To become a member (highly recommended) and receive a year's worth of `DX News', send $24.00 for US, $25.00 for Canadian to: NRC Subscription Center, Box 118, Poquonock, CT 06064-0118 USA. All other countries, write for price information.
Another concise and inexpensive North American directory is the M Street Radio Directory, published annually. The M Street listing includes frequency, power, directionality, format, address, phone number, and other listings less important to DXing. M Street also is indexed by call letter order and by frequency. The M Street directory costs $29.95 + p/h. It can be purchased directly from M Street at 800-248-4242 or +1 212 473 4668 voice, or +1 212 473 4626 fax. The address is M Street Corp., 304 Park Ave S Floor 7, New York, NY 10010 USA.
Another guide for VHF-FM DXers is Bruce Elving's annual FM Atlas. This includes maps showing every VHF-FM transmitter in North America, plus listings by state and frequency indicating power, stereo status, format, subcarriers, and slogans. FM Atlas does not include addresses or phone numbers. It can be purchased directly from Bruce Elving, PO Box 336, Esko MN 55733-0336. It costs $10.95 + $1.05 p/h.
Another listing is the Broadcasting Yearbook, which is available at some larger libraries. The Canadian Almanac and Directory includes call letters, frequency, and addresses for Canadian stations.
For listings outside North America, consult the World Radio TV handbook, published annually and available through your local bookstore or radio specialty mail order outlets such as the Radio Collection and CRB Publishing.
Broadcast band DXing is an exciting way to hear the sounds of cities and towns around the nation and even around the world. It doesn't require hundreds of dollars of expensive equipment or huge antennas on the roof. All it asks of the would-be DXer is time, patience, and knowledge. Have fun!
Bob Foxworth, Garret W. Gengler (antenna ideas), Bob Parnass, Paul Schleck (Superadio information), and Bruce Werner, along with anyone else I may have forgotten.
Scott Fybush can be reached at <fybush at-sign world dot std dot com>
Earl Higgins is a software engineer living in Saint Louis, MO who has been DXing mediumwave and FM for over 20 years. He considers himself primarily a mediumwave DXer, and has logged about 2000 mediumwave stations from the Midwestern USA. He can be reached at <ehiggins at-sign ehiggins dot com>.