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Transcripts for Mann DX, Page Two |
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Carl Mann DX: Carl Mann of Omaha, Nebraska is a veteran journalist and radio hobbyist. Carl has been introducing KNLS listeners to the world of distance listening (DXing) for more than a decade. You can review Carl's scripts by clicking on the subjects listed above.
It was discovered early in the development of radio that radio signals on some frequencies could be received hundreds and even thousands of miles away at certain times of the day or night. It was quickly determined that something in the sky was bouncing the station's signals back down to earth again far over the horizon, allowing for the distant reception. That something turned out to be the ionosphere. Without it, those signals would continue on outward into space. The ionosphere is simply part of the earth's upper atmosphere of rarefied gasses that is affected by sunlight. Rays from the sun activate the atoms of the atmosphere at certain levels, starting at about 50 miles high, so that they lose electrons. An atom without an electron is called an ion, and a layer of ion's can do something interesting with radio waves: It bends them. As a radio signal travels upward toward the sky over the horizon, it will encounter an ionosphere layer at an altitude of about 60 miles. This layer will then bend, or refract, the wave back down to earth, to be heard well over the horizon hundreds of miles away. It's easy to see an example of refraction. Just place a pencil in a glass of water. Instead of appearing straight, the pencil will appear bent at the point it goes into the water. The ionosphere depends on sunlight. So the daytime side of earth is the most affected. Other variations of sunlight replenishment for the ionosphere happen as the seasons go throughout their cycle, and also as the sun itself goes through its 11-year sunspot cycle. So the ionosphere layers are constantly shifting and changing in intensity as it makes possible shortwave reception from distant lands. The ionosphere: a layer of ionized particles in the earth's atmosphere formed by sunlight, that will refract radio signals at certain frequencies so that they will come back down to earth well over the horizon.
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Coordinated Universal Time is abbreviated UTC, even though the letters in the abbreviation are not in that order. UTC is a time zone for worldwide reference used when activities or observations must be synchronized over long distances. Coordinated Universal Time is the actual time at Zero meridian on the globe, that line on maps that goes from the north pole to the south pole, passing right through London, England, and much of Western Africa. The line goes right through the Royal Greenwich Observatory in London, an old timekeeper from which the earlier term Greenwich Mean Time, or GMT, got its name. In 1972 the term UTC was introduced, and was interchangeable with GMT. Then in 1986, UTC completely replaced GMT as the term for the time at the Zero meridian. Sometimes UTC is noted by the letter "Z". Military communications will call it Zulu Time, or simply Zulu. Zulu is the phonetic alphabet word for the letter Z, which stands for Zero Meridian. It's up to the listener to convert UTC to her local time. The 24 basic time zones in the world are established every 15 degrees from both the west and east of the zero meridian. To determine the local time in each zone when UTC is known, simply add the number of hourly time zones if you are east of the meridian, or subtract the hours to the west. For example, New York City and Quito Ecuador are 5 time zones west of London, so their time is always UTC minus 5 hours. Just reverse the procedure to determine UTC from your local time zone. For the New York and Quito example, the user would add 5 hours. When Daylight Savings Time is used, adjust one less hour for the calculation. Many charts and maps are available to help you determine in which time zone you are in relation to UTC. And that's our DX Definition of UTC: Coordinated Universal Time. The time at zero meridian used as a standard when activities or observations over long distances must be synchronized.
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A Heterodyne is the tone, or note, produced when two or more stations are on frequencies very close to each other. The audible tone is created when the difference in the frequency of the two stations falls within an audible range. For example, if two stations are on 9 point 5 mHz, but one is actually running a little higher, on 9 point 501 mHz, a tone of 1000 Hertz will be heard mixing with the stations. The difference between the two is point 001 mHz, or 1000 Hertz. This is an audible frequency, called the beat note. If the offset station were to move closer to the neighboring station, the tone would become lower, because the frequency difference becomes less. When the offset station finally settles in on exactly 9.5, the tone gets so low it finally disappears. This is called the Zero Beat. Heterodynes are common when one of two stations operating on the same frequency is running a little off frequency. Most rules and regulations require broadcasters to remain within 10 Hertz of their assigned frequency. But in some areas of the world frequency deviation is not so closely regulated. Heterodynes are regularly heard on the AM portion of Amateur Bands because the operators there can select any frequency within the band. The heterodyne is an interference to reception, and on a good receiver can be reduced by tuning to the side of the desired signal, selecting the side that is absent of interference. Other tricks to eliminate heterodyne interference include adding audio filters to the output of a receiver, or using a "notch" filter that can be tuned to the heterodyne frequency to suppress it. And that's our DX Definition of. . . Heterodyne: The tone or note created by two or more nearby stations, where the frequency difference between them falls within an audible range.
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The beat frequency is the frequency of the heterodyne generated by two radio signals operating on close frequencies. As an example, two stations assigned to the same frequency produce no beat note. But if one of them is off frequency by 500 Hertz, the two stations will produce a 500 Hertz audio note. This is called the Beat Frequency. For DXers a beat frequency on the shortwave bands is called heterodyne interference. But sometimes heterodynes are useful. A beat frequency is necessary when listening to Morse Code stations. A radio signal keying on and off in code produces no sound. So a second signal is needed nearby to set up a beat frequency. By turning on your receiver's Upper or Lower Sideband mode, a built-in oscillator produces a radio signal that is offset from the tuned station by 2 kHz. When tuning the receiver, this injected signal will beat against the tuned signal at varying frequencies, producing an audio note that goes up or down. In the earlier days of radio, long before single sideband, this internal signal generator for receivers was called the Beat Frequency Oscillator, or BFO. The BFO often had it's own tuning control so that instead of tuning the station to change the beat frequency, the BFO could be tuned. You can see modern day receivers with USB and LSB are just a slightly redesigned Beat Frequency Oscillator. Another use of the heterodyne is in radio and TV receivers. The most famous receiver design, still in use today, is the Superheterodyne receiver designed by Edwin Armstrong during World War I. It uses the heterodyne principal to allow many stages of amplification without requiring the listener to manually retune each stage every time the station is changed. It uses a beat frequency well above the range of sound, audible only to the receiver's circuits. And that's our DX Definition of Beat Frequency: The frequency difference between two radio signals operating on separate but nearby frequencies, setting up a third signal operating at the beat frequency.
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Interval signals are used by international broadcasters and some regional networks around the world; an audio trademark or logo to identify the station. In the early days of international broadcasting, the interval signal was used by the engineers to help as they tuned up the transmitters. Today they are handy identifiers of stations or networks. An interval signal can be a portion of a melody or sound effects. Music and sound effects can be understood by listeners in any language. So interval signals can identify the station regardless of the language being broadcast. The interval signal is most likely to be aired just before the start of the sign on announcements, or between programs, especially during a change in languages. On regional services they are frequently heard during hourly station identification announcements. The melodies selected by the stations are usually something of local significance. For example Radio Austria uses the opening notes of The Blue Danube Waltz, Radio Canada International airs the first 4 notes of their national anthem, O Canada, and the Voice of America plays a portion of the American folk song, Yankee Doodle. More examples include the opening notes of a revolutionary song from Radio Havana. Often, the instruments used in producing the melody can be of significance as well, such as the bells of RAI out of Rome Or the chosen interval signal can be local sounds, like the Kookaburra bird identifying Radio Australia, the jungle drums of the Kenya Broadcasting System, or the peeling of the bells of Big Ben in London heard on the BBC. Many Dxers have been able to identify a weak station in an unknown language simply by recognizing the interval signal,. And that's our DX Definition for Interval Signal: A segment of melody or sound effects used by international and regional broadcasters as an identifier for their service before and during broadcasts.
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Single Sideband is a form of radio transmission that increases the efficiency of communications. On an ordinary receiver, it makes the voice will sound muffled and unintelligible. But careful tuning on a receiver with upper and lower sideband provisions, or Beat Frequency Oscillators on older sets, will bring in the voice clearly. Radio signals are sent out on the transmitter's carrier wave on its given frequency. When sound is introduced on the carrier, it appears as identical, although opposite, sidebands on both sides of the carrier. Since only the sidebands carry the sound, it was discovered that two-thirds of the transmitter power could be saved by filtering out the carrier. And even more power could be saved by filtering out either the upper or lower sideband and just transmitting one sideband. All this saved power can then be placed into the one sideband, making it a more powerful and efficient method of transmission. At the receiver, the carrier has to somehow be restored in order to understand the transmission. So many receivers use an oscillator circuit that can be turned on setting up an internal radio signal which can be tuned to the exact point where the transmitter carrier should be. The oscillator switch is labeled USB and LSB, with the listener selecting the proper sideband. Upper sideband is most commonly used on the higher frequencies. Once correctly tuned in, the sideband transmission is clearly intelligible, and over much greater distances than a full AM signal could deliver. Sideband is primarily used for communication purposes by utility or ham stations but some broadcasters have experimented with using just one sideband plus carrier. Also, before satellites, sideband was used to relay programs to distant transmitter sites in other parts of the world. And that's our DX Definition of single sideband: A method of radio transmission that uses only one of the two sidebands produced by a transmitter, suppressing the carrier, and placing the saved power into the remaining sideband.
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Hertz, spelled H-e-r-t-z as in Zebra, is a measurement of frequency, and determines where on the radio dial a radio station will be found. Frequency is measured in cycles per second, meaning the number of times an electrical current goes from positive to negative and back each second in the transmitting station's antenna. The term cycles was changed to Hertz, in the 1970's in honor of Heinrich Hertz, a German scientist who first discovered radio waves in 1887. Older radio receivers and books may still use the term kilocycles and megacycles, abbreviated kc and mc, but today kiloHertz and megaHertz are abbreviated k, capital H, z, and m, capital H, z. The more cycles per second, the higher the frequency. When a tuning fork vibrates 1000 times per second, it produces an audible tone at one thousand Hertz. Here is where the prefixes Kilo and Mega come in to make it easier to count the cycles, or Hertz per second. Kilo means thousand, so one kiloHertz is that same audible tone of one thousand Hertz. Going higher up into radio frequencies, at 1 thousand kiloHertz, you land in the middle of the AM Medium Wave band. One thousand kiloHertz is the equivalent of one million Hertz. Mega means million. So, if one kiloHertz is one million cycles, then it can also be called one megaHertz. Shortwave frequencies are even higher as you continue to count the cycles, or Hertz, per second. At around 6 megaHertz, you're tuning at 6 thousand kiloHertz, or 6 million Hertz. But as you get into the frequencies in between, for example 6 million one hundred twenty five thousand Hertz, it's easier to say 6 point 125 mHz. And that's the DX Definition of Hertz: A measurement of frequency in cycles per second.
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One of the more popular antennas used on the shortwave frequencies is the dipole antenna. The dipole is designed to resonate at a specific frequency, meaning its length determines the frequency on which it will work best. The antenna will also resonate on multiple frequencies of it's resonant frequency, called harmonics, although to a lesser degree. Also the dipole has directional qualities. It will receive stations whose signals arrive broadside the best, and those arriving off the ends of the antenna will be weakened. The antenna's directional property is called its pattern, and for the dipole it would look like a figure 8, with the dipole running horizontally at the center. The dipole is really two separate wires of equal length. Stretched out, it looks like a big letter "T", with two legs at the top of the T running in opposite directions of each other, and the center feed line forming the stem of the T running down to the receiver. The two lengths of wire don't touch each other. At the coax feedline one wire connects to the receiver using the center conductor, the other to the ground using the outer braid. A full-wave dipole's length matches the wavelength of the frequency you want to maximize. To find the wavelength of the frequency in meters, divide 300 by the frequency in mHz. For 6 mHz, this would be 50 meters. Often this will be too long to be practical. Fortunately a dipole performs nearly as good at one-half the wavelength. Even a quarter-wave dipole gets good results, although most Dxers favor the half-wave dipole. For parts of the world where antenna lengths are measured by the foot instead of meters, one meter equals 3 feet, 3.3 inches. And that's our DX Definition of Dipole: An antenna that uses a pair of wires of equal length, running in opposite directions to each other from one starting point, and has resonant and directional properties dependent upon the total length.
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Sensitivity is your receiver's ability to hear those weak signals that Dxers like to chase down. It is one of two major terms used to describe the quality of a radio receiver, the other being selectivity. The more sensitive a receiver is, the better able it is to hear a weak radio station.. Sensitivity is measured by how many microvolts it takes at your antenna for a station to be heard at a certain level. Radio signals set up very tiny voltages in your antenna to be amplified and turned into sound. These voltages are very weak, usually measured in microvolts. A microvolt is one millionth of a volt, the typical voltage induced in your antenna by a distant station. By contrast, your local radio stations are setting up stronger antenna voltages, measured in millivolts, thousandths of a volt. Typically, a receiver's sensitivity measurement will be listed as the number of microvolts required to set up a certain signal to noise ratio. Many communications receivers have an RF attenuator built in. This is to turn down the sensitivity of the set when tuning to or near very strong signals which can overload the set. Some Dxers try to boost the sensitivity of their receiver by using a broadband amplifier at the antenna input. This is sometimes useful if one cannot use an outdoor antenna and the smaller indoor antenna needs a little help. But adding stages of amplification at the front end of receivers generally is not successful. It often boosts the noise along with the signal, and can create other problems as well such as receiver overload. Generally it is best to stick with the original receiver design, and seek a receiver with good sensitivity specifications to begin with. And that's our DX Definition of sensitivity: The ability of a receiver to pick up and detect weak signals.
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Selectivity is a receiver's ability to separate the station to which it is tuned from nearby stations. It is one of two major terms used to describe the quality of a radio receiver, the other being sensitivity. Selectivity is measured in bandwidth. The bandwidth is the amount of the radio spectrum your radio will hear when set to one frequency. A radio with a typical bandwidth of 12 kiloHertz will hear a station on the tuned frequency, plus other signals up to 6 kHz on either side of that frequency. Of course other signals that are closest to your tuned frequency will be causing the stronger interference. So if you could narrow this bandwidth, those nearby stations would be tuned out more effectively. Some receivers have selectivity filters that can be switched in to make the bandwidth even more narrow. On some sets the switch is labeled "Wide," for a bandwidth of 12 kHz, and "Narrow," for about 6 kiloHertz. Some receivers have, or can be modified to have, bandwidth filters going down to 2.5 kiloHertz making for very selective reception. But there is a limit to how narrow you want the bandwidth. As the station is operates on its carrier frequency, its sidebands are reaching out into the adjacent frequencies on both sides. It is these sidebands that carry the station sound. So as you narrow the receiver's bandwidth, you are also narrowing the quality of the sound, meaning that the full fidelity of music will be lost. Voice fidelity falls in the middle ranges and does not require as wide a bandwidth as music does, so it is less affected by switching in narrow bandwidth filters. And that's our DX Definition of selectivity: The measure of a radio receiver's ability to separate signals on different frequencies
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