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There are different ways to indicate where to find a certain station on a
radio dial. For example, we could say that a station is operating on 9680 kiloHertz (kHz),
9.68 megahertz (MHz), or on 31 meters. And all three ways would be correct! Radio waves are transmitted as a series of cycles, one after the other. The hertz (abbreviated Hz) is equal to one cycle per second. Hertz was named after Heinrich Hertz, a German physicist [1857-1894] who experimentally proved the existence of electromagnetic waves. You may have noticed that the electric power supplied to your home is rated at 60 Hz. Electric power is distributed as alternating current (AC), meaning it goes through a cycle of changing directions of flow. When we say that electric power is "60 Hz," we mean 60 cycles per second (in which time the direction of flow changes 120 times). Radio waves go through far more cycles in a second than electric current, and we need to use bigger units to measure them. One is the kilohertz (kHz), which is equal to 1000 cycles per second. Another common one is the megahertz (MHz), which is equal to 1,000,000 cycles per second----or 1000 kHz. The relationship between these units is like this: 1,000,000 Hertz = 1000 kilohertz = 1 megahertz Radio is usually thought of "beginning" at frequencies of approximately 5 kHz, although most available receivers can only tune down to about 150 kHz. The term "wavelength" is left over from the early days of radio. Back then, frequencies were measured in terms of the distance between the peaks of two consecutive cycles of a radio wave instead of the number of cycles per second. Even though radio waves are invisible, there is a measurable distance between the cycles of electromagnetic fields making up a radio wave. The distance between the peaks of two consecutive cycles is measured in meters. The relationship between a radio signal's frequency and its wavelength can be found by the following formula: wavelength = 300 / frequency in MHz According to this formula, a frequency of 9680 kHz would be equivalent to a wavelength of 30.99 meters, which we would round to 31 meters. Thus, 9680 kHz, 9.68 MHz, and 31 meters all refer to the same operating frequency! As the formula indicates, the wavelength of a radio signal decreases as its frequency increases. This is important because the length or height of various types of antennas must often be a fraction (usually one-quarter or one-half) of the wavelength of the signal to be transmitted or received. This means that most antennas designed for frequencies near 4000 kHz will be physically much larger than antennas designed for frequencies near 30 MHz. Frequencies are seldom given in terms of wavelength anymore. However, certain segments of the shortwave bands are referred to in terms of "meter bands" as a convenient form of shorthand. For example, the term "10-meter band" is used to refer to the ham radio band that extends from 28000 to 29700 kHz. The following is a table of the most common ham radio and shortwave broadcasting "meter bands" found on frequencies below 30 MHz:
You'll notice some inconsistencies in the table above. For example, the 17-meter ham radio band is actually higher in frequency than the 16-meter broadcasting band. These inconsistencies have come about from years of use (misuse?) of a particular "meter band" to refer to a certain range of frequencies. |
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