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Chapter 7. Radio/Wireless > Radio Wave Propagation - Pg. 156

156 Electronics Explained: The New Systems Approach to Learning Electronics Wavelength While we normally use frequency to refer to location of a radio signal in the spec- trum, sometimes you will hear about frequencies or ranges of frequencies desig- nated by wavelength. Wavelength is the actual physical length of a radio wave. It is the amount of space the wave occupies. Specifically, wavelength is the length of one cycle of the sinusoidal variation of the electromagnetic fields. If we could see the electric and magnetic fields, we could measure the distance between the maximum inten- sity points in meters (1 meter 3.28 feet or 39.37 inches). This is the wavelength. Wavelength is also the distance that the radio wave travels in the time of one cycle. You can determine the wavelength of a radio signal with the following simple formula: 300,000,000/f Wavelength is represented by the Greek letter lambda ( ) and frequency in hertz is designated by the letter "f." The value 300,000,000 is the speed of light of a radio wave in meters per second in free space. (This translates to a speed of 186,410 miles per second.) A radio signal with a frequency of 150 MHz is expressed in hertz as 150,000,000 Hz. Therefore, 300,000,000/150,000,000 300/f 2 meters If we give the frequency in megahertz, the formula can be shortened to: For example, we can calculate the wavelength of the shortwave or high- frequency part of the spectrum (3 to 30 MHz) as follows: 300/3 100 meters 300/30 10 meters If you rearrange the formula, you can also find the frequency if you know the wavelength: f f 300/ 300/25 12 MHz What is the frequency of the 25-meter band? RADIO WAVE PROPAGATION Propagation refers to the way in which a radio signal travels from the transmit- ting antenna to the receiving antenna. The way the signal behaves is directly related to its frequency. Low and Medium Frequencies For example, in the low-frequency and medium-frequency ranges, the radio signals tend to hug the Earth as they move from transmitter to receiver. These ground wave signals can only travel relatively short distances because they are