The waves have an amplitude (‘A’), which corresponds to the signal strength. They also have a wavelength (λ), which corresponds to the distance traveled by the wave in one complete up-and-down cycle. In radio work, the wavelength is measured in meters (m), except in the microwave region where centimeters (cm) and millimeters (mm) make more sense. Wavelength can be measured at any pair of points on the wave that are identical: two peaks, two troughs, two zero crossings, as convenient in any specific case. The number of cycles that pass a given point every second is the frequency of the wave. The classic measure of frequency was cycles per second (cps or c/s), but that was changed in 1960 by international consensus to the hertz (Hz), in honor of Heinrich Hertz. But since 1 Hz = 1 cps, there is no practical difference. The hertz is too small a unit for most radio work (although many of our equations are written in terms of hertz). For radio work the kilohertz (kHz) and megahertz (MHz) are used: 1 kHz = 1000 Hz, and 1 MHz = 1 000 000 Hz. Thus, a short-wave frequency of 9.75MHz is 9750 kHz and 9 750 000 Hz.

Wavelength and frequency are related to each other. The wavelength is the reciprocal of frequency, and vice versa, through the velocity constant. In free space, the velocity constant is the speed of light (c), or about 300 000 000 m/s. This is the reason why you often see ‘300’ or its submultiples (150 and 75) in equations. When the frequency is specified in megahertz, then 300 000 000 becomes 300 for one wavelength. The half-wavelength constant is 150, and the quarter-wavelength constant is 75. The relationship is

λmeters= 300 / FMHz

 

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