TC 9-64 _________________________________________________________________________
2-22. Wavelength. A wavelength is the space occupied by one full cycle of a
radio wave at any given instant. Wavelengths are expressed in meters
(1 meter is equal to 3.28 feet). You need to have a good understanding of
successful communications. The relationship between frequency, wavelength,
and antennas are discussed in chapter 4.
2-23. Velocity. The velocity (or speed) of a radio wave radiated into free
space by a transmitting antenna is equal to the speed of light, which is
186,000 miles per second or 300,000,000 meters per second. Because of
various factors, such as barometric pressure, humidity, and molecular
content, radio waves travel inside the earth's atmosphere at a speed slightly
less than the speed of light. Normally, in discussions of the velocity of radio
waves, the velocity referred to is the speed at which radio waves travel in free
space.
2-24. The frequency of a radio wave has nothing to do with its velocity. A
5-megahertz wave travels through space at the same velocity as a
10-megahertz wave. However, the velocity of radio waves is an important
factor in making wavelength-to-frequency conversions, the subject of our next
discussion.
WAVELENGTH-TO-FREQUENCY CONVERSIONS
2-25. Radio waves are often referred to by their wavelength in meters rather
than by frequency. For example, most people have heard commercial radio
stations make announcements similar to the following: "Station WXYZ
operating on 240 meters." To tune receiving equipment that is calibrated by
frequency to such a station, you must first convert the designated wavelength
to its equivalent frequency.
2-26. As discussed earlier, a radio wave travels 300,000,000 meters per
second (speed of light); therefore, a radio wave of 1 hertz would have traveled
a distance (or wavelength) of 300,000,000 meters. Obviously then, if the
frequency of the wave is increased to 2 hertz, the wavelength will be cut in
half to 150,000,000 meters. This illustrates the principle that the higher the
frequency, the shorter the wavelength.
2-27. Wavelength-to-frequency conversions of radio waves are really quite
simple because wavelength and frequency are reciprocals: Either one divided
into the velocity of a radio wave yields the other. Remember, the formula for
wavelength is:
v
v
λ=
or
f=
λ
f
Where:
λ = wavelength in meters
v = velocity of radio of radio wave (in Hz, kHz, or MHz)
f = frequency of radio wave (in Hz, kHz, or MHz)
2-28. The wavelength in meters divided into 300,000,000 yields the frequency
of a radio wave in hertz. Likewise, the wavelength divided into 300,000 yields
the frequency of a radio wave in kilohertz, and the wavelength divided into
300 yields the frequency in megahertz.
2-8
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