where V is the velocity of 300,000,000 meters per second, F is the frequency expressed in Hz, and (Greek letter
lambda) is the wavelength in meters. As the frequency rises, the wavelength shortens. This relationship is
graphically expressed in figure 4-2. The free-space velocity of a radio wave is nearly constant regardless of its
frequency; the wavelength can therefore be found by dividing the velocity by the frequency of the wave.
Example 1: Find the wavelength in meters for a frequency of 3,000 kHz.
= V/F - 300,000,000/3,000,000 = 100 meters.
When the wavelength is known and it is desired to find the frequency, the velocity is divided by
Example 2: Find the frequency in MHz for a radio wave 30 meters long.
F = V/ = 300,000,000/30 = 10 MHz.
e. The amplitude, or strength, of the radio wave is expressed in volts. It represents the height of the wave as
expressed by the distance B in figure 4-2.
f. Two unmodulated RF signals having similar amplitudes but different frequencies are compared in figure
4-3. Both the 2-MHz and 10-MHz signals have the same waveform. The significant difference is the greater
number of cycles in the 10-MHz signal over the same period of time. This figure illustrates the fact that a low-
frequency signal has a longer wavelength than a high-frequency signal.
g. The power radiated is the amount of electrical energy, expressed in watts, radiated by a transmitting
4-4. COMMUNICATION FREQUENCIES
a. Meaning of Radio Wave. A radio wave is an alternating current (ac) of high frequency. The frequencies
that can be used for communication purposes may be conveniently divided into two broad groups: AF and RF.
b. Audio Frequencies. Audio frequencies are those frequencies that lie between 20 and 20,000 Hz which the
human ear can hear. For all practical purposes, the AF range includes those frequencies between about 50 and
(1) The frequencies that are most important in rendering human speech intelligible fall in the range from
approximately 300 to 3,500 Hz. A voice channel must therefore pass at least this frequency range.
(2) In music the range is considerably wider. The fundamental range of a pipe organ is from about 16 to
5,000 Hz, and the highest fundamental note of the flute is about 4,000 Hz.