g. A wavelength is the distance in space occupied by one cycle of a radio wave for a given
period. Wavelengths vary from a few hundredths of an inch at extremely high frequencies, to many
miles at extremely low frequencies. Wavelengths are, however, expressed in meters. For wave 1 in
Figure 1-3, the distance between paints A and E or points B and F is one wavelength.
h. Waves 1 and 2 both have the same wavelength, but different amplitudes. The height of a
wave crest above the reference line is called the amplitude of the wave, which indicates the magnitude
of energy the wave transmits. A continuous series of waves, having the same amplitude and
wavelength, is called a wave train.
i. The number of cycles of a wave train in a unit of time is called the frequency of the wave
train and is measured in hertz (Hz). The frequency of both waves 1 and 2 is four cycles per second. To
honor the German physicist Heinrich Hertz, the term Hz was designated for use in lieu of the term
"cycles per second" when referring to the frequency of radio waves. The frequency of household current
is 60 Hz. The frequency of Army tactical radios is in the millions-of-Hz range.
2. Electromagnetic spectrum. The term spectrum is used to designate the entire range of
electromagnetic waves arranged in the order of their frequencies. Figure 1-4, page 1-6, illustrates the
electromagnetic spectrum. As illustrated, the spectrum represents a continuous array of electromagnetic
waves arranged in the order of their frequencies. Note that the frequencies used for communications
range from very low frequencies (VLF) at 3 kHz through super high frequencies (SHF) at 30 GHz.
These frequencies are called radio frequencies (RF) Microwave signals are in the ultra high frequency
(UHF) through the extremely high frequency (EHF) range.
a. Only a small portion of the spectrum contains visible waves (light) which the human eye can
see. When you look at a rainbow, you see the various colors arranged in their order of frequency. The
electromagnetic field and electromagnetic energy are the means of receiving and transferring energy
from point to point. A radio antenna is used to radiate electromagnetic energy in the form of a radio
wave. Figure 1-5, page 1-7, shows a simple radio communications system. Generated radio waves are
radiated into space in all directions (omnidirectional) from the transmitting antenna, at the speed of light.
Another antenna receives the energy, or signal. Thus, an antenna is a conductor that either radiates or
collects electromagnetic energy.
b. The two fundamental fields associated with every antenna are an induction field and a
radiation field.
(1) The induction field is associated with the energy stored in an antenna. As an antenna
radiates electromagnetic energy, a magnetic field exists around it. Recall how your car radio responds
when you drive past a high voltage transmission line. This interference is caused by the magnetic
transmission line's field acting on your radio receiver. The induction field is considered a local field and
plays no part in transmitting electromagnetic energy.
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