__________________________________________________________ Radio Wave Propagation
CHARACTERISTICS OF RADIO WAVES
2-17. Shape. The basic shape of the wave generated by a transmitter is that
of a sine wave. The wave radiated out into space, however, may or may not
retain the characteristics of the sine wave.
2-18. A sine wave can be one cycle or many cycles. Recall from chapter 1 that
the number of cycles of a sine wave that are completed in 1 second is known
as the frequency of the sine wave. For example, 60 cycles of ordinary house
current occur each second, so house current is said to have a frequency of 60
cycles per second or 60 hertz.
2-19. Frequency. The frequencies falling between 3000 hertz (3 kHz) and
300,000,000,000 hertz (300 GHz) are called radio frequencies (abbreviated
RF) because they are commonly used in radio communications. This part of
the radio frequency spectrum is divided into bands, each band being 10 times
higher in frequency than the one immediately below it. This arrangement
serves as a convenient way to remember the range of each band. The RF
bands are shown in Table 2-1. The usable radio-frequency range is roughly 10
kilohertz to 100 gigahertz.
Table 2-1. Radio Frequency Bands
DESCRIPTION
ABBREVIATION
FREQUENCY
Very low
VLF
3 to 30 kHz
Low
LF
30 to 300 kHz
Medium
MF
300 to 3000 kHz
High
HF
3 to 30 MHz
Very high
VHF
30 to 300 MHz
Ultrahigh
UHF
300 to 3000 MHz
Superhigh
SHF
3 to 30 GHz
Extremely high
EHF
30 to 300 GHz
2-20. Harmonics. Any frequency that is a whole number multiple of a
smaller basic frequency is known as a harmonic of that basic frequency. The
fundamental frequency. A frequency that is twice as great as the
fundamental frequency is called the second harmonic; a frequency three
times as great is the third harmonic; and so on. For example:
First harmonic (fundamental frequency) 3000 kHz
Second harmonic 6000 kHz
Third harmonic
9000 kHz
2-21. Period. The period of a radio wave is simply the amount of time
required to complete one full cycle. If a sine wave has a frequency of 2 hertz,
each cycle has a duration, or period, of one-half second. If the frequency is 10
hertz, the period of each cycle is one-tenth of a second. Because the frequency
of a radio wave is the number of cycles that are completed in one second, you
should be able to see that as the frequency of a radio wave increases, its
period decreases.
2-7
Previous Page
