2-7
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.
The frequencies falling between 3000 hertz (3 kHz) and 300,000,000,000 hertz (300 GHz) are called
RADIO FREQUENCIES (abbreviated rf) since 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
Super high
SHF
3 to 30 GHz
Extremely high
EHF
30 to 300 GHz
Any frequency that is a whole number multiple of a smaller basic frequency is known as a
HARMONIC of that basic frequency. The basic frequency itself is called the first harmonic or, more
commonly, 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
The PERIOD of a radio wave is simply the amount of time required for the completion of 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. Since 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.
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 frequency and wavelength to be able to select the proper antenna(s) for use in successful
