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.

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.