3.1 Waves  
  3.1.2 Sine waves  
A waveform is a representation of how alternating current (AC) varies with time. The most familiar AC waveform is the sine wave, which derives its name from the fact that the current or voltage varies with the mathematical sine function of the elapsed time. The sine wave is unique in that it represents energy entirely concentrated at a single frequency. An ideal wireless signal has a sine waveform, with a frequency usually measured in cycles per second or Hertz (Hz). A million cycles per second is represented by megahertz (MHz). A billion cycles per second represented by gigahertz (GHz). A sine wave has several basic properties, which are illustrated in Figure :
  • Amplitude – The distance from zero to the maximum value of each alternation is called the amplitude. The amplitude of the positive alternation and the amplitude of the negative alternation are the same.
  • Period – The time it takes for a sine wave to complete one cycle is defined as the period of the waveform. The distance traveled by the sine wave during this period is referred to as its wavelength.
  • Wavelength – Indicated by the Greek lambda symbol λ. It is the distance between one value to the same value on the next cycle.
  • Frequency – The number of repetitions or cycles per unit time is the frequency, typically expressed in cycles per second, or Hz.

The inverse relationship between time (t), the period in seconds, and frequency (f), in Hz, is indicated by the following formulas:

t = 1/f

f = 1/t

One full period or cycle of a sine wave is said to cover 360 degrees (360°). It is possible for one sine wave to lead or lag another sine wave by any number of degrees, except zero or 360. When two sine waves differ by exactly zero° or 360°, the two waves are said to be in phase. Two sine waves that differ in phase by any other value are out of phase, with respect to each other.

Transmission on a medium can be changed or modulated, to impress information upon it. Likewise, demodulation can be used to recover the information. As it applies to radio frequency (RF) communications, modulation involves impressing the characteristics of one waveform onto a second waveform by varying the amplitude, frequency, phase, or other characteristic of the second, or carrier, waveform.

Use the following activities to see the relationships among the amplitude, the frequency, and the phase of a sine wave.

 
 

Interactive Activity

Interactive Activity: Amplitude and Frequency

This activity demonstrates a moving sine wave that allows for amplitude and frequency changes.

  

 

Interactive Activity

Interactive Activity: Amplitude, Frequency, and Phase

This activity draws a dynamic wave calculated from user input.