<P> In radio communication systems, information is carried across space using radio waves . At the sending end, the information to be sent, in the form of a time - varying electrical signal, is applied to a radio transmitter . The information signal can be an audio signal representing sound from a microphone, a video signal representing moving images from a video camera, or a digital signal representing data from a computer . In the transmitter, an electronic oscillator generates an alternating current oscillating at a radio frequency, called the carrier because it serves to "carry" the information through the air . The information signal is used to modulate the carrier, altering some aspect of it, "piggybacking" the information on the carrier . The modulated carrier is amplified and applied to an antenna . The oscillating current pushes the electrons in the antenna back and forth, creating oscillating electric and magnetic fields, which radiate the energy away from the antenna as radio waves . The radio waves carry the information to the receiver location . </P> <P> At the receiver, the oscillating electric and magnetic fields of the incoming radio wave push the electrons in the receiving antenna back and forth, creating a tiny oscillating voltage which is a weaker replica of the current in the transmitting antenna . This voltage is applied to the radio receiver, which extracts the information signal . The receiver first uses a bandpass filter to separate the desired radio station's radio signal from all the other radio signals picked up by the antenna, then amplifies the signal so it is stronger, then finally extracts the information - bearing modulation signal in a demodulator . The recovered signal is sent to a loudspeaker or earphone to produce sound, or a television display screen to produce a visible image, or other devices . A digital data signal is applied to a computer or microprocessor, which interacts with a human user . </P> <P> The radio waves from many transmitters pass through the air simultaneously without interfering with each other . They can be separated in the receiver because each transmitter's radio waves oscillate at a different rate, in other words each transmitter has a different frequency, measured in kilohertz (kHz), megahertz (MHz) or gigahertz (GHz). The bandpass filter in the receiver consists of a tuned circuit which acts like a resonator, similarly to a tuning fork . It has a natural resonant frequency at which it oscillates . The resonant frequency is set equal to the frequency of the desired radio station . The oscillating radio signal from the desired station causes the tuned circuit to oscillate in sympathy, and it passes the signal on to the rest of the receiver . Radio signals at other frequencies are blocked by the tuned circuit and not passed on . </P> <P> Radio waves are nonionizing radiation, which means they do not have enough energy to separate electrons from atoms or molecules, ionizing them, or break chemical bonds, causing chemical reactions or DNA damage . The main effect of absorption of radio waves by materials is to heat them, similarly to the infrared waves radiated by sources of heat such as a space heater or wood fire . The oscillating electric field of the wave causes polar molecules to vibrate back and forth, increasing the temperature; this is how a microwave oven cooks food . However, unlike infrared waves, which are mainly absorbed at the surface of objects and cause surface heating, radio waves are able to penetrate the surface and deposit their energy inside materials and biological tissues . The depth to which radio waves penetrate decreases with their frequency, and also depends on the material's resistivity and permittivity; it is given by a parameter called the skin depth of the material, which is the depth within which 63% of the energy is deposited . For example the 2.45 GHz radio waves (microwaves) in a microwave oven penetrate most foods approximately 2.5 to 3.8 cm (1 to 1.5 inches). Radio waves have been applied to the body for 100 years in the medical therapy of diathermy for deep heating of body tissue, to promote increased blood flow and healing . More recently they have been used to create higher temperatures in hyperthermia treatment, to kill cancer cells . Looking into a source of radio waves at close range, such as the waveguide of a working radio transmitter, can cause damage to the lens of the eye by heating . A strong enough beam of radio waves can penetrate the eye and heat the lens enough to cause cataracts . </P>

Where is the radio wave located on the electromagnetic spectrum