Radio frequency is the frequency or rate of oscillation within a range of about 3 cycles per second to 300 billion cycles per second. This range corresponds to the frequency of alternating current electrical signals used to produce and detect radio waves. Since most of this range is beyond the vibration rate that most mechanical systems can respond to, radio frequency usually refers to oscillations in electrical circuits or electromagnetic transmission.

Electromagnetic waves occur in the radio frequency portion of the electromagnetic spectrum. A common use is to transport information through the atmosphere or outer space without wires. Radio waves are distinguished from other kinds of electromagnetic waves by their wavelength, a relatively long wavelength in the electromagnetic spectrum.

Radio waves were first predicted by mathematical work done in 1865 by James Clerk Maxwell. Maxwell noticed wave like properties of light and similarities in electrical and magnetic observations and proposed equations that described light waves and radio waves as waves of electromagnetism that travel in space. In 1887 Heinrich Hertz demonstrated the reality of Maxwell’s electromagnetic waves by experimentally generating radio waves in his laboratory. Many inventions followed, making practical use of radio waves to transfer information through space.

The transmission of radio waves is affected by many variables. Atmospheric moisture, the stream of particles from the sun called solar wind, and time of day will all have an effect on the signal. All radio waves are partially absorbed by atmospheric moisture. Atmospheric absorption reduces the strength of radio signals over long distances.

Ultra high frequency signals of 30 to 300 million cycles per second are used for radio and television broadcasting. They are generally more degraded by moisture than lower bands of transmission. The layer of the Earth’s atmosphere called the ionosphere is filled with charged particles that can reflect radio waves. The reflection of radio waves can be helpful in transmitting a radio signal over long distances as the wave repeatedly bounces from the sky to the ground.

Microwaves are electromagnetic waves with wavelengths ranging from 300 to 3000 million cycles per second, and are used for cell phone transmissions. Microwaves contain insufficient energy to chemically change substances by ionization and so are an example of nonionizing radiation. Due to this fact, it has not yet conclusively been shown that microwaves have any biological effects.

During World War II, it was observed that individuals in the radiation path of microwave radar installations observed clicks and buzzing sounds in response to the transmissions. It was through this observation that it became known that microwaves could cause the perception of sounds in the human brain by inducing an electric current in the body’s hearing centers.

Research by NASA in the 1970s showed that this effect occurs as a result of thermal expansion of parts of the human ear around the cochlea, even at low power density. Later, signal modulation was found to produce sounds or words that appeared to originate intracranially. It was studied for its possible use in communications but has not been developed due to the possible hazardous biological effects of microwave radiation.

The first American to publish on microwave effects was Allan H. Frey, in 1961. In his experiments, the subjects were discovered to be able to hear appropriately pulsed microwave radiation from a distance of over 300 feet from the transmitter. This was accompanied by side effects such as dizziness, headaches, and a pins and needles sensation.

Extremely low frequency 3–300 Hz Power grids, communication with submarines
Ultra low frequency 300–3000 Hz Communication within mines
Very low frequency 3–30 kHz Submarines, avalanche beacons, heart rate monitors, geophysics
Low frequency 30–300 kHz Navigation, time signals, AM longwave broadcasts
Medium frequency 300–3000 kHz AM broadcasts
High frequency 3–30 MHz Shortwave broadcasts, amateur radio and aviation communications
Very high frequency 30–300 MHz FM, television broadcasts, aircraft communications
Ultra high frequency 300–3000 MHz Television, microwave ovens, cell phones, LAN, Bluetooth, GPS
Super high frequency 3–30 GHz Microwave devices, wireless LAN, most modern Radars
Extremely high frequency 30–300 GHz Radio astronomy, high-speed microwave radio relay

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