Radio frequency (RF) is a rate of oscillation in the range of about 30 kHz to 300 GHz, which corresponds to the frequency of electrical signals normally used to produce and detect radio waves. RF usually refers to electrical rather than mechanical oscillations, although mechanical RF systems do exist (see mechanical filter and RF MEMS).
In order to receive radio signals an antenna must be used. However, since the antenna will pick up thousands of radio signals at a time, a radio tuner is necessary to tune in to a particular frequency (or frequency range). This is typically done via a resonator – in its simplest form, a circuit with a capacitor and an inductor forming a tuned circuit. The resonator amplifies oscillations within a particular frequency band, while reducing oscillations at other frequencies outside the band. Often the inductor or the capacitor of the tuned circuit is adjustable allowing the user to change the frequencies at which it resonates. The resonant frequency of a tuned circuit is given by the formula
where f0 is the frequency in hertz, L is inductance in henries, and C is capacitance in farads.
Special properties of RF electrical signals
Electrical currents that oscillate at RF have special properties not shared by direct current signals. One such property is the ease with which they can ionize air, creating a conductive path through it. This property is exploited by 'high frequency' units used in electric arc welding, although strictly speaking these machines do not typically employ frequencies within the HF band. Another special property is that RF current cannot penetrate deeply into electrical conductors but flows along the surface of conductors; this is known as the skin effect. Another property is the ability to appear to flow through paths that contain insulating material, like the dielectric insulator of a capacitor. The degree of effect of these properties depends on the frequency of the signals.
Radio frequency (RF) energy has been used in medical treatments for over 75 years, generally for minimally invasive surgeries, using radiofrequency ablation and coagulation, including the treatment of sleep apnea. Magnetic resonance imaging (MRI) uses radio frequency waves to generate images of the human body.
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