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FCC : Federal Communications Commission. The US government body that regulates the use of the radio spectrum.

F/D : The ratio of focal length to diameter of a parabolic dish antenna.

Feedline : The wire or cable that joins a transmitter or receiver to an antenna. Also known as a transmission line.

FET : Field-effect transistor, a semiconductor in which current between source and drain is controlled by voltage applied to a high-impedance gate

Field day : An annual event in which portable stations are deployed, contacts made and new or prospective operators encouraged to get on the air.

Field Strength Meter : A devie used to measure the strength of an electromagnetic field.

Filter : An electric circuit (often called a network) that allows certain frequencies to pass but rejects (attenuates) other unwanted frequencies.

FM (Frequency Modulated or Frequency Modulation): The frequency of the carrier signal is changed (modulated) in direct proportion to the instantaneous amplitude of an input signal, usually the voice of the operator. The resulting transmitted signal is of constant amplitude, with increasing amounts of power removed from the carrier and displaced to the sidebands as the modulating signal increases in amplitude.

Fox Hunt : Also known as ARDF (Amateur Radio Direction Finding). A contest to locate a hidden transmitter.

Frequency: The number of complete waves that pass a reference point per second. Expressed in Hertz (Hz)

FSK - Frequency Shift Keying: A digital mode in which the signal is broken into "chunks". Each chunk is then shifted in frequency slightly above or below the central carrier. Effectively, frequency modulation used to carry a digital signal.

Fuse : A device that self destructs when too much current passes through it - used for circuit protection.