FCC Commercial Radio key terms

A continuous radio-frequency signal, generated by an oscillator, that is modulated to carry information. It has constant amplitude and frequency before modulation is applied and occupies a narrow bandwidth.

The process of varying one or more properties of a carrier wave, such as amplitude, frequency, or phase, in step with an information signal so that the information can be transmitted over the air.

A modulation method in which the strength of the carrier wave rises and falls in step with the audio signal. It produces a carrier plus two sidebands, each carrying the same information.

The band of frequencies above or below the carrier created by modulation. The upper sideband holds the sum of carrier and audio frequencies; the lower sideband holds the difference.

An amplitude modulation mode in which the carrier and one sideband are removed, leaving only one sideband. It concentrates power in a narrow bandwidth and is common in marine and land mobile HF service.

An operating condition in which the carrier is reduced or removed after modulation, so transmitter power goes into the sidebands instead of an unmodulated carrier. Receivers must then reinsert a carrier to demodulate the signal.

A modulation method in which the carrier frequency shifts up and down in step with the audio signal while amplitude stays constant. Noise on the amplitude is largely rejected, giving quieter reception.

The amount a frequency-modulated carrier shifts away from its assigned center frequency at any instant. Maximum deviation is limited by rule because excessive deviation spreads the signal into adjacent channels.

For amplitude modulation, the ratio of audio amplitude to carrier amplitude, usually expressed as a percentage. Values above 100 percent cause overmodulation, splatter, and distortion of the received signal.

Driving a transmitter beyond the modulation level it can handle cleanly, producing excessive bandwidth, distortion, and interference on nearby frequencies. It is a common cause of complaints and rule violations.

A receiver oscillator mixed with an incoming signal to produce an audible tone, used to make single sideband and continuous wave signals intelligible. Its setting must be adjusted to clarify voice.

The tendency of an FM receiver to respond only to the stronger of two signals on the same frequency, suppressing the weaker one. This allows reuse of frequencies at sufficient distance.

The way radio waves travel from transmitting antenna to receiving antenna. It depends on frequency, time of day, season, and ionospheric conditions, and determines usable range.

A radio wave that follows the earth's surface, bending slightly with the curvature. It supports reliable short-range communication at lower frequencies and is relatively independent of ionospheric conditions.

A radio wave that travels upward and is refracted back to earth by the ionosphere. Sky-wave paths allow long-distance HF communication but vary greatly with time of day and solar activity.

Layers of ionized gas high in the atmosphere that refract radio waves. Ionization rises with sunlight and solar activity, so usable frequencies change between day and night.

The ring-shaped area between the end of reliable ground-wave coverage and the nearest point where sky waves return to earth, where reception may be weak or absent on a given frequency.

The highest frequency that the ionosphere will reflect straight up at a given time. Signals above it pass through the layer instead of returning, so operators must choose lower frequencies.

The highest frequency usable for sky-wave communication between two specific points at a given time. Above it, signals punch through the ionosphere rather than bending back to the receiver.

A measure of how well a transmitter, feed line, and antenna are matched. A ratio near 1:1 means little reflected power; high values indicate mismatch, wasted power, and possible transmitter damage.

The transmission line carrying radio-frequency energy between the transmitter or receiver and the antenna. Coaxial cable is common; losses rise with frequency and with poor impedance match.

An adjustable matching network placed between the transmitter and feed line to present a proper load impedance. It reduces reflected power but does not itself radiate or replace a resonant antenna.

A non-radiating resistive load that absorbs transmitter power for testing and adjustment. Using one lets an operator tune or service equipment without putting a signal on the air.

A logarithmic unit comparing two power or signal levels. A 3 dB change doubles or halves power, 10 dB is a tenfold change, and 20 dB is a hundredfold change in power.

An increase in signal power or voltage, often stated in decibels. Antenna gain describes how much more power is radiated in a favored direction compared with a reference antenna.

Loss of signal strength as energy travels through cable, connectors, or the air. It is expressed in decibels and grows with distance and, for cable, with frequency.

An antenna that concentrates radiation or reception in particular directions rather than equally all around. It provides gain but must be aimed, and its pattern affects coverage and interference.

The orientation of the electric field of a radiated wave, usually vertical or horizontal. Transmitting and receiving antennas should share polarization; a mismatch causes substantial signal loss.

The width of the frequency range actually containing a transmitted signal's energy. Emission designators and power limits are set against it, and excessive width causes adjacent-channel interference.

Interference caused by two or more transmitters operating on the same frequency and heard at the same time. It is reduced by geographic separation, power control, and directional antennas.

Interference reaching a receiver from a transmitter on a nearby frequency, often because of excessive modulation, poor filtering, or a receiver with weak selectivity.

The authorization issued for a radio station, specifying the frequencies, power, emission types, and operating conditions permitted. Operating outside its terms is a violation of the rules.

A personal authorization showing the holder is qualified to operate or maintain certain radio stations. It is held by an individual and is separate from the license granted to the station itself.

The licensed person named as accountable for a station's proper operation and rule compliance. Duties include supervising unlicensed operators and keeping required records available.

The specific channel or frequency band a station is authorized to use, together with any power and emission limits. Transmitting elsewhere without authority is prohibited.

The required announcement of a station's call sign and any other specified information, made at prescribed intervals and in the prescribed form during transmission.

The unique letter-and-number identifier assigned to a licensed station. It is used in identification announcements, in log entries, and in contacts with other stations.

Signals sent when a vessel, aircraft, or person is in grave and imminent danger and needs immediate help. They have absolute priority over all other traffic on the frequency.

The spoken distress signal, from the French for 'help me', used to announce grave and imminent danger. It precedes the vessel name and position and calls for immediate assistance.

The spoken urgency signal used when a station has an important message concerning safety but no immediate danger to life. It ranks below mayday but above routine traffic.

Specified minutes in each half hour, commonly the third and sixth, when radio silence is observed on certain maritime distress frequencies so weak distress calls can be heard.

The practice of keeping a receiver tuned to a designated frequency, such as a distress and calling channel, during specified hours so calls for help are not missed.