ACMA Standard Theory Exam key terms

A logarithmic way of expressing the ratio between two power or voltage levels. A gain of 3 dB roughly doubles power, while 10 dB means ten times the power.

The ability of a component to store charge in an electric field, measured in farads. Capacitors block steady direct current while passing alternating current more easily as frequency rises.

The property of a coil that opposes changes in current by storing energy in a magnetic field, measured in henries. Inductive reactance rises as frequency increases.

The opposition that a capacitor or inductor presents to alternating current, measured in ohms. It varies with frequency and does not dissipate power the way resistance does.

The total opposition a circuit presents to alternating current, combining resistance with reactance. It is expressed in ohms and determines how much current flows for a given applied voltage.

The condition where a circuit's inductive and capacitive reactances cancel, leaving only resistance. It occurs at one particular frequency and lets a tuned circuit favour that frequency over others.

The relationship stating that current through a resistance equals the voltage across it divided by that resistance (I = V/R). It underpins almost every calculation of voltage, current and resistance.

The average power delivered to the antenna during one radio-frequency cycle at the crest of the modulation envelope. It is the standard limit used for amateur transmissions.

The process of varying a property of a radio-frequency carrier, such as amplitude, frequency or phase, so that it can carry information like voice or data.

A receiver's ability to reject signals on nearby frequencies while accepting the wanted one. Good selectivity depends mainly on the shape of the filters in the intermediate-frequency stages.

The smallest input signal a receiver can turn into usable output at a stated signal-to-noise ratio. It is usually quoted in microvolts or dBm.

A fixed frequency to which a receiver converts incoming signals before filtering and detection. Using a constant frequency lets sharp, stable filters do the selectivity work.

A circuit stage that combines an incoming signal with a local oscillator to produce sum and difference frequencies, allowing conversion to a fixed intermediate frequency.

A measure of how well a load matches a feedline, being the ratio of maximum to minimum voltage along the line. A value near 1:1 indicates a good match.

The impedance a transmission line presents to a travelling wave, set by its physical dimensions and insulation. It is typically 50 ohms for amateur coaxial cable.

A device that converts between a balanced antenna connection and an unbalanced feedline such as coaxial cable, helping to stop current flowing on the outside of the coax.

The increase in radiated signal in a particular direction compared with a reference antenna, expressed in dBi or dBd. Gain comes from concentrating energy, not from adding power.

A straight wire antenna about half a wavelength long that is fed at its centre. It is the standard reference antenna and has a feedpoint impedance near 70 ohms at resonance.

A feedline with a centre conductor surrounded by insulation, a braided shield and an outer jacket. It is convenient and flexible but has higher loss than open-wire line.

The distance a radio wave travels in one complete cycle, equal to the speed of light divided by the frequency. It sets practical antenna dimensions and influences propagation behaviour.

Layers of ionised gas high in the atmosphere, produced largely by solar radiation, that can refract radio waves back to Earth and enable long-distance communication on the HF bands.

The shortest distance from a transmitter at which a sky wave returns to Earth for a given frequency and ionospheric condition. Inside that gap no sky-wave signal is heard.

The portion of a radio signal that travels along or near the Earth's surface, following its curvature. It is strongest at lower frequencies and over good-conductivity ground such as seawater.

Refraction of VHF and UHF signals within layers of the lower atmosphere where temperature or humidity changes sharply, allowing contacts far beyond the normal horizon.

Variation in received signal strength caused by changing propagation paths, especially on HF. Signals may rise and fall slowly or flutter rapidly as the ionosphere changes.

A general-purpose instrument for measuring voltage, current and resistance, usually with extra ranges such as continuity or diode test. It is the basic tool for checking circuits.

An instrument that plots signal voltage against time on a screen, allowing waveform shape, amplitude, frequency and timing to be observed directly.

A non-radiating resistor, usually 50 ohms, that absorbs transmitter power during testing so adjustments and measurements can be made without sending a signal over the air.

A test instrument that measures the frequency of an applied signal by counting cycles over a known time interval and displaying the result digitally.

A signal at an integer multiple of a fundamental frequency, produced by non-linear circuits. Unwanted harmonics can interfere with other services and must be filtered or suppressed.

Any unwanted radio-frequency output outside the necessary bandwidth of a transmission, such as harmonics, mixing products or parasitic oscillations, which must be kept within regulated limits.

The generation of unwanted signals at sum and difference frequencies when two or more strong signals pass through a non-linear stage, often causing interference that cannot be tuned out.

Conductive screening around cables, circuits or equipment that confines radio-frequency energy and prevents external fields from coupling in, reducing both emissions and susceptibility.

The ability of equipment to operate correctly in its electromagnetic environment without causing or suffering unacceptable interference. It is achieved through suppression, filtering, shielding and good installation practice.

Connecting equipment chassis and antenna systems to a low-resistance earth point so fault currents and static charges are safely diverted, reducing shock and fire risk.

The amount of radio-frequency energy absorbed by the human body near an antenna. Operators must keep antennas away from people and observe power and distance limits.

A deliberate weak link in a circuit that melts and breaks the current path when current exceeds a rated value, protecting wiring and equipment from overheating or fire.