ACMA Advanced Amateur Radio Theory key terms

A logarithmic ratio used to express power or voltage gain and loss, allowing very large ranges to be compressed into manageable numbers and cascaded stages to be added rather than multiplied.

The frequency at which a circuit's inductive and capacitive reactances are equal in magnitude and cancel, leaving only resistance and producing maximum or minimum response depending on the circuit configuration.

A dimensionless measure of how sharply a resonant circuit selects frequency, equal to the ratio of stored energy to energy lost per cycle; higher Q means narrower bandwidth and lower loss.

The total opposition a circuit presents to alternating current, combining resistance with reactance and expressed as a complex quantity with magnitude and phase angle in ohms.

The frequency-dependent opposition to alternating current offered by inductors and capacitors, storing and returning energy rather than dissipating it, measured in ohms and carrying a phase shift.

The tendency of alternating current to concentrate near the surface of a conductor as frequency rises, increasing effective resistance and motivating silver plating, stranded wire or tubing in RF circuits.

A three-terminal semiconductor device in which a small base current controls a larger collector current, widely used for amplification and switching and characterised by current gain and transconductance.

A semiconductor device in which voltage applied to a gate controls current flow through a channel, offering very high input impedance and low intermodulation, making it popular in RF amplifiers and mixers.

The steady direct current or voltage applied to a device to establish its operating point, setting class of operation and determining linearity, efficiency and distortion in amplifiers.

A letter designation (A, AB, B or C) describing how long a amplifying device conducts over the signal cycle, trading linearity against efficiency and determining suitable applications.

Spurious products created when two or more signals pass through a nonlinear stage, appearing at sums and differences of the input frequencies and often falling inside receive bands where they cause interference.

The span between the weakest signal a receiver can usefully detect and the strongest it can handle before distortion or blocking occurs, often quoted as a two-tone or blocking figure in decibels.

The total noise power present in a receiver or measurement system from thermal, atmospheric and device sources, setting the lower limit against which signal strength is compared.

The degradation in signal-to-noise ratio a stage introduces, expressed in decibels; the first stage of a receiver dominates overall sensitivity, so low-noise front ends matter most.

The minimum input signal a receiver requires to produce usable output at a stated signal-to-noise ratio, usually quoted in microvolts or dBm for a particular mode.

A receiver's ability to reject signals on frequencies adjacent to the wanted one, determined chiefly by filter shape factor and bandwidth in the intermediate-frequency stages.

A feedback system that adjusts receiver gain according to incoming signal strength, keeping audio output roughly constant and preventing overload on strong signals while preserving weak ones.

The impedance a uniform transmission line presents to a travelling wave, determined by conductor geometry and dielectric, and independent of line length; common values are 50 and 75 ohms.

The ratio of maximum to minimum voltage or current along a line, indicating mismatch between line and load; a value of 1:1 means a perfect match and higher values mean reflected power.

The complex ratio of reflected to incident voltage at a discontinuity, ranging from zero for a matched load to one for an open or short circuit, from which standing wave ratio is derived.

The ratio of signal propagation speed in a transmission line to the speed of light in vacuum, typically between 0.6 and 0.9, used to calculate physical length for phasing and matching stubs.

A transmission line with a centre conductor surrounded by a tubular shield and dielectric, providing a shielded unbalanced feed with predictable impedance and low radiation from the line itself.

A device that converts between balanced and unbalanced transmission lines or loads, preventing unwanted current on the outside of a coaxial shield and preserving antenna radiation patterns.

A balanced antenna of two collinear radiating elements fed at the centre, with a half-wave version presenting roughly 70 ohms and a figure-eight pattern broadside to the wire.

The increase in signal strength an antenna or amplifier provides relative to a reference, expressed in decibels and reflecting directivity and efficiency rather than any added power.

The product of transmitter power delivered to the antenna and antenna gain relative to an isotropic radiator, used to express effective radiated power and to assess exposure and interference potential.

Long-distance radio communication by refraction of signals from charged layers in the upper atmosphere, varying with solar activity, time of day, season and frequency.

The highest frequency that the ionosphere reflects at vertical incidence for a given layer and time, above which signals escape to space rather than returning to Earth.

The highest frequency usable for ionospheric communication between two specific points at a given time, roughly the critical frequency divided by the cosine of the incidence angle.

An amplitude modulation variant in which the carrier and one sideband are removed before transmission, concentrating power in the remaining sideband for greater efficiency and narrower occupied bandwidth.

A number describing the degree of modulation applied to a carrier, such as deviation ratio in frequency modulation or modulation depth in amplitude modulation, governing bandwidth and distortion.

The peak shift of a frequency-modulated carrier away from its resting frequency, which together with modulating frequency determines occupied bandwidth by Carson's rule.

The width of the frequency range containing nearly all of a transmitted signal's power, used by regulators to limit spectral spread and by operators to avoid adjacent channel interference.

Unwanted wideband energy produced by overdriving or overmodulating a transmitter, spreading interference onto adjacent channels and often caused by excessive audio drive or incorrect bias.

The reduction in signal power caused by placing a device such as a filter, connector or cable in a circuit, usually expressed in decibels and added along the signal path.

A non-radiating resistive termination used to test or tune a transmitter at full power, converting RF energy to heat and preventing interference to other users.

The average power supplied to the antenna during one radio-frequency cycle at the crest of the modulation envelope, the standard limit measure for amplitude-modulated and single-sideband emissions.

The ability of equipment to operate satisfactorily in its environment without introducing intolerable interference to other equipment, achieved through suppression, shielding, filtering and good installation practice.

Radio-frequency current flowing equally on all conductors of a feed line or cable, often causing the line to radiate and produce interference; controlled with chokes and baluns.

The rate at which radio-frequency energy is absorbed per unit mass of body tissue, measured in watts per kilogram and used to establish exposure limits for the general public and workers.

An exposure location where the person present is aware of and can control their exposure, such as an operator's own station, allowing higher permissible limits than uncontrolled public areas.

A radiocommunication service for self-training, intercommunication and technical investigation carried out by duly authorised persons with no pecuniary interest in its operation.

Interference that endangers the functioning of a radionavigation or safety service or seriously degrades, obstructs or repeatedly interrupts a radiocommunication service operating in accordance with regulations.

A tier of amateur authorisation granting specified frequency bands, power limits and operating privileges, with progression typically requiring further examination and demonstrating greater technical competence.

A unique identifier assigned to a licensed station, structured with a national prefix, a numeral and a suffix that together identify the country and the individual operator.