ACMA Standard Theory Exam flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Ohm's law

Voltage equals current multiplied by resistance (V = I × R); rearranged, I = V/R and R = V/I for a purely resistive circuit.

RMS of sine wave

The root-mean-square value of a sinusoidal voltage or current equals its peak value divided by the square root of two, about 0.707 times the peak.

Capacitive reactance

Opposition a capacitor offers to alternating current, calculated as Xc = 1/(2πfC); it decreases as frequency or capacitance increases.

Decibel power ratio

Power gain in decibels is ten times the logarithm base ten of output power divided by input power; 3 dB represents approximately double the power.

Kirchhoff's current law

At any circuit node, the algebraic sum of currents entering and leaving equals zero, so total incoming current equals total outgoing current.

Series resonance

A series LC circuit resonates when inductive and capacitive reactances are equal, at f = 1/(2π√LC); impedance is then minimum and current maximum.

RC time constant

For a resistor and capacitor in series, the time constant is R × C; after one time constant a charging capacitor reaches about 63 percent of final voltage.

Transformer turns ratio

Secondary voltage divided by primary voltage equals secondary turns divided by primary turns; current ratio is inverse, and impedance ratio is turns ratio squared.

Superheterodyne intermediate frequency

A receiver mixer converts incoming radio frequency to a fixed intermediate frequency equal to the difference between local oscillator and signal frequencies.

AM modulation index

For amplitude modulation, modulation index equals modulating voltage divided by carrier voltage; values above one cause overmodulation and splatter.

Carson's rule

Approximate FM bandwidth equals twice the sum of peak frequency deviation and highest modulating frequency: BW ≈ 2(Δf + fm).

Single sideband bandwidth

An SSB voice signal transmits one sideband and suppressed carrier, typically occupying about 3 kHz, and needs a product detector or BFO to recover audio.

Half-wave dipole length

A practical half-wave dipole is about 143 divided by frequency in MHz, in metres; its feedpoint impedance near resonance is roughly 73 ohms.

Voltage standing wave ratio

VSWR equals (1 + |Γ|)/(1 − |Γ|), where Γ is the reflection coefficient; 1:1 means no reflected power and 2:1 reflects about 11 percent.

Velocity factor

The velocity factor of a transmission line is the ratio of signal speed in the line to the speed of light, typically 0.66 to 0.95 for common cables.

Antenna gain dBi versus dBd

Gain in dBd is referenced to a half-wave dipole, while dBi is referenced to an isotropic radiator; dBd equals dBi minus about 2.15 dB.

Sky-wave propagation

HF signals refract from ionospheric layers, mainly F2, allowing beyond-horizon communication; the maximum usable frequency depends on ionisation and path geometry.

Ground-wave propagation

Vertically polarised signals at low and medium frequencies follow the earth's surface beyond the horizon, with attenuation increasing as frequency rises.

Radio horizon distance

For an antenna height h in metres, the approximate line-of-sight radio horizon is 3.57 times the square root of h, in kilometres.

Sporadic E layer

Patchy intense ionisation in the E layer can refract VHF signals over long distances for short periods, often in summer, causing unexpected contacts.

Ammeter connection

An ammeter is connected in series with the circuit so current flows through it; it should have very low resistance to avoid disturbing the measurement.

Zero dBm reference

Zero dBm equals one milliwatt delivered to a 50-ohm load, corresponding to about 0.224 volts RMS or 0.316 volts peak.

Oscilloscope 10x probe

A 10x probe divides the measured voltage by ten and presents about ten times higher input impedance, so the displayed amplitude must be multiplied by ten.

Spectrum analyzer resolution bandwidth

Resolution bandwidth is the filter width used in a spectrum analyzer; narrower settings resolve closer signals but slow the sweep and raise displayed noise.

Common-mode interference

Common-mode currents flow in the same direction on both conductors and return through earth or chassis; a balun or common-mode choke suppresses them.

Ferrite bead suppression

A ferrite bead placed on a cable adds high-frequency series impedance, absorbing radio-frequency energy and reducing conducted and radiated interference.

Intermodulation products

When two signals f1 and f2 pass through a nonlinear stage, mixing produces products such as 2f1 − f2 and 2f2 − f1 that can fall in-band as interference.

Ground loop

A ground loop occurs when equipment connects to earth at multiple points, forming a loop that picks up current and causes hum or interference.

RF power density

Far-field power density from an antenna is approximately PG/(4πd²), where P is power, G is numeric gain and d is distance from the antenna.

Capacitor discharge safety

High-voltage capacitors can retain dangerous charge after power is removed; discharge them through a suitable resistor and verify with a meter before touching.

Lightning protection

Antenna systems should be bonded to earth and fitted with surge arrestors; disconnect feedlines during storms and never handle antennas during lightning.

Lithium battery fire risk

Lithium-ion cells can overheat or catch fire if short-circuited, crushed or overcharged; use correct chargers and isolate damaged cells in a safe area.