ACMA Advanced Amateur Radio Theory flashcards

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

Decibel power formula

Power ratio in decibels equals 10 log10 of the ratio of two powers. A 3 dB increase doubles power, 10 dB multiplies it by ten, and 20 dB by one hundred.

Series resonance condition

A series LC circuit resonates when inductive and capacitive reactance are equal, giving minimum impedance at f = 1/(2 pi sqrt(LC)). Above resonance the circuit is inductive.

Loaded Q and bandwidth

Loaded Q equals resonant frequency divided by the 3 dB bandwidth. Higher Q produces a narrower passband, sharper selectivity, and longer ringing after excitation.

RMS of sine wave

The root-mean-square value of a sine wave is 0.707 times its peak value. RMS is the value that produces the same heating in a resistor as an equal DC current.

Transistor saturation region

In saturation the collector-emitter voltage is near 0.2 V and the device behaves almost as a closed switch. Further base current does not increase collector current.

Crystal oscillator stability

A quartz crystal's high mechanical Q makes its resonant frequency very stable against temperature and load changes, giving far better stability than LC oscillators.

Full-wave rectifier ripple

A full-wave or bridge rectifier produces output ripple at twice the mains frequency, which is easier to filter than the mains-frequency ripple of a half-wave rectifier.

Class C amplifier

A Class C stage conducts for much less than 180 degrees, giving high efficiency but heavy distortion. It suits constant-envelope RF service such as FM or CW, followed by filtering.

Receiver noise figure

Noise figure expresses how much a receiver degrades signal-to-noise ratio. Lower noise figure improves weak-signal sensitivity, and the first stage dominates overall performance.

Image frequency

In a superheterodyne receiver the image is a signal offset from the local oscillator in the same way as the wanted signal, separated by twice the intermediate frequency.

Product detector

A product detector mixes the received signal with a locally generated carrier to recover audio from SSB or CW. An envelope detector cannot correctly demodulate suppressed-carrier signals.

Automatic gain control

AGC reduces receiver gain when strong signals arrive, keeping audio output roughly constant. Slow attack and release avoid pumping on speech peaks while still protecting the audio stage.

Characteristic impedance

Characteristic impedance is the ratio of voltage to current for a wave travelling on a transmission line. It depends on conductor spacing and dielectric, not on line length.

Voltage standing wave ratio

VSWR equals (1 + reflection coefficient magnitude) divided by (1 minus that magnitude). A perfect match gives 1:1, while total reflection gives infinite VSWR.

Quarter-wave transformer

A quarter-wavelength line transforms load impedance to Zo squared divided by the load impedance. It also inverts reactance, turning an inductive load capacitive at the input.

Velocity factor

Velocity factor is the ratio of wave speed on a line to the speed of light. Physical length equals electrical length multiplied by velocity factor, typically 0.66 to 0.85 for coax.

Free-space path loss

Path loss in dB equals 32.44 plus 20 log10 of frequency in MHz plus 20 log10 of distance in km. Doubling distance adds about 6 dB of loss.

Maximum usable frequency

The MUF is the highest frequency that ionospheric refraction supports between two points. It rises with solar activity and falls at night as ionisation decreases.

SSB bandwidth and power

Single sideband suppresses the carrier and one sideband, occupying about 3 kHz. For the same peak envelope power it concentrates energy where information is carried, unlike AM.

FM modulation index

The FM modulation index is peak deviation divided by modulating frequency. Carson's rule estimates occupied bandwidth as twice the sum of deviation and highest modulating frequency.

Key clicks

Abrupt on-off keying creates wideband sidebands heard as clicks. Shaping rise and fall times to a few milliseconds reduces occupied bandwidth and interference to nearby stations.

Frequency-shift keying

FSK shifts the carrier between two frequencies to represent binary states. RTTY uses this method, and the tone spacing determines the occupied bandwidth and required receiver filter.

Directional wattmeter

A directional wattmeter samples forward and reflected power separately. VSWR is computed from the square roots of those readings, revealing feedline and antenna mismatch.

Dummy load rating

A dummy load provides a matched 50-ohm termination for testing without radiation. It must dissipate the full transmitter power and remain low-reactance across the operating band.

Common-mode choke

A choke balun adds impedance to currents flowing on the outside of a coaxial shield. This reduces feedline radiation on transmit and common-mode noise pickup on receive.

Spectrum analyzer resolution

Resolution bandwidth determines how closely spaced signals can be distinguished and sets the displayed noise floor. Narrower settings resolve closer signals but slow the sweep.

Power density formula

In the far field, power density equals EIRP divided by 4 pi d squared. Doubling distance quarters the density, a reduction of about 6 dB.

Far-field boundary

The far field begins at roughly twice the square of the largest antenna dimension divided by wavelength. Beyond it, fields behave as plane waves and the simple density formula applies.

Duty cycle and exposure

Exposure limits are time-averaged, so intermittent transmission lowers average exposure. A 50 percent duty cycle halves the time-averaged power density compared with continuous operation.

Station grounding

Bonding equipment to a single low-impedance ground reduces ground loops, common-mode currents, and conducted interference. Grounding also provides a path for lightning and fault currents.

Third-party traffic

Third-party traffic is a message passed on behalf of a person other than the licensed operators. International handling requires both administrations to permit such traffic.

Station identification

Licensed stations identify with their assigned callsign at the start and end of a contact and at intervals set by national rules, using the mode in use.