Greece AM Theory flashcards

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

Ohm's law

The current through a conductor is directly proportional to voltage and inversely proportional to resistance: V = I × R. Used to calculate any one of the three quantities when the other two are known.

Electrical power formula

Power in watts equals voltage times current (P = V × I). Combined with Ohm's law it also gives P = I²R and P = V²/R for resistive circuits.

RMS value of sine wave

The root-mean-square value of a sinusoidal voltage or current equals the peak value divided by the square root of two, approximately 0.707 times the peak. It produces the same heating effect as an equal DC value.

Decibel power ratio

A power ratio expressed in decibels equals ten times the base-10 logarithm of the ratio. A gain of 3 dB roughly doubles power; a loss of 3 dB halves it.

LC resonance frequency

An inductor and capacitor resonate when their reactances are equal in magnitude, at f = 1/(2π√(LC)). At resonance the circuit impedance is resistive and current is maximal in a series circuit.

Capacitive reactance

The opposition a capacitor offers to alternating current, Xc = 1/(2πfC), measured in ohms. It falls as frequency or capacitance increases, so a capacitor blocks DC and passes high frequencies.

Wavelength from frequency

Wavelength in metres equals 300 divided by frequency in megahertz. This quick formula gives the free-space wavelength used for antenna and band calculations.

Half-wave dipole length

A practical half-wave dipole is approximately 143 divided by frequency in megahertz, in metres. The shortening from the theoretical half wavelength accounts for end effects and conductor thickness.

Standing wave ratio formula

SWR equals (1 + |Γ|)/(1 − |Γ|), where Γ is the voltage reflection coefficient (ZL − Z0)/(ZL + Z0). A perfect match gives 1:1; higher values indicate reflected power and possible transmitter stress.

Ionospheric D layer absorption

The lowest ionospheric layer exists mainly by day and absorbs HF signals, with stronger absorption at lower frequencies. It fades at night, which is why distant lower-band signals improve after dark.

AM modulation index

The modulation index m equals (Vmax − Vmin)/(Vmax + Vmin) of the envelope. Values above 1 cause overmodulation and splatter; AM bandwidth is twice the highest modulating frequency.

Carson's rule

The bandwidth of a frequency-modulated signal is approximately twice the sum of peak deviation and highest modulating frequency. It shows that wide deviation needs wider channel spacing.

Characteristic impedance of feed line

The impedance a uniform line presents to a travelling wave, set by conductor spacing and dielectric, typically 50 ohms for amateur coax. It does not depend on line length.

Superheterodyne receiver block diagram

RF amplifier, mixer, local oscillator, IF filter and amplifier, demodulator, then audio amplifier. Converting to a fixed intermediate frequency allows sharp, stable filtering and gain.

Mixer output frequencies

A mixer multiplies two signals and produces the sum and difference of their frequencies. The difference between local oscillator and received signal becomes the intermediate frequency.

Image frequency

An unwanted signal spaced twice the intermediate frequency from the wanted signal that also converts to the IF. RF filtering before the mixer is the main defence against it.

Crystal oscillator stability

A quartz crystal resonator sets frequency far more precisely than an LC circuit, typically within a few parts per million. Stability is quoted in ppm of the operating frequency.

Class C amplifier

An amplifier conducting for less than half the input cycle, giving high efficiency but severe distortion. It suits constant-envelope modes such as FM and CW, usually followed by a filter.

Harmonic spurious emission

Radiation at integer multiples of the carrier, generated by nonlinear amplifier stages. A low-pass filter after the final amplifier suppresses harmonics before they reach the antenna.

Oscilloscope volts per division

The vertical scale setting that converts displayed height into voltage. Peak-to-peak voltage equals the number of divisions times the volts/division, adjusted for any probe attenuation.

Frequency from period

Frequency equals one divided by the period, f = 1/T. Reading a waveform's period from an oscilloscope time base gives the frequency directly.

SWR meter placement

An SWR meter is inserted in the feed line between transmitter and antenna, where it samples forward and reflected power. It must match the line impedance, usually 50 ohms.

Dummy load requirements

A dummy load must present the transmitter's design impedance, typically 50 ohms resistive, and dissipate the full carrier power. It allows testing without radiating a signal.

Frequency counter accuracy

Counter accuracy depends on its internal time-base reference and is stated in parts per million, plus a one-count ambiguity. A disciplined or GPS-locked reference improves it.

Common-mode current interference

RF flowing on the outside of a coaxial shield or on mains leads radiates and causes interference or RF burns. Ferrite chokes and baluns reduce it.

Ferrite choke suppression

A ferrite bead or clamp on a cable adds impedance to unwanted RF currents without affecting the wanted signal. It is widely used to cure interference on audio and mains leads.

RF exposure assessment

Exposure is judged by power density or field strength at accessible locations, with frequency-dependent limits. Increasing distance from the antenna and reducing power lower exposure.

Mains protective earth

The protective earth conductor bonds equipment chassis to ground so fault currents trip protection devices. It must never be disconnected or used to carry signal current.

ITU Regions

The ITU Radio Regulations divide the world into three regions for frequency allocation. Greece lies in Region 1, together with Europe, Africa and the Middle East.

Greek amateur call sign structure

Greek amateur call signs use the prefixes SZ, SV or SW, followed by a district number and a suffix. The prefix and number identify the licence category and region of issue.

Amateur service definition

The ITU defines the amateur service as radiocommunication for self-training, intercommunication and technical investigation, by duly authorised persons with no pecuniary interest.

Station identification requirement

An amateur station must transmit its assigned call sign at the beginning and end of each contact and at reasonable intervals during it. Identification must be clearly readable.