Greece AM Theory key terms

The relationship stating that current through a conductor equals the voltage across it divided by its resistance (I = U/R). It also lets you calculate voltage as current times resistance, or resistance as voltage divided by current.

The property of a material or component that opposes the flow of electric current, converting electrical energy into heat. It is measured in ohms and depends on material, length and cross-sectional area.

The rate at which electrical energy is transferred or dissipated in a circuit. In a resistive load it equals voltage multiplied by current, and can also be written as current squared times resistance.

The effective value of an alternating voltage or current: the DC value that would produce the same heating effect in a resistor. For a sine wave it equals the peak value divided by the square root of two.

The maximum instantaneous magnitude reached by an alternating voltage or current during one cycle. For a sine wave it is the root mean square value multiplied by the square root of two.

A passive two-terminal component manufactured to provide a defined resistance, used to limit current, divide voltage or set bias points. Its value is marked by a colour code or printed digits, and it has a power rating.

A component that stores electric charge between two conducting plates separated by an insulator. Its capacitance is measured in farads, and it blocks direct current while passing alternating current, with reactance falling as frequency rises.

A component, usually a coil, that stores energy in a magnetic field when current flows through it. Its inductance is measured in henries, and its reactance rises with frequency, so it opposes changes in current.

The opposition a capacitor or inductor presents to alternating current, measured in ohms, without dissipating energy. Capacitive reactance falls as frequency rises, while inductive reactance rises with frequency.

A semiconductor component that conducts current easily in one direction and blocks it in the other. It is used for rectification, reverse-polarity protection and voltage regulation in its Zener form.

A three-terminal semiconductor device that can amplify signals or act as an electronic switch. In bipolar types the base controls current between collector and emitter; in field-effect types the gate controls voltage.

A circuit, typically using diodes, that converts alternating current into pulsating direct current. A following filter, usually a capacitor, smooths the pulses into usable DC for powering equipment.

Equipment that converts mains or battery voltage into the stable DC voltages an amateur station needs. It usually contains a transformer, rectifier, filter and regulator, and its current rating must exceed the load demand.

A connection in which components form a single path, so the same current flows through each of them and the total voltage divides among them. Total resistance is the sum of the individual resistances.

A connection in which components are placed across the same two points, so each has the full supply voltage and the currents add. Total resistance is always lower than the smallest individual resistance.

The physical distance between two successive points of the same phase on a radio wave, equal to the speed of light divided by frequency. It determines antenna and feed-line dimensions.

The number of complete cycles of an alternating signal occurring in one second, measured in hertz. Radio frequencies are grouped into bands, and each amateur band has its own propagation and antenna characteristics.

The way radio waves travel from transmitter to receiver, including ground-wave, sky-wave via the ionosphere, and line-of-sight paths. It varies with frequency, time of day, season and solar activity.

Layers of ionised gas in the upper atmosphere that can refract and reflect high-frequency radio waves back to Earth. Its ionisation, driven by solar radiation, changes through the day and the solar cycle.

The shortest ground distance between transmitter and receiver for which a sky-wave signal returns to Earth at a given frequency. Inside that distance no sky-wave is heard, though ground-wave coverage may reach.

The condition in which an antenna's inductive and capacitive reactances cancel, leaving a purely resistive feed-point impedance. At resonance the antenna radiates efficiently at its design frequency.

The transmission line that carries radio-frequency energy between the transmitter or receiver and the antenna. Coaxial cable is common; its characteristic impedance and loss determine how much power reaches the antenna.

The ratio of maximum to minimum voltage along a feed line, showing how well the load impedance matches the line. A value of 1:1 means a perfect match; high values indicate reflected power and possible equipment stress.

The speed at which a signal travels along a transmission line, expressed as a fraction of the speed of light in vacuum. It is used to calculate physical lengths for feed lines, stubs and phasing harnesses.

The process of varying a property of a radio-frequency carrier, such as amplitude, frequency or phase, to carry information. The receiver demodulates the signal to recover the original audio or data.

One of the two bands of frequencies above and below the carrier produced by amplitude modulation. Single-sideband transmission sends only one of them, saving power and bandwidth compared with full carrier AM.

A circuit that generates a continuous periodic signal at a chosen frequency, usually by positive feedback around an amplifier. Its stability and purity determine the frequency accuracy of a transmitter or receiver.

A circuit stage that combines two input signals to produce sum and difference frequencies. In receivers it converts the incoming radio frequency to a fixed intermediate frequency for easier filtering and amplification.

A circuit that increases the amplitude or power of a signal, using active devices such as transistors or valves. Radio-frequency power amplifiers boost the transmitter output, while receiver amplifiers raise weak signals.

A network of inductors, capacitors or resonators that passes signals in chosen frequency ranges and attenuates others. Filters select sidebands, reject interference and define the bandwidth of a receiver or transmitter.

Equipment that generates a radio-frequency carrier, modulates it with information and amplifies it to the required power for radiation by an antenna. Its stages typically include oscillator, buffer, driver and final amplifier.

Equipment that selects, amplifies and demodulates an incoming radio signal to recover the transmitted information. Modern designs often use a superheterodyne architecture with one or more intermediate frequencies.

A general-purpose measuring instrument combining voltmeter, ammeter and ohmmeter functions, often with continuity and diode tests. It is used to check supply voltages, component values and circuit continuity safely.

A test instrument that displays a signal's voltage waveform against time on a screen. It reveals amplitude, frequency, shape and distortion, and is useful for tracing faults in transmitter and receiver circuits.

An instrument inserted in the feed line that measures forward and reflected power to indicate the standing wave ratio. It helps verify antenna matching and detect feed-line or connector faults.

A device that measures the radio-frequency power delivered by a transmitter, usually by sensing voltage and current or heat in a dummy load. It confirms output power and compliance with licensed limits.

An instrument that counts cycles of an input signal over a known gate time to display its frequency digitally. It is used to set oscillators and check transmitter frequency accuracy.

A resistive load, typically 50 ohms, that absorbs transmitter power and converts it to heat instead of radiating it. It allows off-air testing and adjustment without causing interference.

Unwanted energy that degrades or disrupts a wanted radio signal, whether generated by other transmitters, electrical equipment or the affected station's own equipment. Its causes include harmonics, spurious emissions and poor screening.

A signal at an integer multiple of a fundamental frequency, produced by non-linear circuits or over-driven amplifiers. Harmonics can radiate from an antenna and cause interference far from the intended operating frequency.

A filter that passes signals below a chosen cut-off frequency and attenuates higher ones. Fitted at the transmitter output, it suppresses harmonics and reduces the risk of interfering with other services.

The ability of equipment to operate correctly in its electromagnetic environment without causing or suffering unacceptable interference. Good screening, earthing and filtering are key practical measures.

Connecting equipment chassis and station wiring to a common low-resistance earth point for safety and to reduce unwanted radio-frequency currents. It helps protect operators from electric shock and limits interference.

The passage of electric current through the body, which can cause burns, muscle contraction, heart rhythm disturbance or death. Mains voltages and charged high-voltage capacitors in equipment are the main hazards.

The exposure of the human body to radio-frequency electromagnetic fields, which can cause heating of tissue at high levels. Amateur stations must keep antennas and operating positions within recognised exposure limits.

The international treaty text that allocates frequency bands to radio services, defines interference obligations and sets operating rules. National amateur regulations, including Greece's, are built on its provisions.

A radiocommunication service for self-training, intercommunication and technical investigation, operated by duly authorised persons without pecuniary interest. Its users must hold a licence and follow the conditions attached to it.

The unique identifier assigned to an amateur station, used at the start and end of contacts and at intervals during them. It shows the station's nationality and, in many countries, the licence class.

A general invitation transmitted to any station listening, requesting a contact. It is followed by the caller's call sign, and answered by another station identifying itself in reply.

A two-way radiocommunication between amateur stations, established after a call is answered. Operators exchange call signs, signal reports and other information, and must identify themselves throughout.

A written confirmation of a two-way contact, exchanged by post or electronically between amateur stations. It records the call signs, date, time, band and mode, and is used for awards and operating evidence.