Greece AM Theory cheat sheet

Ohm's law: V = I × R; power: P = V × I = I²R = V²/R. Units: volt (V), ampere (A), ohm (Ω), watt (W).

Series: R_total = R1 + R2 + ...; parallel: 1/R_total = 1/R1 + 1/R2 + ... Two in parallel: R1R2/(R1+R2).

Capacitive reactance Xc = 1/(2πfC), inductive reactance XL = 2πfL; both in ohms and frequency-dependent.

Resonance in an LC circuit at f = 1/(2π√(LC)); at resonance XL = Xc and impedance is purely resistive.

RMS of a sine wave = peak/√2 ≈ 0.707 × peak; peak = 1.414 × RMS. Average of a full sine cycle is zero.

Decibel definitions: power gain dB = 10 log10(P2/P1); voltage gain dB = 20 log10(V2/V1). +3 dB ≈ double power, +6 dB ≈ double voltage.

Impedance Z combines resistance and reactance: Z = √(R² + X²); phase angle θ = arctan(X/R).

Ideal transformer: Vp/Vs = Np/Ns and Ip/Is = Ns/Np; power in equals power out (ignoring losses).

Three common passive components: resistor (opposes current, dissipates heat), capacitor (stores energy in electric field, blocks DC), inductor (stores energy in magnetic field, opposes current change).

Wavelength λ (m) = 300 / frequency (MHz). Example: 145 MHz → λ ≈ 2.07 m; 7 MHz → λ ≈ 42.9 m.

Free-space path loss increases with frequency and distance; VHF/UHF are essentially line-of-sight, HF relies on ionospheric refraction.

HF propagation: D layer absorbs by day (higher absorption at lower frequencies), E and F layers refract; F2 layer supports long-distance skip.

Ionospheric layers vary with solar cycle and time of day; maximum usable frequency (MUF) rises with solar activity and daytime ionisation.

Half-wave dipole length ≈ 143/f(MHz) metres; quarter-wave vertical ≈ 71.5/f(MHz) metres (with end/velocity factor corrections).

Feed line characteristic impedance (e.g. 50 Ω coax) depends on conductor geometry and dielectric, not on length; loss rises with frequency and SWR.

SWR = (1 + |Γ|)/(1 − |Γ|), where reflection coefficient Γ = (ZL − Z0)/(ZL + Z0); SWR = 1:1 is a perfect match.

AM modulation index m = (Vmax − Vmin)/(Vmax + Vmin); sidebands at fc ± fm, bandwidth = 2 × fm.

FM: constant amplitude, information in frequency deviation; Carson's rule bandwidth ≈ 2(Δf + fm); FM is more resistant to amplitude noise than AM.

Transmitter chain: oscillator → buffer → driver/amplifier stages → output filter → antenna; the oscillator sets frequency stability.

Crystal oscillators give far better stability than free-running LC oscillators; stability is quoted in ppm (parts per million).

Superheterodyne receiver: RF amplifier → mixer → local oscillator → IF amplifier/filter → detector/demodulator → audio amplifier.

Mixer produces sum and difference frequencies: f_IF = |f_LO − f_RF| (or sum); image frequency = f_RF + 2·f_IF for high-side injection.

IF filter sets selectivity; a narrower IF filter improves adjacent-channel rejection but reduces audio bandwidth.

Class A amplifier conducts over the whole cycle (low distortion, low efficiency ~25–30%); Class C is high efficiency but nonlinear, used in FM/CW finals.

Spurious emissions include harmonics (multiples of the carrier) and mixer products; output low-pass filters suppress harmonics.

Automatic gain control (AGC) keeps audio level roughly constant as received signal strength varies.

Multimeter: voltage measured in parallel, current in series; select DC/AC range and correct jack before measuring.

Oscilloscope: probe ×10 divides signal by 10 and raises input impedance; time/div sets horizontal scale, volts/div sets vertical scale.

Frequency measured as f = 1/T, where T is the period read from the oscilloscope time base.

SWR meter is inserted in the feed line between transmitter and antenna; it reads forward and reflected power and derives SWR.

Power meters (dummy load + wattmeter) measure true output power; reflected power ratio gives SWR via SWR = (√Pf + √Pr)/(√Pf − √Pr).

Frequency counter accuracy is limited by its time-base reference; accuracy is often stated in ppm plus a ±1 count error.

Dummy load must match the transmitter impedance (typically 50 Ω) and dissipate the full transmit power; never transmit into an open or shorted load.

RF probe/diode detector measures peak RF voltage, which can be converted to RMS for a sine wave by dividing by √2.

Common amateur interference causes: harmonics and spurious emissions, overmodulation/splatter, poor shielding, common-mode currents on feed lines, corroded connectors.

Suppression methods: low-pass/band-pass filters at the transmitter, ferrite chokes on cables, bypass capacitors, proper grounding and shielded enclosures.

TV/radio interference (TVI/BCI) is often front-end overload of the receiver; distance, filters and reduced power help.

RF exposure is assessed by power density; limits are frequency-dependent and set out in ICNIRP-based national rules; keep antennas away from occupied areas.

Electrical safety: mains earth (PE) must never be removed; use RCD/GFCI protection and fuse the mains side of power supplies.

High voltages in valve/linear amplifiers persist after switch-off; discharge capacitors before servicing.

Lightning protection: disconnect antennas and ground feed lines when storms approach; never operate during electrical storms.

Common-mode RF in the shack is indicated by RF burns or 'hot' equipment; a choke/balun at the antenna feed and single-point grounding reduce it.

The ITU Radio Regulations divide the world into three Regions; Greece is in Region 1, which governs HF band allocations for European amateurs.

Amateur service is defined by the ITU as a service for self-training, intercommunication and technical investigation, carried out by duly authorised persons without pecuniary interest.

In Greece amateur radio is regulated by national law (Presidential Decree 1244/1979 as amended) and administered by the Ministry responsible for communications.

Greek amateur call signs consist of the prefix SZ, SV or SW followed by a district number and suffix letters; the prefix indicates the licence class/district.

Licence classes in Greece (Category 1 and Category 2) differ in permitted bands, modes and maximum power; Category 1 has wider privileges.

Calls must include the call sign at the start and end of a contact and at reasonable intervals, especially during long transmissions.

Amateur transmissions must not carry commercial traffic, broadcasting, or encrypted messages; third-party traffic rules depend on the countries involved.

Power limits and permitted frequency bands are set by the national licence; operation outside allocated amateur bands is prohibited.

International identification: prefix + national call sign; when operating abroad, the host country's prefix is added (e.g. SV/… or the host prefix).