Radio fundamentals
Waves, the spectrum, how signals travel, and how information rides on them.
Waves and the spectrum
The physics every radio is built on.
Electromagnetic wavesElectric and magnetic fields travelling together at the speed of light.Frequency and wavelengthTwo ways to describe the same wave, tied together by the speed of light.The radio spectrumLF to microwaves: where the bands sit and what each is good for.PolarizationThe orientation of a wave's electric field, and why antennas must match it.DecibelsThe logarithmic scale for gain, loss and signal strength.Noise and signal-to-noise ratioWhere noise comes from and why SNR decides what you can copy.
How signals travel
Why a signal reaches across town, the world, or nowhere at all.
Ground wave and surface waveLow-frequency signals that follow the curve of the Earth.Line of sight and the radio horizonWhy VHF and UHF reach about as far as you can see, and a little further.The ionosphereThe D, E and F layers: what they are and what they do to radio waves.Skywave, hops and the skip zoneHow HF signals return to Earth hundreds or thousands of miles away.MUF, LUF and the best frequencyThe highest and lowest usable frequencies for a path.The solar cycle and propagation indicesSunspots, solar flux, and the A and K indices.Grey line and long pathDawn-and-dusk enhancement, and signals that go the long way around.Sporadic EShort-lived patches of ionization that open VHF over long distances.Tropospheric ducting and scatterWeather that bends VHF and UHF well past the horizon.Meteor scatterBouncing signals off ionized meteor trails.Aurora propagationReflecting VHF signals from the northern lights.Near-vertical incidence skywaveSteep-angle HF for reliable regional contacts.Space weatherFlares, CMEs and storms, and how they help or wreck HF.
Signals and modulation
How information is put onto a radio wave and taken off again.
What modulation isPutting information onto a carrier wave.Amplitude modulation (AM)Varying carrier strength: sidebands, bandwidth and efficiency.Frequency modulation (FM)Varying carrier frequency: deviation, capture effect and noise immunity.Single sideband (SSB)Dropping the carrier and one sideband for efficient HF voice.Digital modulation: FSK, PSK and QAMHow bits become tones and phase shifts.Bandwidth, symbol rate and data rateHow much spectrum a signal needs and how much it can carry.Error detection and correctionParity, CRC and forward error correction.Spread spectrumDirect sequence, frequency hopping and why they resist interference.Time domain, frequency domain and FourierSeeing a signal as its sine-wave ingredients.Sampling and the Nyquist limitTurning a continuous signal into numbers, and aliasing.