Electromagnetic waves
Radio waves are electromagnetic waves. Their electric and magnetic fields are at right angles to each other and to the direction of travel. In free space, they travel at about 300 million metres per second.
Electromagnetic spectrum
The spectrum includes radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays. Radio engineering uses frequency bands across the radio and microwave regions.
Frequency bands
Common band names include VLF, LF, MF, HF, VHF, UHF, SHF and EHF. HF is 3–30 MHz. VHF is 30–300 MHz. UHF is 300 MHz–3 GHz. Amateur allocations occupy portions of these ranges.
Wavelength
Wavelength is the distance travelled during one cycle. Frequency and wavelength are inversely related.
λ = c/f | λ(m) ≈ 300/f(MHz)
Ground waves
Ground waves travel along the Earth's surface. Their range depends on frequency, ground conductivity, terrain and antenna arrangement. Lower frequencies often follow the Earth's curvature more effectively.
Sky waves
Sky waves travel towards the ionosphere and may return to Earth. They support long-distance HF communication beyond the normal line of sight.
Space waves
Space waves travel through the lower atmosphere between antennas. They are important at VHF, UHF and microwave frequencies. They are often limited by the radio horizon, though refraction and terrain can change coverage.
Ionosphere
The ionosphere contains charged particles created mainly by solar radiation. It affects radio waves through refraction, absorption and other interactions. Conditions change with solar activity, season, time of day and location.
D, E and F regions
The D region mainly absorbs lower HF signals during daylight and weakens at night. The E region can support some sky-wave paths and sporadic-E events. The F region is central to long-distance HF propagation. It may split into F1 and F2 during the day.
Critical frequency
Critical frequency is the highest frequency returned vertically by a given ionospheric layer under the prevailing conditions. It changes with ionisation. It is not the same as the maximum usable frequency for a slant path.
MUF and LUF
Maximum usable frequency is the highest frequency that supports a specified ionospheric path. Lowest usable frequency is the lowest frequency that remains usable after absorption and noise are considered. The best operating frequency lies between these limits.
Day and night changes
Solar radiation increases ionisation during daylight. The D region often causes more absorption by day. At night, D-region absorption falls, while the F region can continue to support long-distance paths. The best band can therefore change after sunset.
Skip distance and skip zone
Skip distance is the distance from the transmitter to the first ground point reached by a returning sky wave. The skip zone is an area between ground-wave coverage and the first sky-wave return where reception may be weak.
Fading
Fading is a change in received signal strength. It can result from changing ionospheric conditions or multiple signal paths. Selective fading affects parts of a signal differently. Flat fading affects the signal more evenly.
Tropospheric propagation
The troposphere is the lowest atmospheric layer. Changes in temperature and humidity can alter radio-wave refraction. Ducting can carry VHF, UHF or microwave signals beyond their usual range.
Sporadic E
Sporadic-E propagation occurs when small, intense ionised regions form in the E region. It can support unusual long-distance VHF reception, including FM broadcast reception.