Radio Wave Propagation
How signals travel
The General exam expects you to explain ionospheric propagation, ground wave, sky wave, line-of-sight, and the effects of the sun. The ionosphere has layers (D, E, F1, F2); the D layer absorbs lower-frequency signals during the day, the E layer supports short skip, and the F layer supports long-distance skip. Maximum usable frequency (MUF) rises with solar activity and falls at night; the lowest usable frequency (LUF) is limited by absorption and noise.
Solar and geomagnetic effects
Know that sunspots increase ionization and improve HF propagation, especially on higher bands. Solar flares can cause sudden ionospheric disturbances that black out HF communications on the daylight side. Geomagnetic storms degrade propagation, particularly at high latitudes, and can cause auroral propagation that sounds distorted and fluttery. The 11-year solar cycle is a common question target.
Scatter and unusual modes
Tropospheric ducting occurs when temperature inversions trap VHF/UHF signals and can extend range hundreds of kilometers. Sporadic E supports occasional long-distance VHF contacts. Meteor scatter and aurora scatter are also tested. Know that ground wave works best at low frequencies and over good soil or salt water, and that VHF/UHF is essentially line-of-sight, extended somewhat by refraction and diffraction.
Worked example
A question asks why 20 meters works well during the day but 80 meters works better at night. Answer: the D layer absorbs 80-meter signals during the day, while at night the D layer disappears and the lower F layer supports 80-meter skip; 20 meters needs the higher ionization that daytime sunlight produces.
Common traps
Confusing MUF (highest usable frequency) with LUF. Assuming sunspots always help - they help HF but can worsen VHF auroral conditions. Forgetting that D-layer absorption is a daytime phenomenon.