Propagation and Antenna Systems
Propagation questions ask you to predict whether a link will work and why: HF by ionospheric refraction, VHF by line of sight, and which antenna choices help or hurt.
HF skywave
- Solar radiation ionises layers: D (daytime absorption), E, F1 and F2. Refraction in the F layer returns the signal to earth; the D layer mainly absorbs, which is why the lower bands are noisy by day.
- The MUF is the highest frequency that refracts for a given path; the LUF is the lowest that survives absorption and noise. Operate between them: higher frequencies by day and over long distances, lower frequencies at night.
- Between the ground-wave limit and the first skywave return lies the skip zone, where nothing is heard.
VHF and antennas
- VHF is essentially line of sight. The radio horizon extends beyond the visible horizon: distance in nautical miles is about 1.23 times the square root of antenna height in feet.
- Antenna gain comes from directivity. VHF aeronautical antennas are vertically polarised; a polarisation mismatch costs several dB. Keep VSWR low and the radiator clear of metal structures.
- Ducting, sporadic E and heavy precipitation can extend range or raise noise.
Worked example. An aircraft at 35 000 ft and a station antenna at 100 ft: aircraft horizon = 1.23 x square root of 35 000 = 1.23 x 187 = about 230 nm; station horizon = 1.23 x 10 = about 12 nm; total about 242 nm. Beyond that, expect no VHF contact.
Traps. Using only one height in the horizon formula; choosing a high HF band at night; assuming HF works by ground wave over 1 000 nm; ignoring the skip zone when planning frequencies.