Radio propagation, frequencies and basic radio theory
Frequency, wavelength and bands
Frequency and wavelength are related by λ = c/f. VHF (156–174 MHz) is essentially line-of-sight, so antenna height dominates range. MF (around 2 MHz) uses groundwave for short/medium range and some skywave at night. HF (3–30 MHz) uses skywave via the ionosphere; the D layer absorbs by day, while E and F layers refract. Higher frequencies need a higher MUF; the LUF rises with absorption. Day/night changes alter usable HF bands.
GMDSS frequencies to know
VHF Ch70 (156.525 MHz) is DSC only; Ch16 (156.800 MHz) is radiotelephony distress/safety. MF DSC is 2187.5 kHz and radiotelephony 2182 kHz. HF DSC: 4207.5, 6312, 8414.5, 12577 and 16804.5 kHz. HF radiotelephony pairs include 4125, 6215, 8291, 12290 and 16420 kHz. EPIRB 406 MHz; SART 9 GHz; NAVTEX 518 kHz.
Worked example
A VHF antenna is 30 m above sea level and a coast station antenna is 60 m. Use radio horizon in nautical miles ≈ 1.23√h(m): ship 6.7 nm, coast 9.5 nm, total about 16 nm. Reasoning: VHF is line-of-sight; doubling antenna height increases range only by √2, not double. Terrain and antenna gain change the result, but the geometry is the key.
Traps
Do not use HF skywave for a guaranteed short-range VHF-style link. Do not assume 2182 kHz is usable in daylight at long range. Do not confuse Ch16 with Ch70; Ch70 carries DSC, not voice. A higher frequency is not always better-MUF and absorption matter.