Radio Wave Fundamentals and Propagation
Core concepts
Radio navigation depends on four linked ideas: frequency and wavelength, the modes of propagation, the behaviour of the ionosphere, and the practical consequences of power and path loss. Wavelength equals the speed of light divided by frequency, so a 300 kHz NDB signal has a wavelength of about 1000 m while a 1 GHz GNSS signal has a wavelength of about 30 cm. Low and medium frequencies follow the ground wave, hugging the terrain with relatively stable amplitude and phase; this is why NDB and ADF work at long range near the surface. High frequencies can be refracted by the ionosphere (sky wave), which is useful for long-distance communication but a source of bearing error for navigation. VHF and above are essentially line-of-sight, so their usable range grows roughly with the square root of the sum of the heights of transmitter and receiver (the radio horizon).
Traps in questions
Watch for the distinction between ground wave and sky wave, and remember that sky wave is a nuisance for ADF, not a benefit. Questions often test that VHF is not affected by ionospheric refraction but is affected by terrain and by ducting in anomalous conditions. Another trap is confusing attenuation with absorption: precipitation weakens signals above roughly 3 GHz far more than at VHF. Also expect questions on polarisation, on the effect of frequency on antenna size, and on why DME and SSR use the UHF band.
Rule of thumb
For line-of-sight VHF, usable range in nautical miles is approximately 1.23 times the square root of the height in feet (summed for both antennas). A 100 ft receiver and a 400 ft transmitter give about 1.23 × (10 + 20) ≈ 37 NM.