Radio Wave Propagation and Fundamentals
Radio aids begin with the behaviour of electromagnetic waves. Know the relationship c = f x lambda, so that higher frequency means shorter wavelength and different propagation behaviour. DGCA questions test the standard frequency bands (VLF through EHF) and typical uses: VHF for line-of-sight communication and navigation, UHF for DME, SSR and radar, SHF for weather radar and GNSS. Understand polarisation, and that VHF/UHF are effectively line-of-sight, with range limited by the radio horizon, which extends slightly beyond the optical horizon due to atmospheric refraction (rule of thumb: range in nautical miles is about 1.23 x sqrt(height in feet), or 1.23(sqrt h1 + sqrt h2) for two aerials).
At HF, ionospheric refraction allows skywave beyond the horizon, with skip distance, skip zone and critical frequency concepts. At VHF and above, anomalous propagation (ducting, super-refraction) can extend range unpredictably. Know free-space path loss, signal-to-noise ratio, and the effects of absorption, fading and multipath. Be clear on modulation types: AM for VHF voice and ILS/VOR, DSB/SSB for HF, pulse for DME and radar.
Trap: confusing the radio horizon with the geometric horizon, or forgetting that VHF range depends on both aircraft and ground aerial heights. Another trap: assuming HF is always usable; skywave depends on time of day, season and solar activity.
Worked example: an aircraft at 10,000 ft and a ground station aerial at 100 ft. Range = 1.23(sqrt10000 + sqrt100) = 1.23(100 + 10) = about 135 nm.