Antennas, Wave Propagation and Microwave Engineering
This domain links antenna parameters, propagation modes and microwave hardware. Expect gain, beamwidth, Friis link-budget, VSWR, waveguide and fading questions.
Core rules
- Directivity and gain: G = ηD; gain in dBi = 10 log10(G).
- Half-power beamwidth and gain are inversely related: narrower beam, higher gain.
- Friis transmission: Pr = Pt Gt Gr (λ/4πd)²; in dB, Pr(dBm) = Pt(dBm) + Gt + Gr − FSPL.
- FSPL(dB) = 32.44 + 20 log10(d in km) + 20 log10(f in MHz).
- VSWR = (1 + |Γ|)/(1 − |Γ|); VSWR 1:1 is a perfect match.
- Propagation modes: ground wave, skywave (ionospheric), space wave/LOS. Microwave uses LOS, waveguides and dish antennas.
- Microwave tubes: klystron (amplifier), magnetron (oscillator), TWT (wideband); solid-state: Gunn, IMPATT.
Worked example
Pt = 10 W = 40 dBm, Gt = Gr = 20 dBi, d = 10 km, f = 2400 MHz. FSPL = 32.44 + 20 log10(10) + 20 log10(2400) = 32.44 + 20 + 67.6 = 120.0 dB. Pr = 40 + 20 + 20 − 120 = −40 dBm. This is 0.1 µW, so receiver sensitivity and fade margin must be checked.
Traps that cost marks
- Mixing dBm, dBW and watts without converting.
- Using gain and directivity interchangeably when efficiency is given.
- Forgetting that FSPL uses distance in km and frequency in MHz in the shortcut formula.
- Ignoring polarization mismatch and feeder loss.
- Treating skywave as reliable at all frequencies; it matters mainly at HF.
Exam approach
Convert all powers to dBm, gains to dBi, then add and subtract. State the fade margin required for reliable operation.