Short note: Features of microwave frequency band

Microwave bands (300 MHz–300 GHz, λ = 1 m to 1 mm) lie between radio waves and infrared. Their main features come from the short wavelength.

• Electrically large circuits: circuit size is comparable to λ, so distributed (transmission-line) analysis is used instead of lumped KVL/KCL.
• Wave-based description: S-parameters, reflection coefficient and VSWR replace voltage and current.
• Wide bandwidth: large information capacity, for example 100 MHz is only 1% of 10 GHz.
• Line-of-sight propagation: microwaves pass through the ionosphere (used for satellites) and are not reflected back like HF waves.
• High-gain, small antennas: horns, parabolic dishes and patch arrays give narrow beams. Gain = 4πAe/λ².
• Skin effect: current flows in a thin surface layer, which raises conductor loss.
• Transit-time limits: ordinary tubes and transistors fail, so klystrons, magnetrons, TWTs, Gunn and IMPATT diodes are used.
• Radiation from discontinuities: bends, gaps and open ends radiate, so shielding and careful layout are needed.
• Atmospheric effects: rain attenuation above about 10 GHz, and absorption peaks at 22 GHz (water vapour) and 60 GHz (oxygen).
• Molecular resonance: used for heating (2.45 GHz ovens) and spectroscopy.
• Reflection from targets: objects comparable to λ reflect well, which is the basis of radar.

Applications: radar, satellite and terrestrial links, mobile phones, Wi-Fi, GPS, ovens, and medical diathermy.