--- title: "Short note: Features of microwave frequency band" subject: "RF and Microwave Engineering" chapter: 1 key: "semester_7/rf_and_microwave_engineering:microwave-band-features-note" source: https://www.bctengineer.com/q/semester_7/rf_and_microwave_engineering/microwave-band-features-note site: BCT Engineering --- # Short note: Features of microwave frequency band RF and Microwave Engineering · Chapter 1: Introduction - Asked in: 2074 Bhadra, 6 marks; 2074 Magh, 4 marks (asked 2 times) - Video: [Microwave Frequency Bands and Applications of Microwave Frequency Bands in Microwave Engineering](https://www.youtube.com/watch?v=8Sdp1PYeboU) — Engineering Funda - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/rf_and_microwave_engineering/microwave-band-features-note - Whole chapter: https://www.bctengineer.com/md/semester_7/rf_and_microwave_engineering/chapter-1 ## Question Write a short note on features (behavior) of microwave frequency bands. ## Short answer Microwave bands have electrically large circuits requiring distributed analysis, S-parameters and VSWR, wide bandwidth, line-of-sight propagation, small high-gain antennas, strong skin effect, transit-time limits and radiation from discontinuities. Rain attenuation above 10 GHz, oxygen/water absorption peaks, molecular resonance and radar reflection are also characteristic. ## Answer 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. At which frequencies are the atmospheric absorption peaks for water vapour and oxygen? A. 22 GHz water vapour, 60 GHz oxygen B. 60 GHz water vapour, 22 GHz oxygen C. 2.45 GHz water vapour, 10 GHz oxygen D. 10 GHz for both Answer: A. Water vapour absorbs strongly near 22 GHz and oxygen near 60 GHz. (Key point: Atmospheric absorption) 2. How do microwaves differ from HF waves in ionosphere behaviour? A. Both are reflected back B. Microwaves pass through the ionosphere; HF is reflected back C. Microwaves are absorbed; HF passes through D. Neither interacts with it Answer: B. Microwaves penetrate the ionosphere, which enables satellite links; HF is reflected. (Key point: Ionosphere) 3. Why do bends, gaps and open ends matter at microwave frequencies? A. They radiate, so shielding and careful layout are needed B. They increase DC resistance only C. They have no effect D. They create lumped capacitors with ideal behaviour Answer: A. Discontinuities radiate at microwave frequencies. (Key point: Radiation) 4. Why is reflection from targets the basis of radar at microwave frequencies? A. Targets much larger than lambda absorb all energy B. Objects comparable to lambda reflect well C. Microwaves cannot be reflected D. Only metals at DC reflect Answer: B. Objects of size comparable to wavelength reflect strongly, giving good radar returns. 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