--- title: "Microwave radiation hazards based on radiation fields" subject: "RF and Microwave Engineering" chapter: 7 key: "semester_7/rf_and_microwave_engineering:radiation-fields-hazards" source: https://www.bctengineer.com/q/semester_7/rf_and_microwave_engineering/radiation-fields-hazards site: BCT Engineering --- # Microwave radiation hazards based on radiation fields RF and Microwave Engineering · Chapter 7: Microwave Antennas and Propagation - Asked in: 2072 Magh, 5 marks; 2069 Bhadra (old course), 8 marks (asked 2 times) - Video: [MICROWAVE APPICATIONS & RADIATION HAZARDS](https://www.youtube.com/watch?v=Sw8_84uzw6k) — Prof. Barapate's Tutorials - 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/radiation-fields-hazards - Whole chapter: https://www.bctengineer.com/md/semester_7/rf_and_microwave_engineering/chapter-7 ## Question Compare RF radiation fields and explain how they are hazardous to living body (microwave radiation hazards based on the radiation fields). ## Short answer RF fields: reactive near field r<0.62√(D³/λ) stores energy, E/H not 377; radiating near field to 2D²/λ forms pattern; far field is plane wave, E/H=377Ω. Reactive near field causes high SAR, burns and shock; near-field main beam heats eyes and testes; far-field S=EIRP/4πr² usually falls below limits. ## Answer RF radiation fields around an antenna are classified by distance r from the antenna (D = largest dimension, λ = wavelength): ```text Antenna |<-- R1 -->|<------- R2 ------->| far field [D] | reactive | radiating near | (Fraunhofer) | near | (Fresnel) | R1 = 0.62√(D³/λ) R2 = 2D²/λ ``` | Region | Range | Features | |---|---|---| | Reactive near field | r < 0.62√(D³/λ) | Stored energy, E and H not in phase, very high fields | | Radiating near field (Fresnel) | 0.62√(D³/λ) to 2D²/λ | Pattern changes with distance, fields not simple | | Far field (Fraunhofer) | r > 2D²/λ | Plane wave, E/H = 377 Ω, S = EIRP/4πr² | #### Comparison | Point | Reactive near | Radiating near | Far field | |---|---|---|---| | Energy | Stored | Radiated, pattern forming | Radiated | | E/H ratio | Not 377 Ω | Varies | 377 Ω | | Field decay | 1/r² or 1/r³ | Irregular | 1/r | | Measurement | E and H separately | Complex | Power density only | | Exposure | Highest | High | Calculable, usually lowest | #### How they are hazardous - **Reactive near field:** strong E and H fields close to the antenna can deliver very high local SAR, causing burns and RF shock; workers on a live tower are at risk. Even a small antenna (phone) gives high local SAR to the head. - **Radiating near field:** in the main beam of a BTS or radar dish, power density can exceed the limit; tissues heat, the eye lens and testes are most at risk (cataract, sterility). - **Far field:** power density S = PtGt/(4πr²) falls with distance; for the public it is usually below ICNIRP reference levels, but long exposure to high-power radars or broadcast stations can still cause heating. - In all zones, the body absorbs energy as heat (SAR = σE²/ρ). Above about 4 W/kg whole-body SAR, core temperature rises about 1 °C; limits are 0.4 W/kg (workers) and 0.08 W/kg (public). Hence exclusion zones are set within the near field and the main beam, and exposure in the far field is checked against standards. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which describes the reactive near field? A. E and H in phase with E/H = 377 ohm and 1/r decay B. Stored energy, E and H not in phase, very high fields, and measured separately C. A fixed pattern with exposure calculable by S = EIRP/4 pi r^2 D. A region where no energy is present Answer: B. The reactive near field stores energy; E and H are not in phase and the E/H ratio is not 377 ohm. (Key point: Reactive near field features) 2. Why is the reactive near field the most hazardous region? A. Strong E and H fields give very high local SAR, causing burns and RF shock B. It is the only region with ionising radiation C. It has the lowest field strength but the longest exposure D. Because power density here is always below the limits Answer: A. Very strong local fields give high SAR and burns, so workers on a live tower and phone users (head) are at risk. (Key point: Hazard in reactive near field) 3. Where in the radiating near field is exposure most likely to exceed limits? A. Behind the antenna B. Within the main beam of a BTS or radar dish C. Only at ground level far from the mast D. Nowhere, because pattern is fixed Answer: B. In the main beam power density can exceed the limit, heating tissues; the eye lens and testes are most at risk. (Key point: Hazard in main beam) 4. What is true of far-field exposure? A. It is highest of all regions B. S = PtGt/(4 pi r^2) falls with distance and is usually below ICNIRP levels for the public, though long exposure to high-power radars can still heat C. It cannot be calculated D. E and H must be measured separately Answer: B. In the far field exposure is calculable and normally lowest, but high-power sources can still cause heating. 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