--- title: "GSM BTS propagation fields and non-ionizing SAR" subject: "RF and Microwave Engineering" chapter: 7 key: "semester_7/rf_and_microwave_engineering:gsm-bts-fields-sar" source: https://www.bctengineer.com/q/semester_7/rf_and_microwave_engineering/gsm-bts-fields-sar site: BCT Engineering --- # GSM BTS propagation fields and non-ionizing SAR RF and Microwave Engineering · Chapter 7: Microwave Antennas and Propagation - Asked in: 2081 Baisakh, 4+4 marks - 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/gsm-bts-fields-sar - Whole chapter: https://www.bctengineer.com/md/semester_7/rf_and_microwave_engineering/chapter-7 ## Question Classify microwave propagation fields of a GSM BTS and its radiation parameters. Explain what non-ionizing SAR is and how it becomes hazardous to human body. ## Short answer GSM BTS fields are reactive near field r<0.62√(D³/λ), radiating near field to 2D²/λ, and far field with S=EIRP/4πr². SAR=σE²/ρ measures absorbed power; above about 4 W/kg whole-body core temperature rises about 1°C. ICNIRP/IEEE limits: whole-body 0.08 W/kg public, 0.4 W/kg occupational, local 2 W/kg public. ## Answer #### Propagation fields of a GSM BTS A GSM BTS antenna (900/1800 MHz panel, length D ≈ 1–2 m) produces three field regions: ```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 | Example: 900 MHz (λ = 0.333 m), D = 2 m → R1 = 0.62√(8/0.333) = 3.04 m, R2 = 2×4/0.333 = 24 m. **Radiation parameters used to describe BTS exposure:** - **EIRP** = Pt·Gt (W or dBm): effective radiated power in the main beam. - **Power density** S = EIRP/(4πr²) (W/m²) in the far field. - **Electric field** E = √(377·S) (V/m), **magnetic field** H = E/377 (A/m). - **Antenna gain, beamwidth and tilt** decide where the main beam reaches the ground. - **SAR** (W/kg) for absorbed power in the body. #### Non-ionising SAR and its hazard Microwave radiation is **non-ionising**: each photon carries too little energy to break chemical bonds. Its effect on the body is measured by the **Specific Absorption Rate**: ```text SAR = σ|E|²/ρ (W/kg) ``` where σ is tissue conductivity, E the internal rms field and ρ the tissue density. How it becomes hazardous: 1. Absorbed power becomes heat. Whole-body SAR above about 4 W/kg raises core temperature by about 1 °C, causing heat stress. 2. High local SAR heats organs with poor blood flow: eye lens (cataract), testes (temporary sterility); very high levels cause burns. 3. Limits: ICNIRP/IEEE whole-body 0.08 W/kg (public), 0.4 W/kg (occupational); local 2 W/kg (public, 10 g). 4. Near a BTS, workers in the near field or main beam can exceed limits; the public at ground level is usually far below them. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. For a 900 MHz BTS panel with D = 2 m (lambda = 0.333 m), what are R1 and R2? A. R1 = 24 m, R2 = 3.04 m B. R1 = 3.04 m, R2 = 24 m C. R1 = 0.33 m, R2 = 2 m D. R1 = 12 m, R2 = 48 m Answer: B. R1 = 0.62 sqrt(8/0.333) = 3.04 m and R2 = 2 x 4/0.333 = 24 m. (Key point: BTS example) 2. How are EIRP, power density and electric field related in the far field? A. EIRP = Pt Gt, S = EIRP/(4 pi r^2), E = sqrt(377 S) B. EIRP = Pt/Gt, S = EIRP x 4 pi r^2, E = 377 S C. EIRP = Pt + Gt, S = EIRP/r, E = S/377 D. EIRP = Gt, S = EIRP/r^3, E = S Answer: A. EIRP is power times gain; spreading over a sphere gives S, and E = sqrt(377 S) because E^2/377 = S. (Key point: EIRP and field relation) 3. What does non-ionising mean in the context of SAR? A. Photons are too weak to break chemical bonds; the effect is measured by absorbed power per kg B. The radiation is not absorbed at all C. The radiation always ionises the water in tissue D. SAR is zero below 1 GHz Answer: A. Microwave photons cannot break bonds, so the hazard is heating, quantified by SAR = sigma E^2/rho. (Key point: Meaning of non-ionising SAR) 4. Who is most likely to exceed limits near a BTS? A. The public at ground level B. Workers in the near field or main beam; the public at ground level is usually far below limits C. Nobody, limits cannot be exceeded D. Only users far outside the beam Answer: B. Near-field and main-beam positions can exceed limits, which is why they are exclusion zones for workers. (Key point: Who is at risk near BTS) ## Related in RF and Microwave Engineering - [BTS radiation fields at 900 and 1800 MHz](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Abts-900-1800-radiation-fields) — RF and Microwave Engineering, chapter 7 - [Hazardous and safe zone for 2600 MHz BTS](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Abts-2600mhz-safe-zone) — RF and Microwave Engineering, chapter 7 - [Microwave radiation hazards based on radiation fields](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Aradiation-fields-hazards) — RF and Microwave Engineering, chapter 7, asked 2× - [SAR and microwave radiation hazards](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Asar-radiation-hazards) — RF and Microwave Engineering, chapter 7, asked 2× - [Hazards and device to map BTS radiation zones](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Ahazards-bts-radiation-zone-device) — RF and Microwave Engineering, chapter 7 - [Short note: Microwave radiation fields](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Amicrowave-radiation-fields-note) — RF and Microwave Engineering, chapter 7 ## Related in other subjects - [Touching energized microwave oven body](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Amicrowave-oven-earthed-unearthed) — Industrial Power Distribution and Illumination, chapter 2 - [Infinitesimal dipole fields and basic parameters](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Ainfinitesimal-dipole-fields-parameters) — Propagation and Antenna, chapter 1