--- title: "BTS radiation fields at 900 and 1800 MHz" subject: "RF and Microwave Engineering" chapter: 7 key: "semester_7/rf_and_microwave_engineering:bts-900-1800-radiation-fields" source: https://www.bctengineer.com/q/semester_7/rf_and_microwave_engineering/bts-900-1800-radiation-fields site: BCT Engineering --- # BTS radiation fields at 900 and 1800 MHz RF and Microwave Engineering · Chapter 7: Microwave Antennas and Propagation - Asked in: 2082 Baisakh, 4+2+2 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/bts-900-1800-radiation-fields - Whole chapter: https://www.bctengineer.com/md/semester_7/rf_and_microwave_engineering/chapter-7 ## Question Compare radiation fields around a mobile BTS operating at 900 MHz and 1800 MHz frequency bands with their quarter-wavelength antennas. Explain how microwave EMR becomes hazardous to humans; and mention its IEEE safety standard. ## Short answer At 900 MHz λ=33.33 cm and D=λ/4=8.33 cm; at 1800 MHz λ=16.67 cm and D=4.17 cm, so all field regions are half as large at 1800 MHz. Microwaves heat tissue; above 4 W/kg SAR core temperature rises about 1°C. IEEE C95.1-2019 sets whole-body SAR 0.08 W/kg public, 0.4 W/kg occupational, local 2 W/kg public, and S=f/200 W/m². ## Answer #### Radiation fields for λ/4 antennas ```text λ = c/f 900 MHz: λ = 3×10⁸/9×10⁸ = 33.33 cm, D = λ/4 = 8.33 cm 1800 MHz: λ = 3×10⁸/1.8×10⁹ = 16.67 cm, D = λ/4 = 4.17 cm ``` | Boundary | Formula | 900 MHz | 1800 MHz | |---|---|---|---| | Reactive near (large-antenna formula) | 0.62√(D³/λ) | 2.58 cm | 1.29 cm | | Reactive near (small antenna) | λ/2π | 5.31 cm | 2.65 cm | | Far-field start | 2D²/λ | 4.17 cm | 2.08 cm | For a small antenna (D < λ), the formula 2D²/λ gives a distance smaller than λ/2π, so the reactive near field ends at about **λ/2π** and the far field starts beyond about one wavelength (~33 cm at 900 MHz, ~17 cm at 1800 MHz). Comparison: - All field regions at **1800 MHz are half as large** as at 900 MHz, because λ and D are both halved. - At 1800 MHz the energy is absorbed nearer the skin; at 900 MHz it penetrates deeper. - Real BTS panel antennas are much larger (≈ 1–2 m), so their far field begins tens of metres away. #### How microwave EMR becomes hazardous The body absorbs RF energy, which turns into heat (SAR = σE²/ρ). Above about 4 W/kg, core temperature rises by about 1 °C. Organs with low blood flow, such as the eyes and testes, are damaged first (cataract, sterility); near-field exposure can cause burns. Possible non-thermal effects (headache, sleep disturbance) are still being studied. #### IEEE safety standard **IEEE C95.1-2019** (limits for human exposure, 0 Hz–300 GHz): - Whole-body SAR: 0.08 W/kg (unrestricted/public), 0.4 W/kg (restricted/occupational). - Localised SAR (10 g): 2 W/kg (public), 10 W/kg (occupational). - Public power density for 400–2000 MHz is S = f/200 W/m² (f in MHz), the same as ICNIRP: 4.5 W/m² at 900 MHz and 9 W/m² at 1800 MHz. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What are the wavelength and quarter-wave antenna length at 900 MHz? A. lambda = 33.33 cm, D = 8.33 cm B. lambda = 16.67 cm, D = 4.17 cm C. lambda = 33.33 cm, D = 33.33 cm D. lambda = 3.33 cm, D = 0.83 cm Answer: A. lambda = c/f = 3x10^8/9x10^8 = 33.33 cm and D = lambda/4 = 8.33 cm. (Key point: Quarter-wave antenna size) 2. For a small antenna (D < lambda), where does the reactive near field end? A. At about lambda/2pi, because 2D^2/lambda gives a smaller distance B. At 2D^2/lambda C. At 10 m regardless of frequency D. At D Answer: A. For small antennas 2D^2/lambda is smaller than lambda/2pi, so the reactive near field ends near lambda/2pi (5.31 cm at 900 MHz). (Key point: Small antenna reactive field) 3. How do the field regions at 1800 MHz compare with those at 900 MHz for lambda/4 antennas? A. Twice as large B. Half as large, since lambda and D are both halved C. The same D. Four times larger Answer: B. Halving lambda and D halves all the boundaries; at 1800 MHz energy is absorbed nearer the skin. (Key point: Comparison of bands) 4. Which is the IEEE C95.1-2019 whole-body SAR for the public and for workers? A. 0.4 W/kg public, 0.08 W/kg workers B. 0.08 W/kg public (unrestricted), 0.4 W/kg workers (restricted) C. 2 W/kg public, 10 W/kg workers D. 4 W/kg for both Answer: B. IEEE C95.1-2019 uses 0.08 W/kg for unrestricted and 0.4 W/kg for restricted environments. 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