Rooftop LTE Site: Power Density, Exclusion Zone and a Feeder Replacement
A telecom operator has a rooftop site on a 30 m building in Guwahati with three sector antennas for 1800 MHz LTE. Each antenna is fed 40 W at the input and has 18 dBi gain with a 4° downtilt. The antennas are mounted on a 1.5 m parapet at the roof edge, and a metal water tank platform stands 6 m away from the antenna faces; the building's plumber uses it routinely. Residents of the opposite building, 22 m away and at the same height, have complained about radiation. The ICNIRP general-public reference level at 1800 MHz is 9 W/m² and the occupational level is 45 W/m². A rigging team is scheduled the next morning to replace a feeder, which requires a technician to stand within 1.5 m of the antenna face with the sector still carrying traffic. The operator's safety manual requires a permit for work at height and lockout/tagout for any work inside the RF exclusion zone.
1. Compute the EIRP and the power density at 22 m and at 6 m. Is the residents' complaint supported by the calculation? What limits the validity of the calculation near the antenna?
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EIRP
40 W = 46 dBm. EIRP = 46 dBm + 18 dBi = 64 dBm, i.e. 10^6.4 mW ≈ 2512 W.
Power density, far-field plane-wave formula S = EIRP / (4πd²):
- At 22 m: S = 2512 / (4π × 484) = 2512 / 6082 = 0.41 W/m², about 4.6% of the 9 W/m² general-public level. Even allowing for pattern and reflection effects, the residents' complaint is not supported at that distance and height; it is also further reduced by the 4° downtilt.
- At 6 m: S = 2512 / (4π × 36) = 2512 / 452 = 5.6 W/m² - still below 9 W/m², but about 62% of the limit, which is not a comfortable place to stand routinely.
Limit of the validity: the formula assumes a plane wave in the far field. The boundary is roughly D²/λ; for a 1.3 m antenna at 1800 MHz (λ ≈ 0.167 m) that is about 10 m. The 6 m water tank is therefore inside the near field, where the simple 1/d² law does not hold and local hot spots can exceed the calculated figure. Compliance there must be established from the manufacturer's near-field data or, better, by measurement with a calibrated broadband meter. Solving the formula for 9 W/m² gives a calculated boundary of about 4.7 m from the antenna face in the main beam - a planning figure, not a substitute for measurement.
2. Define the RF exclusion zone for this site and state the controls you would insist on before the feeder replacement starts.
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Exclusion zone: the volume in front of each antenna, in the main beam, out to at least 5 m from the antenna face (measured boundary 4.7 m, rounded up for uncertainty) and bounded by the sector's horizontal and vertical beamwidth. Treat anything inside that volume as an occupational area requiring control, and the water tank platform at 6 m as a monitored area pending measurement.
Controls to insist on before work starts:
- Eliminate first: ask the NOC to mute or bar the sector for the duration; a de-energised antenna has no exclusion zone at all. If the sector cannot be muted, the work needs RF-specific controls, not just height controls.
- Lockout/tagout on the RF source, or a documented power-reduction plan approved by the NOC if muting is refused.
- Permit to work covering both work at height and RF, signed before access; a rescue plan and a second person at roof level.
- Personal RF monitor worn by everyone on the roof, with an alarm threshold set at the occupational action level; a calibrated survey meter to confirm the actual level at the work position before climbing.
- Fall protection: full-body harness on a certified anchor, helmet, no lone working, tools tethered.
- Access control: barriers and warning signage at the roof door, and a check that nobody with an implanted medical device joins the team.
The overriding judgement: a 1.5 m working position in front of a live 40 W, 18 dBi antenna is well inside the exclusion zone and must not be permitted on the strength of a calculation alone.
3. The plumber needs routine access to the water tank platform. Propose a permanent solution and the evidence you would collect to show the site is compliant.
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Permanent options, in order of preference:
- Engineering the field away: increase downtilt or re-aim the sector, lower the antenna, or reduce power so that the calculated and measured 9 W/m² boundary falls inside the antenna's own mounting area and cannot reach the platform. This removes the hazard rather than managing it.
- Physical separation: relocate the tank access to the far side, or erect a fixed railing and locked gate at the 5 m line so the platform cannot be occupied within the exclusion zone.
- Administrative: only as a supplement - signage, a rooftop access register and a rule that the sector is muted before any roof work.
Evidence to collect:
- A site RF survey with a calibrated broadband meter (frequency range covering 100 kHz–3 GHz), recorded at the antenna face, at 5 m, at the water tank and at the neighbouring building, with date, instrument serial number and calibration due date.
- The calculation sheet with the EIRP, antenna pattern and the derived compliance boundary.
- The site's ICNIRP compliance documentation and a photograph of the barriers and signage.
- A short method statement and the access procedure for the plumber, signed by the building owner and the operator.
- A schedule for a repeat survey whenever the antenna configuration, power or tilt changes.
That package converts a one-off calculation into an auditable compliance record and gives the building owner something concrete in answer to the residents' complaint.