RF Exposure Compliance at a Rooftop Vertical
An advanced licensee installs a 5/8 λ vertical for 2 m on a rooftop. The antenna's feedpoint is 3 m above the roof, and the roof is accessible to the licensee and family. The station runs 100 W FM (continuous carrier) into a 5/8 λ vertical with 6 dBi gain. The antenna's elevation pattern has a main lobe at about 5° above the horizon, but there is significant downward radiation. The licensee uses the FCC's OET-65 formula and the uncontrolled-environment limit of 1 mW/cm² at 144 MHz (power density averaged over 6 minutes). The antenna is 2 m from the edge of the roof, and a neighbor's balcony is 8 m away at the same height.
1. Estimate the power density at 2 m from the antenna and determine whether the station complies with the uncontrolled limit.
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EIRP: 100 W × 10^(6/10) = 100 × 3.98 = 398 W EIRP. For a 5/8 λ vertical, the gain is about 6 dBi, and the pattern is roughly omnidirectional in azimuth with a low-angle lobe. Using the far-field formula S = EIRP/(4πr²): at r = 2 m, S = 398/(4π × 4) = 398/50.3 = 7.9 W/m² = 0.79 mW/cm².
Compare to limit: The uncontrolled limit at 144 MHz is 1 mW/cm² (per OET-65, f/1500 for 1.5–30 MHz, 1 mW/cm² for 30–300 MHz, 5 mW/cm² for 300–1500 MHz, etc.). 0.79 mW/cm² is just under 1 mW/cm², so it is marginally compliant in the main beam at 2 m. However, this is a free-space estimate; ground reflection and the roof can increase the field. Also, the 6-minute averaging applies; FM is continuous, so the average equals the peak. A 20% safety margin is advisable, and the licensee should verify with a calibrated meter.
At the neighbor's balcony (8 m): S = 398/(4π × 64) = 398/804 = 0.495 W/m² = 0.05 mW/cm² - well below the limit. The neighbor is not at risk from the main lobe, but the downward radiation and reflections should be checked.
2. What does the licensee need to do to ensure compliance, and what records should be kept?
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- Control access: The roof must be restricted so that persons cannot stand within the calculated compliance distance. The compliance distance for 1 mW/cm² is r = √(EIRP/(4πS)) = √(398/(4π × 10)) = √(3.17) = 1.78 m in free space. Add a safety factor (e.g., 2×) and restrict access to at least 3.5 m from the antenna, or fence the antenna.
- Signage and procedures: Post RF hazard signs at roof access points, and include RF exposure in the station log. Family members should be informed.
- Reduce power or duty cycle if access cannot be controlled: 50 W would halve the power density to 0.4 mW/cm², giving more margin.
- Measure with a calibrated RF meter (e.g., a broadband field strength meter) at the roof edge and at the neighbor's balcony, with the transmitter keyed. Record the measurements, date, power, and antenna configuration.
- Records: Keep a written RF exposure evaluation (calculations and measurements), the antenna gain and pattern data, the power and frequency, and the date. Retain for at least the period required by the regulator (often the life of the licence or several years). Review if the antenna or power changes.
- Neighbor notification: If the neighbor's balcony is accessible, ensure the power density there is below the limit; 0.05 mW/cm² is far below, so no action is needed, but document it.
3. How would the analysis change if the mode were SSB instead of FM?
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SSB has a much lower average power than FM for the same PEP. A 100 W PEP SSB signal has an average power of roughly 20–30 W depending on speech processing and the duty cycle of the voice waveform (typically 20–30% for SSB). The 6-minute averaging in OET-65 allows the use of average power, so the power density would drop by about 5–7 dB compared with FM. The compliance distance would shrink accordingly: for 25 W average, EIRP ≈ 100 W, and r = √(100/(4π × 10)) = 0.89 m. This means the station would be comfortably compliant at 2 m. However, the licensee must still consider the PEP for the transmitter's ratings and the possibility of a stuck carrier or a tune-up carrier. The analysis should use the worst-case mode (FM or a continuous carrier) for access control, and note that SSB operation has additional margin. The records should state the mode and the assumed duty cycle.