Servicing a live 50 W VHF repeater within 4 m of the antenna
A technician must replace a corroded connector on a rooftop VHF repeater installation operating near 155 MHz. The transmitter runs 50 W into a 6 dBd collinear on a short mast; the work point is 4 m from the antenna, roughly in the horizontal plane of the radiator. The client insists the repeater stay on the air because it carries a volunteer fire service's paging traffic, and there is no standby machine. The site's RF safety plan, based on the applicable Canadian exposure code, cites a power-density reference level of 2 W/m² at this frequency for areas accessible to the general public, and the site is classed as controlled only while a trained person supervises and access is restricted. The technician's own radiation monitor is being recalibrated and is not available. The tower also has guy wires and a long earthed coax run, and the technician will be working alone.
1. Compute the EIRP and the power density at the 4 m work point, and judge whether the calculation alone justifies working while the transmitter is on.
Read the worked answer
EIRP: 50 W = 47 dBm. Converting gain: 6 dBd = 8.15 dBi. EIRP = 47 + 8.15 = 55.15 dBm ≈ 327 W.
Power density (isotropic assumption):
S = EIRP / (4πd²) = 327 / (4π × 4²) = 327 / 201 ≈ 1.6 W/m²
That is about 81% of the 2 W/m² reference level - a margin of under 1 dB.
Why that is not a green light:
- The isotropic formula spreads power uniformly; the collinear concentrates it, and at the work point near the radiator's horizontal plane the real density in front of the antenna can be several times the calculated value.
- The far-field formula is not valid within about a wavelength-to-few-wavelengths of a large radiator; 155 MHz has a 1.9 m wavelength, so 4 m is close enough that near-field structure and the antenna's own current distribution matter.
- No allowance is made for ground reflections, nearby metalwork, or the technician's body, which can locally increase absorption.
- The measurement device that would confirm the estimate is unavailable.
A calculation barely under the limit, made with optimistic assumptions and without a monitor, is not adequate evidence for live work.
2. What controls should be applied before the connector is replaced?
Read the worked answer
Apply the hierarchy of controls, not a single measure.
- Eliminate the exposure. Arrange a short coordinated outage - even ten minutes - with the fire service notified, and use the time for the connector replacement plus any other needed work. Paging traffic can be covered by a paging schedule or an alternate transmitter if one exists.
- Substitute or reduce. If outage is genuinely impossible, reduce power to the minimum that keeps the service usable. Dropping from 50 W to 5 W cuts power density by 10 dB, from about 1.6 W/m² to roughly 0.16 W/m², and simultaneously shrinks the controlled area.
- Engineering controls. Terminate into a dummy load and work off-air, or temporarily lower the mast or re-aim the antenna away from the work point.
- Administrative controls. If any live work remains, restrict access, post signage, require a second trained person as safety watch with an independent means of shutting the transmitter down, and follow a written RF work procedure with time limits.
Working alone, without a monitor, on an unverified calculation, is the combination to refuse.
3. What documentation and site-obligation issues must be closed out afterwards?
Read the worked answer
The exposure assessment, the chosen controls and the measured or calculated result should be recorded and kept with the site file, because the reference level used, the antenna gain and the transmitter power are all inputs that can change.
Close out items:
- Note the outage or power-reduction period and who authorised it, including notification to the fire service.
- Record the antenna type and gain after the work; if a different antenna or higher gain is fitted later, the exposure assessment and the station's authorisation parameters may need review, and the site plan must be updated.
- Confirm earthing and bonding of the coax, mast and guy wires; induced RF voltage on long earthed conductors and guy wires is a shock and burn hazard in its own right, and weather-seal the new connector.
- Re-check the site's signage and access control so the controlled-area boundary matches the actual power and antenna in use.
- Confirm that no transmitter adjustments moved frequency, power or emission characteristics outside the authorised or certified values, and record the final test of the repeater before it returns to normal service.