Stairwell Coverage in a Concrete High-Rise
An 18-story reinforced-concrete hotel uses a 700 MHz P25 trunked system for fire operations. The existing in-building system is a 5 W Class A BDA with a roof-mounted donor antenna and a single server antenna feeding coax risers. Baseline testing shows guest floors at -90 to -94 dBm with DAQ 3.0, but stairwells read -106 to -110 dBm and DAQ 1.0-2.0; the elevator cab is -98 dBm. The AHJ requires 99% coverage in critical areas at the adopted threshold and DAQ 3.0. The installing contractor proposes raising BDA gain by 10 dB and reusing the same antenna locations, arguing that the general-floor margin proves the system has headroom. The building has a 12-hour standby power requirement and no spare fiber. The owner wants a fix that will pass acceptance without causing donor-site interference.
1. Evaluate the contractor's proposal to increase BDA gain by 10 dB. What will it fix and what will it not fix?
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The 10 dB gain increase is the wrong first move. The BDA is already delivering -90 to -94 dBm on open guest floors, so the downlink budget has only about 1-5 dB of margin to the -95 dBm threshold; +10 dB would likely drive the BDA into AGC or overload and would raise uplink noise at the donor site. Stairwells are shielded by concrete and closed doors, so the missing 12-15 dB is a coverage-distribution problem, not a total-power problem. Increasing gain also increases both uplink and downlink equally and can reduce donor-to-server isolation, creating oscillation or noise amplification. Before changing gain, verify donor signal level, antenna isolation, and that the BDA is not already at maximum usable gain. The correct conclusion is that stairwells need dedicated radiating elements or leaky coax, with the gain re-balanced after the RF path is fixed. A gain increase alone will not reliably meet 99% critical-area coverage.
2. Outline a corrective design and the RF calculations you would perform to meet 99% critical-area coverage.
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Add dedicated stairwell coverage. For an 18-story building, place vertically polarized ceiling or wall antennas in each stairwell at approximately every third floor, or use a vertical leaky-coax run if the shaft is accessible. Feed them from the server through a tapped riser, using directional couplers so each antenna receives the design power. Perform an RF budget: allowed path loss equals antenna EIRP minus the -95 dBm threshold minus a design margin, typically 10 dB. Include 20-30 dB concrete penetration loss and body loss; do not rely on floor antennas transmitting through the stairwell wall. Keep each antenna's ERP within the license and the BDA's per-port limits. Recheck uplink: a 3 W portable at the stairwell landing must reach the donor with acceptable DAQ; adjust uplink gain only after downlink is balanced. Verify donor-to-server isolation is at least 15 dB above operating gain to avoid oscillation. Then test 99% of critical areas, including elevator cabs and stair landings.
3. What acceptance evidence should be presented to the AHJ to show the corrected system complies?
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Acceptance evidence should include: a grid test of every general floor and 99% of critical areas using a calibrated public-safety receiver or radio, recording DAQ 3.0 or better and signal at or above the adopted threshold; inbound and outbound tests, because downlink-only data do not prove the portable can reach the donor; worst-case locations such as stairwell tops, elevator cabs, basements, and closed tenant spaces; BDA commissioning records showing gain, isolation, and noise-floor contribution; a 12-hour battery load test with the actual load; and as-built drawings plus a statement that the system does not degrade the donor network. The stairwell data should show 99% of tested locations at DAQ 3.0, not just an average. If any critical location fails, add or relocate an antenna and retest that area. The AHJ should witness or review the test method before occupancy.