This free interactive 3D model shows how a radio-frequency (RF) signal spreads through a multi-story building and where it weakens into dead zones. It illustrates a public-safety distributed antenna system (DAS) — donor antenna, bidirectional amplifier (BDA), and interior antennas — used to give firefighters and police reliable in-building radio coverage. It's a visual learning tool for low-voltage designers, AHJ reviewers, RF engineers, and students learning NFPA 1221/72 public-safety radio requirements.
The viewer maps RF signal strength and the DAS that fixes weak coverage:
• Coverage zones — areas colored by signal strength (dBm), from strong to weak. • Dead zones — pockets (stairwells, basements, core rooms) where the outside signal can't reach. • Donor antenna — a rooftop antenna aimed at the public-safety tower to pull in the off-air signal. • BDA (bidirectional amplifier / signal booster) — amplifies the donor signal in both directions. • Interior DAS antennas — distributed through the building to re-radiate the boosted signal. • Floor-by-floor levels so you can see coverage change with height and shielding.
Radio signals lose strength as they travel and pass through walls, floors, and metal — this is path loss. Concrete, steel, and Low-E glass attenuate signals heavily, so the public-safety signal from an outdoor tower often can't penetrate building cores, basements, and stairwells, leaving dead zones. A DAS solves this: a rooftop donor antenna captures the tower signal, a BDA amplifies it (uplink and downlink), and a network of interior antennas redistributes it so coverage meets the required signal level (typically −95 dBm) across the floor area. NFPA 1221/72 and IFC Chapter 51 set the coverage percentages and signal thresholds that AHJs enforce.
1. Drag to orbit the building, scroll to zoom, and pan to move around. 2. Watch the coverage coloring to find strong areas versus dead zones. 3. Click the donor antenna, BDA, and interior antennas to see how the DAS chain delivers signal to weak areas.
A DAS is a network of antennas distributed throughout a building, fed from a common signal source, that re-radiates radio signal so coverage is uniform indoors. Public-safety DAS specifically rebroadcasts the local first-responder radio band so firefighters and police have reliable communication everywhere in the building, including stairwells and basements.
A BDA (bidirectional amplifier) is the active component of a DAS. It amplifies the radio signal in both directions — downlink (tower to interior antennas) and uplink (handheld radios back to the tower) — so a weak off-air signal becomes strong enough to cover the inside of the building.
Radio signals weaken as they pass through construction materials. Concrete, steel framing, metal decking, and energy-efficient Low-E glass all absorb or reflect RF energy. Deep interior spaces, stairwells, elevator shafts, and basements are shielded on all sides, so the outdoor signal drops below a usable level, creating dead zones.
Codes such as NFPA 1221/72 and IFC Chapter 51 require new and many existing buildings to provide a minimum in-building signal level (commonly −95 dBm) over a set percentage of floor area — often 90% general areas and 99% in critical areas like stairwells and fire command rooms. The AHJ tests coverage and requires a DAS where the building fails.
dBm is a measure of absolute RF signal power on a logarithmic scale referenced to one milliwatt. Higher (less negative) numbers mean a stronger signal: −70 dBm is strong, −95 dBm is the typical minimum acceptable for public-safety coverage, and −110 dBm or lower is effectively a dead zone.