Can a Walkie-Talkie Cover a 50-Storey Building?

What the physics says

A typical commercial portable radio transmits 4 watts (36 dBm) on UHF, around 400–470 MHz. For reliable digital (DMR) voice you want the received signal at the other handset to be no worse than about −95 dBm, and you should hold a fade margin of at least 10 dB on top of that. So the total loss the signal can tolerate between two handsets, after accounting for antenna gains and roughly 3 dB of body loss at each end, is on the order of 120 dB.

In free space that is an enormous distance. Inside a reinforced concrete high-rise it is not. Each post-tensioned concrete floor slab typically knocks 10–20 dB off a UHF signal, and the M&E risers, lift cores, and services between floors often make it worse. Take a middle figure of 15 dB per slab: after just five or six floors of vertical separation you have burned 75–90 dB in slabs alone, before any horizontal distance, walls, or interference. In practice, handset-to-handset range in a concrete tower is around 5 to 8 floors — sometimes fewer where slabs are thick or floors are dense with services. A security officer in the basement carpark and a technician on level 40 might as well be in different countries.

Why "just add a repeater" only half-solves it

The next step people reach for is a repeater: a fixed 25–40 W base station, usually placed mid-building, that receives on one frequency and retransmits on another. A repeater on, say, level 25 genuinely helps — it doubles the vertical budget, because each handset only has to reach the repeater, not the far handset.

But run the same slab arithmetic from level 25: the repeater's signal still has to punch through 20-plus slabs to reach the basement or the roof. Even with the repeater's higher power on the downlink, the limiting path is almost always the uplink — the 4 W portable trying to get back to the repeater through those same slabs. A repeater alone typically gives you solid coverage for perhaps 10–15 floors either side of its antenna. On a 50-storey building with basements, that still leaves dead zones at both ends, and the basement is usually where you need coverage most: carparks, plant rooms, loading bays, fire pump rooms.

You can add a second repeater, but now you are managing frequency coordination and handoff behaviour between two coverage islands, and IMDA licenses each station. It gets expensive and fragile quickly.

What actually works: a DAS

The correct engineering answer for a 50-storey tower is a distributed antenna system (DAS). Instead of asking one antenna to fight through 50 slabs, you run a coaxial trunk up the ELV riser and tap off a small amount of power at each floor — through directional couplers — feeding one or more low-profile ceiling antennas per floor. Every handset is then never more than one slab and a short horizontal run from an antenna.

The design discipline matters. A proper DAS for a tower this size means a link budget computed in both directions (downlink and uplink — the uplink is usually what limits it), a coupler schedule staged so every antenna port lands within about ±3 dB of its neighbours, and an antenna placement plan per floor checked against the worst point on that floor, not the average. Done properly, 95% area coverage at −95 dBm or better is a normal, verifiable target — including basements, stairwells, and lift lobbies where fire safety requirements often apply. Done badly — splitters cascaded ad hoc, no uplink budget, no port balance — you get a system that demos well beside the riser and dies at the far corner of every floor.

Typical scope for a 50-storey design: one 40 W DMR repeater at a mid-level ELV room, a 7/8" main trunk in the riser, directional couplers at each floor, one to three antennas per floor plate depending on floor area and layout, and dedicated runs for basements and lift shafts, which behave differently enough to design separately. The whole system runs on one licensed repeater channel pair, which keeps the IMDA licensing simple compared with multiple repeaters.

The honest checklist

If someone quotes you a handful of high-powered walkie-talkies for a 50-storey building, walk away. If they quote a single repeater with one antenna and promise full coverage, ask to see the uplink budget for the basement — there usually isn't one. What you should expect from a competent proposal: stated assumptions, a two-way link budget, a floor-by-floor tap schedule, an antenna layout you could hand to a contractor, and a site survey before anything is priced firm.

Suneast Pte Ltd designs and installs in-building radio coverage systems (DAS) for high-rise commercial, hotel, and industrial buildings in Singapore. Every design is produced with full downlink and uplink link budgets, floor-by-floor coupler schedules, and IMDA licensing handled as part of the project, and is verified on-site against the coverage target after installation.