How much signal does a two-way radio lose passing through each reinforced-concrete floor slab or basement wall?

As a working rule at UHF (400–470 MHz, the band most Singapore two-way radios run on), expect roughly 15–20 dB of loss through each reinforced-concrete (RC) floor slab and 20–30 dB through a basement or shear wall. The loss is not perfectly additive: the first slab costs the most, and by the second or third floor the signal is travelling as much through stairwells, lift shafts and service risers as through the concrete itself. Two RC floors typically swallow 30–40 dB — enough on its own to drop a healthy handset signal below the −95 dBm you need for reliable digital voice.

Those are the numbers I use for a first-pass sanity check before anyone runs a proper survey. Below is where they come from and, more importantly, when the loss means you need a repeater or an in-building distributed antenna system (DAS) rather than a better handset.

Why reinforced concrete is so hard on UHF radio

Two things attenuate the signal, and the second matters more than most people expect. The concrete itself absorbs energy — dry structural concrete (around a C-30/37 mix) attenuates roughly 26 dB per metre of thickness at 400 MHz, so a 200 mm slab loses about 5 dB in the concrete alone. Wet or freshly poured concrete is worse, pushing that figure up as water content rises.

The bigger problem is the steel. A slab or wall is not plain concrete — it is a dense rebar mesh, and that mesh behaves like a partial Faraday cage. At 450 MHz the wavelength is about 0.67 m, and rebar spaced at 150–200 mm screens the signal far more effectively than the concrete mass alone would suggest. Double-mesh basement walls, common under Singapore developments, are the worst offenders because you get two screening layers plus greater thickness.

There is one piece of good news buried in the physics: lower UHF actually penetrates concrete better than higher bands. Attenuation through RC rises sharply with frequency — the same wall that costs you 20-odd dB at 400 MHz can cost 60+ dB at 2.4 GHz. That is precisely why professional two-way radio sits at 400–470 MHz and not up where Wi-Fi lives.

Floor-by-floor: how the loss stacks up

If the loss were simply additive you could multiply 18 dB by the number of floors and be done. It is not. Field measurements consistently show the first floor traversal costing around 25–28 dB, but two floors totalling roughly 40 dB rather than 50-plus — the signal finds leakage paths through openings, stairwells and risers, so the marginal cost per floor drops off. That is also why coverage in a high-rise so often collapses suddenly rather than fading gently: you are fine until the accumulated slab-plus-wall loss finally crosses the threshold, then two floors later you have nothing.

Here are the hard numbers I design against:

• RC floor slab (UHF 400–470 MHz): ~15–20 dB per slab

• First floor traversal: ~25–28 dB (the single most expensive step)

• Two floors total: ~40 dB (not ~50 — loss is sub-additive)

• Basement / shear / party wall: ~20–30 dB

• Dry structural concrete: ~26 dB per metre thickness at 400 MHz

• Wet concrete: attenuation climbs, up to ~35 dB through a saturated section

• Reliable DMR voice threshold: −95 dBm at the handset

• Typical portable RX sensitivity: ~−119 dBm (12 dB SINAD)

The gap between a strong in-room signal and that −95 dBm floor is your entire budget. A handset that reads −60 dBm in an open lobby has about 35 dB of headroom — two RC floors, or one basement wall plus a slab, and it is gone.

Basement walls and lift shafts are the worst case

Basements combine everything that hurts UHF: thick double-mesh RC walls, no windows or façade openings to leak signal through, and often a slab directly overhead as well. A single below-grade level can impose 25–30 dB, and a second basement is usually a dead zone outright. Lift cars are a separate problem — a steel box in a concrete shaft can add 20–30 dB on top of whatever the shaft walls already cost, which is why guards routinely lose contact mid-ride. If you want the detail on those specific black spots, see our note on why radios fail in basement carparks and stairwells.

When does the loss mean you need a repeater or DAS?

The decision is a link-budget one, not a gut call. Once the concrete between your handset and the repeater (or the nearest covered area) adds up to more than your available margin — in practice, two or more RC floors, or any occupied basement level, or a lift shaft — a higher-power radio will not save you. Bumping a portable from 4 W to 5 W buys you about 1 dB; you are trying to recover 30 or 40. The fix is to put the signal source inside the shielded space: an on-site repeater feeding antennas distributed through the building on a DAS, or a bi-directional amplifier (BDA) where an off-air donor signal is viable. That way each antenna only has to cover one floor or one basement, and no single path ever has to punch through more than a slab or two. How that coverage is then measured and signed off is covered in our explainer on how in-building radio coverage is tested on a grid survey.

Related questions

How many floors can a walkie-talkie cover before it needs a repeater? In open-plan RC construction, direct handset-to-handset typically holds for two to three floors before the accumulated slab loss crosses −95 dBm. Beyond roughly four floors, or across any basement, plan for a repeater or DAS rather than relying on radio-to-radio range.

Does a higher-power radio fix concrete signal loss? No. Doubling transmit power adds only 3 dB, and the legal 4–5 W ceiling for UHF portables gives you nothing against the 30–40 dB a couple of RC floors impose. The answer is distributing the signal inside the building, not shouting louder from outside it.

Is UHF or VHF better through concrete? UHF (400–470 MHz) generally penetrates dense reinforced concrete and rebar mesh better than VHF for in-building use, which is why nearly all commercial building radio systems in Singapore are UHF. VHF can win over long open outdoor distances, but indoors UHF is the right band.

How Suneast solves this

Suneast designs and installs in-building two-way radio coverage for Singapore buildings — offices, hotels, hospitals, carparks and industrial sites — where reinforced concrete and basements would otherwise create dead zones. We start from a measured link budget and a floor-by-floor coverage survey, then engineer a repeater or BDA-fed DAS with antennas placed so no signal path ever has to fight through more concrete than it can afford. The result is coverage that holds to the −95 dBm design threshold across stairwells, basements and lift lobbies, and that passes a grid survey at handover.