Single repeater or full DAS: how many storeys or how big a basement before one antenna stops being enough?

A single repeater with one antenna is usually enough for a building of about four to six storeys with an open floor plate, or a single shallow basement with a clear line of sight along the ramp. Beyond that, or wherever the radios must work behind reinforced-concrete cores, stairwells and below-grade levels, one antenna runs out of uplink margin and you need antennas distributed through the building, which is what a DAS is. The limit is almost always the portable's 4 W uplink, not the repeater's 40 W downlink.

What does a single repeater antenna actually do?

It radiates from one point, so every floor and room is served by whatever signal survives the path from that point. In free space at 450 MHz the path loss is roughly 85.5 dB plus 20 log of the distance in kilometres, so 100 m costs about 65.5 dB. The building then adds its own loss on top: reinforced-concrete slabs and cores commonly cost 10 to 20 dB each at UHF, and a brick or drywall partition a few dB. These are typical figures; the real values come from a survey of the actual structure.

How far does the link budget stretch?

Using standing design defaults (verify on site): repeater 40 W (46 dBm), 1.5 dB feeder loss, 3 dBi antenna, portable 4 W (36 dBm), 3 dB body loss, 0 dBi portable antenna, design threshold -95 dBm at the portable.

• Downlink: 46 - 1.5 + 3 = 47.5 dBm EIRP. Reaching -95 dBm at a body-worn portable allows 47.5 - 3 + 95 = 139.5 dB of path loss.

• Uplink: 36 - 3 = 33 dBm from the portable, plus 3 dBi less 1.5 dB feeder at the repeater. Taking -100 dBm at the repeater as the design threshold (an assumption, set 19 dB above the -119 dBm datasheet sensitivity to leave margin), the allowed path loss is 33 + 1.5 + 100 = 134.5 dB.

• The uplink is the limiting path, by about 5 dB. Raising repeater power does nothing for it.

Take off a 10 dB fade margin and you have roughly 125 dB to spend. After 70 to 80 dB of distance and clutter loss, that leaves room for perhaps three to five concrete slabs or a couple of cores. That is where the four-to-six-storey figure comes from, and it assumes the antenna sits mid-height, as covered in our note on repeater antenna placement, rooftop or mid-level.

How big can a basement be before one antenna fails?

A single basement level with a ramp opening toward the antenna can work to around 60 to 100 m of open run. Columns, ramp walls and parked vehicles break up the line of sight and cost several dB each. Once the carpark has a second level, a bend, or a plant room, the signal has to cross a slab or turn a corner and the uplink margin disappears. Stairwells and lift lobbies behind RC cores are the first places to drop out, which is the pattern described in our article on dead zones in basement carparks and stairwells.

When is the answer a full DAS?

Treat any of these as a trigger: more than about six storeys; more than one basement level; mandatory coverage in stairwells, the fire command centre or plant rooms; or an acceptance test written to a percentage-of-floor-area criterion. A DAS lowers the path loss per radio by putting antennas on each floor or zone, typically with a coupler on the riser feeding each floor. Cheaper intermediate options exist, such as a second receive antenna feeding the repeater, but they fix only the uplink, as explained in voting receivers versus in-building DAS.

Related questions

Can I add a second antenna to the same repeater instead of a DAS? Yes, for two or three nearby zones, using a splitter and extra cable, but each split costs about 3.5 dB and both antennas share one repeater.

Does a taller building always need a DAS? Not strictly. Height matters because of the slabs between the antenna and the radio, so a tall open-atrium building behaves differently from a stacked-floor tower. A survey settles it.

Who decides, the vendor or the building owner? The owner should require a written coverage criterion and a two-way (uplink and downlink) survey, then let the design follow.

How Suneast solves this

Suneast surveys the building in both directions, computes the uplink and downlink budgets, and only then recommends a single repeater with an antenna, an enhanced receive system or a full DAS. Systems are designed, supplied, installed and commissioned in Singapore, with IMDA licensing handled as part of the project.