Leaky Feeder or Discrete Antennas on a DAS for a Multi-Storey Car Park or Road Tunnel?

Use leaky feeder (radiating cable) where the space is a long, narrow, enclosed tube — a road tunnel, a service tunnel, a long ramp or a single-lane corridor — because the cable radiates uniformly along its whole length and follows the bends the vehicles do. Use discrete antennas on a distributed antenna system (DAS) where the space is a wide, open floor plate — a typical multi-storey car park deck — because a handful of antennas cover more square metres per dollar than cable does at UHF. Most real Singapore car parks are a hybrid: discrete antennas on the open decks, and a short radiating-cable run through the enclosed ramps, exhaust plenums and any tunnel link where an antenna cannot see down the length.

What leaky feeder is, and where it wins

Leaky feeder is a coaxial cable with slots cut in its outer conductor so RF leaks in and out along the entire run — effectively a continuous antenna hundreds of metres long. In a tunnel that geometry is exactly right: the signal decays smoothly with distance instead of dropping into nulls, and it follows curves that would put a fixed antenna out of line-of-sight. The trade-off is loss. The cable attenuates the signal as it travels, so you insert an in-line amplifier every few hundred metres, and at UHF you need a physically larger, more expensive cable to keep per-metre loss down. Installation is slower and dearer than hanging antennas because the cable must run continuously and be supported off the wall at a controlled distance. I reach for feeder in road and service tunnels, long single-bore vehicle ramps, and narrow enclosed corridors under roughly 4–6 m wide — anywhere uniform coverage along a line matters more than covering open area.

Where discrete antennas on a DAS win

An open car park deck is not a tube — it is a wide slab with columns, and there discrete antennas are cheaper and easier. A ceiling-mounted 2 dBi omni radiates outward across a broad radius, and two or three per deck typically blanket an open level. The working numbers on a standard UHF design:

• UHF two-way radio and DMR sit at 400–470 MHz; half-inch plenum coax loses about 4.5 dB per 100 m at 450 MHz.

• Leaky feeder needs an in-line amplifier roughly every 350–500 m to overcome cable loss.

• A 40 W (46 dBm) on-site repeater feeding the riser can deliver about +30 dBm EIRP per antenna port on an open deck.

• The design threshold for reliable DMR voice is −95 dBm at the portable, with a minimum 10 dB fade margin (15 dB for mission-critical/public-safety).

• Directional coupler taps are staged (typical 6, 10, 13, 15, 20 dB) so every antenna port lands within about ±3 dB of the others.

You cannot get that area efficiency from cable: radiating a wide floor from a single line of feeder wastes most of the signal into the slabs above and below. Antennas also suit the common case where the same DAS has to serve office, retail or plant floors off one head-end.

The honest answer for most Singapore car parks: hybrid

In practice I rarely design a large car park as pure one or the other. The decks get discrete antennas fed off the ELV riser; the enclosed spiral ramp, the tunnel link to the adjacent block and the exhaust ducts get a radiating-cable segment on the same system. The decision is driven by shape, not preference: if a person standing at the far point cannot see an antenna because the structure curves away, that segment wants feeder; if they are on an open floor, it wants antennas. Both feed from the same repeater and share one uplink path — and the uplink (a 4 W portable talking back to the repeater) is very often the limiting path, not the downlink, so it must be budgeted separately whichever medium you choose. For the underlying reason coverage collapses in these spaces to begin with, see our explainer on why radios fail in basement carparks and stairwells.

Related questions

Is leaky feeder more expensive than antennas? Per metre of cable, yes — and it needs in-line amplifiers every 350–500 m. But over a long narrow tunnel it can work out cheaper and far more uniform than chaining enough antennas to cover the same line, so compare on coverage-per-dollar for the actual geometry, not on cable price alone.

Can I run both leaky feeder and discrete antennas on the same DAS? Yes. A single repeater and head-end can feed antenna drops on the open decks and a radiating-cable segment through the ramps and tunnels, keeping everything on one uplink and one licence. This is the normal design for a large car park.

How do I prove the car park actually passes after installation? A grid walk-test measures received signal at portable height across the floor, against the −95 dBm threshold and the required fade margin — see our guide on how in-building radio coverage is measured and passed.

How Suneast approaches this

Suneast designs and builds in-building two-way radio coverage for car parks, tunnels and mixed-use developments across Singapore. We start from the floorplan and the geometry — deciding leaky feeder versus discrete antennas segment by segment — then produce a full link budget (downlink and uplink), an antenna and coupler-tap schedule the contractor can build from, and a post-installation grid survey that verifies coverage against the design threshold and fade margin. The result is a system that is buildable, auditable, and holds up when the client's consultant recomputes the numbers.