Should we fit a receiver multicoupler and preamplifier at the repeater, or go straight to a DAS?
Fit the preamplifier only if your problem is purely uplink — staff can hear the repeater clearly but the repeater cannot hear them. A well-sited low-noise amplifier recovers roughly 4 to 9 dB of effective receiver sensitivity, and that is all it will ever give you. A distributed antenna system is worth 40 dB or more, in both directions, because it removes the concrete from the path instead of trying to amplify through it. The test that decides between them takes about ten minutes with a portable and a signal meter.
What a receive preamplifier actually buys you
The physics is cascade noise figure, and the position of the amplifier in the chain is everything. Take a typical Singapore rooftop repeater: the receiver sits in the ELV room, fed by roughly 40 m of 1/2" coax at 4.5 dB/100 m (about 1.8 dB at 450 MHz), a duplexer at around 1.5 dB, and two mated connector pairs at 0.5 dB each. That is 4.3 dB of loss sitting ahead of a receiver with a noise figure near 6 dB, so the system noise figure is about 10.3 dB.
Move a low-noise amplifier up to the antenna — noise figure 1.5 dB, gain 18 dB — and the Friis cascade gives a system noise figure of roughly 1.95 dB. The improvement is about 8.3 dB. That is a real number and it matches what the land mobile literature reports for tower-top amplifiers: measured effective sensitivity gains of 4 to 9 dB, with the larger figures on the longer feeder runs. Put the same amplifier at the bottom of the cable, between duplexer and receiver, and you recover almost nothing — the feeder loss has already been added to the noise figure and no amount of downstream gain undoes it.
Now translate 8.3 dB into coverage. At an indoor path-loss exponent of about 3.5, it extends uplink distance by a factor of 1.73 — roughly 70% further across an open floorplate. Through structure it is far less impressive. A reinforced-concrete floor slab costs about 10 to 20 dB at UHF, so 8.3 dB buys you around half a floor. If your guards are three basement levels down, a preamplifier is not the answer.
When a preamplifier makes things worse
An LNA improves the ratio of signal to receiver-generated noise. It does nothing for signal-to-external-noise, because it amplifies both equally. On a dense CBD rooftop with LED drivers, VFDs, PoE switches and neighbouring transmitters raising the noise floor well above thermal, the site is externally noise-limited and the preamplifier delivers no usable gain at all. Worse, it narrows dynamic range and makes the receiver more prone to intermodulation and overload from strong nearby carriers. Always fit a bandpass filter ahead of the LNA, and measure the site noise floor before you specify one.
A receiver multicoupler is a different tool again. Its job is to let several receivers share one antenna, using an internal amplifier to offset the split loss. It is the right answer for a multi-channel site — a hotel running four talkgroups, a mall running six — where you want one clean receive antenna rather than a forest of them. It improves channel-to-channel consistency and site tidiness. It does not put signal into a basement carpark.
The numbers that decide it
Preamplifier gain in effective sensitivity: 4-9 dB, uplink only
Reinforced-concrete floor slab loss at 450 MHz: 10-20 dB per slab
Basement wall plus soil: 20-30 dB
Design threshold for reliable DMR voice at the portable: -95 dBm
Fade margin, standard commercial: 10 dB; mission-critical: 15 dB
Raw link imbalance, 40 W repeater vs 4 W portable: 10 dB, before antenna gain and 3 dB body loss
The ten-minute test
Take a portable into the worst area and read the downlink level from the repeater. If it sits better than -95 dBm but you still cannot get a call in, the problem is link imbalance and a receiver-side fix is genuinely worth costing. If the downlink is already below -95 dBm in that area, stop — the portable cannot hear the repeater either, and improving the repeater's ears changes nothing about that. Buildings fail this test far more often than they pass it, which is why receiver-side fixes are oversold.
The honest sequencing is: fix the uplink imbalance where the downlink is already healthy, and build a DAS where it is not. On a large site both can be true in different zones — a mast-head LNA serving the outdoor and upper-floor population, a DAS serving the basements and lift cores. That is a cheaper total build than a DAS everywhere, and defensible at audit.
Related questions
Does a preamplifier need an IMDA licence? A receive-path amplifier does not change your transmitted ERP, so it does not by itself alter your licence position, but the repeater it serves still needs its IMDA station licence and type-approved equipment. See our note on IMDA licensing for in-building repeaters and boosters.
Where exactly should the LNA be mounted? As close to the antenna as the mounting allows, ahead of the feeder run, with a bandpass filter in front of it and DC injected over the coax via a bias tee. Every dB of cable between antenna and LNA is a dB added straight to the system noise figure.
Can voting or satellite receivers do the same job more cheaply? Sometimes. Multiple distributed receive-only sites feeding a voter fix uplink coverage without a full DAS backbone, but they only fix the uplink — the downlink still has to reach the portable, so the same -95 dBm test applies.
How Suneast approaches this
We measure before we specify. On a coverage complaint we run a bidirectional survey — downlink level at the portable and uplink success from the same points — plus a noise-floor measurement at the repeater site, because those three readings determine whether the fix is a filter and an LNA, a multicoupler, or a distributed antenna system. Where the answer is a DAS we design it with a full link budget and coupler tap schedule so the contractor can build it and the client's consultant can recompute it. If you want the background on what a DAS is and when a building genuinely needs one, start with our explainer on distributed antenna systems in Singapore buildings.
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