How Many Repeaters Does a Site Need? Size by Coverage, Not by Area
The question arrives as "we have 20 hectares, how many repeaters?" and the honest answer is that hectares do not determine it. Six variables do, and only one of them is distance. Here is how the count is actually derived, what one repeater covers, and what to send a supplier so the answer is a design rather than a guess.
| Specification | What it changes.. | What to measure.. |
|---|---|---|
| Variable | What it changes | What to measure |
| Distance between users | Whether direct handheld-to-handheld reaches at all | Furthest two points that must talk, measured on a plan |
| Obstruction | Costs more range than distance does: concrete, steel, earth, racking | How many walls or floors between the two furthest users |
| Concurrent talk paths | How many conversations the network must carry at once | Peak number of groups active at the same time |
| Antenna height and position | Usually worth more than transmit power | Whether a mast, roof or hill is available near the middle |
| Licence and channel plan | Decides the band, the bandwidth and whether a repeater is legal to run | Whether you hold an assignment, and on which band |
| Site count | One repeater covers one area well; separate sites need linking | How many discrete areas must talk to each other |
| Published RF power | 5 W VHF / 4 W UHF on current Motorola portables, so extra watts are rarely the binding limit | The datasheet for the exact model, not the marketing page |
| Receiver sensitivity | 0.14 uV digital at 5% BER, which is about -124 dBm into 50 ohm | Use it as the floor, then plan margin above it rather than chasing power |
| Loss per doubling of distance | Path loss rises with 20*log10(distance), so every extra 6 dB halves the reach | Ask what each obstruction, wall or feeder run costs in dB, not in metres |
Coverage Sizing
Start with what a single repeater does. It takes the signal from handhelds that cannot hear each other and re-transmits it from one point with height and gain, so the practical coverage is set by where that point is, not by how big the site is. On open ground with a reasonable antenna height, one repeater can hold a site of a few kilometres across. The same repeater inside a multi-storey building with concrete cores, steel racking and a basement may cover a fraction of that, because the loss is in the structure rather than the distance. This is why the area figure people send first is the least useful number in the enquiry. Two sites of identical size - one a container yard, one a food plant with metal cladding and cold stores - can differ by a factor of several in repeater count. If a supplier answers a hectare count with a repeater count and no further questions, they are quoting a guess with a price attached.
Repeater Planning
The count comes out of five things you can actually describe: the furthest pair of users that must talk, how many walls or floors sit between them, how many groups are active at peak rather than how many radios exist, whether a mast or roof is available near the middle of the site, and whether you hold a frequency assignment. Send those five and the number becomes arguable rather than arbitrary. Where one repeater is genuinely not enough, the usual next step is not a second repeater in the same building but a different topology: distributed antenna or leaky feeder inside a structure, or linked repeaters where two separate sites both need local coverage and occasional cross-talk. Ask for the reasoning next to each number in the BOM, and ask what happens to the design if the site changes - phase two of a construction project, or a new warehouse on the same land. A design you cannot re-run cheaply is a design that will be re-quoted at full price. The published figures are worth knowing because they show where the limit actually sits. Current Motorola portables are specified at 5 W VHF and 4 W UHF, with digital sensitivity of 0.14 uV at 5% BER - about -124 dBm into 50 ohm on the R7 datasheet. Between those two numbers there is roughly 160 dB of link budget, which in free space would suggest a distance no site ever achieves. What eats it is the site: every additional 6 dB of loss halves the reach, and a wall, a floor slab or 30 m of thin feeder can each cost several dB. That is why the repeater question is really a question about the site, not about the power.
Data Source: Standard DMR Tier II system behaviour (2-slot TDMA on 12.5 kHz) and the repeater documentation for the six units in the RadioSysHub catalogue (Motorola SLR 5500, DR3000; Hytera RD98X, RD98xS, HR106X, HR655). No coverage figures are published by the manufacturers, and none are asserted here. Power and sensitivity values quoted in this article come from the Motorola R7 datasheet (https://www.motorolasolutions.com/content/dam/msi/docs/mototrbo/r7/r7-datasheet-na.pdf), which publishes 5 W VHF / 4 W UHF and 0.14 uV digital sensitivity. The -124 dBm figure is arithmetic on that sensitivity into 50 ohm, not a manufacturer statement.
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