Reader Placement and Coverage: Designing the Read Zone Before Buying Antennas

Reader placement is usually decided by where the network drop already exists, then justified afterwards. That produces sites where one door reads at 99 % and the next at 88 %, with identical hardware.

Coverage is a three-dimensional problem: a read zone is a volume where the tag receives enough power and returns enough signal. Antenna pattern, height, tilt, polarization and multipath all shape it, and a floor plan shows none of them.

Why start from the process, not the floor plan?

The process defines the dwell time, the tag population, the orientation of the tags and the acceptable miss rate. Those four numbers decide the antenna count, the power and the geometry.

A dock door with pallets passing at walking speed has about 1.5 seconds of dwell. A conveyor at 2 m/s gives 0.5 seconds. A shelf reader has hours. Designing all three the same way is how sites end up over-equipped at the shelf and under-equipped at the conveyor.

• Define dwell time per zone before choosing hardware.
• Define the worst-case tag, not the average one.
• Define the miss rate the business can absorb, and design to it.

What does an antenna actually cover?

A fixed UHF antenna has a beam, typically 60–70 degrees wide in both planes for a panel, and a gain that falls off toward the edges. The usable read volume is smaller than the beam because the tag also has to backscatter enough power to be decoded.

Practical numbers are instructive. A 9 dBi panel at 2 W EIRP gives a reliable zone of roughly 4–6 metres in the main beam, tapering quickly outside it. A 6 dBi patch gives 2–3 metres. A circular antenna costs 3 dB and therefore about 30 % of that distance.

• Beam width: 60–70 degrees typical, 30 degrees for narrow panels.
• Usable zone: smaller than the beam, and roughly conical.
• Doubling distance requires four times power: design for several antennas, not one big one.

How do height and tilt change coverage?

Height and tilt are the two levers that shape the zone without changing the hardware. Height sets the vertical footprint; tilt aims the main beam at the tag plane.

For a dock door, side antennas at 1.5–2.2 metres, tilted slightly downward and inward, cover the pallet body. For a conveyor, a single antenna 0.6–1 metre above the belt, aimed straight down, gives the tightest zone. For overhead inventory, 3–4 metres with a slight tilt covers a rack face.

• Too high: the zone becomes a narrow cone that misses low cartons.
• Too low: the zone is dominated by near-field effects and reads unevenly.
• Tilt more than 30 degrees away from the tag plane usually wastes power.

How should multipath be used rather than feared?

In a warehouse, most received power arrives via reflections. Multipath creates nulls and peaks, which is why two antennas at the same height can produce different results.

The practical response is diversity: mount antennas at different heights and angles so that a null for one is not a null for the other. Add a second antenna rather than increasing power, because power does not fill a null.

• Two antennas at different heights beat one antenna at double power.
• Avoid mounting directly opposite a large metal surface.
• Expect 3–6 dB variation across a pallet face and design margin for it.

How do portals, conveyors and shelves differ?

Each has a different geometry and a different failure mode.

• Portal: antennas on both sides, 1.5–2.2 m high, opposing and slightly offset, circular polarization, power balanced so neither side dominates.
• Conveyor tunnel: antennas above and beside the belt, linear polarization, tuned to the known tag face, short dwell so every decibel counts.
• Shelf: one antenna per bay or per two bays, low power, tight zone, so the adjacent bay is not read.
• Handheld: no placement problem, but a workflow problem — the operator must sweep at a consistent speed and distance.

How do you verify coverage after installation?

Never sign off on a layout drawing. Measure it.

• Tag 20 items per zone at the intended position, then read five times.
• Rotate the pallet 90 degrees and repeat; orientation failure is the most common surprise.
• Walk the perimeter of the zone and record where reads stop.
• Re-measure after the first week of live operation, when real pallets replace test ones.

If the drop-off point is 2 metres short of the design, the problem is geometry or power, not the reader brand.

Bottom line

Design the zone from dwell time, tag population and acceptable miss rate, then choose antenna count, height and tilt. Use diversity instead of power, plan polarization deliberately, and verify with real pallets at real speed. Coverage is engineered, not inherited from the network cabling plan.