Antenna Placement & Optimization

Antenna placement is the #1 factor in RFID system performance — more important than tag sensitivity or reader power. A $5,000 reader with poorly placed antennas will underperform a $500 reader with well-placed ones. The goal is to create a well-defined read zone (the 3D space where tags are reliably read) while minimizing stray reads from outside the target area.

A real-world example: moving a dock door antenna from 2.5m height to 2.0m height and tilting it 15° downward improved read rates from 87% to 99.2% on a major logistics deployment. Small positioning changes create large performance differences because RF signal strength follows the inverse-square law — double the distance means ¼ the signal power.

Antenna polarization determines the orientation of the electromagnetic waves. This is one of the most important decisions in system design because it directly controls whether tags in various orientations will be readable.

The read zone is the 3D volume where tags can be reliably read. It's shaped like a cone or lobe extending from the antenna face, with dimensions determined by antenna gain, reader TX power, and tag sensitivity. A 9 dBic antenna at 30 dBm power with an NXP UCODE 9 tag (-22.1 dBm sensitivity) creates a read zone approximately 8–10 meters deep and 3–4 meters wide at the far end.

Near-field vs Far-field: UHF RFID antennas work in two regions. The near-field (within ~35cm at 920 MHz) uses magnetic coupling for very short, controlled reads — perfect for POS stations where you want to read only items on the counter. The far-field (beyond 35cm) uses electromagnetic propagation for most RFID applications. Near-field antennas are specifically designed with confined read zones for item-level encoding and point-of-sale.

Power guidelines: 33 dBm for maximum range (~10m, dock doors). 30 dBm for standard range (~6–8m, general use). 25 dBm for medium range (~3–5m, conveyor belts). 20 dBm for short range (~1–2m, point-of-sale). 15 dBm for near-field (~0.5m, shelf readers). Always start with lower power and increase until you achieve your target read rate — excess power causes stray reads.

33 dBm → ~10m   dock doors, max range
30 dBm → ~6-8m  general warehouse
25 dBm → ~3-5m  conveyor belts
20 dBm → ~1-2m  point-of-sale
15 dBm → ~0.5m  shelf / near-field

VSWR (Voltage Standing Wave Ratio) measures how efficiently power transfers from the reader to the antenna. A perfect match is 1:1 (all power radiated). Anything above 2:1 means significant power is reflected back to the reader, reducing performance and potentially damaging the PA amplifier over time. Most commercial RFID antennas achieve 1.2–1.5:1 VSWR across the operating band.

Common VSWR problems: Damaged or kinked RF cables (replace if VSWR exceeds 2:1). Wrong connector type (use RP-TNC or SMA as specified). Antenna mounted directly on metal surface without spacer (use 15mm+ standoffs). Water ingress in outdoor connectors (use weatherproof RP-TNC with boots). Cable length exceeding 10m without low-loss cable (use LMR-400 or equivalent for runs over 5m).

Always verify VSWR across your entire operating band (920–925 MHz for Vietnam). An antenna may show excellent 1.2:1 VSWR at 920 MHz but degrade to 2.5:1 at 925 MHz — which means poor performance on half your FHSS channels.

Most production deployments use multiple antennas per reader. Nextwaves readers support up to 32 antenna ports. Key considerations: Spacing — typically 1–2 meters apart for dock doors, with beam overlap of 15–20% for complete coverage. Mounting angle — 15–45° inward tilt for portal applications to focus the read zone on the doorway. Antenna sequencing — the reader switches between antennas automatically to prevent simultaneous transmission from overlapping zones.

Portal configuration example (dock door): Mount 4 antennas — 2 on each side of the door at 1.5m and 2.5m heights, tilted 30° inward. Use linear polarization aimed at pallet faces. Set reader to Session S2 with Q=6 for fast-moving forklifts. This gives 99%+ read rates on standard pallet loads of 48–100 tagged cases.

Conveyor tunnel example: Mount 4 circular polarized antennas in a square arrangement around the belt — top, bottom, left, right. Set Session S1 for single-pass reading. Power at 25 dBm to confine the read zone to the tunnel. This prevents reading tags on adjacent conveyors.

CONFIGURE_ANTENNA_ENABLE payload (4 bytes):

Ports 1-4:    0x0F 0x00 0x00 0x00  (0b00001111)
Ports 1,3:    0x05 0x00 0x00 0x00  (0b00000101)
Port 1 only:  0x01 0x00 0x00 0x00  (0b00000001)

Bit 0=ANT1  Bit 1=ANT2  ...  Bit 31=ANT32

Metal surfaces are the #1 interference source in warehouses. They reflect RF signals, creating dead zones and multipath interference. Solution: mount antennas on non-metallic surfaces or use 50mm+ standoffs from metal structures. Orient antennas so the main lobe doesn't hit metal walls or racking directly.

Water and liquids absorb UHF radio waves heavily. A case of water bottles between the antenna and tagged pallet can block reads entirely. Solution: position antennas so the RF path avoids liquid containers, or increase power by 3–6 dB to compensate for the absorption loss.

Other readers operating nearby can cause interference. Dense Reader Mode (DRM) and FHSS help, but additional measures include: configuring non-overlapping channel masks between adjacent readers, using directional antennas to limit spill-over, and implementing TDMA scheduling if your middleware supports it.

Keep antennas ≥1m from fluorescent lights (RF noise source) and ≥2m from Wi-Fi access points. While Wi-Fi operates at 2.4/5 GHz (different from UHF 920 MHz), poorly shielded equipment can generate broadband harmonics.