Key Takeaways
- Forward link: reader to tag
- Reverse link: tag to reader
- Field tuning: from 4 m to 7 m
01Forward link: reader to tag
The forward link is the path from reader transmit power to tag rectification. The Friis equation gives free-space received power: Pr = Pt × Gt × Gr × (λ/4πd)². With 30 dBm transmit, 6 dBi antenna gain, 2 dBi tag antenna gain, 915 MHz, at 6 m: tag received power = 30 + 6 + 2 - 47.3 = -9.3 dBm. A typical chip sensitivity of -18 dBm leaves 8.7 dB of forward margin at 6 m, extending the theoretical limit to ~11 m.
But the theoretical 11 m is almost unreachable in practice. Three reasons: multipath fading can drop received power by 10-20 dB; polarisation mismatch (a circular-polarised antenna vs a linear-polarised tag loses 3 dB); human-body shadowing loses ~8-15 dB. Combined, 6 m of measured range is already an excellent design. Raising transmit power also yields diminishing returns — every 6 dB (4× power) only doubles range, and most countries cap UHF reader EIRP at 36 dBm (4 W).
- Forward: Pr = Pt + Gt + Gr - 20log(4πd/λ)
- At 6 m tag receives ~-9.3 dBm, 8.7 dB margin
- Multipath 10-20 dB, polarisation mismatch 3 dB, body shadow 8-15 dB
- EIRP cap is 36 dBm (4 W) in most countries
02Reverse link: tag to reader
The reverse link is the true bottleneck. The tag is not a transmitter — it backscatters the reader carrier. Reflection efficiency is set by modulation depth, typically -10 to -15 dB. Continuing the example: the tag receives -9.3 dBm; after backscatter the power returning to the reader antenna is about -9.3 - 12 - 47.3 + 6 = -62.6 dBm. The reader sensitivity of -85 dBm looks like 22 dB of margin, but backscatter signal bandwidth is narrow and the practical SNR is limited by reader LO phase noise and adjacent-channel interference.
- Reverse must add tag modulation loss -10 to -15 dB
- Reverse power at 6 m ~-62.6 dBm
- Phase noise and adjacent-channel interference limit practical SNR
- Reverse link is the real bottleneck, not transmit power
03Field tuning: from 4 m to 7 m
A real warehouse project: a fixed reader with a 6 dBi circular-polarised antenna transmitting 30 dBm had an initial read range of only 4 m. Diagnosis found tags attached to cartons with metal foil, plus antenna polarisation mismatch. Optimisation steps: 1) switch to an 8 dBi circular-polarised antenna (+2 dB gain); 2) use anti-metal tags (reflection efficiency +5 dB); 3) adjust antenna tilt to avoid ground reflection (multipath +4 dB). Combined, read range reached 7.2 m — an 80% improvement.
This case shows that simply raising transmit power is inefficient — a 3 dB lift extends range only 1.4× and hits regulatory ceilings. But optimising antenna gain, tag type and deployment position converts each dB of gain directly into range, with no regulatory limit. SpidersRFID's deployment team's experience: 80% of "read-range shortfall" issues are rooted not in reader power but in tag selection and antenna placement.
- Switch to 8 dBi antenna: +2 dB
- Use anti-metal tags: +5 dB
- Adjust tilt: +4 dB
- Read range 4 m → 7.2 m, 80% improvement
