RFID Link Budget Calculation
Engineering Read Range from First Principles
Engineering guide to calculating RFID read range from transmit power, cable loss, antenna gain, path loss, and tag sensitivity.
RFID Link Budget Calculation
An RFID link budget is a systematic accounting of every gain and loss in the signal path between reader and tag. It answers the question: will a reliable RF link exist at the desired distance? Getting the link budget right before installation saves expensive field troubleshooting.
Forward and Reverse Links
An RFID system has two distinct signal paths:
- Forward link (reader → tag): reader transmits; tag harvests energy and demodulates commands. The limiting factor is whether the tag receives enough power to activate and respond.
- Reverse link (tag → reader): tag backscatters a modulated signal; reader demodulates it. The limiting factor is whether the reader's receiver can detect the weak backscattered signal.
In passive UHF, the forward link typically dominates — tags are power-limited long before the reader's receiver sensitivity becomes the bottleneck.
EIRP and Transmitted Power
EIRP (Effective Isotropic Radiated Power) is the product of transmitter power and antenna gain, normalised to an isotropic reference antenna. Regulators specify maximum EIRP rather than transmitter power because the combination of transmitter power and antenna gain determines actual field strength.
| Region | Max EIRP (UHF RFID) | Frequency Band |
|---|---|---|
| US (FCC Part 15) | 4 W (36 dBm) | 902–928 MHz |
| Europe (etsi-302-208-term/" class="glossary-term-link" data-term="ETSI EN 302 208" data-definition="European UHF RFID radio standard." data-category="Standards & Protocols">ETSI EN 302 208) | 2 W (33 dBm ERP ≈ 33.15 dBm EIRP) | 865.6–867.6 MHz |
| Japan | 250 mW (24 dBm) | 920–928 MHz |
| China | 2 W (33 dBm) | 840–845 / 920–925 MHz |
EIRP (dBm) = Transmitter output power (dBm) + Antenna gain (dBi) − Cable/connector losses (dB)
A typical portal reader at 1 W output driving a 6 dBi antenna through 1 dB of cable loss produces EIRP = 30 + 6 − 1 = 35 dBm — within FCC limits.
Path Loss
Free-space path loss (FSPL) describes signal attenuation with distance due to spherical spreading of the wavefront:
FSPL (dB) = 20·log₁₀(d) + 20·log₁₀(f) + 20·log₁₀(4π/c)
At 915 MHz, FSPL at 3 m ≈ 51 dB. At 6 m ≈ 57 dB. Real deployments add multipath, absorption by liquids/metals, and near-field effects.
Tag Sensitivity
Tag sensitivity is the minimum power the tag IC needs to wake up and respond. It is specified as a received power level at the tag antenna terminals.
| IC Generation | Typical Sensitivity | Notes |
|---|---|---|
| Gen 2 v1 (older) | −17 to −15 dBm | e.g., Alien Higgs 3 |
| Gen 2 v1 (modern) | −20 to −18 dBm | e.g., Impinj Monza R6 |
| Gen 2 v2 (crypto) | −18 to −16 dBm | Crypto logic adds power draw |
Tag sensitivity must be combined with tag antenna gain: received power = EIRP − FSPL + tag antenna gain − polarisation mismatch loss.
Read Range Estimate
Using the Friis transmission equation, maximum theoretical read range for a passive tag:
Read range (m) = (λ / 4π) × √(EIRP × G_tag / P_sensitivity)
Where λ = wavelength (0.328 m at 915 MHz), G_tag = tag antenna gain (linear), P_sensitivity = minimum activation power (linear).
A tag with −18 dBm sensitivity, 2 dBi antenna gain, driven by 36 dBm EIRP gives theoretical read range ≈ 9 m. Real-world range is typically 60–80 % of theoretical due to multipath and detuning.
Practical Link Budget Table
| Parameter | Value | Notes |
|---|---|---|
| Reader transmit power | +30 dBm | 1 W |
| Reader antenna gain | +6 dBi | Circular polarised |
| Cable/connector loss | −1 dB | 3 m LMR-200 |
| EIRP | 35 dBm | |
| FSPL at 3 m (915 MHz) | −51 dB | |
| Tag antenna gain | +2 dBi | Dipole-based inlay |
| Polarisation mismatch | −3 dB | Circular→linear worst case |
| Received power at tag | −17 dBm | |
| Tag sensitivity | −18 dBm | Monza R6 |
| Forward link margin | +1 dB | Marginal — consider higher gain antenna |
Use the Link Budget Calculator and Read Range Calculator to model your specific deployment parameters.
See also: RFID Antenna Selection Guide, Dense Reader Mode Optimisation, RFID on Metal and Challenging Materials.
よくある質問
Our guides cover a range of experience levels. Getting Started guides introduce RFID fundamentals. Implementation guides help engineers design RFID solutions for specific industries. Advanced guides cover topics like dense reader mode, anti-collision algorithms, and EPC encoding schemes.
Most getting-started guides require only a basic UHF RFID reader (such as the Impinj Speedway or ThingMagic M6e) and a few sample tags. Some guides reference desktop USB readers for development. All hardware requirements are listed at the beginning of each guide.