Maximum Input Power

It is common to include the parameter Maximum Input Power on the datasheet for UHF RFID antennas. Comparing datasheets from different manufacturers shows that there is a great discrepancy in the antenna’s rating ranging from 3 Watts to 10 Watts.

In the following, we will explain what Maximum Input Power stands for and what it says about the antenna’s performance.

What is Maximum Input Power

The Maximum Input Power defines the maximum power which can be transmitted to one antenna port.

That parameter defines the antenna’s ability to withstand the power that is transmitted into one antenna port – usually through an UHF RFID reader - without damaging it by overvoltage. The figure is specified in Watts. By including this parameter into the antenna’s datasheet, the manufacturer ensures a guaranteed performance under the stated figure, though most antennas may be able to handle more.

Most Readers Have Restricted Maximum Output Power

Even though most antennas can handle more Watts, it is most unlikely that they will have to for two reasons:

Firstly, most UHF RFID readers available in the market have a maximum output (transmitter) power of 33 dBm. 33 dBm corresponds to 2 Watts power. So any antenna with a maximum input power of 3 Watts is more than sufficient.

Secondly, the power regulations for UHF RFID equipment is specified worldwide by GS1.

The document shows that most countries allow a maximum of 4W EIRP (e.g., USA, Argentina, New Zealand, etc.) or 2W ERP (e.g., the Netherlands, France, Greece, Germany, etc.). Some countries like India allow 4W ERP.

The following calculations show what it means for the power transmitted to the antenna:

Power is usually specified as ERP (Effective Radiated Power) or EIRP (Effective Isotropic Radiated Power).

The EIRP is calculated using the transmitter’s output power, cable losses and the antenna’s gain in dBi.

Equation to determine the EIRP:

30 dBm transmitter power - 0 dB cable loss + 6 dBi antenna = EIRP = 36 dBm, which is 4 Watts.

Equation to determine the ERP: EIRP - 2.15

Example: 36 dBm – 2.15 = 33.85 dBm, which is 2.42 Watts.

Conclusion: Transmitted Power Always Below 3 Watts

The above factors show that there will be no requirement for an UHF RFID reader antenna to be able to withstand transmitted power more than 2 Watts (33 dBm reader output power). It also clarifies, that this parameter is not an indicator for the antenna’s tag reading performance, but guarantees the ability to perform without damaging under the GS1 standards of transmitted power in each country.

If you would like to determine the output power for your UHF RFID application and make sure that the antenna matches your requirements, we recommend this handy calculator.