### RFID Signal Power

**RFID systems use the decibel (dB**) to describe antenna gain, cables losses and power output for all hardware specifications and regulations. There are different regulations for each frequency in every country that must be taken into consideration when designing, installing and maintaining an RFID system.

**The wrong power selection has serious legal and health implications!**

The Decibel is a ratio between two signal strength levels and is 1/10^{th} of a Bel. The Bel is named after Alexander Graham Bell (hence the letter B is capitalized). Bell devised these *logarithmic scale measurements* while inventing the telephone. You can learn more about this in our RFID Signal Power article.

**Bel = log (P2/P1)**

dB is also a logarithmic measurement and gives simple numbers for large-scale variations in signal strength. This is very useful as you can easily calculate the RFID system gain and losses by adding and subtracting whole numbers.

- dB=10*log (
**P2/P1)**

**The dB unit allows big variations in signal strengths/levels to be handled with simple math**.

Gain is positive and loss is negative.

- A 3dB gain/loss is equal to a 2 times increase or decrease in signal level. So if a cable has a 3dB loss, it loses 50% of its signal strength by the time it gets to the other end of the cable.
- A 10 dB gain/loss is equal to a 10 times increase or decrease in signal level. If a cable has a 10dB loss, then it loses 90% of its signal strength by the time it gets to the other end of the cable.
- A 20dB gain/loss is equal to a 100 times increase or decrease in signal level. So if a cable has a 20dB loss, then it will lose 99% of its signal strength by the time it gets to the other end of the cable.

The Radio Frequency **POWER LEVEL** is expressed in watts or in dBm. dBm refers to the power in decibel referenced to one milliwatt (1mW). The relation between dBm and watts is given by the formula:

**P(dBm) = 10 x Log (P/1mW)**

dBm | Watt |

0 | 0.001 |

10 | 0.01 |

20 | 0.1 |

30 | 1.0 |

40 | 10.0 |

The value 30 dBm = 1.0 Watt should be remembered for future use.

- P (dBi; references an isotropic radiator) = 1 - log (Pr/Pi), where Pi is power received from an isotropic antenna.
- P (dBd; references a dipole antenna) = 10 Log (Pr/Pd), where Pd is power received from a dipole antenna.

**Basic electromagnetic (EM) propagation modes are**:

- Free space
**line of sight transmission**– satellite communications

**Reflection**– most commonly used for land mobile communications

**Refraction**– bending the EM waves when it passes through media such as water, plastic, and other materials that have different refractive indexes.

**Diffraction**– bending the EM waves around an object.

**Attenuation is the reduction in amplitude and intensity of a signal**. As the EM wave goes farther, is reflected, refracted or diffracted the signal strength drops dramatically.

An Isotropic source radiates power equally in all directions – laboratory and theory only just to assist you in understanding theory versus practicality.

As the EM signal travels from the transmit antenna to the receiving antenna it loses strength.

While the previous information may seem somewhat complicated, rest assured that it will become clearer and clearer in the next few lessons.