Electromagnetic Radiation and Protection in RFID Applications

In the application of radio frequency identification technology ( RFID ), a common concern is whether the electromagnetic radiation associated with it will cause some degree of harm to the human body? The same problem also occurs when using small radio stations, wireless walkie-talkies, radio and television transmitters, mobile phones, wireless communication base stations, household microwave ovens, televisions, radios, MP3/MP4 players, computers, ultraviolet disinfection and other equipment.

Whether we like it or not, the entire universe is filled with electromagnetic radiation of various frequencies. Broadly speaking, any substance with a temperature higher than absolute zero (-273.15°C) will generate thermal radiation due to the movement of its internal molecules, atoms, and electrons. Thermal radiation is also electromagnetic radiation in nature. Therefore, each of us is also an electromagnetic radiator.

Artificial electromagnetic radiation phenomena constitute a new landscape of the electromagnetic environment for human survival. The coverage of radio, television and mobile communication networks has become a typical representative of the diffuse artificial electromagnetic phenomena in the air of densely populated areas. Although these electromagnetic radiations are invisible to the human eye and the human body is not sensitive to them, radios, televisions and mobile communication phones can receive and confirm the existence of these electromagnetic radiations that are almost everywhere in the space.

To consider the impact of electromagnetic radiation on the human body, we need to analyze which electromagnetic radiation (or what conditions electromagnetic radiation meets) will have adverse effects on the human body, and which electromagnetic radiation people can safely withstand without fear or worry.

Electromagnetic spectrum

The electromagnetic spectrum takes the instantaneous frequency of electromagnetic radiation (energy, signal) as a variable, and incorporates all electromagnetic radiation into a one-dimensional number axis. Through appropriate annotation, a graph can be formed, which is called an electromagnetic spectrum graph, as shown in Figure 1. There are two common forms of the number axis representation of the electromagnetic spectrum:

1. Linear scale: In this case, it is a ray axis with an origin. The origin of the ray axis corresponds to a direct current with a frequency of "0", and the direction of the ray axis points to the direction of increasing frequency. The concept of the electromagnetic spectrum diagram of the linear scale is very clear, but its ability to represent the frequency range is limited. As shown in Figure 1 (a).

2. Logarithmic scale: At this time, the origin of the number axis corresponds to the frequency of "1", the "negative direction" points to the direct current with a frequency of "0" (generally omitted), and the "positive direction" points to the direction of increasing frequency. The logarithmic scale has the effect of "compressing the large and enlarging the small", and the frequency range is very strong. As shown in Figure 1 (b).

Figure 1 Electromagnetic spectrum

With the help of electromagnetic spectrum diagram, people can easily grasp and analyze electromagnetic radiation (energy, signal) at the macro scale (reduction) and micro scale (magnification). As a result, various methods of representing electromagnetic spectrum diagrams have been formed. Figure 2 shows the frequency band representation of electromagnetic spectrum diagram.

Figure 2 Frequency band representation of electromagnetic spectrum diagram (Note: reference figure)

Parameters of electromagnetic radiation

To analyze the impact of electromagnetic radiation on human health, we need to understand some basic parameters. Parameters related to electromagnetic radiation can be divided into three categories: "electrical parameters", "radiation parameters" and "radiation impact parameters".

1. Electrical parameters. Basic parameters that reflect electromagnetic radiation, including:

Frequency (F): The frequency range for studying the effects of electromagnetic radiation on the human body is shown in Figure 3. Typically, the research focuses on two segments: radio frequency radiation (30kHz~300MHz) and microwave radiation (300MHz~300GHz);

Figure 3 Frequency range of research on the impact of electromagnetic radiation on the human body (Note: cited figure)

Power (P): The power indicates the strength of electromagnetic radiation. It can be divided into instantaneous power, average power, equivalent radiated power (the product of radiated power and antenna gain), and other related concepts.

Modulation mode: The modulation mode is the variation of the amplitude, frequency and phase of the radiated electromagnetic signal. For example, ASK, FSK, PSK, etc.

2. Radiation parameters. Indicates the spatial propagation and distribution characteristics of electromagnetic radiation, including:

Radiation field: It is divided into three categories: near field, mid-field and far field. The near field is within one wavelength of the electromagnetic radiator, the far field is outside one wavelength, and the mid-field is about one wavelength. The magnetic field plays a major role in the near field, and the electric field plays a major role in the far field.

Polarization: Indicates the direction of the electric field or magnetic field vector in the electromagnetic radiation space. It can be divided into linear polarization and elliptical polarization (including circular polarization). When the polarization of electromagnetic radiation transmission and reception matches, the maximum received signal can be obtained.

3. Radiation impact parameters. Characteristic parameters that indicate electromagnetic radiation and human health, including:

Exposure limit: It indicates the maximum intensity of electromagnetic radiation diffused in space that the human body can withstand, that is, power density. Common units are: μW/cm2, mW/cm2, W/m2. The relationship between power density and field strength is: S=E×H, where S is the power density of electromagnetic radiation transmitted in the air, E is the electric field strength (V/m), and H is the strong field strength (A/m). In a vacuum, E/H=377Ω.

Radiation dose: It indicates the continuous exposure to electromagnetic radiation and reflects the accumulation of electromagnetic radiation. The value is expressed as "power density × duration".

Specific absorption rate: refers to the electromagnetic radiation power absorbed by a biological body per unit mass, that is, the absorbed dose rate.

As shown in Figure 3, the effects of electromagnetic radiation on organisms (including humans) can be divided into two categories: ionizing effects and non-ionizing effects. Ionizing effects can cause electrons in organisms to break free from the structural constraints of atoms or molecules, causing serious damage to cells, including mutations such as cancer. Non-ionizing effects are generally manifested as thermal effects (standing waves in the body cause fever, causing damage to the body) and non-thermal effects (psychological effects, etc.).