Sick: RFID Application Notes—A Brief Discussion on Common Units and Antennas

Common units

dB vs. dBm

First of all, it is necessary to know that dB is a decibel and is defined as 10lg (output power/input power), that is, take the logarithmic value of two powers, for example, if an amplifier amplifies the signal power by 1000 times, it is converted to dB, then the gain is 30dB.

Let's take a few examples:

+6dB is 4x magnification (log4 is about 0.6)

+10dB is 10x magnification

-6dB is reduced to 0.25 (i.e. 1/4)

-10dB is reduced to 0.1 (i.e., 1/10)

For example, 0dB means that it does not get bigger and smaller.

We can often simply use the word Gain, which means gain, in professional materials, which is usually measured in dB.

dBm vs. dBW

Looking back at the unit of dB, it is easy to express when the difference between two signals is hundreds of times or even hundreds of millions of times, considering that power is usually expressed in watts and watts, then dBm is generally used as the unit of power in RF applications.

dBm Definition:

　　0dBm=1mW;

　　0dBW=1W;

therefore

Sick: RFID Application Notes—A Brief Discussion on Common Units and Antennas

　　Since 1W = 1000mW, we can conclude that:

+30dBm = 0dBW (i.e. power 1W)

-30dBW = 0dBm (i.e. power 1mW)

In particular, dBm is commonly used in RFID, which we can commonly convert to:

　　1mW=0dBm;10mW=10dBm;

　　2mW=3dBm;0.1mW=-10dBm。

If you are interested, you can deduce the power corresponding to -40dBm to 40dBm, and you can tell two answers first: 40dBm corresponds to 10W, and 10dBm corresponds to 10mW.

Q: Why do you see dBi, dBd, and even dBic in the antenna, what do these mean?

A: These are all important reference values for antennas, let's understand them step by step.

About antennas

The first thing to know is that antennas are passive devices that do not produce energy on their own, and the concept of gain here refers to the ability to efficiently concentrate energy in some specific direction to radiate or receive electromagnetic waves. The antenna gain is generated by the superposition of oscillators, and the length of the antenna increases with the increase of gain, and the higher the gain, the better the directionality, that is, the more concentrated the energy and the more convergent the lobe.

The radiation range of the antenna usually contains two or more "lobes", which can be understood as the coverage of radio frequency, the main lobe with the largest intensity, and the rest are side lobes, as shown in the following diagram:

　　We can often see the antenna radiation pattern, the ratio of the maximum intensity in this graph to the intensity of the reference antenna is logarithmic, which refers to the directional gain, and the unit of gain is dBi, which is understood here as db isotropic, isotropic is omnidirectional, and dB is the ratio, that is, the gain of the omnidirectional ratio. For example, a dipole antenna (half-wave oscillator) has a gain of 2.15 dBi.

Since a perfect omnidirectional reference antenna cannot be manufactured, and the theoretical half-wave oscillator antenna is similar to the actual dipole antenna gain, the dipole antenna is also commonly used as a reference antenna, in this case the antenna gain is measured in dBd, understood as dB dipole, and dipole is a dipole.

The following diagram is a schematic diagram of the radiation of an ideal isolated wave source, the theoretical half-wave oscillator radiation and the gain of a directional antenna:

　　We can see in the Sick antenna that some units are dBiC, and this unit is usually used for circularly polarized antennas, which can be summarized as follows:

The gain of the linearly polarized antenna is dBi, which is defined as the gain relative to the dot circle antenna;

The gain of the circularly polarized antenna is dBiC, which is converted to dBiC = dBi + 3dB;

The dipole antenna gain is dBd, which is converted to dBd = dBi - 2.15.

Q: What do the "3dB aperture" and "opening angle" mean in the parameter table?

　　As we mentioned earlier, the antenna gain is passive, the antenna does not add additional energy, but only redistributes it so that more energy is radiated in a certain direction than the omnidirectional antenna, which is not the same as the gain of RF. If the antenna is positive in some directions, it must be negative in other directions, so that the gain that the antenna can achieve is balanced with the range it can cover, which is also in line with the fundamental laws of physics.

In general, UHF RFID uses 3dB (i.e., half the power density) lobe width to define the opening angle, or aperture, sometimes called the opening angle or half-power angle, or beamwidth, etc. The narrower the lobe width, the better the directionality, the farther the distance, but the less horizontal coverage.

Source: Sick

Sick: RFID Application Notes—A Brief Discussion on Common Units and Antennas

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