2023-12-11
Now it is the era of the Internet of Things, as one of the key technologies, RFID technology is promoting the development of the entire Internet of things, especially in the fields of intelligent transportation, logistics warehousing, intelligent libraries, unmanned retail, etc., and has achieved relatively good achievements. And the antenna is an indispensable part of RFID, so what is the RFID antenna? Today, from the antenna principle, directivity, gain and other aspects of a comprehensive understanding.
What is an antenna?
An antenna is a converter that converts a traveling wave traveling on a transmission line into an electromagnetic wave traveling in an unbounded medium (usually free space), or vice versa. A part of a radio device used to transmit or receive electromagnetic waves.
Principle of antenna radiation electromagnetic wave
When the wire carries alternating current, it will radiate electromagnetic waves, and its radiating ability is related to the length and shape of the wire. If the distance between the two wires is very close, the electric field is bound between the two wires, so the radiation is very weak; By spreading the two wires apart, the electric field spreads into the surrounding space, thus increasing the radiation. When the length of the wire is much less than the wavelength of the radiated electromagnetic wave, the radiation is very weak; When the length of the wire can be compared with the wavelength of the electromagnetic wave of the radiation, the current on the wire is greatly increased, forming a strong radiation. The above straight wire that can produce significant radiation is usually called the oscillator, and the oscillator is a simple antenna.
The longer the wavelength of the electromagnetic wave, the larger the antenna size. The greater the power to be radiated, the larger the antenna size;
Directivity of the antenna
Antenna radiation electromagnetic wave is directional, at the transmitting end of the antenna, directivity refers to the ability of the antenna to radiate electromagnetic waves in a certain direction, for the receiving end, it means the ability of the antenna to receive electromagnetic waves from different directions.
The antenna pattern is a function pattern between the antenna radiation characteristics and space coordinates, so the analysis of the antenna pattern can analyze the radiation characteristics of the antenna, that is, the ability of the antenna to transmit (or receive) electromagnetic waves in all directions of space. Usually used in the vertical plane and horizontal plane to represent the radiation (or received) electromagnetic wave power of the curve is the antenna directivity.
Antenna pattern
By making corresponding changes to the internal structure of the antenna, the directivity of the antenna can be changed, and different types and characteristics of the antenna can be formed.
Several symmetrical arrays of oscillators can control the radiation and further concentrate the signal in the horizontal direction, thus producing a pattern similar to the "flat doughnut".
Planar reflectors placed on one side of the array can form a sector covering the antenna. The horizontal diagram below illustrates the role of the reflector: the reflector reflects power in a unilateral direction, increasing the gain.
Lobe width of the antenna
lobe
The pattern usually has two or more lobes, the lobe with the greatest radiation intensity is called the main lobe, and the rest is called the secondary lobe or sidelobe.
Antenna lobe
On both sides of the maximum radiation direction of the main lobe, the Angle between two points where the radiation intensity is reduced by 3dB (the power density is reduced by half) is defined as the lobe width (also known as the beam width or main lobe width or half power Angle).
The narrower the width of the lobe, the better the directivity, the farther the operating distance, the stronger the anti-interference ability. There is also a lobe width, that is, a 10dB lobe width, which, as the name suggests, is the Angle between two points in the pattern where the radiation intensity is reduced by 10dB (the power density is reduced to one-tenth).
Antenna gain
Antenna gain describes quantitatively the degree to which an antenna concentrates the input power. From the direction diagram, the narrower the main lobe and the smaller the secondary lobe, the higher the gain. In engineering, antenna gain is used to measure the ability of the antenna to send and receive signals in a specific direction. Increasing the gain can increase the coverage area of the network in a certain direction, or increase the gain margin within a certain range. Under the same conditions, the higher the gain, the farther the wave travels.
The gain is the ratio of the power density of the signal generated by the actual antenna and the ideal radiation unit at the same point in space under the condition of equal input power. It quantitatively describes the degree to which an antenna concentrates the input power. The gain is obviously closely related to the antenna pattern. The narrower the main lobe and the smaller the sublobe, the higher the gain.
The physical meaning of gain can be understood in this way: in order to produce a signal of a certain size at a certain point over a certain distance, if an ideal non-directional point source is used as the transmitting antenna, the input power is required to be 100/20=5W, while a directional antenna with a gain of G = 13 dB = 20 is used as the transmitting antenna. In other words, the gain of an antenna, in terms of its radiation effect in the maximum radiation direction, is a multiple of the input power amplification compared with an ideal non-directional point source. The gain of the half-wave symmetric oscillator is G = 2.15 dBi. The four half-wave symmetrical oscillators are arranged up and down the vertical line to form a vertical quad array with a gain of about G=8.15dBi(the unit dBi indicates that the comparison object is an ideal point source of uniform radiation in all directions).
If the half-wave symmetric oscillator is used as the comparison object, the unit of gain is dBd. The gain of the half-wave symmetric oscillator is G=0dBd (because it is compared with itself, the ratio is 1, and the logarithm is zero). ; The gain of a vertical quaternary matrix is about G=8.15-2.15=6dBd.
The gain of an antenna compared to a symmetric oscillator is expressed by dBd, and the gain of an antenna compared to an isotropic radiator is expressed by dBi
Upper sidelobe suppression
For wireless coverage antennas, which serve wireless users on the ground, the radiation pointed at the sky is meaningless. Therefore, people often require the first side lobe above the main lobe of the vertical plane to be as weak as possible, which is the so-called upper side lobe suppression.
In the direction diagram, the ratio of the maximum value of the front and back lobes is called the front and back ratio.
Polarization of antenna
The polarization of an antenna is determined by the polarization of an electromagnetic wave. The polarization direction of an electromagnetic wave is usually described by the spatial direction of its electric field vector, that is, the trajectory depicted by the change of the orientation of its electric field vector in space over time when viewed along the propagation direction of an electromagnetic wave at a certain position in space. If this trajectory is a straight line, it is called linear polarization, if it is a circle, it is circular polarization, and if it is an ellipse, it is elliptical polarization. Generally speaking, the direction of antenna polarization is the direction of electric field.
The combination of vertically polarized and horizontally polarized antennas, or the combination of +45° polarized and -45° polarized antennas, constitutes a new antenna - dual-polarized antenna. The dual-polarized antenna has two connectors.
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