1. Basic principles of electromagnetic wave generation According to Maxwell's electromagnetic field theory, a changing electric field will produce a changing magnetic field in the space around it, and a changing magnetic field will produce a changing electric field. In this way, the changing electric field and the changing magnetic field are interdependent, mutually exciting, alternately generated, and propagated from near to far in space at a certain speed. The periodically changing magnetic field excites the periodically changing electric field, and the periodically changing electric field excites the periodically changing magnetic field. Electromagnetic waves are different from mechanical waves. Their propagation does not need to rely on any elastic medium. They only propagate by the mechanism of "changing electric field produces changing magnetic field, and changing magnetic field produces changing electric field". When the frequency of electromagnetic waves is low, they can only be transmitted through tangible conductors; when the frequency gradually increases, the electromagnetic waves will overflow outside the conductor and can transmit energy to the outside without a medium. This is a kind of radiation. In low-frequency electrical oscillations, the mutual change between magnetism and electricity is relatively slow, and almost all of its energy is reflected back to the original circuit without any energy radiated out. However, in high-frequency electrical oscillations, the mutual change between magnetism and electricity is very fast, and it is impossible for energy to be reflected back to the original oscillation circuit. Therefore, electric energy and magnetic energy are propagated into space in the form of electromagnetic waves as the electric field and magnetic field change periodically. According to the above theory, every section of wire through which high-frequency current flows will have electromagnetic radiation. Some wires are used for transmission, so it is not desirable to have too much electromagnetic radiation loss energy; some wires are used as antennas, so it is hoped that energy can be converted into electromagnetic waves as much as possible and emitted. So there are transmission lines and antennas. Whether it is an antenna or a transmission line, it is the application of electromagnetic wave theory or Maxwell's equations in different situations. For a transmission line, the structure of the wire should be able to transmit electromagnetic energy without radiating outward; for an antenna, the structure of the wire should be able to transmit electromagnetic energy as much as possible. Wires of different shapes and sizes have different efficiencies when transmitting and receiving radio signals of a certain frequency. Therefore, to achieve ideal communication effects, appropriate antennas must be used! The study of what kind of wire structure can achieve efficient transmission and reception has formed the science of antennas. When high-frequency electromagnetic waves propagate in the air, if they encounter a conductor, they will be inductively acted upon, generating high-frequency currents in the conductor, allowing us to use wires to receive radio signals from far away. 2. Antennas In wireless communication systems, it is necessary to convert the waveguide energy from the transmitter into radio waves, or convert radio waves into waveguide energy. The device used to radiate and receive radio waves is called an antenna. The modulated high-frequency current energy (or waveguide energy) generated by the transmitter is transmitted to the transmitting antenna via the feeder, and is converted into a certain polarized electromagnetic wave energy through the antenna and sent out in the desired direction. After arriving at the receiving point, the receiving antenna converts the electromagnetic wave energy of a certain polarization from a specific direction in space into modulated high-frequency current energy, which is then transmitted to the input end of the receiver via the feeder. In summary, the antenna should have the following functions: 1. The antenna should be able to convert as much waveguide energy as possible into electromagnetic wave energy. This first requires that the antenna is a good electromagnetic open system, and secondly that the antenna is matched with the transmitter or receiver. 2. The antenna should make the electromagnetic waves as concentrated as possible in a certain direction, or accept the waves coming from a certain direction to the maximum extent, that is, the direction has directionality. 3. The antenna should be able to transmit or receive electromagnetic waves of specified polarization, that is, the antenna has appropriate polarization. 4. The antenna should have enough working frequency band. These four points are the most basic functions of the antenna, and several parameters can be defined based on them as the basis for designing and evaluating the antenna. The system that connects the antenna and the transmitter or receiver is called the feeder system. The form of the feeder is divided into wire transmission line, coaxial transmission line, waveguide or microstrip line according to the frequency. Therefore, the so-called feed line is actually a transmission line. Electrical parameters of antennas The basic functions of antennas are energy conversion and directional radiation. The so-called electrical parameters of antennas are quantities that can quantitatively characterize their energy conversion and directional radiation capabilities. 1. Directivity of antennas Measures the ability of antennas to radiate energy in the desired direction. Main lobe width: The main lobe width is a physical quantity that measures the extent of the maximum radiation area of the antenna. The wider the better. Side lobe level: The side lobe level refers to the level of the first side lobe that is closest to the main lobe and has the highest level. In fact, the side lobe area is the area that does not need to radiate, so the lower its level, the better. (The main lobe and side lobes of the antenna radiation are similar to the spectrum of a square wave signal) Front-to-back ratio: The front-to-back ratio refers to the ratio of the level in the maximum radiation direction (forward) to the level in the opposite direction (backward). The larger the front-to-back ratio, the smaller the antenna's backward radiation (or reception). The calculation of the front-to-back ratio F / B is very simple--- F / B = 10 Lg {(forward power density) / (backward power density)} Directivity: At a certain distance from the antenna, the ratio of the radiation power flux density of the antenna in the maximum radiation direction to the radiation power flux density of an ideal non-directional antenna with the same radiation power at the same distance. This is the most important indicator of directivity, which can accurately compare the directivities of different antennas and represents the electrical parameters of the antenna's beaming energy. 2. Antenna efficiency Antenna efficiency is defined as the ratio of the antenna's radiated power to the input power. The radiation resistance R of an antenna is often used to test the ability of an antenna to radiate power. The radiation resistance of an antenna is a virtual quantity, which is defined as follows: suppose there is a resistor R. When the current passing through it is equal to the maximum current on the antenna, the power lost is equal to its radiation power. Obviously, the level of radiation resistance is an important indicator for measuring the radiation ability of an antenna, that is, the larger the radiation resistance, the stronger the radiation ability of the antenna. 3. Gain coefficient The gain coefficient is a parameter that comprehensively measures the energy conversion and directional characteristics of an antenna. It is defined as: the product of the directional coefficient and the antenna efficiency, recorded as: D is the directional coefficient, and is the antenna efficiency. It can be seen that the higher the sum of the antenna directional coefficients, the higher the gain coefficient. Physical meaning: The gain coefficient of an antenna describes the multiple by which the antenna amplifies the output power in the maximum radiation direction compared to an ideal non-directional antenna. It can also be understood in a popular way as the ratio of a signal of a certain size generated by a directional antenna to an ideal omnidirectional antenna (whose radiation is equal in all directions) at a certain point at a certain distance. For example: If an ideal non-directional point source is used as a transmitting antenna, an input power of 100W is required, while when a directional antenna with a gain of G = 13 dB = 20 is used as a transmitting antenna, the input power only needs 100 / 20 = 5W. In other words, the gain of an antenna, in terms of its radiation effect in the maximum radiation direction, is the multiple by which the input power is amplified compared to an ideal non-directional point source. 4.Polarization direction Polarization characteristics refer to the law of how the direction of the electric field vector of the antenna in the direction of maximum radiation changes over time. The polarization direction is the direction of the electric field of the antenna. The polarization modes of the antenna include linear polarization (horizontal polarization and vertical polarization) and circular polarization (left-hand polarization and right-hand polarization). How to understand linear polarization? First, imagine the classic electromagnetic wave propagation diagram. The electric field propagates as a sine wave in a plane, and the magnetic field also propagates as a sine wave in the orthogonal plane of the electric field. When we look at the electric field from the starting point along the propagation direction, what we see is a short line. This polarization is linear polarization. So how do we determine the direction of linear polarization? When a high-frequency current passes through the antenna, a high-frequency voltage will be generated on the antenna, forming a high-frequency electric field. The direction of this electric field is generally consistent with the direction of the antenna, that is, the polarization direction of linear polarization is consistent with the direction of the antenna. If the antenna is a wire erected in the horizontal direction, the electric field generated is also in the horizontal direction, and it is called a "horizontally polarized" antenna; if the antenna is a wire erected perpendicular to the ground, the electric field generated is also in the vertical direction, and it is called a "vertically polarized" antenna. (Usually, the antenna with a straight wire structure is linearly polarized) How to understand circular polarization? It is still the classic electromagnetic wave propagation diagram, but the electric field size remains unchanged at this time, but the direction rotates around the x-axis unchanged, but the projection on any plane is a sine wave, which is a bit similar to our signal processing in which the amplitude remains unchanged but the phase is constantly changing. At this time, looking at the electric field from the origin to the propagation direction, what you see is a circle, and this polarization is circular polarization. Of course, rotating to the left is left-hand polarization, and rotating to the right is right-hand polarization. (Usually, the antenna with a spiral structure is circularly polarized) Only when the polarization direction of the receiving antenna is consistent with the polarization direction of the received electromagnetic wave can the largest signal be sensed. Based on this principle, we can infer the following conclusions. For linear polarization, when the polarization direction of the receiving antenna is consistent with the linear polarization direction (electric field direction), the induced signal is the largest (the projection of the electromagnetic wave in the polarization direction is the largest); as the polarization direction of the receiving antenna deviates more and more from the linear polarization direction, the induced signal becomes smaller (the projection continues to decrease); when the polarization direction of the receiving antenna is orthogonal to the linear polarization direction (magnetic field direction), the induced signal is zero (the projection is zero). The linear polarization method has high requirements on the direction of the antenna. Of course, under actual conditions, the reflection and refraction encountered during the propagation of electromagnetic waves will cause the polarization direction to deflect. Sometimes a signal can be received by both horizontal and vertical antennas, but in any case, the polarization direction of the antenna is often an important issue that needs to be considered. For circular polarization, no matter what the polarization direction of the receiving antenna is, the induced signal is the same, and there will be no difference (the projection of the electromagnetic wave in any direction is the same). Therefore, the use of circular polarization reduces the system's sensitivity to the antenna's orientation (the orientation here is the antenna's orientation, which is different from the orientation of the directional system mentioned above). Therefore, circular polarization is used in most occasions. To use a vivid metaphor, linear polarization is like a snake crawling on the ground, and circular polarization is like a snake winding around a stick. Another metaphor is that if you take a rope and swing it up and down, the wave transmitted by the rope is in the form of linear polarization; if you keep drawing circles, the wave transmitted is circularly polarized. 5. Bandwidth The electrical parameters of the antenna are all related to the frequency, that is, the above electrical parameters are designed for a certain operating frequency. When the operating frequency deviates from the design frequency, it often causes changes in the antenna parameters. When the operating frequency changes, the relevant electrical parameters of the antenna should not exceed the specified range. This frequency range is called the bandwidth, or simply the bandwidth of the antenna. 6. Input impedance For the transmitter, the antenna is a load. How to make the antenna absorb the most energy requires solving a matching problem. Only when the impedance of the antenna itself is equal to the impedance of the transmitter can the maximum transmission power be obtained! For high-frequency signals, the antenna is a very long wire. The time it takes for the high-frequency signal to flow from the feed point to the antenna end and reflect back from the end is enough to cause a large difference in the amplitude and phase of the voltage and current of each part of the antenna, resulting in different impedances due to different lengths, structures and feed point positions of the antenna. For example, a center-fed dipole oscillator, when the length of each arm is one-quarter of a wavelength, presents a pure resistance of about 50 to 75 ohms, which is easy to match directly with the feed cable and transmitter. When conditions limit the length of the antenna and it is impossible to trim it to an appropriate value, inductors, capacitors and other reactive elements should generally be added to the antenna circuit to offset the reactance presented by the antenna itself. Sometimes, an impedance transformer is also required to transform the antenna impedance to the required value of the transmitting circuit. The equipment composed of these additional components is called an "antenna tuner" or "antenna matcher". 7. Effective length Effective length is another important indicator to measure the radiation capacity of an antenna. The effective length of an antenna is defined as follows: the equivalent length of the antenna when the current distribution on the antenna is uniform, while keeping the field strength value in the maximum radiation direction of the actual antenna unchanged. The longer the effective length, the stronger the radiation capacity of the antenna. There is an example in the book to strengthen the perceptual understanding: the field strength in the maximum radiation direction of a short vibrator with a length of 2h and uneven current distribution is equal to the field strength in the maximum radiation direction of a vibrator with a length of h and uniform current distribution. In other words, the effective length of the short vibrator is h. Receiving antenna theory When high-frequency electromagnetic waves propagate in the air, if they encounter a conductor, they will be inductively generated, generating high-frequency current in the conductor, so that we can use wires to receive radio signals from far away. The wire used to receive electromagnetic waves is generally called a "receiving antenna." 1. Effective receiving area The effective receiving area is an important indicator to measure the ability of an antenna to receive radio waves. It is defined as: when the antenna is aligned with the direction of the incoming wave with the maximum receiving direction, the average power transmitted by the receiving antenna to the matching load is PLmax, and assuming that this power is intercepted by an area perpendicular to the direction of the incoming wave, this area is called the effective receiving area of the receiving antenna. The larger the effective receiving area, the stronger the antenna's ability to receive radio waves. 2. Equivalent noise temperature The equivalent noise temperature of the receiving antenna is an important electrical parameter that reflects the performance of the antenna in receiving weak signals. The process of the receiving antenna sending the noise power received from the surrounding space to the receiver is similar to the process of the noise resistor sending the noise power to the resistor network connected to it. Therefore, the receiving antenna is equivalent to a resistor with a temperature of Ta. The higher Ta is, the greater the noise sent by the antenna to the receiver, and vice versa. 3. Transmission Line Transmission line is a general term for various forms of transmission systems used to transmit microwave information and energy. Its function is to guide electromagnetic waves to transmit in a certain direction, so it is also called a waveguide system. The electromagnetic waves it guides are called guided waves. Transmission line is also a conductor, but unlike antennas, it does not want electromagnetic waves to radiate when propagating here. Therefore, the structure of the transmission line made of metal is to radiate energy as little as possible. Taking the most common coaxial cable as an example, there is a wire in the middle and a circle of circular wire on the outside. The electromagnetic waves propagate in such a space and do not radiate out. Effective length Effective length is another important indicator to measure the radiation capacity of an antenna. The effective length of an antenna is defined as follows: the equivalent length of the antenna when the current distribution on the antenna is uniform, while keeping the field strength value in the maximum radiation direction of the actual antenna unchanged. The longer the effective length, the stronger the radiation capacity of the antenna. There is an example in the book to strengthen the perceptual understanding: the field strength of a short vibrator with a length of 2h and uneven current distribution in the maximum radiation direction is equal to the field strength of a vibrator with a length of h and uniform current distribution in the maximum radiation direction. In other words, the effective length of the short vibrator is h. Receiving antenna theory When high-frequency electromagnetic waves propagate in the air, if they encounter a conductor, they will be inductively generated, generating high-frequency current in the conductor, so that we can use wires to receive radio signals from far away. The wire used to receive electromagnetic waves is generally called a "receiving antenna." 1. Effective receiving area The effective receiving area is an important indicator to measure the ability of an antenna to receive radio waves. It is defined as: when the antenna is aligned with the direction of the incoming wave with the maximum receiving direction, the average power transmitted by the receiving antenna to the matching load is PLmax, and assuming that this power is intercepted by an area perpendicular to the direction of the incoming wave, this area is called the effective receiving area of the receiving antenna. The larger the effective receiving area, the stronger the antenna's ability to receive radio waves. 2. Equivalent noise temperature The equivalent noise temperature of the receiving antenna is an important electrical parameter that reflects the performance of the antenna in receiving weak signals. The process of the receiving antenna sending the noise power received from the surrounding space to the receiver is similar to the process of the noise resistor sending the noise power to the resistor network connected to it. Therefore, the receiving antenna is equivalent to a resistor with a temperature of Ta. The higher Ta is, the greater the noise sent by the antenna to the receiver, and vice versa. 3. Transmission Line Transmission line is a general term for various forms of transmission systems used to transmit microwave information and energy. Its function is to guide electromagnetic waves to transmit in a certain direction, so it is also called a waveguide system. The electromagnetic waves it guides are called guided waves. Transmission line is also a conductor, but unlike antennas, it does not want electromagnetic waves to radiate when propagating here. Therefore, the structure of the transmission line made of metal is to radiate energy as little as possible. Taking the most common coaxial cable as an example, there is a wire in the middle and a circle of circular wire on the outside. The electromagnetic waves propagate in such a space and do not radiate out.
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