According to Maxwell's electromagnetic field theory, a changing electric field produces a changing magnetic field in its surrounding space, and a changing magnetic field produces a changing electric field. Thus, the changing electric field and the changing magnetic field are interdependent, mutually excited, alternately generated, and propagated in space from near and far at a certain speed. A periodically varying magnetic field excites a periodically varying electric field, and a periodically varying electric field excites a periodically varying magnetic field. Electromagnetic waves are different from mechanical waves. Their propagation does not depend on any elastic medium. It only propagates by the mechanism of "changing the electric field to produce a changing magnetic field and changing the magnetic field to produce a changing electric field". When the electromagnetic wave frequency is low, it can be transmitted mainly by the tangible electric conductor; when the frequency is gradually increased, the electromagnetic wave will overflow outside the conductor, and the medium can transmit energy without the medium. This is a type of radiation. In the low-frequency electrical oscillation, the mutual change between the magnetrons is relatively slow, and almost all of the energy is returned to the original circuit without energy radiation. However, in high-frequency electrical oscillations, the magnetoelectric interaction becomes very fast, and the energy cannot be reversed back to the original oscillating circuit, so that the electric energy and the magnetic energy propagate to the space in the form of electromagnetic waves as the electric field and the magnetic field change periodically. According to the above theory, each section of the wire that flows through the high-frequency current will have electromagnetic radiation. Some wires are used for transmission, and it is not desirable to have too much electromagnetic radiation to lose energy; some wires are used as antennas, and it is desirable to convert energy into electromagnetic waves as much as possible. Then 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 equation in different situations. For transmission lines, the structure of such a conductor should be capable of transmitting electromagnetic energy without radiating outward; for an antenna, the structure of such a conductor should be capable of transmitting electromagnetic energy as much as possible. Different shapes and sizes of wires have different efficiencies when transmitting and receiving radio signals of a certain frequency. Therefore, in order to achieve the desired communication effect, an appropriate antenna must be used! Studying what kind of structure of the wire can achieve efficient transmission and reception, it also forms the knowledge of the antenna. High-frequency electromagnetic waves propagate through the air. In the case of a conductor, induction occurs, and high-frequency current is generated in the conductor, allowing us to receive radio signals from a distant location. In a wireless communication system, it is necessary to convert guided wave energy from a transmitter into a radio wave or convert the radio wave into guided wave energy, and a device for radiating and receiving radio waves is called an antenna. The modulated high-frequency current energy (or guided wave energy) generated by the transmitter is transmitted to the transmitting antenna via the feeder, and is converted into a polarized electromagnetic wave energy through the antenna and goes out in a desired direction. After reaching the receiving point, the receiving antenna converts the electromagnetic energy of a certain polarization from a specific direction of space into the modulated high-frequency current energy, which is sent to the receiver input via the feeder. In summary, the antenna should have the following functions: 1. The antenna should be able to convert the guided wave energy into electromagnetic wave energy as much as possible. This first requires the antenna to be a good electromagnetic open system and secondly to match the antenna to the transmitter or receiver. 2. The antenna should be such that the electromagnetic waves are concentrated as much as possible in a certain direction, or the maximum acceptance of the incoming waves in a certain direction, that is, the direction is directional. 3. The antenna should be capable of transmitting or receiving electromagnetic waves of defined polarization, ie the antenna is properly polarized. 4. The antenna should have sufficient operating frequency band. These four points are the most basic functions of the antenna, and several parameters can be defined as the basis for designing and evaluating the antenna. A system that connects an antenna to a transmitter or receiver is called a feeder system. The form of the feeder is divided into a wire transmission line, a coaxial transmission line, a waveguide or a microstrip line, etc. depending on the frequency. Therefore, the so-called feeder is actually the transmission line. Antenna electrical parameters The basic function of an antenna is energy conversion and directional radiation. The so-called electrical parameters of an antenna are the quantities that can quantitatively characterize its energy conversion and directional radiation. Directionality of the antenna Measure the ability of an antenna to radiate energy in the desired direction. Main lobe width: The main lobe width is a physical quantity that measures the extent of the largest radiating area of ​​the antenna. The wider the better. Sidelobe level: The sidelobe level refers to the level of the first sidelobe closest to the main lobe and having the highest level. In fact, the sidelobe region is a region that does not require radiation, so the lower the level, the better. (The main lobe of the antenna radiates a spectrum similar to the square wave signal) Before and after ratio: The front-to-back ratio is the ratio of the maximum radiation direction (forward) level to its opposite (backward) level. The larger the front-to-back ratio, the smaller the backward radiation (or reception) of the antenna. The calculation of front and back ratio F / B is very simple --- F / B = 10 Lg {(forward power density) / (backward power density)} Direction coefficient: The ratio of the radiated power flow density of the antenna in the direction of maximum radiation to the radiated power flow density at the same distance from the ideal non-directional antenna of the same radiated power at a distance from the antenna. This is the most important indicator of directionality. It can accurately compare the directionality of different antennas and represent the electrical parameters of the antenna bundle energy. 2. Antenna efficiency Antenna efficiency is defined as the ratio of antenna radiated power to input power.
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First, the basic principle of electromagnetic wave generation