1、 Basic principle of electromagnetic wave generation

According to Maxwell’s electromagnetic field theory, the changing electric field will produce a changing magnetic field in its surrounding space, and the changing magnetic field will produce a changing electric field. In this way, the changing electric field and the changing magnetic field depend on each other, excite each other, generate alternately, and spread out in space at a certain speed.

A periodically changing magnetic field excites a periodically changing electric field, and a periodically changing electric field excites a periodically changing magnetic field.

Electromagnetic wave is different from mechanical wave. Its propagation does not depend on any elastic medium. It only depends on the mechanism of “changing electric field produces changing magnetic field, and changing magnetic field produces changing electric field”.

When the frequency of electromagnetic wave is low, it can be transmitted mainly by tangible conductor; When the frequency increases gradually, the electromagnetic wave will spill out of the conductor and transfer energy without a medium, which is a kind of radiation. In low-frequency electrical oscillation, the mutual change between magnetoelectricity is relatively slow, and almost all of its energy returns to the original circuit without energy radiation. However, in the high-frequency electric oscillation, the magnetoelectric interaction is very fast, and the energy cannot return to the original oscillation circuit. Therefore, the electric energy and magnetic energy propagate to space in the form of electromagnetic wave with the periodic change of electric and magnetic field.

According to the above theory, each section of wire flowing through high-frequency current will have electromagnetic radiation. Some wires are used for transmission, so they don’t want too much electromagnetic radiation to lose energy; Some wires are used as antennas, hoping to convert energy into electromagnetic waves as much as possible. So there are transmission lines and antennas. Whether it is antenna or transmission line, it is the application of electromagnetic wave theory or Maxwell equation in different cases. For transmission lines, the structure of such wires should be able to transmit electromagnetic energy without outward radiation; For the antenna, the structure of this wire should be able to transfer electromagnetic energy as much as possible. When wires of different shapes and sizes transmit and receive radio signals of a certain frequency, the efficiency varies a lot. Therefore, in order to achieve ideal communication effect, appropriate antennas must be used!

The knowledge of antenna is formed by studying what kind of wire structure can achieve efficient transmission and reception.

High frequency electromagnetic wave propagates in the air. If it meets a conductor, it will induce and generate high-frequency current in the conductor, so that we can use wires to receive radio signals from afar.

2、 Antenna

In the wireless communication system, it is necessary to convert the guided wave energy from the transmitter into radio wave, or convert the radio wave into guided wave energy. The device used to radiate and receive radio wave is called antenna. The modulated high-frequency current energy (or guided wave energy) generated by the transmitter is transmitted to the transmitting antenna through the feeder, which will be converted into some polarized electromagnetic wave energy and go out in the required direction. After reaching the receiving point, the receiving antenna converts the electromagnetic wave energy of a certain polarization from a specific direction of space into modulated high-frequency current energy, which is transmitted to the input of the receiver through the feeder.

To sum up, the antenna shall have the following functions:

1. The antenna shall be able to convert the guided wave energy into electromagnetic wave energy as much as possible. Firstly, the antenna is required to be a good electromagnetic open system, and secondly, the antenna is required to match with the transmitter or receiver.

2. The antenna shall concentrate the electromagnetic wave in the determined direction as much as possible, or accept the incoming wave in the determined direction to the greatest extent, that is, the direction is directional.

3. The antenna shall be able to transmit or receive electromagnetic waves with specified polarization, that is, the antenna has appropriate polarization.

4. The antenna shall have sufficient working frequency band.

These four points are the most basic functions of the antenna, according to which several parameters can be defined as the basis for the design and evaluation of the antenna.

The system that connects the antenna with the transmitter or receiver is called the feeder system. The form of feeder is divided into conductor transmission line, coaxial transmission line, waveguide or microstrip line with different frequency. Therefore, the so-called feeder is actually a transmission line.

Electrical parameters of antenna

The basic function of antenna is energy conversion and directional radiation. The so-called electrical parameters of antenna are the quantities that can quantitatively characterize its energy conversion and directional radiation ability.

1. Directivity of 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 to measure the degree of the maximum radiation 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 with the highest level. In fact, the sidelobe region is a region that does not need radiation, so the lower its level, the better.

(the main lobe and side lobe radiated by the antenna are similar to the spectrum of square wave signal)

Front to back ratio:

Front to back ratio refers to the ratio of the maximum radiation direction (forward) level to its opposite direction (backward) level. The larger the front to rear ratio, the smaller the backward radiation (or reception) of the antenna. The calculation of the front to rear ratio f / B is very simple — f / b = 10 LG {(forward power density) / (backward power density)}

Directional coefficient:

At a certain distance from the antenna, the ratio of the radiation power flow density of the antenna in the maximum radiation direction to the radiation power flow density of the ideal non directional antenna with the same radiation power at the same distance. This is the most important index of directivity. It can accurately compare the directivity of different antennas and represent the electrical parameters of antenna cluster energy.

2. Antenna efficiency

Antenna efficiency is defined as the ratio of antenna radiation power to input power.

The radiation resistance R of the antenna is often used to test the ability of the antenna to radiate power. The radiation resistance of the antenna is a virtual quantity, which is defined as follows: a resistance R is set. 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 radiation resistance is an important index to measure the radiation capacity of the antenna, that is, the greater the radiation resistance, the stronger the radiation capacity of the antenna.

3. Gain coefficient

Gain coefficient is a parameter that comprehensively measures antenna energy conversion and directional characteristics. It is defined as the product of directional coefficient and antenna efficiency, which is recorded as:

D is the direction coefficient and antenna efficiency.

It can be seen that the higher the antenna direction coefficient and, the higher the gain coefficient.

Physical significance: the gain coefficient of the antenna describes the magnification of the output power in the maximum radiation direction compared with the ideal non directional antenna. It can also be understood as the ratio of a directional antenna to an ideal omnidirectional antenna (whose radiation is equal in all directions) to produce a signal of a certain size at a certain point at a certain distance.

For example, if an ideal non directional point source is used as the transmitting antenna, 100W input power is required, while when a directional antenna with a gain of G = 13 dB = 20 is used as the transmitting antenna, the input power only needs 100 / 20 = 5W. In other words, the gain of an antenna, in terms of the radiation effect in its maximum radiation direction, is a multiple of the input power amplification compared with an ideal point source without directivity.

4. Polarization direction

Polarization characteristic refers to the law that the direction of electric field vector changes with time in the maximum radiation direction of antenna.

The polarization direction is the direction of the antenna electric field. The polarization mode of the antenna is wired polarization, wired polarization (horizontal polarization and vertical polarization) and circular polarization (left-handed polarization and right-handed polarization).

How to understand linear polarization? First, imagine the classic electromagnetic wave propagation diagram. The electric field propagates as a sine wave in one plane, and the magnetic field 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, we see a short line, and this polarization is linear polarization. So how to determine the direction of linear polarization? When high-frequency current passes through the antenna, high-frequency voltage will be generated on the antenna to form a high-frequency electric field. The direction of the 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 direction of the antenna is horizontal, it is also called the “horizontal direction” of the antenna; If the antenna is a conductor erected perpendicular to the ground, the generated electric field is also vertical, which is called “vertical polarization” antenna（ Generally, the antenna with linear conductor structure is linear polarization)

How to understand circular polarization? It is also the classic electromagnetic wave propagation diagram, but the electric field size remains unchanged at this time, but the direction rotates and changes around the x-axis, but the projection on any plane is a sine wave, which is a bit similar to our signal processing, the radian remains unchanged, but the phase position is constantly changing. At this time, when you look at the electric field from the origin to the propagation direction, you see a circle, and this polarization is circular polarization. Of course, rotating to the left is left-handed polarization, and rotating to the right is right-handed polarization（ Generally, spiral antenna is circularly polarized)

The maximum signal can be induced only when the polarization direction of the receiving antenna is consistent with that of the received electromagnetic wave. According to 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 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 decreases); When the polarization direction of the receiving antenna is orthogonal to the linear polarization direction (magnetic field direction), the induced signal is zero (projection is zero). The linear polarization mode has high requirements for the direction of the antenna. In any case, the polarization of the antenna can be considered. Of course, it can be reflected by the antenna in a vertical direction, but sometimes it can also be considered that the antenna will be deflected in the actual direction.

For circular polarization, the induced signals are the same regardless of the polarization direction of the receiving antenna (the projection of electromagnetic waves in any direction is the same). Therefore, the circular polarization method reduces the sensitivity of the system to the azimuth of the antenna (the azimuth here is the azimuth of the antenna, which is different from the azimuth of the directional system mentioned above). Therefore, circular polarization is adopted in most occasions.

As a figurative analogy, linear polarization is similar to a snake bending and crawling on the ground, and circular polarization is similar to a snake winding around a wooden stick. To make another analogy, you take a rope and swing it up and down. The wave transmitted by the rope is in the form of linear polarization; Constantly draw a circle, and the transmitted wave is circularly polarized.

5. Bandwidth

The electrical parameters of the antenna are related to the frequency, that is, the above electrical parameters are designed for a certain working frequency. When the working frequency deviates from the design frequency, the antenna parameters often change. When the working frequency changes, the relevant electrical parameters of the antenna shall not exceed the specified range. This frequency range is called frequency band width, which is referred to as the bandwidth of the antenna for short.

6. Input impedance

For the transmitter, the antenna is a load. How to make the antenna absorb the most energy, we must solve a matching problem. Only when the impedance of the antenna itself is equal to that of the transmitter can the maximum transmission power be obtained!

For high-frequency signals, the antenna is a long wire. The time taken for the high-frequency signal to flow from the feed point to the antenna end point and reflect back from the end point is enough to cause great differences in the amplitude and phase of voltage and current in each part of the antenna, resulting in different lengths, structures and positions of feed points of the antenna, and different impedance. For example, the dipole oscillator fed by the center presents a pure resistance of about 50 to 75 Ω when the length of each arm is one quarter of the wavelength, which is easy to match directly with the feeding cable and transmitter.

When conditions are limited and it is impossible to trim the length of the antenna to an appropriate value, reactance elements such as inductance and capacitance shall be added to the antenna circuit to offset the reactance presented by the antenna itself. Sometimes, an impedance transformer shall be added to transform the antenna impedance to the required value of the transmitting circuit. The equipment composed of these additional elements is called “antenna tuner” or “antenna matcher”.

7. Effective length

Effective length is another important index to measure the radiation capacity of antenna.

The effective length of the antenna is defined as follows: the equivalent length of the antenna when the current distribution on the antenna is uniform under the condition of keeping the field intensity 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 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 that of a vibrator with a length of H and uniform current distribution in the maximum radiation direction. That is, the effective length of the short vibrator is h.

Receiving antenna theory

High frequency electromagnetic wave propagates in the air. If it meets a conductor, it will induce and generate high-frequency current in the conductor, so that we can use wires to receive radio signals from afar. The wires used to receive electromagnetic waves are generally called “receiving antennas”.

1. Effective receiving area

Effective receiving area is an important index to measure the ability of an antenna to receive radio waves. It is defined as: when the antenna receives with the maximum receiving direction aligned with the incoming wave 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 incoming wave direction, 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 receiving antenna is an important electrical parameter to reflect the performance of receiving weak signal.

The process that the receiving antenna sends the noise power received from the surrounding space to the receiver is similar to the process that the noise resistor transmits the noise power to the resistor network connected to it. Therefore, the receiving antenna is equivalent to a resistance with a temperature of TA. The higher TA, the greater the noise sent by the antenna to the receiver, and vice versa.

3、 Transmission line

Transmission line is the general name of 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 guided wave system. The electromagnetic wave guided by it is called guided traveling wave.

Transmission line is also a kind of conductor, but unlike antenna, it does not want radiation when electromagnetic wave propagates here. Therefore, the structure of transmission lines made of metal is not to radiate energy as much as possible.

Take the most common coaxial cable as an example. There is a wire in the middle and a ring of wire outside. Electromagnetic waves propagate in such a space without radiation.

Responsible editor: CT