Shulou(Shulou.com)11/24 Report--

Original title: "Mengxin Notes-Antenna (principle)"

Said in front of 1. As an important part of the microwave system, the antenna should also become an important part of perfecting the basic theory of every "electromagnetic CAEer". From the beginning of its establishment, the purpose is to try to establish a more perfect and diversified electromagnetic knowledge system, so as to help readers be more determined and calm when solving more and more complex system problems.

The origin of this paper begins with the systematic study of bilibili's training video of Microwave Technology and Antenna, followed by the targeted study of academic monographs such as Microwave Technology and Antenna, Antenna and Antenna Engineering Handbook. Combined with the retrieval study of a large number of antenna degree papers, I studied for 2 months in my spare time, and then spent more than half a month on digestion-absorption-output, and finally written.

This article will take into account the system and details, science popularization and professional, theory and practice, systematic and popular introduction of all kinds of antennas, hoping to help antenna beginners and assist antenna engineers to check leaks and fill gaps. Limited to the author's professional and cognitive level, if the article is biased, please correct it.

two。 The content of this article will be carried out from four aspects:

Low-level cognition: whether you are an engineer in antenna design or a layman who is only slightly interested in antennas, you should have the most basic understanding of antennas: "what is an antenna? what is the function of the antenna? and how does it achieve this effect?" As the "bottom cognition", it is the key to open the door of "antenna world".

Analysis theory: as a highly engineering microwave device with a history of more than one hundred years, antenna can not be analyzed and designed without a solid "mathematical foundation", which can be divided into two theoretical groups: 1) based on maxwell equation, combined with a series of extended principles, the basic principle of electromagnetic field, which is the basis for the analysis of all electromagnetic problems. 2) the basic theory of electromagnetic wave radiation is composed of four basic radiation unit analysis theories, which are like antennas to analyze the building blocks of this Lego mansion, which greatly simplifies the analysis process.

Engineering parameters: what kind of design is a good design? you can't just rely on "perceptual cognition". You need professional evaluation parameters and quantitative indicators. This part will introduce several important engineering parameters for evaluating the quality of antenna design. It includes the "circuit parameters" of the antenna as the terminal of the microwave system and the "spatial parameters" as the beginning of the space electromagnetic wave radiation.

Antenna family: after the foundations of "Analysis Theory" and "Engineering parameters" have been built, we begin to formally analyze the radiation characteristics and mechanism of a large number of antennas. According to the structural characteristics of the antenna, this paper introduces the analysis methods and radiation mechanism of eight common antennas, which are divided into two categories: wire antenna and surface antenna. The analysis will be based on several main engineering parameters of the antenna, combined with the current and near-field distribution on the antenna. The characteristics and radiation mechanism of each antenna will be shown in the most intuitive way possible.

First, underlying cognition in daily life, antennas can be seen everywhere, from household routers and mobile phone equipment to communication base stations and even radio telescopes. Antenna can be regarded as a very important part of the information age. So what is an antenna? What's the use of it? And how to achieve these functions?

All kinds of common antennas, such as the broadcast horn at the head of the village, are the terminals of the sound transmission system, which converts the sound in the electrical signal state in the wire into sound waves that travel freely in space; another example is that the nerve dendrites are the terminals of the nervous system, converting sensory electrical signals on nerve cells into neurotransmitters in a free state. Antenna is the terminal of microwave system, which realizes wireless communication by converting the microwave signal bound to the cable into electromagnetic wave propagated in free space. It is the link between microwave system and free space.

The terminal of a sound transmission system

The terminal of the nervous system

The antenna is the terminal of the microwave system without losing its generality. taking the simple dipole antenna in the antenna family as an example, we introduce how the antenna converts the bound electromagnetic wave signal into the free radiation electromagnetic wave signal. Several antennas on the routers commonly used in our home are usually variants of the dipole antenna.

The router antenna shows that the electromagnetic signal in the parallel double wires is bound to the periphery of the wire and can only be transmitted over long distances along the direction of the parallel wire, just as a train is bound to the railway track.

High-speed rail is bound to the rail when the parallel transmission line gradually splits, the electromagnetic waves that are bound to it can not be caught, and begin to radiate into the free space, as shown in the following figure. (the picture is taken from the official account "Wireless Deep Sea")

Dipole antenna radiation mechanism 2. analysis theory antenna analysis theory includes two aspects: 1): the basic principle of electromagnetic field; 2) the basic theory of electromagnetic wave radiation.

The basic principle of electromagnetic field

The basic principles of electromagnetic fields are mainly related to the basic theories of antenna analysis: 1) maxwell equations; 2) Green's function and superposition principle; 3) reciprocity principle; 4) Huygens principle; 5) dual principle; 6) Babine principle; 7) Mirror principle; 8) scaling principle.

Basic theory of electromagnetic wave radiation

Electromagnetic wave radiation theory mainly introduces four basic radiation models: 1) basic electric oscillator radiation; 2) basic magnon radiation; 3) basic gap radiation; 4) basic facet radiation. These four basic radiation models are the basis of antenna radiation analysis with various forms and different structures. The focus of the theoretical introduction is the radiation analysis process of the basic electric oscillator, and the analysis process of the following three basic models are inextricably linked with the basic electric oscillator.

Basic analysis theory 1. The basic principle of electromagnetic waves Maxwell equation

In 1758, Coulomb obtained "Coulomb's Law" through experiments, and its most intuitive physical phenomenon is "the same sex repel and the opposite sex attract". At the same time, it also gives the quantitative relationship between "repulsive force" or "attraction" and the amount of electric charge. thus leads to the concept of "electric field", that is, the force of one electric charge on another charge is realized by the "electric field" generated by the electric charge in the surrounding space. The greater the charge, the stronger the electric field, and the stronger the effect on another charge, which is actually the same as the "aura". Powerful people will produce a strong aura around them, thus affecting the people around them. The stronger the aura.

In 1820, Oster discovered the magnetic effect of electric current, that is, the moving charge (current) can produce a magnetic field in the space around it. In the following months, Biot, Saval and Ampere gave the quantitative relationship between magnetic field and electric current, namely Biot-Savart formula and "Ampere Loop Theorem".

In 1831, Faraday discovered the phenomenon of "magnetoelectricity" through experiments, that is, the law of electromagnetic induction: a changing magnetic field can produce electric current. Later, a talented physicist Maxwell introduced the concept of "displacement current". It shows that the changing electric field can also produce magnetic field, that is, "electromagnetism".

At this point, the relationship between electricity and magnetism is basically clear: electric charge produces electric field, the movement of electric charge (current) causes the change of electric field, the changing electric field will produce magnetic field, and the changing magnetic field will produce electric field. In this way, the electric field and magnetic field excite each other, thus propagating to infinity in the form of electromagnetic waves.

On the basis of summarizing the previous test results and relying on his solid mathematical foundation, Maxwell quantitatively unifies "electricity" and "magnetism", which is known as the greatest physical formula-Maxwell equation, and its birth lays the most important foundation for the construction of the whole electromagnetic building. As one of the microwave devices, the antenna is naturally based on this.

Green's function and superposition principle

As shown in the figure, Green's function is the distribution of electromagnetic field generated by a point source in space (that is, the solution of the wave equation). When the field of a point source (i.e. Green's function) is known, the solution of any source distribution can be obtained by the superposition principle. We can use the light field distribution of a small LED bulb as an analogy. The light intensity produced by a small light bulb in space can be expressed as Green's function, and the light intensity produced by the LED array composed of N small light bulbs can be expressed as the superposition of the light intensity produced by each small light bulb.

A small light bulb

Reciprocity principle of N small light bulbs

The reciprocity theorem of electromagnetic field is a theorem about two groups of sources. Under certain dielectric conditions, the reciprocal relationship between the two groups of sources is established: 1) the current obtained at the antenna B terminal after a given excitation is equal to the current obtained at the antenna A terminal when the antenna B is excited by the same excitation; 2) the radiation characteristics of the same antenna used as radiation, including far-field pattern, antenna effective length and antenna input impedance, are the same as the reception characteristics of the antenna when it is used for reception.

Reciprocity theorem is widely used in practical engineering, for example, in antenna testing, many antennas used as transmitting antennas in practical engineering are not convenient to continue to test their emitted electromagnetic waves. At this time, they can be used as receiving antennas to measure their radiation characteristics. According to the reciprocity theorem, the test results are the same.

What needs to be explained in the antenna test is that the establishment of the reciprocity theorem has certain requirements for the dielectric properties, and for the materials that do not meet the requirements (ferrite or plasma materials), the reciprocity theorem is not true.

Huygens equivalent principle

The Huygens equivalent principle shows that if the field on the surface S of the closed body is known, the field outside S can be calculated by the field on the S plane, that is, the electromagnetic field distribution outside the S plane produced by the real source in S can be generated by the equivalent source on the S plane. That is, the electromagnetic field distribution "E" and "H" generated outside the dotted line of the following two graphs are consistent. The equivalence principle can help us to analyze the surface antennas in the form of horn antenna and paraboloid antenna more simply.

Equivalent principle dual principle

In the Maxwell equations, as long as the magnetic charge density and magnetic current density are introduced, the classical Maxwell equation is transformed into a generalized form with complete symmetry, and the corresponding field also has a similar electromagnetic correspondence.

According to the symmetrical relationship between "electricity" and "magnetism", the dual relation encounters any electromagnetic relation and replaces all the electromagnetic parameters above the dotted line in the equation with the parameters below the dotted line, and the equation is still valid.

Babine complementarity principle

The Babine complementarity principle describes the relationship between the solutions of two electromagnetic field problems with complementary structures and dual sources. In antenna engineering, the radiation characteristics of complementary antennas are easily derived from the known radiation characteristics of antennas by using Babine complementarity principle. It has a similar use in scattering analysis.

The radiation characteristics of the slot antenna on the infinite conductor screen and the complementary dipole antenna complement each other. This method can be used to analyze the radiation characteristics of the slot antenna.

Mirror image principle

In many radiation-scattering problems, there are metal plates with large electrical size near the source. At this time, the radiation field above the metal can be solved according to the mirror image principle.

In the mirror image principle, when the infinite ideal conductive (magnetic) plane is removed, there should be not only the corresponding mirror source but also the corresponding mirror scatterer in the lower half of the space. The position and shape of the mirror scatterer and the original scatterer are symmetrical in an infinite conductive (magnetic) plane. It should be noted that the mirror principle is equivalent only for the half-space field distribution of the real source, but not for the half-space field where the mirror source exists. When the current and magnetic current are placed vertically and horizontally on the infinite PEC and PMC, the characteristics of the mirror source are shown in the following figure.

Image scaling principle of different sources

When analyzing electromagnetic problems, we often pay attention to the electrical size of the target () rather than the physical size, which is mainly due to the scaling principle, which describes the similarity of the field distribution of two radiation-scattering problems with different physical sizes but with the same electrical size, which is the theoretical basis for indoor RCS testing using the scaled model instead of the full-size model.

The condition that the scaling principle is established is that it is necessary to keep the dielectric parameters (permittivity and permeability) unchanged with frequency. In the practical radiation-scattering problem, if the conductivity in the medium is 0 and the conductivity in the metal structure tends to ∞, the permittivity of the material can be regarded as not changing with the frequency. In this case, the radiation pattern and antenna input impedance in the scaling model are very close to the real scale pattern and antenna input impedance. By using the scaling principle of the field, the field distribution of large-scale radiation-scattering problems can be simulated accurately in the laboratory.

However, when the dielectric is a lossy medium, that is, the conductivity is not zero, or the conductivity of the metal conductor in the antenna feeder is not ∞, the permittivity of the material will change with the change of frequency, so the scaling principle is not valid, and the input impedance measured by the scaling principle is larger than the pattern error.

two。 Basic theory of electromagnetic wave radiation

Antenna analysis theory there are two methods to solve the spatial electromagnetic field distribution according to Maxwell equation, one is "direct method", the other is "indirect method". The so-called "direct method" is to calculate the distribution of space electromagnetic field directly according to the distribution of "current" and "magnetic current" on the target. This method can be seen in the previous issue, "electromagnetic CAE designer, is it necessary to understand computational electromagnetism?" The so-called "indirect method" is to simplify the wave equation by introducing intermediate variables magnetic dislocation "A" and electric standard "", and then through the differential relationship between electric field "E" and magnetic field "H" and potential function to complete the solution of electric field and magnetic field distribution. its basic idea is to reduce the difficulty of solving a complex problem by solving it step by step. The specific implementation is as follows:

If there are only charge sources and current sources in the field, there are:

Then the magnetic field B can be expressed as a curl of a vector, that is:

If you bring it into the maxwell equations, you will have:

Or

If the curl is 0, a scalar potential can be defined.

Instead of the complex vector in parentheses above, there are:

At this point, we establish the electromagnetic field "E", "H" and the intermediate variable "A" and "

By using this relation, we can replace all the "E" and "H" in the maxwell equation, thus establishing "A" and "A".

"satisfied wave equation:

It is known that the form of wave equation satisfied by intermediate variables is much simpler than that satisfied by "E" and "H". Is it necessary for you, designer of electromagnetic CAE, to understand computational electromagnetism? According to the article, the solution of the equation can be quickly obtained with the help of Green's function:

After the spatial distribution of intermediate variables is obtained, the distribution of electromagnetic field in space can be solved by using the relationship between "E", "H" and intermediate variables.

Radiation of basic electric oscillator

Distribution form of field

Basic electric oscillator: also known as current element or electric dipole, refers to an infinitely small linear current unit, that is, its length l is much smaller than the working wavelength.

The spherical coordinate system places the current element along the Z axis of the spherical coordinate, with the center at the origin, as shown in the figure, because the current on the current element is an ideal line current.

By using the solution formula of magnetic vector potential "A", the following can be obtained:

It is known that because the direction of the current "J" is along the z direction, the direction of the magnetic vector potential "A" is only the component of the z direction. According to the relationship between "H" and "A" defined in the above formula

It can be calculated that the distribution of "H" is:

That is, the magnetic field "H" is only "

"Direction component, and then according to maxwell equation

The distribution of the space electric field "E" can be obtained.

That is, the electric field "E" only has "R" and "R".

"the component of the direction. at this point, the calculation of the spatial electromagnetic field distribution of the current element is completed.

Power flow

With the passage of time, the electromagnetic field generated by the current element propagates from the field source to the outer space, forming an electromagnetic wave. The Poynting vector represents the power flow density in the process of electromagnetic wave propagation, which is defined as:

It is known that the Poynting vector is a real number along the "R" direction, while along the "

"the direction is an imaginary number, indicating that there is indeed a flow of energy along the radial direction in the process of electromagnetic wave propagation, while along the circumferential direction, the energy is constantly transformed in the form of" electric field energy storage "and" magnetic field energy storage ". The real power density is expressed as:

Division of the field

The distribution of the electromagnetic field is closely related to the distance "R" from the current element, and the distance determines the main form of the field distribution.

Near field

Near field refers to

That is,

(but

), in this region, the expressions of electric and magnetic fields can be approximately expressed as:

It is known that the magnetic field is a pure real number and the electric field is a pure imaginary number, so the Poynting vector is a pure imaginary number, indicating that the form of energy transfer in the near field is mainly in the form of mutual conversion between "electric field energy storage" and "magnetic field energy storage".

Far field

The far-field area refers to

That is,

In this region, the forms of electric and magnetic fields are mainly composed of

If the term determines that the higher order term can be ignored, the expressions of electric and magnetic fields can be approximated as follows:

It is known that the electric field has only "" directional component, and the magnetic field has only "" directional component, and its expression is only different. "

"times, and the electric field and magnetic field are both pure imaginary numbers, then the Poynting vector is pure real, that is, the energy transfer form of the far field is mainly electromagnetic radiation. for the study of the electrical performance of the antenna, we mainly investigate the radiation characteristics of the far field, and its electric field pattern is the" spindle shape "shown in the figure below. the far field intensity distribution of the magnetic field is consistent with the electric field, but the difference is that the direction is perpendicular to the electric field direction.

Radiation of basic magnon in oscillator pattern

According to the dual principle, the radiation characteristics of the basic magnon can be obtained directly through the field distribution form of the "basic electric oscillator".

Compared with the far field distribution of the basic electric oscillator, the pattern factor of the electric field and the magnetic field (reflecting the shape of the pattern) does not change, but the direction of the electric field changes along the "" direction, while the direction of the magnetic field becomes along the "" direction.

Radiation from a basic gap

The equivalence of the basic gap is based on the Babine complementarity principle, and the basic gap is complementary to the basic electric oscillator. In order to obtain the radiation characteristics of the basic gap, we first calculate the radiation characteristics of the basic electric oscillator.

Because the complementary basic electric oscillator is an infinitely thin sheet oscillator, its cross-sectional perimeter can be regarded as 2d, so the surface current density and current on the basic electric oscillator can be expressed as:

Therefore, the radiation field of the complementary basic electric oscillator is:

The radiation characteristics of the basic magnon dual to the basic electric oscillator are as follows:

It is known that the radiation of the basic magnon depends on the tangential electric field "Et" on the surface of the magnon, and for the basic gap, if the applied voltage on the gap is

, there are:

Comparing the pattern characteristics of the magnon, we can know that: 1) the radiation pattern of the basic slot antenna is consistent with the radiation pattern of the basic magnon; 2) the direction of the electric field and magnetic field is replaced.

Radiation of basic surface elements

The equivalent rational surface antenna usually consists of a metal surface S1 and a primary radiator. Assuming that the closed surface S divides the space into two regions, where the region Ⅰ contains the source and the region Ⅱ does not contain the source, the radiation problem of the surface antenna is transformed into the radiation of the aperture S2. According to the Huygens-Fresnel principle, the aperture surface is divided into many facets, which are called Huygens elements.

The equivalent current and magnetic current on the Huygens element are:

The current moment and magnetic current moment on the surface element are respectively:

On the E plane (yoz plane), the radiated electric field consists of two aspects: the electric field produced by the equivalent current and the electric field produced by the equivalent magnetic current source, in which the radiated electric field produced by the equivalent oscillator is:

The radiated electric field produced by the equivalent basic magnon is:

Taking into account

And

And

And dS=dxdy, then the total radiation electric field can be expressed as:

Similarly, the far-region radiation field on the H plane can be obtained as follows:

This shows that the Huygens element has a completely consistent form of radiation field in the far region of E plane and H plane, which can be written as follows:

The pattern is shown in the figure. It can be seen that, unlike the symmetrical direction of the dipole array, the maximum gain of the fundamental element points in one direction.

Basic surface element direction figure 3. Engineering parameters as shown in the figure, when studying the functional characteristics of an antenna, as a link between microwave devices and free space, we need to pay attention not only to its "circuit parameters" as microwave components, but also to its "field parameters" as electromagnetic waves radiating into free space. at the same time, the connection between the two is the structural physical parameters that determine the current distribution on it. This is why the antenna analysis needs to use the field analysis method based on "Maxwell equation" and the analysis method of "equivalent circuit" at the same time.

Classification of antenna engineering parameters 1. Pattern correlation parameters Antenna pattern, as one of the most important parameters of antenna design, directly determines the radiation orientation performance of the antenna. The narrower the beam, the better the directional performance of the antenna, on the contrary, the wider the beam, the better the omni-directional performance of the antenna. Two different forms of pattern distribution are used in different situations. The relevant parameters of the pattern are as follows:

Gain

Main lobe width

Sidelobe level

Sidelobe level

Antenna pattern composition 2. Efficiency

Because the conductors and dielectrics in the actual antenna introduce a certain ohmic loss, the radiation power Pr of the antenna is generally less than the input power Pin of the antenna, and the efficiency of the antenna is defined as the ratio between the two (Pd is the power of loss).

3. Gain coefficient

In order to comprehensively measure the energy conversion and directional performance of the antenna, the pattern coefficient and the antenna efficiency are usually linked, and a new characteristic parameter, the gain coefficient, is introduced. It is defined as the ratio of the power density of the antenna at a certain point in the maximum radiation direction of the far region to the power density of the non-directional antenna with the same input power at the same point.

4. Effective length

In order to measure the radiation capacity of the antenna, the effective length of the antenna is usually introduced. The effective length of the antenna is defined as the antenna length when the current on the antenna is assumed to be uniformly distributed under the condition of keeping the field strength in the maximum radiation direction of the actual antenna unchanged. It is a parameter that connects the field strength in the maximum radiation direction of the antenna with the current of the antenna.

5. Input impedance

For a wire antenna, its input impedance is defined as the ratio of complex voltage to complex current at the input of the antenna, which is a complex number, including the real part (resistance) and the imaginary part (reactance):

6. Antenna polarization

Electromagnetic wave is a vector, the vector has direction in addition to size, if the arrow is used to represent the electric field vector (length represents the amplitude of the electric field, pointing to represent the direction). As shown in the figure, with the transmission of electromagnetic waves, if the end of the arrow is always drawing a straight line, it is linear polarization, similarly, drawing a circle is circular polarization, and drawing an ellipse is elliptical polarization. The polarization of the antenna refers to the polarization of the electromagnetic wave emitted by the antenna. (the picture is from the Internet)

Three polarization modes of electromagnetic waves 4. After introducing the basic theory of the antenna, the antenna family began to use these theories to analyze several common antennas, which are mainly divided into two types according to the structure of the antenna: 1) wire antenna; 2) surface antenna. The wire antenna is mainly introduced: 1) dipole antenna; 2) helical antenna; 3) directional antenna; 4) non-frequency varying weather. The surface antenna mainly introduces: 1) horn antenna; 2) paraboloid antenna; 3) slot antenna; 4) microstrip antenna. We hope to use the analysis theory mentioned above to briefly explain the analysis methods of different antennas, and at the same time, combined with the full-wave simulation software, according to the performance characteristics of different antennas, we hope to vividly display the engineering parameters of different antenna. try to quickly establish the most intuitive understanding of these common antennas.

1. Wire antenna dipole antenna

Analytical method.

As can be seen from the radiation characteristics of the basic electric oscillator above, the length is

The far-field "electric field" and "magnetic field" distribution of the electric oscillator are as follows:

The length is

The far field distribution of the half wave electric oscillator can be made use of the "electric field" and "magnetic field" distribution line segment of the basic oscillator.

"obtained by integral:

Radiation mechanism

The current distribution on the half-wave oscillator is shown in the figure, the amplitude of the current is cosine distribution, and the direction of the current oscillates back and forth along the direction of the wire, which is the main cause of electromagnetic radiation.

Current distribution of half-wave dipole antenna

The current distribution (quantization) is known from the far-field radiation pattern of the basic electric oscillator: the pattern functions of the electric field and the magnetic field are the same, but the difference is that the direction of the electric field is parallel to the current, while the direction of the magnetic field is perpendicular to the current. The E-plane pattern is spindle-shaped and the H-plane pattern is circular.

Dipole antenna pattern helical antenna

By adjusting the parameters such as diameter d and pitch h of the spiral antenna, three kinds of antennas with completely different pattern forms can be achieved: 1) end-firing type; 2) side-shooting type; 3) conical radiation.

The radiation characteristic of the helical antenna is related to the ratio of the diameter to the perimeter of the helix.

The maximum radiation direction of the antenna is circumferential, which is lateral, and this mode is normal mode. Because it is too thin, the helical antenna is approximate to a monopole antenna; when

That is, the circumference of a circle of the helical antenna is about one wavelength, and the maximum radiation direction of the antenna is end-shooting along the axis of the helix, and this mode is an axial mode, in which the current phase on the adjacent circle on the helix is approximately in the same phase. the radiation electric field is superimposed along the axial direction to form the radiation characteristics of the end fire.

The maximum radiation direction of the antenna will deviate from the helix axis, and the pattern will become conical, thus forming a side-projecting antenna.

Under the classification of spiral antennas, we will focus on the radiation characteristics and broadband characteristics of side-fired and end-fired spiral antennas.

Helical antenna-normal mode

For the side-incidence spiral antenna, the pattern is similar to that of the monopole antenna, and compared with the current distribution of the monopole antenna, it can be found that the current distribution is similar to that of the monopole antenna, which means that its bandwidth will be relatively narrow.

Pattern of side-projectile spiral antenna

Current distribution

Voltage standing wave ratio helical antenna-axial mode

When the circumference of the helix is about one wavelength, the pattern of the antenna is end-firing as shown in the figure. Different from the side-fired spiral antenna, the end-fired spiral antenna has a wide bandwidth, which can be obtained from the current distribution on it.

When the spiral antenna works in the low frequency band, the current distribution on it is standing wave.

Current distribution

With the increase of the working frequency, the current distribution on the helix changes significantly. The current distribution on the helix is decomposed into the outflow current and the reflected current. Because the reflected current attenuates rapidly after the terminal reflection, the reflected current pair will not affect the incident current distribution, so that most of the middle region of the helix is dominated by the outflow wave and the VSWR is very small. It can maintain a very low VSWR in a wide frequency range.

High frequency current distribution

Decomposition of electric current

Wideband matched directional antenna

Yagi antenna is an important guiding antenna, which is jointly developed by Yagi and Uda of Tohoku University in Japan. It is widely used in meter wave and decimeter band communications, radar, television and other radio technical equipment. The basic structure of Yagi antenna includes three: 1) active vibrators; 2) reflectors; 3) deflectors, all vibrators are arranged in a plane and parallel to each other, and their midpoints are fixed on a metal rod. Except that the feed point of the active oscillator is insulated from the metal rod, the passive oscillator and the metal rod are short-circuited, because the metal rod is perpendicular to each vibrator, so the metal rod does not induce current and does not participate in radiation.

Analysis method of basic structure composition

The induced electromotive force method is a basic method for analyzing Yagi antenna. from the point of view of John Klaus's Antenna, the reflected oscillator and the directed oscillator are regarded as the "parasitic units" of the active oscillator, and the current on it is generated by the field induction of the active oscillator.

As shown in the figure, the relationship between the length of the oscillator and the impedance, because the mutual impedance with the length of the oscillator is not very dramatic, the Yagi antenna oscillator is about half a wavelength, and the mutual impedance between the vibrators mainly depends on the distance between the vibrators. The spacing is generally 0.15-0.4 wavelength, and the self-impedance mainly depends on the length of the oscillator itself: 1) when the length of the passive oscillator is greater than half-wavelength, the passive oscillator becomes "perceptual". As can be seen from the figure, R11 > 0 and X11 > 0

That is, the current phase on the passive oscillator is ahead of the active oscillator.

According to the array theory, the maximum radiation direction points to the direction of phase lag, and the phase difference caused by the wave path difference between adjacent elements in this direction exactly offsets the phase difference of the current. The electromagnetic field of the active oscillator and the passive oscillator is superimposed in the same direction to get the maximum value, so the field along the parasitic direction to the feed unit is larger than that in the opposite direction, and this "parasitic unit" is the reflector of the Yagi antenna. 2) when the length of the passive oscillator is less than half the wavelength, the impedance of the passive oscillator is capacitive, R11 > 0 and X11.

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