The ratio of the total input power of the antenna is called the maximum gain factor of the antenna. It is a more comprehensive reflection of the antenna's effective utilization of the total radio frequency power than the antenna directivity coefficient. It is also expressed in decibels. It can be inferred by mathematics that the maximum antenna gain coefficient is equal to the product of the antenna directivity coefficient and the antenna efficiency.
1. Related concepts
1) Antenna efficiency
It refers to the ratio of the power radiated by the antenna (that is, the power that effectively converts the electromagnetic wave part) and the active power input to the antenna. It is a value that is always less than 1.
2) Antenna polarized wave
When electromagnetic waves propagate in space, if the direction of the electric field vector remains fixed or rotates according to a certain rule, this electromagnetic wave is called polarized wave, also known as antenna polarized wave, or polarized wave. Usually can be divided into plane polarization (including horizontal polarization and vertical polarization), circular polarization and elliptical polarization.
3) Polarization direction
The electric field direction of a polarized electromagnetic wave is called the polarization direction.
4) Polarization plane
The plane formed by the polarization direction and the propagation direction of the polarized electromagnetic wave is called the polarization plane.
5) Vertical polarization
The polarization of radio waves often uses the earth as the standard plane. Any polarized wave whose polarized plane is parallel to the normal plane of the earth (vertical plane) is called a vertically polarized wave. The direction of the electric field is perpendicular to the earth.
6) Horizontal polarization
All polarized waves whose polarized plane is perpendicular to the normal plane of the earth are called horizontally polarized waves. The direction of the electric field is parallel to the earth.
7) Planar polarization
If the polarization direction of the electromagnetic wave remains in a fixed direction, it is called planar polarization, or linear polarization. In the component of the electric field parallel to the earth (horizontal component) and the component perpendicular to the surface of the earth, its spatial amplitude has any relative magnitude, and planar polarization can be obtained. Both vertical polarization and horizontal polarization are special cases of planar polarization.
8) Circular polarization
When the angle between the polarization plane of the radio wave and the normal plane of the earth changes periodically from 0 to 360°, that is, the magnitude of the electric field does not change, and the direction changes with time, the trajectory of the end of the electric field vector is on a plane perpendicular to the direction of propagation When the projection is a circle, it is called circular polarization. When the horizontal and vertical components of the electric field have the same amplitude and the phase difference is 90° or 270°, circular polarization can be obtained. Circular polarization, if the polarization plane rotates with time and is in a right spiral relationship with the propagation direction of electromagnetic waves, it is called right circular polarization; on the contrary, if it is in a left spiral relationship, it is called left circular polarization.
9) Elliptical polarization
If the angle between the polarization plane of the radio wave and the normal plane of the earth changes periodically from 0 to 2π, and the trajectory at the end of the electric field vector is projected as an ellipse on a plane perpendicular to the direction of propagation, it is called elliptical polarization. When the amplitude and phase of the vertical component and the horizontal component of the electric field have arbitrary values (except when the two components are equal), elliptical polarization can be obtained.
2. Antenna type
1) Long wave antenna, medium wave antenna
It is a collective term for transmitting antennas or receiving antennas that work in the long-wave and medium-wave bands. Long and medium waves propagate by ground and sky waves, while sky waves are continuously reflected between the ionosphere and the earth. According to this propagation characteristic, long and medium wave antennas should be able to generate vertically polarized waves. Among the long and medium wave antennas, vertical, inverted L, T, and umbrella vertical ground antennas are widely used. Long and medium wave antennas should have a good ground net. Long and medium wave antennas have many technical problems, such as small effective height, small radiation resistance, low efficiency, narrow passband, and small directivity coefficient. In order to solve these problems, the antenna structure is often very complicated and very large.
2) Shortwave antenna
Transmitting or receiving antennas that work in the short-wave band are collectively referred to as short-wave antennas. The short wave is mainly propagated by the sky wave reflected by the ionosphere, and it is one of the important means of modern long-distance radio communication. There are many forms of shortwave antennas, among which symmetrical antennas, in-phase horizontal antennas, double-wave antennas, angular antennas, V-shaped antennas, diamond antennas, fishbone antennas, etc. are most used. Compared with long-wave antennas, short-wave antennas have a large effective height, large radiation resistance, high efficiency, good directivity, high gain, and bandwidth.
3) Ultrashort wave antenna
The transmitting and receiving antennas that work in the ultrashort wave band are called ultrashort wave antennas. Ultrashort waves mainly rely on space waves to propagate. There are many forms of such antennas, among which the most widely used are Yagi antennas, disc-cone antennas, bi-cone antennas, and "batwing" TV transmitting antennas.
4) Microwave antenna
Transmitting or receiving antennas that work in meter wave, decimeter wave, centimeter wave, millimeter wave and other wave bands are collectively referred to as microwave antennas. Microwaves mainly rely on space waves to propagate. In order to increase the communication distance, the antenna is set up relatively high. Among microwave antennas, parabolic antennas, horn parabolic antennas, horn antennas, lens antennas, slot antennas, dielectric antennas, periscope antennas, etc. are widely used.
5) Directional antenna
A directional antenna refers to an antenna that emits and receives electromagnetic waves in one or several specific directions is particularly strong, while transmitting and receiving electromagnetic waves in other directions is zero or very small. The purpose of using a directional transmitting antenna is to increase the effective utilization of radiated power and increase confidentiality; the main purpose of using a directional receiving antenna is to increase the anti-interference ability.
6) Non-directional antenna
Antennas that radiate or receive electromagnetic waves uniformly in all directions are called non-directional antennas, such as whip antennas for small communication devices.
7) Broadband antenna
An antenna whose directivity, impedance, and polarization characteristics remain almost unchanged over a wide band is called a broadband antenna. Early broadband antennas include diamond antennas, V-shaped antennas, double-wave antennas, disk cone antennas, etc., and new broadband antennas include log-period antennas.
8) Tuning antenna
An antenna with a predetermined directivity only in a very narrow frequency band is called a tuned antenna or a tuned directional antenna. Generally, a tuned antenna maintains its directivity only in the 5% band near its tuning frequency, while at other frequencies, the directivity changes very drastically, causing communication damage. Tuned antennas are not suitable for shortwave communications with variable frequencies. In-phase horizontal antennas, folded antennas, zigzag antennas, etc. are all tuned antennas.
9) Vertical antenna
A vertical antenna refers to an antenna placed perpendicular to the ground. It has two forms, symmetrical and asymmetrical, and the latter is widely used. Symmetrical vertical antennas are often center-fed. The asymmetric vertical antenna is fed between the bottom of the antenna and the ground, and its maximum radiation direction is concentrated in the ground direction when the height is less than 1/2 wavelength, so it is suitable for broadcasting. Asymmetrical vertical antennas are also called vertical grounded antennas.
10) Inverted L antenna
An antenna formed by connecting a vertical down conductor to one end of a single horizontal wire. Because its shape resembles the reverse of the English letter L, it is called an inverted L-shaped antenna. The word Γ in the Russian alphabet is exactly the reverse of the English letter L. Therefore, it is more convenient to call a Γ-type antenna. It is a form of vertically grounded antenna. In order to improve the efficiency of the antenna, its horizontal part can be composed of several wires arranged on the same horizontal plane. The radiation generated by this part is negligible, while the vertical part generates radiation. Inverted L antennas are generally used for long-wave communications. Its advantages are simple structure and convenient erection; its disadvantages are large floor area and poor durability.
11) T-shaped antenna
In the center of the horizontal wire, connect a vertical down wire, the shape is like the English letter T, so it is called a T-shaped antenna. It is the most common type of vertically grounded antenna. The horizontal part of the radiation is negligible, and the vertical part produces radiation. In order to improve efficiency, the horizontal part can also be composed of multiple wires. The characteristics of the T-shaped antenna are the same as the inverted L-shaped antenna. It is generally used for long wave and medium wave communications.
12) Umbrella antenna
On the top of a single vertical wire, lead several inclined conductors in various directions. The antenna formed in this way is shaped like an open umbrella, so it is called an umbrella antenna. It is also a form of vertically grounded antenna. Its characteristics and uses are the same as inverted L-shaped and T-shaped antennas.
13) Whip antenna
The whip antenna is a flexible vertical rod antenna whose length is generally 1/4 or 1/2 wavelength. Most whip antennas do not use ground wires but use ground nets. Small whip antennas often use the metal shell of a small radio as a ground net. Sometimes in order to increase the effective height of the whip antenna, some small radial blades can be added to the top of the whip antenna or an inductance can be added to the middle end of the whip antenna. The whip antenna can be used for small communication devices, walkie talkies, car radios, etc.
14) Symmetrical antenna
The two parts of the same length but the center is disconnected and connected to feed the wire, can be used as transmitting and receiving antennas, the antenna formed in this way is called a symmetrical antenna. Because antennas are sometimes called vibrators, symmetrical antennas are also called symmetrical vibrators, or dipole antennas. A symmetrical oscillator with a total length of half a wavelength is called a half-wave oscillator, also called a half-wave dipole antenna. It is the most basic unit antenna and is also the most widely used. Many complex antennas are composed of it. The half-wave vibrator has simple structure and convenient power feeding, and is widely used in short-distance communication.
15) Cage antenna
It is a wide-band weakly directional antenna. It is formed by replacing the single-wire radiator in the symmetrical antenna with a hollow cylinder surrounded by several wires. Because the radiator is a cage, it is called a cage antenna. The cage antenna has a wide working band and is easy to tune. It is suitable for short-distance trunk communication.
16) Angular antenna
It belongs to a category of symmetrical antennas, but its two arms are not arranged in a straight line, forming an angle of 90° or 120°, so it is called an angular antenna. This kind of antenna is generally horizontal, and its directivity is not significant. In order to obtain wide-band characteristics, the double arms of the angular antenna can also adopt a cage structure, which is called an angular cage antenna.
17) Folding antenna
A symmetrical antenna that bends the vibrator into parallel is called a folded antenna. There are several forms of double-line folded antenna, three-line folded antenna and multi-line folded antenna. When bending, the currents at the corresponding points on each line should be in phase. From a distance, the entire antenna looks like a symmetrical antenna. However, compared with a symmetrical antenna, the folded antenna has enhanced radiation. The input impedance increases to facilitate coupling with the feeder. The folded antenna is a tuned antenna with a narrow working frequency. It is widely used in shortwave and ultrashort wave bands.
18) V-shaped antenna
It is composed of two wires at an angle to each other, shaped like an antenna of the English letter V. Its terminal can be open circuit or connected to a resistor, the size of which is equal to the characteristic impedance of the antenna. The V-shaped antenna is unidirectional, and the maximum emission direction is in the vertical plane of the diagonal direction. Its disadvantages are low efficiency and large footprint.
19) Diamond antenna
It is a broadband antenna. It consists of a horizontal rhombus suspended on four pillars. One acute angle of the rhombus is connected to the feeder, and the other acute angle is connected to a terminal resistance equal to the characteristic impedance of the rhombus antenna. It is unidirectional in the vertical plane pointing to the direction of the terminal resistance.
The advantages of the diamond antenna are high gain, strong directivity, wide use band, easy installation and maintenance; the disadvantage is that it covers a large area. After the rhombus antenna is deformed, there are three forms of double rhombus antenna, feedback rhombus antenna and folded rhombus antenna. Diamond antennas are generally used for large and medium-sized shortwave receiving stations.
20) Disk cone antenna
It is an ultrashort wave antenna. At the top is a disc (ie, radiator), fed by the core of the coaxial line, and at the bottom is a cone, connected to the outer conductor of the coaxial line. The function of the cone is similar to that of an infinite ground. Changing the inclination angle of the cone can change the maximum radiation direction of the antenna. It has an extremely wide frequency band.
21) Fishbone antenna
Fishbone antenna, also called side-fire antenna, is a special short-wave receiving antenna. It consists of connecting a symmetrical oscillator at a certain distance on two assembly lines, and these symmetrical oscillators are all connected to the assembly line through a small capacitor. At the end of the assembly line, that is, the end facing the communication direction, a resistor equal to the characteristic impedance of the assembly line is connected, and the other end is connected to the receiver through the feeder. Compared with the diamond antenna, the fishbone antenna has the advantages of small side lobes (that is, strong reception in the main lobe direction and weak reception in other directions), small interaction between the antennas, and small footprint; the disadvantage is efficiency Low, installation and use are more complicated.
22) Yagi antenna
Also called the steering antenna. It consists of several metal rods, one of which is a radiator, the longer one behind the radiator is a reflector, and the shorter ones in the front are directors. The radiator usually uses a folded half-wave oscillator. The maximum radiation direction of the antenna is the same as the direction of the director. The advantages of the Yagi antenna are simple structure, light weight and sturdiness, and convenient power feeding; the disadvantages are narrow frequency band and poor anti-interference. It is used in ultrashort wave communication and radar.
23) Sector antenna
It has two forms: metal plate type and metal wire type. Among them, it is a fan-shaped metal plate type and a fan-shaped metal wire type. This kind of antenna enlarges the antenna cross-sectional area, so the antenna frequency band is widened. The wire sector antenna can use three, four or five metal wires. Sector antennas are used for ultrashort wave reception.
24) Biconical antenna
The biconical antenna is composed of two cones with opposite cone tips, and power is fed at the cone tips. The cone can be made of metal surface, metal wire or metal mesh. Just like a cage antenna, as the cross-sectional area of the antenna increases, the antenna frequency band also widens. Biconical antennas are mainly used for ultrashort wave reception.
25) Parabolic antenna
The parabolic antenna is a directional microwave antenna, which is composed of a parabolic reflector and a radiator. The radiator is installed on the focal point or focal axis of the parabolic reflector. The electromagnetic wave emitted by the radiator is reflected by the parabola to form a very directional beam.
The parabolic reflector is made of metal with good conductivity. There are four main methods: rotating paraboloid, cylindrical paraboloid, cut-off rotating paraboloid and elliptical edge paraboloid. The most commonly used are rotating paraboloid and cylindrical paraboloid. The radiator generally uses half-wave oscillators, open waveguides, slotted waveguides, etc.
The parabolic antenna has the advantages of simple structure, strong directivity, and wide working frequency band. The disadvantages are: because the radiator is located in the electric field of the parabolic reflector, the reflector has a large reaction effect on the radiator, and it is difficult for the antenna and the feeder to match well; the back radiation is large; the degree of protection is poor; and the manufacturing accuracy is high. This antenna is widely used in microwave relay communication, tropospheric scatter communication, radar and television.
26) Horn parabolic antenna
The horn parabolic antenna consists of two parts, the horn and the parabola. The parabola covers the horn, and the apex of the horn is located at the focal point of the parabola. The horn is a radiator, which radiates electromagnetic waves to the parabola, and the electromagnetic waves are reflected by the parabola and focused into a narrow beam to be emitted. The advantages of the horn parabolic antenna are: the reflector has no reaction to the radiator, and the radiator has no shielding effect on the reflected electric wave. The antenna and the feeding device are better matched; the back radiation is small; the protection degree is high; the working frequency band is very wide; the structure is simple. Horn parabolic antennas are widely used in trunk relay communications.
27) Horn antenna
Also known as horn antenna. It is composed of a uniform waveguide and a horn-shaped waveguide with a gradually increasing cross-section. There are three types of horn antennas: sector horn antenna, pyramid horn antenna and conical horn antenna. The horn antenna is one of the most commonly used microwave antennas and is generally used as a radiator. The advantage is the bandwidth of the working frequency; the disadvantage is that the volume is large, and for the same caliber, its directivity is not as sharp as the parabolic antenna.
28) Horn lens antenna
It is composed of a horn and a lens mounted on the horn diameter, so it is called a horn lens antenna. Refer to the lens antenna for the principle of the lens. This antenna has a relatively wide working frequency band and has a higher degree of protection than the parabolic antenna. It is widely used in microwave trunk communication with more channels.
29) Lens antenna
In the centimeter band, many optical principles can be used for antennas. In optics, the lens can be used to make the spherical wave radiated by the point light source placed on the focal point of the lens to become a plane wave after being refracted by the lens. The lens antenna is made using this principle. It consists of a lens and a radiator placed at the focal point of the lens. There are two types of lens antennas: dielectric decelerating lens antenna and metal accelerating lens antenna. The lens is made of low-loss high-frequency medium, thick in the middle and thin around it. The spherical wave emitted from the radiation source is decelerated as it passes through the dielectric lens. Therefore, the deceleration path of the spherical wave in the middle part of the lens is long, and the path deceleration in the surrounding part is short. Therefore, the spherical wave becomes a plane wave after passing through the lens, that is, the radiation becomes directional. The lens is made up of many metal plates with different lengths placed in parallel. The metal plate is perpendicular to the ground, and the closer the metal plate is, the shorter. Electric waves in parallel metal plates
Accelerated when spreading. When the spherical wave emitted from the radiation source passes through the metal lens, the closer to the edge of the lens, the longer the accelerated path, and the shorter the accelerated path in the middle. Therefore, the spherical wave after passing through the metal lens becomes a plane wave.
Lens antenna has the following advantages:
1. The side lobes and back lobes are small, so the pattern is better;
2. The precision of manufacturing the lens is not high, so the manufacturing is more convenient. Its disadvantages are low efficiency, complex structure and high price. Lens antennas are used in microwave relay communications.
30) Slotted antenna
One or several narrow slots are cut on a large metal plate and fed by coaxial lines or waveguides. The antenna formed in this way is called a slot antenna, or a slit antenna. In order to obtain unidirectional radiation, the back of the metal plate is made into a cavity, and the slot is directly fed by the waveguide. The slotted antenna has a simple structure and no protruding parts, so it is particularly suitable for use on high-speed aircraft. Its disadvantage is that it is difficult to tune.
31) Dielectric antenna
The dielectric antenna is a round rod made of low-loss and high-frequency dielectric material (usually polystyrene), and one end of it is fed by a coaxial line or a waveguide. 2 is the extension of the inner conductor of the coaxial line, forming a vibrator to excite electromagnetic waves; 3 is the coaxial line; 4 is the metal sleeve. The role of the sleeve is not only to clamp the dielectric rod, but also to reflect electromagnetic waves, so as to ensure that the electromagnetic waves are excited by the inner conductor of the coaxial line and propagate to the free end of the dielectric rod. The advantages of dielectric antennas are small size and sharp directivity; the disadvantage is that the dielectric is lossy, so the efficiency is not high.
32) Periscope antenna
In microwave relay communication, the antenna is often placed on a very high bracket, so a long feeder line is required to feed the antenna. Too long a feeder will cause many difficulties, such as complex structure, large energy loss, and distortion due to energy reflection at the feeder connector. In order to overcome these difficulties, a periscope antenna can be used. The periscope antenna consists of a lower mirror radiator installed on the ground and an upper mirror reflector installed on a bracket. The lower mirror radiator is generally a parabolic antenna, and the upper mirror reflector is a flat metal plate. The lower mirror radiator emits electromagnetic waves upward, which are reflected by the metal plate. The advantages of the periscope antenna are low energy loss, low distortion, and high efficiency. Mainly used in microwave relay communication with small capacity.
33) Helical antenna
It is an antenna with a spiral shape. It is composed of a metal spiral wire with good electrical conductivity. It is usually fed by a coaxial wire. The core wire of the coaxial wire is connected to one end of the spiral wire. The outer conductor of the coaxial wire is connected to the grounded metal mesh (or plate). connection. The radiation direction of the spiral antenna is related to the circumference of the spiral. When the circumference of the spiral is much smaller than a wavelength, the direction of the strongest radiation is perpendicular to the spiral axis; when the circumference of the spiral is on the order of a wavelength, the strongest radiation appears in the direction of the spiral axis.
34) Antenna Tuner
An impedance matching network that connects the transmitter and the antenna is called an antenna tuner. The antenna input impedance changes greatly with frequency, while the transmitter output impedance is constant. If the transmitter is directly connected to the antenna, when the transmitter frequency changes, the impedance between the transmitter and the antenna will not match, which will reduce the radiation. power. Using the antenna tuner, the impedance between the transmitter and the antenna can be matched, so that the antenna has the maximum radiation power at any frequency. Antenna tuners are widely used in ground, vehicle, shipboard and aviation shortwave radio stations.
35) Log periodic antenna
It is a broadband antenna, or a frequency-independent antenna. Among them, it is a simple log-periodic antenna, and its dipole length and spacing are in accordance with the following relationship: τ dipole is fed by a uniform two-wire transmission line, and the transmission line needs to switch positions between adjacent dipoles . This kind of antenna has a characteristic: all the characteristics at the frequency f will be repeated at all frequencies given by τⁿf, where n is an integer. These frequencies are all equally spaced on the logarithmic scale, and the period is equal to the logarithm of τ. The name of the log-periodic antenna comes from this. Log periodic antennas simply repeat the radiation pattern and impedance characteristics periodically. However, if τ is not much smaller than 1, the change of its characteristics in one cycle is very small, so it is basically independent of frequency. There are many types of log-period antennas, including log-period dipole antennas and monopole antennas, log-period resonant V-shaped antennas, log-period helical antennas and other forms. Among them, the most common is the log-period dipole antenna. These antennas are widely used in shortwave and above shortwave bands.
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