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The basic principle of antenna

The basic principle of an antenna is that high-frequency currents generate changing electric and magnetic fields around it. According to Maxwell's theory of electromagnetic field, "changing electric fields generate magnetic fields, and changing magnetic fields generate electric fields". As the excitation continues, wireless signal propagation is realized.

Gain coefficient

The ratio of the total input power of the antenna is called the maximum gain coefficient of the antenna. It is a more comprehensive reflection of the antenna's effective utilization of the total RF power than the antenna's directivity coefficient. And expressed in decibels. It can be proved mathematically that the maximum gain coefficient of antenna is equal to the product of antenna directivity coefficient and antenna efficiency.

The efficiency of the antenna

It is the ratio of the power radiated by the antenna (that is, the power that effectively converts the electromagnetic wave part) to the active power input to the antenna. It's always less than 1.

Antenna polarization wave

Electromagnetic wave travels in space, if the direction of the electric field vector remains stationary or rotating according to certain rule, this was called polarization wave, also known as the antenna polarization wave, or polarized wave. Usually can be divided into plane polarization (including horizontal polarization and vertical polarization), circular polarization and elliptic polarization.

The polarization direction

The direction of the electric field of a polarized electromagnetic wave is called the polarization direction.

The polarization surface

The plane formed by the polarization direction and the propagation direction of the polarized electromagnetic wave is called the polarization plane.

Vertical polarization

The polarization of radio waves, often with the earth as the standard surface. The polarization wave whose polarization surface is parallel to the earth normal plane (vertical plane) is called the vertical polarization wave. The direction of its electric field is perpendicular to the earth.

Horizontal polarization

The polarization wave which is perpendicular to the normal surface of the earth is called horizontal polarization wave. The direction of its electric field is parallel to the earth.

The plane of polarization

If the polarization direction of the electromagnetic wave remains in a fixed direction, it is called plane polarization, also known as linear polarization. The plane polarization can be obtained in the components of the electric field parallel to the earth (horizontal component) and perpendicular to the surface of the earth, whose spatial amplitudes have arbitrary relative magnitudes. Both vertical and horizontal polarization are special cases of plane polarization.

Circular polarization

When the Angle between the polarization plane and the geodetic normal plane of radio waves changes from 0 to 360° periodically, that is, the electric field size is unchanged, the direction changes with time, and the trajectory of the end of the electric field vector is projected as a circle on the plane perpendicular to the propagation direction, it is called circular polarization. Circular polarization can be obtained when the horizontal and vertical components of the electric field have equal amplitudes and phase differences of 90° or 270°. Circular polarization, if the polarization surface rotates with time and has a right spiral relationship with the electromagnetic wave propagation direction, it is called right circular polarization; On the contrary, if a left spiral relationship, said left circular polarization.

The elliptical polarized

If the Angle between the radio wave polarization plane and the geodetic normal plane changes periodically from 0 to 2π, and the trajectory of the end of the electric field vector is projected as an ellipse on the plane perpendicular to the propagation direction, it is called elliptic polarization. When the amplitude and phase of the vertical and horizontal components of the electric field have arbitrary values (except when the two components are equal), the elliptic polarization can be obtained.

Long wave antenna, medium wave antenna

It is a general term for transmitting or receiving antennas working in long and medium wave bands. Long and medium waves propagate as ground waves and sky waves, which are continuously reflected between the ionosphere and the earth. According to this propagation characteristic, long and medium wave antennas should be able to produce vertically polarized waves. In the long and medium wave antenna, the vertical type, inverted L type, T type and umbrella type vertical ground antenna are widely used. Long and medium wave antennas should have good ground network. There are many technical problems in long and medium wave antenna, such as small effective height, low radiation resistance, low efficiency, narrow pass band and small directionality coefficient. In order to solve these problems, the antenna structure is often very complex and very large.

Shortwave antenna

The transmitting or receiving antennas operating in the short wave band are collectively called short wave antennas. Short wave is mainly transmitted by the sky wave reflected by the ionosphere and is one of the important means of modern long distance radio communication. There are many forms of shortwave antenna, among which the most widely used are symmetric antenna, in-phase horizontal antenna, double wave antenna, angular antenna, V-shaped antenna, rhombus antenna, fishbone antenna and so on. Compared with the long-wave antenna, the short-wave antenna has the advantages of higher effective height, higher radiation resistance, higher efficiency, better directionality, higher gain and wider passband.

Ultrashort wave antenna

The transmitting and receiving antennas operating in the ultrashort wave band are called ultrashort wave antennas. Ultrashort waves travel mainly by space waves. There are many forms of this kind of antenna, among which the most used Yaki antenna, dish conical antenna, double conical antenna, "bat wing" TV transmitting antenna and so on.

Microwave antenna

The transmitting or receiving antennas working in the wave bands of meter wave, decimeter wave, centimeter wave and millimeter wave are collectively referred to as microwave antennas. Microwave mainly depends on space wave propagation, in order to increase the communication distance, the antenna is set up higher. In the microwave antenna, the widely used paraboloid antenna, horn paraboloid antenna, horn antenna, lens antenna, slotted antenna, dielectric antenna, periscope antenna and so on.

Directional antenna

Directional antenna is a kind of antenna that transmits and receives electromagnetic waves in one or several specific directions especially strongly, while transmits and receives electromagnetic waves in other directions is zero or very small. The purpose of using directional transmitting antenna is to increase the effective utilization of radiation power and increase the secrecy. The main purpose of using directional receiving antenna is to increase the anti-interference ability.

Non directional antenna

The antenna that radiates or receives electromagnetic wave uniformly in all directions is called non-directional antenna, such as whip antenna used in small communication machine, etc.

Wide band antenna

An antenna whose directionality, impedance and polarization properties remain almost constant over a wide band is called a wideband antenna. The early wideband antenna has rhombus antenna, V antenna, double wave antenna, disk cone antenna, etc., the new wideband antenna has logarithmic period antenna, etc.

Tuning the antenna

An antenna that has a predetermined directionality only in a very narrow frequency band is called a tuned antenna or a tuned directional antenna. Typically, the directionality of a tuned antenna remains constant only up to 5 percent OF the band near its tuning frequency, while at other frequencies the directionality changes so much that communication is disrupted. Tuned antennas are not suitable for short-wave communications with variable frequencies. The same - phase horizontal antenna, folded antenna and zigzag antenna are all tuned antennas.

Vertical antenna

Vertical antenna refers to the antenna placed perpendicular to the ground. It has symmetric and asymmetric forms, and the latter is more widely used. Symmetrical vertical antennas are usually center fed. The asymmetric vertical antenna feeds 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. Asymmetric vertical antenna is also called vertical ground antenna.

Pour L antenna

An antenna formed by connecting a vertical lead to one end of a single horizontal wire. Because of its shape like the English letter L upside down, it is called an inverted L antenna. The γ of the Russian letter is the reverse L of the English letter. Therefore, γ type antenna is more convenient. 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, and the radiation produced by this part can be ignored, while the radiation produced by the vertical part is. Inverted L antennas are generally used for long wave communication. Its advantages are simple structure and convenient erection; Disadvantages are large footprint, poor durability.

T antenna

In the center of the horizontal wire, a vertical lead is connected, which is shaped like the English letter T, so it is called T-antenna. It is the most common type of vertically grounded antenna. The horizontal part of the radiation is negligible, the radiation is produced by the vertical part. In order to improve efficiency, the horizontal section can also be composed of more than one wire. The T - shaped antenna has the same characteristics as the inverted L - shaped antenna. It is generally used for long wave and medium wave communication.

Umbrella antenna

On the top of a single vertical wire, several tilted conductors are led down in all directions, so that the antenna shape is 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 - and T-shaped antennas.

Whip antenna

Whip antenna is a flexible vertical rod antenna, which is generally 1/4 or 1/2 wavelength in length. Most whip antennas use a net instead of a ground wire. Small whip antennas often use the metal shell of a small radio station as a ground network. Sometimes in order to increase the effective height of the whip antenna, some small spoke blades can be added to the top of the whip antenna or inductance can be added to the middle end of the whip antenna. Whip antenna can be used for small communication machine, chat machine, car radio, etc.

Symmetric antenna

Two wires of EQUAL length, disconnected AT the center and connected to a feed, can be used as transmitting and receiving antennas, such an antenna is called a symmetric antenna. Because antennas are sometimes called oscillators, symmetric antennas are also called symmetric oscillators, or dipole antennas. A symmetric oscillator with a total length of half a wavelength is called a half-wave oscillator, also known as a half-wave dipole antenna. It is the most basic element antenna and the most widely used. Many complex antennas are composed of it. The half-wave oscillator has simple structure and convenient feeding. It is widely used in near field communication.

Cage antenna

It is a wide band weak directional antenna. It is a hollow cylinder surrounded by several wires instead of a single wire radiation body in a symmetric antenna, because the radiation body is cage shaped, it is called cage antenna. The operating band of cage antenna is wide and easy to tune. It is suitable for close range trunk line communication.

Horn antenna

Belongs to a kind of symmetric antenna, but its two arms are not arranged in a straight line, and into 90° or 120° Angle, so called angular antenna. This kind of antenna is generally horizontal device, its directionality is not significant. In order to obtain the wide band characteristics, the two arms of the angular antenna can also adopt the cage structure, called the angular cage antenna.

Is equivalent to the antenna

Bending the oscillators into parallel symmetric antennas is called a folded antenna. There are several forms of double-wire converted antenna, three-wire converted antenna and multi-wire converted antenna. When bending, the current at the corresponding point on each line should be in the same phase. From a distance, the whole antenna looks like a symmetric antenna. But compared with the symmetrical antenna, the radiation of the converted antenna is enhanced. The input impedance increases to facilitate the coupling with the feeder. The folded antenna is a tuned antenna with a narrow operating frequency. It is widely used in short wave and ultrashort wave bands.

V antenna

An antenna consisting of two wires at an Angle to each other in the shape of the letter V. The terminal can be open or connected with a resistance equal to the characteristic impedance of the antenna. The V-shaped antenna is unidirectional and the maximum transmitting direction is in the vertical plane along the Angle line. Its disadvantages are low efficiency and large footprint.

Rhombic antenna

It's a wide band antenna. It consists OF a horizontal DIAMOND HANGING ON four pillars, one of the diamond is connected to the feeder at an acute Angle, and the other is connected to a terminal resistance equal to the characteristic impedance of the diamond antenna. It is unidirectional in the vertical plane pointing in the direction of the terminal resistance.

The advantages of rhombus antenna are high gain, strong directionality, wide band, easy to set up and maintain; The disadvantage is the large footprint. After the rhomboid antenna is deformed, there are three forms of double rhomboid antenna, reply rhomboid antenna and fold rhomboid antenna. Rhombus antenna is generally used in medium and large short wave receiver stations.

Dish cone antenna

It's an ultrashort wave antenna. The top is a disc (radiation body), fed by the core line of the coaxial line, and the bottom is a cone, connected to the outer conductor of the coaxial line. The effect of the cone is similar to that of the infinite ground. Changing the tilt Angle of the cone can change the maximum radiation direction of the antenna. It has an extremely wide frequency band.