Microwave antenna and various antenna concepts analysis
Transmitting or receiving antennas that work in meter wave, decimeter wave, centimeter wave, millimeter wave and other wave bands are collectively called 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.
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The Cassegrain antenna is a commonly used antenna in microwave communication, which evolved from a parabola. The Cassegrain antenna consists of three parts, namely the main reflector, the sub-reflector and the radiation source. The main reflector is a paraboloid of revolution, and the secondary reflector is a hyperboloid of revolution. Structurally, one focal point of the hyperboloid coincides with the focal point of the paraboloid, the focal axis of the hyperboloid coincides with the focal axis of the paraboloid, and the radiation source is located on the other focal point of the hyperboloid. It is a reflection of the electromagnetic wave emitted by the radiation source by the sub-reflector, which reflects the electromagnetic wave to the main reflector, and then obtains a directional plane wave beam after reflection by the main reflector to achieve directional emission.
When the radiator is located at the real focus F1 of the hyperboloid of revolution, the rays emitted by F1 after being reflected by the hyperboloid are equivalent to the rays directly emitted from the virtual focus of the hyperboloid. Therefore, as long as the virtual focus of the hyperboloid coincides with the focus of the paraboloid, the rays reflected by the secondary reflection surface to the main reflection surface can be reflected by the paraboloid as a plane wave and radiate out.
Compared with the parabolic antenna, the Cassegrain antenna changes the radiation mode of the feed from the parabolic feedforward mode to the backfeed mode, which makes the structure of the antenna more compact and easier to manufacture. In addition, the Cassegrain antenna can be equivalent to a parabolic antenna with a long focal length, and this long focal length can make the distance of the antenna from the focal point to the point of the mouth close to a constant, so the influence of space attenuation on the radiation of the feeder is small , Making the efficiency of the Cassegrain antenna higher than the standard parabolic antenna.
Transmitting or receiving antennas that work in the short-wave band are collectively referred to as short-wave antennas. Shortwave is mainly propagated by sky waves reflected by the ionosphere, and 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 the most widely 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.
Antennas are widely used in radio systems such as communications, broadcasting, television, radar, and navigation. They play a role in spreading radio waves and are essential devices for effectively radiating and receiving radio waves.
The current rapid development of antenna communication knowledge and technology, as well as many international research directions for antennas, have prompted the birth of new antennas. Array antenna is a kind of research direction. The so-called array antenna is not simply arranging the antenna into an array that we are familiar with, but its composition is in the form of an array. In the case of transmitting antennas, simple radiation sources such as points Sources, symmetrical dipole sources are common radiation sources that make up array antennas. They form an array according to different parameters such as antenna feed current, spacing, and electrical length in a straight line or a more complex form to obtain the best radiation directivity. This is the charm of the array antenna, which can adjust the directional performance of the radiation according to the needs. This has produced smart antennas such as those used in modern mobile communications. I believe that in the near future, these high-tech antennas will Will bring us the same high-quality communication environment.
Log periodic antenna
It is a broadband antenna, or a frequency-independent antenna. For a single log-periodic antenna, its dipole length and spacing meet the following relationship:
The 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 τf, 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-period 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.
"Log Periodic Antenna"
Log periodic antenna
Mobile phone antenna
The radio frequency signal power output by the radio transmitter is transmitted to the antenna through the feeder (cable), and radiated by the antenna in the form of electromagnetic waves. After the electromagnetic wave arrives at the receiving location, it is followed by the antenna (receiving only a small part of the power) and sent to the radio receiver through the feeder. It can be seen that the antenna is an important radio device that transmits and receives electromagnetic waves. Without an antenna, there is no radio communication.
There are many varieties of antennas for use in different situations such as different frequencies, different uses, different occasions, and different requirements.
Comparison of internal and external antennas
At present, there are mainly two types of mobile phone antennas: built-in and external antennas, and built-in antennas must be objectively weaker than external antennas. The antenna is installed as far away from the ground and buildings as possible. When the antenna is close to the reference ground, most of the energy will be concentrated between the antenna and the reference ground and cannot be transmitted smoothly. Therefore, the antenna transmission requires an "as open as possible" space . The mobile phone circuit board is the reference ground of the mobile phone antenna. Keeping the antenna away from other circuits of the mobile phone is the key to improving the transmission efficiency of the mobile phone antenna.
However, due to the limitations of the actual environment and the requirements of everyone who pursues the convenience of carrying, the design of the mobile phone must be compromised in the electrical aspect. In fact, the gain of the receiving and sending circuits of all GSM mobile phones can be automatically adjusted according to environmental changes, and the related losses can be automatically compensated through reasonable parameter settings. Therefore, for the mobile phone as a whole, when the signal is relatively good, the difference between the inner antenna and the outer antenna cannot be seen.
There is a difference. When the signal is very weak, the signal dead point threshold of the external antenna, especially the long antenna, will be higher than that of the internal antenna. That is, in theory, the internal antenna mobile phone is easier to lose signal in a weak signal environment.
For radiation problems, the decrease in antenna efficiency must be compensated with a large transmission power. Under the same conditions, the radiation of the inner antenna will be greater than that of the outer antenna. However, the actual radiation received by the human body is related to the structure of the whole machine, and the internal antenna mobile phone can also offset the influence of radiation on the human body by reasonably arranging the antenna position.
The radiation of the mobile phone is mainly brought by the antenna transmitting module of the mobile phone. The antenna of the mobile phone is made very thick, and its function is to reduce the resistance of transmission.
It can be said that the mobile phone antenna is the radiation source of the mobile phone, and it is impossible to stick the so-called antimagnetic sticker on the listener, because this will change the magnetic field around the antenna, causing the signal of the antenna to change, making the call impossible.
Dual-polarization antenna is a new type of antenna technology that combines +45° and -45° polarization directions orthogonal to each other antenna and works in the transceiver duplex mode at the same time, so its most prominent advantage is to save a single directional The number of base station antennas; generally, the directional base station (three sectors) of the GSM digital mobile communication network uses 9 antennas, and each sector uses 3 antennas (space diversity, one transmitter and two receivers). If dual-polarized antennas are used, each Only one antenna is needed for each sector; at the same time, due to the polarization orthogonality of ±45° in dual-polarized antennas, the isolation between +45° and -45° two antennas can meet the requirements of intermodulation pair antenna isolation Degree requirements (≥30dB), so the space between dual-polarized antennas only needs 20-30cm; in addition, dual-polarized antennas have the advantages of electrical adjustable antennas, and dual-polarized antennas are used in mobile communication networks with the same electrical adjustable Like the antenna, it can reduce call loss, reduce interference, and improve the quality of service of the entire network. If a dual-polarized antenna is used, because the dual-polarized antenna does not have high requirements for erection and installation, there is no need to requisition the land to build a tower, only a 20cm diameter iron pole is needed to fix the dual-polarized antenna on the iron pole according to the corresponding coverage direction That is to say, thereby saving infrastructure investment, and at the same time making the layout of the base station more reasonable, and the selection of the base station site is easier.
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.
An omnidirectional antenna, that is, it shows uniform radiation in 360° on the horizontal pattern, which is usually said to be non-directional, and shows a beam with a certain width on the vertical pattern. In general, the smaller the lobe width, The greater the gain. The omnidirectional antenna is generally used in the mobile communication system with the station type of the suburban district system, and the coverage is large.
The so-called mechanical antenna refers to a mobile antenna that uses mechanical adjustment of the downward tilt angle.
The so-called electrically adjustable antenna refers to a mobile antenna that uses electronic adjustment of the downward tilt angle.
A transceiving antenna used by the mobile base station BTS. That is, the antenna that transmits and receives to users (mobile phones).
Antennas that radiate or receive electromagnetic waves uniformly in all directions are called non-directional antennas, such as whip antennas for small communication devices.
It is an antenna shaped like the English letter V composed of two wires at an angle to each other. 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.
The dielectric antenna is a round rod made of low-loss high-frequency dielectric material (usually polystyrene), and one end of it is fed by a coaxial line or a waveguide.
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.
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.
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. For the principle of the lens, refer to the lens antenna. 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.
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.
In the center of the horizontal wire, connect a vertical down wire, the shape is like the English letter T, so it is called 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.
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.
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 caused by energy reflection at the feeder connector. In order to overcome these difficulties, a periscope antenna can be used. The periscope antenna is composed of a lower mirror radiator placed 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.
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 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 produced by this part is negligible, while the vertical part produces radiation.
Inverted L antennas are generally used for long-wave communications. Its advantages are simple structure and convenient erection; its disadvantages are large floor space and poor durability.
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 inductor can be added to the middle of the whip 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. Asymmetric vertical antennas are also called vertical ground antennas.
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.
A smart antenna is a two-way antenna installed on the site of a base station. It obtains directivity through a set of fixed antenna units with programmable electronic phase relationships, and can simultaneously obtain