What is half-wave radiator ?
A half-wave radiator (in English: half-wave radiator) is a receiving device used in medium-sized and large shortwave receiving stations. The main types include disc-conical antennas, fishbone antennas, octagon antennas, fan-shaped antennas, double-conical antennas, and parabolic antennas, etc. It achieves shortwave signal reception through different structural designs and has the characteristics of compact size.
Half-wave radiator classification introduction
Cone-shaped antenna
It is a type of ultra-short wave antenna. The top part is a disc (i.e. the radiation body), which is fed by the center line of the coaxial cable. Below it is a cone, which connects to the outer conductor of the coaxial cable. The function of the cone is similar to that of an infinitely large ground, and by changing the inclination angle of the cone, the maximum radiation direction of the antenna can be altered. It has an extremely wide frequency band.
Fishbone-shaped antenna
The fishbone-shaped antenna, also known as the edge-fed antenna, is a specialized shortwave receiving antenna. It is composed of a pair of connecting lines, with an identical dipole connected at regular intervals along each line. These dipoles are all connected to the connecting lines through a very small capacitor. At the end of the connecting line, that is, the end facing the communication direction, a resistor with the same characteristic impedance as the connecting line is connected. The other end is connected to the receiver through a feeder.
Compared with the rhombic antenna, the advantages of the fishbone-shaped antenna are that its side lobes are small (meaning that it has strong reception capability in the main lobe direction and weak reception in other directions), there is little mutual influence among the antennas, and it occupies less space; the disadvantages are low efficiency, and installation and use are relatively complex.
Yagi antenna
Also known as a directional antenna. It is composed of several metal rods. One of them is the radiator, the longer one behind it is the reflector, and the shorter ones in front are the directors. The radiator is usually a folded half-wave dipole. The maximum radiation direction of the antenna is the same as the direction of the director. The advantages of an octagon antenna are simple structure, lightweight, sturdy, and convenient for feeding; the disadvantages are narrow bandwidth and poor anti-interference performance. It is used in ultra-short wave communication and radar.
Fan-shaped antenna
It comes in two forms: metal plate type and metal wire type. This antenna increases the cross-sectional area of the antenna, thereby widening the antenna frequency band. The wire-type fan-shaped antenna can be made with three, four or five metal wires. Fan-shaped antennas are used for ultra-short wave reception.
Double-conical antenna
The double-conical antenna is composed of two cones with their apexes facing each other, and it is fed at the apexes. The cones can be made of metal surfaces, metal wires or metal meshes. Just like the loop antenna, as the cross-sectional area of the antenna increases, the frequency band also widens. The double-conical antenna is mainly used for ultra-short wave reception.
Parabolic Antenna
A parabolic antenna is a directional microwave antenna, consisting of a parabolic reflector and a radiator. The radiator is installed at the focus or the focal axis of the parabolic reflector. The electromagnetic waves emitted by the radiator are reflected by the parabolic surface, forming a highly directional beam. The parabolic reflector is made of a metal with good conductivity, and there are the following four main types: rotating parabolic, cylindrical parabolic, cut-rotating parabolic, and elliptical-edge parabolic. The most commonly used types are rotating parabolic and cylindrical parabolic. The radiator usually adopts half-wave oscillators, open waveguides, and slot waveguides.
Parabolic antennas have the advantages of simple structure, strong directionality, and wide working frequency band. However, their disadvantages are as follows: Since the radiator is located in the electric field of the parabolic reflector, the reflector has a significant reaction on the radiator, making it difficult to achieve good matching between the antenna and the feeder; the back radiation is large; the protection degree is poor; and the manufacturing accuracy is high. These antennas are widely used in microwave relay communication, tropospheric scattering communication, radar and television.
Horn parabolic antenna
The horn parabolic antenna consists of a horn and a parabolic section. The parabolic section is placed over the horn, and the vertex of the horn is located at the focus of the parabolic section. The horn acts as a radiator, emitting electromagnetic waves towards the parabolic section. These waves are reflected by the parabolic section and focused into a narrow beam for emission.
The advantages of the horn parabolic antenna are as follows: The reflector has no effect on the radiator, the radiator does not obstruct the reflected radio waves, the antenna has a good match with the feeder device; the back radiation is small; the protection degree is high; the working frequency band is very wide; and the structure is simple. The horn parabolic antenna is widely used in trunk line relay communication.
Horn antenna
Also known as horn antenna. It is composed of a uniform waveguide and a horn-shaped waveguide with gradually increasing cross-section. There are three forms of horn antennas: sector-shaped horn antenna, conical horn antenna and cylindrical horn antenna.
The horn antenna is one of the most commonly used microwave antennas, typically serving as a radiator. Its advantages include a wide operating frequency band; however, its disadvantage is that it is relatively large in size. Moreover, for the same aperture size, its directionality is not as sharp as that of a parabolic antenna.
Horn lens antenna
It is composed of a horn and a lens mounted on the aperture of the horn. Therefore, it is called a horn lens antenna. The principle of the lens can be found in the lens antenna. This type of antenna has a relatively wide operating frequency band and has higher protection performance than parabolic antennas. It is widely used in microwave trunk communication with a large number of channels.
Lens Antenna
In the centimeter wave band, many optical principles can be applied to the field of antennas. In optics, by using lenses, the spherical waves emitted by a point light source placed at the focus of the lens can be refracted by the lens and transformed into plane waves. The lens antenna is made based on this principle. It consists of a lens and a radiator placed at the focus of the lens.
There are two types of lens antennas: the medium deceleration lens antenna and the metal acceleration lens antenna.
The medium deceleration lens is made of low-loss high-frequency medium, which is thick in the middle and thin around the edges. When the spherical wave emitted from the radiation source passes through the medium lens, it is slowed down. Therefore, the path of the spherical wave that is slowed down in the middle part of the lens is longer, while the path of the spherical wave that is slowed down in the peripheral part is shorter. Thus, after passing through the lens, the spherical wave becomes a planar wave, meaning that the radiation becomes directional.
The metal acceleration lens is composed of many metal plates of different lengths placed parallelly. The metal plates are perpendicular to the ground, and the shorter the metal plate is closer to the center. The electromagnetic wave is accelerated when propagating in the parallel metal plates. When the spherical wave emitted from the radiation source passes through the metal lens, the longer the path of acceleration it receives as it approaches the edge of the lens, while the shorter the path of acceleration in the center. Therefore, the spherical wave after passing through the metal lens becomes a planar wave.
The lens antenna has the following advantages: 1. The side lobes and rear lobes are small, thus the direction pattern is better; 2. The manufacturing accuracy of the lens is not high, so the manufacturing process is relatively simple. Its disadvantages are low efficiency, complex structure and high price.
The lens antenna is used in microwave relay communication.
Slotted Antenna
By cutting one or several narrow slots on a large metal plate and feeding it with a coaxial line or a waveguide, an antenna is formed. This type of antenna is called a slotted antenna or a crack antenna. To achieve unidirectional radiation, a cavity is made at the back of the metal plate, and the slots are directly fed by the waveguide. The slotted antenna has a simple structure without protrusions, making it particularly suitable for use on high-speed aircraft. However, its disadvantage is that tuning is difficult.
Dielectric antenna
A dielectric antenna is a cylindrical rod made of low-loss high-frequency dielectric material (typically polystyrene). One end of the rod is fed with a coaxial line or waveguide. The function of the sleeve is not only to hold the dielectric rod but also to reflect electromagnetic waves, thereby ensuring that the electromagnetic waves excited by the inner conductor of the coaxial line propagate towards the free end of the dielectric rod.
The advantages of the dielectric antenna are its small size and sharp directionality; its disadvantages are that the dielectric has loss, resulting in low efficiency.
Periscope antenna
In microwave relay communication, antennas are often placed on very high supports, so feeding the antenna requires very long feedlines. Long feedlines can cause many problems, such as complex structure, high energy loss, and distortion caused by energy reflection at the feedline joints. To overcome these difficulties, a periscope antenna can be used. The periscope antenna is placed on the ground.