What are the Different Antenna Types?

An antenna is a transformer that transforms a guided wave propagating on a transmission line into an electromagnetic wave propagating in an unbounded medium (usually free space), or vice versa. A component used in radio equipment to transmit or receive electromagnetic waves. Engineering systems such as radio communications, broadcasting, television, radar, navigation, electronic countermeasures, remote sensing, radio astronomy, etc., all use electromagnetic waves to transmit information, relying on antennas to work. In addition, in terms of transmitting energy by electromagnetic waves, non-signal energy radiation also requires an antenna. Generally, antennas are reversible, that is, the same antenna can be used as both a transmitting antenna and a receiving antenna. The basic characteristics of the same antenna as transmitting or receiving are the same. This is the reciprocity theorem of the antenna.

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An antenna is a transformer that transforms guided waves propagating on a transmission line into propagating in an unbounded medium (usually free space).

The antenna is made by Russian scientists

We know that radio, communications, radar, navigation, radio, and television all use radio waves to transmit information, and they all need to radiate and receive radio waves. In radio equipment, the device used to radiate and receive radio waves is called an antenna. Antennas provide the required coupling between a transmitter or receiver and the medium on which radio waves are propagated. Antennas, like transmitters and receivers, are also an important part of radio equipment.

The antenna radiates radio waves and receives radio waves. However, it is not radio waves that the transmitter sends to the antenna through the feeder. The receiving antenna cannot send radio waves directly to the receiver through the feeder. The energy conversion process must be performed. Below we take the radio communication equipment as an example to analyze the signal transmission process, and then explain the energy conversion effect of the antenna.

When a high-frequency current flows through a conductor, an electric field and a magnetic field are generated in the surrounding space. press

1.According to the nature of work

The measurement of most common antennas is to determine the radiation characteristics of its far field, such as the pattern (amplitude, phase, polarization), sidelobe level, gain, bandwidth, etc. This section will define the basic concepts of these measurements.

Figure 4 shows a typical configuration for measuring radiation characteristics. The basic step is to place a pair of transmitting or receiving source antennas in a far-field position relative to the antenna under test (AUT). The antenna under test is set up on a rotatable platform, and the antenna under test is rotated to collect a large number of directional pattern sampling values. To achieve the measurement of antenna radiation characteristics. Because the antenna is an electromagnetic open system, the test environment will affect the measurement results. Therefore, it is necessary to choose a test site reasonably and try to achieve a non-reflective environment, such as building a microwave darkroom.

In the antenna measurement, the working state of the antenna under test may be a transmitting state or a receiving state. This can be flexibly selected according to the content of the measurement, the measuring equipment, the site conditions and other factors. According to the antenna reciprocity principle, the results of measuring the antenna parameters in the two working states should be consistent.

However, in actual measurement, the principle of reciprocity must be applied under certain conditions.

(1) The antenna must be linear and passive, such as a satellite TV receiving antenna, whose feed is integrated with the high-frequency head (LNB) and cannot be used for transmission.

(2) The impedance matching of the transceiver system should be good. Although there are multiple reflections between the antenna under test and the source antenna, this effect is not serious due to the attenuation of free space propagation. The source antenna, the feeder, the signal source, and the antenna, the feeder, and the receiver to be tested. The impedance matching between them is an important condition to meet the principle of reciprocity.

(3) When the antenna is swapped, there are no active devices on the transmit and receive branches, such as power amplifiers, low-noise amplifiers, and mixers.

An antenna is an energy conversion device. A transmitting antenna converts a guided wave into a space radiation wave, and a receiving antenna converts a space radiation wave into a guided wave. Therefore, a transmitting antenna can be regarded as a source of radiated electromagnetic waves, and the field strength distribution around it is generally a function of the distance and angular coordinates from the antenna. Generally, the field area around the antenna is divided into an induction field area, a radiation near field area, and a radiation far field area according to the distance from the antenna, as shown in FIG. 5.

The antenna radiation characteristic measurement method is shown in Figure 6. The far field method can be divided into outdoor field, indoor field and compact field; the near field method can be divided into plane, spherical and cylindrical near field test methods.

1. Far-field method

The far-field method is also called the direct method. The obtained far-field data does not require calculation and post-processing to be a pattern. However, it often requires a long distance to test the characteristics of the antenna, so most far-field methods are performed in outdoor test sites. The outdoor field is divided into an elevated field and an inclined field, collectively referred to as a free space test field. The main disadvantage is that it is easily affected by external interference and site reflection. The far-field method is called an indoor field if it is performed in a dark room. Because of the large space required, indoor venues tend to be costly.

The compact field is classified as a far-field test field, but it does not use a large test field. Instead, it uses a parabolic antenna and a feed source. The feed source is placed in the focal area of the parabolic antenna. The wave reflected by the paraboloid is a plane wave. In this way, the antenna under test is in the plane wave area. The processing precision of the compact field equipment is very high. Changing the working frequency band requires replacing the feed source, which is relatively expensive.

2. Near-field method

The near-field measurement technology is a technology that uses a probe with known characteristics to sample the amplitude and phase characteristics of a field on a surface in the near-field area of the antenna, and obtains the far-field radiation characteristics of the antenna through strict mathematical transformation. According to the shape of the sampling surface, the near-field test field is divided into three types, that is, a flat test field, a cylindrical test field, and a spherical test field.

The main advantage of near-field measurement technology is that the required space is small, and high-precision measurements can be performed in a microwave darkroom, eliminating the difficulty of building a large microwave darkroom. The measurement is minimally affected by the surrounding environment and guarantees smooth operation all day. The amount of information measured is large, and by sampling on a surface in the near-field region, the far-field amplitude phase and polarization characteristics in any direction of the antenna can be accurately obtained. Near-field measurement techniques are discussed in detail in Chapter 7.

The back emission of the antenna is based on the principle of coherent addition of cavity waves. The resonant cavity is composed of a main reflector, a sub-reflector and a feed source. The radiation from the feed of the slow-wave structure strikes the main reflector, and then it is reflected back by the main reflector, and is reflected again at the fork of the sub-reflector, so it is formed in the resonant cavity along its axial direction. Standing wave field. The condition for forming a standing wave field is that the distance between the main and sub reflectors is an integer multiple of ^ / 2. Cause

The figure below shows two other cases of single polarization: + 45 ° polarization and -45 ° polarization, which are only used in special occasions. In this way, there are four kinds of unipolarity, see the figure below. Vertically and horizontally polarized

There are many critical parameters that affect antenna performance, and can usually be adjusted during antenna design, such as resonance frequency, impedance, gain, aperture or radiation pattern, polarization, efficiency, and bandwidth. In addition, the transmitting antenna has a maximum rated power, while the receiving antenna has noise suppression parameters.

Base station antenna, repeater antenna and indoor antenna commonly used in mobile communication.

17.1 Antenna 17.1 Overview

The cable connecting the antenna to the transmitter output (or receiver input) is called a transmission line or feeder. The main task of a transmission line is to efficiently transmit signal energy. Therefore, it should be able to transmit the signal power from the transmitter to the input of the transmitting antenna with minimal loss, or the signal received by the antenna to the receiver with minimal loss. The input terminal, at the same time, itself should not pick up or produce stray interference signals, so the transmission line must be shielded.

By the way, when the physical length of the transmission line is equal to or greater than the wavelength of the transmitted signal, the transmission line is also called a long line.

17.2 Antenna 17.2 Transmission Line Type

Coaxial cable transmission line There are generally two types of ultra-short-wave transmission lines: parallel two-wire transmission lines and coaxial cable transmission lines; microwave-wave transmission lines include coaxial cable transmission lines, waveguides, and microstrips. A parallel two-wire transmission line consists of two parallel wires. It is a symmetrical or balanced transmission line. This kind of feeder has a large loss and cannot be used in the UHF band. The two conductors of a coaxial cable transmission line are a core wire and a shielded copper network. Because the copper network is grounded, the two conductors are asymmetrical to the ground, so they are called asymmetric or unbalanced transmission lines. The coaxial cable has a wide operating frequency range and low loss, and it has a certain shielding effect on electrostatic coupling, but it is powerless to interfere with magnetic fields. When using, do not run in parallel with the line with strong current, and do not approach the low-frequency signal line.

17.3 Antenna 17.3 matching concept

What is matching? Simply put, when the load impedance ZL connected to the feeder terminal is equal to the feeder characteristic impedance Z0, it is called that the feeder terminal is matched and connected. During matching, only the incident wave to the terminal load exists on the feeder, and there is no reflected wave generated by the terminal load. Therefore, when the antenna is used as the terminal load, the matching can ensure that the antenna obtains all signal power. As shown in the figure below, when the antenna impedance is 50 ohms, it is matched with a 50 ohm cable, and when the antenna impedance is 80 ohms, it is not matched with a 50 ohm cable.

If the diameter of the antenna element is relatively large, the change of the input impedance of the antenna with frequency is small, and it is easy to keep matching with the feeder. At this time, the working frequency range of the antenna is wider. Otherwise, it is narrower.

In actual work, the input impedance of the antenna is also affected by surrounding objects. In order to make the feeder line match the antenna well, it is necessary to adjust the local structure of the antenna appropriately or install a matching device when measuring the antenna.

17.4 Antenna 17.4 reflection loss

It has been pointed out earlier that when the feeder and antenna are matched, there are no reflected waves on the feeder, only incident waves, that is, only the waves traveling in the direction of the antenna are transmitted on the feeder. At this time, the voltage amplitude and current amplitude everywhere on the feeder are equal, and the impedance at any point on the feeder is equal to its characteristic impedance.

When the antenna and the feeder do not match, that is, when the antenna impedance is not equal to the characteristic impedance of the feeder, the load can only absorb part of the high-frequency energy transmitted on the feeder, but not all of it. The unabsorbed part of the energy will be reflected back to form Reflected wave.

For example, in the picture on the right, because the impedance of the antenna and the feeder are different, one is 75 ohms, and the other is 50 ohms. The impedances do not match. The result is

17.5 Antenna 17.5 VSWR

In the case of mismatch, there are both incident and reflected waves on the feeder. Where the incident and reflected waves have the same phase, the voltage amplitudes are added to the maximum voltage amplitude Vmax to form an antinode. Where the incident and reflected waves are at opposite phases, the voltage amplitude is subtracted to the minimum voltage amplitude Vmin to form a node. The amplitudes of other points are between antinodes and nodes. This synthetic wave is called a traveling standing wave.

The ratio of the amplitude of the reflected wave voltage to the incident wave voltage is called the reflection coefficient and is recorded as R

Reflected wave amplitude (ZL-Z0)

Incident wave amplitude (ZL + Z0)

The ratio of the antinode voltage to the amplitude of the node voltage is called the standing wave coefficient, which is also called the voltage standing wave ratio, and is recorded as VSWR.

Antinode voltage amplitude Vmax (1 + R)

Nodal voltage amplitude Vmin (1-R)

The closer the terminal load impedance ZL and the characteristic impedance Z0, the smaller the reflection coefficient R, and the closer the standing wave ratio VSWR to 1, the better the matching.

17.6 Antenna 17.6 Balancer

Signal sources or loads or transmission lines can be divided into balanced and unbalanced types according to their relationship to ground.

If the voltage between the two ends of the signal source is equal to the ground and the polarity is opposite, it is called a balanced signal source, otherwise it is called an unbalanced signal source. It is called a balanced load, otherwise it is called an unbalanced load; if the impedance between the two conductors of the transmission line and the ground is the same, it is called a balanced transmission line, otherwise it is an unbalanced transmission line.

Coaxial cables should be connected between the unbalanced signal source and the unbalanced load, and parallel two-wire transmission lines should be connected between the balanced signal source and the balanced load, so as to effectively transmit signal power, otherwise their balance or unbalanced The balance will be disrupted and it will not work properly. If you want to use an unbalanced transmission line to connect with a balanced load, the usual method is to install a "balanced-unbalanced" conversion device between the grain producers, which is generally called a balanced converter.

7.1 Half-wavelength balanced converter

Also called "U" shaped tube balanced converter, it is used for the connection between the unbalanced feeder coaxial cable and the balanced load half-wave symmetrical oscillator. The "U" tube balance converter also has a 1: 4 impedance conversion effect. The characteristic impedance of coaxial cables used in mobile communication systems is usually 50 ohms, so in the YAGI antenna, a half-wave-reduced oscillator is used to adjust the impedance to about 200 ohms to achieve the final impedance matching with the main feeder 50 ohm coaxial cable .

7.2 quarter-wavelength balanced-unbalanced

The balance-unbalance conversion between the balanced input port of the antenna and the unbalanced output port of the coaxial feeder is realized by utilizing the property that the quarter-wave short-circuited transmission line terminal is open at high frequencies.