Some concepts related to antennas

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Some concepts related to antennas [Copy link]

Antenna gain ( dB dBi dBd )

In the practical application of wireless communication, in order to effectively improve the communication effect and reduce the input power of the antenna, the antenna will be made into various structures with radiation directionality to concentrate the radiation power, which leads to the concept of " antenna gain " . Simply put, antenna gain refers to the degree to which an antenna concentrates the input RF power. Obviously, the gain of the antenna is closely related to its radiation pattern. The narrower the main lobe and the smaller the side lobe, the higher the gain of the antenna. The radiation patterns of antennas with different structures are very different. In the field of communication technology, like other parameters such as power and level, antenna gain is also expressed by a simplified method of relative comparison and logarithm. The specific calculation method is: when the same radiation field strength is generated in a certain direction to a certain position, the ratio of the input power of the lossless ideal reference antenna to the input power of the antenna to be considered is taken and multiplied by 10 ( G = 10lg ( reference Pin/ considered Pin) ), which is called the gain of the antenna in the direction of that point. The commonly used units for measuring antenna gain are dBi and dBd . And the dB we often say is dBi . For dBi , the benchmark is an ideal point source antenna, that is, a real " point " as a comparison benchmark for antenna gain. The radiation of an ideal point source antenna is omnidirectional, and its radiation pattern is an ideal sphere. The electromagnetic wave radiation intensity of all points on the same sphere is the same; for dBd , the benchmark is an ideal dipole antenna. Because the dipole antenna is directional, there is a fixed constant difference of 2.15 between the two, that is, 0dBd=2.15dBi=2.15dB . It should be noted that the commonly referred to " omnidirectional antenna " is not a strict term. The omnidirectional antenna should refer to the omnidirectional in three-dimensional space, but the engineering community often calls the antenna with a circular radiation pattern in a certain plane an omnidirectional antenna, such as a whip antenna, which has a circular main lobe in the radial direction, but still has an axial side lobe. Common antenna gain: whip antenna 6-9dBi , GSM base station Yagi antenna 15-17dBi , parabolic directional antenna can easily achieve 24dBi

Directivity function Directivity pattern Directivity coefficient

The radiation of any actual antenna is directional. The mathematical expression that describes the relative distribution of the electromagnetic field intensity radiated by the antenna in space at a certain distance from the antenna is called the antenna's directivity function. The directivity function is represented by a graph, which is a directional diagram. Because the radiation field of the antenna is distributed throughout the space, the antenna's directional diagram is a three-dimensional stereogram. Although computers can now be used to draw three-dimensional directional diagrams of complex antennas, the directional diagram on the so-called "principal plane" is still commonly used.

The radiation pattern also includes multiple lobes, which are called main lobe, double lobe and back lobe.

1. Main lobe width: the angle between the two half-power points (where the power drops to half of the maximum value or the field strength drops to 0.707 times the maximum value) in the maximum radiation direction of the main lobe and the center.

2. Sidelobe level: the logarithm of the ratio of the power density in the direction of maximum radiation of the sidelobe to the power density in the direction of maximum radiation of the main lobe (the sidelobe level usually refers to the first sidelobe closest to the main lobe and with the highest level)

3. Front-to-back suppression ratio: The logarithm of the ratio of the power density in the maximum radiation direction of the back lobe to the power density in the maximum radiation direction of the main lobe is called the front-to-back suppression ratio.

Directivity coefficient

In order to quantitatively describe the strength of the antenna directivity, or to compare the directivities of different antennas, the ratio of the power density of a point in the far field in the direction of maximum radiation of the antenna to the power density of an ideal non-directional point antenna with the same radiation power at the same point is called the directivity coefficient of the antenna and is expressed as D.

Radiation efficiency Radiation efficiency and gain relationship

The radiation efficiency of an antenna indicates whether the antenna can effectively convert energy. It is defined as the ratio of the radiated power of the antenna to the power input to the antenna.

The power loss of the transmitting antenna is usually the heat loss of the antenna conductor, the dielectric material loss, and the inductive loss near the objects near the antenna.

The gain factor is calculated using the input power, while the directivity factor is calculated using the radiated power. So the gain factor is equal to the directivity factor multiplied by the radiation efficiency

Input impedance polarization

The so-called antenna input impedance refers to the ratio of the high-frequency voltage at the antenna input to the high-frequency current at the input (the voltage and current are both vectors)

Antenna polarization is divided into linear polarization, circular polarization, and elliptical polarization. Linear polarization is further divided into horizontal polarization and vertical polarization; circular polarization is divided into left-hand circular polarization and right-hand circular polarization.

Antenna bandwidth

Whether it is a transmitting antenna or a receiving antenna, they always have to work within a certain frequency range (bandwidth). In general communication systems, the bandwidth of the antenna is usually specified by the standing wave ratio. Mobile communications generally define the working bandwidth of the antenna under the condition of standing wave ratio SWR ≤ 1.5 ; when the requirements are higher, it is specified as the working bandwidth under the condition of standing wave ratio SWR ≤ 1.2 . It should be known that when working within a certain frequency range (bandwidth), all electrical indicators of the antenna will change, but the changes that occur are allowed, and the communication system can work normally.

Balance and Imbalance

Signal sources, loads or transmission lines can be divided into two categories, balanced and unbalanced, according to their relationship to the ground. If the voltage between the two ends of the signal source and the ground is equal in magnitude and opposite in polarity, it is called a balanced signal source, otherwise it is called an unbalanced signal source; if the voltage between the two ends of the load and the ground is equal in magnitude and opposite in polarity, 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 used to connect the unbalanced signal source and the unbalanced load, and parallel two-wire transmission lines should be used to connect the balanced signal source and the balanced load, so that the signal power can be transmitted effectively, otherwise their balance or imbalance will be destroyed and they cannot work normally. If you want to connect an unbalanced transmission line to a balanced load, the usual way is to install a "balanced-unbalanced" conversion device between the two conductors, generally called a balun.

Standing wave ratio Return loss Reflection coefficient Traveling wave coefficient

Standing wave ratio: It is the inverse of the traveling wave coefficient, and its value is between 1 and infinity. A standing wave ratio of 1 indicates perfect matching; a standing wave ratio of infinity indicates total reflection and complete mismatch. In mobile communication systems, the standing wave ratio is generally required to be less than 1.5, but in actual applications, the VSWR should be less than 1.2

Return loss: It is the reciprocal of the absolute value of the reflection coefficient, expressed in decibels. The return loss value ranges from 0dB to infinity. The larger the return loss, the worse the match, and the larger the return loss, the better the match. 0 means full reflection, and infinity means perfect match.

Where the incident wave and the reflected wave have the same phase, the voltage amplitudes add up to the maximum voltage amplitude V max, forming an antinode; and where the incident wave and the reflected wave have opposite phases, the voltage amplitudes subtract to the minimum voltage amplitude V min, forming a node. The amplitude values at other points are between the antinode and the node. This composite wave is called a traveling standing wave.

The ratio of the reflected wave voltage to the incident wave voltage amplitude is called the reflection coefficient, denoted by R

Reflected wave amplitude (ZL-Z0)

R = ───── = ───────

Incident wave amplitude (ZL + Z0)

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

Antinode voltage amplitude Vmax (1 + R)

VSWR = ─────── = ────

Nodal voltage amplitude Vmin (1 - R)

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