liwen01 2024.10.01
preamble
Maxwell predicted the existence of electromagnetic waves, Hertz confirmed Maxwell's prediction through experiments, Marconi invented the wireless telegraphy system based on the principle of wireless electromagnetic waves, and since then mankind has entered the era of wireless communication systems.
Antennas are an essential component of a communication system, which serves to convert electrical signals into electromagnetic wave signals for transmitting, as well as to convert received electromagnetic wave signals into electrical signals.
In WiFi applications, WiFi antennas are closely related to WiFi performance, including but not limited to parameters such as antenna orientation, polarization, gain, and operating frequency range.
(i) Fundamentals of secondary school physics
To understand how antennas work, you need to review the basics of secondary school physics, which will only be outlined here without going into detail. Some of these basics can be found in the first article content of thewifi basic (a): radio waves and WIFI signal interference, attenuation》
(1) Maxwell's equations
James Clerk Maxwell Maxwell's system of equations, which collectively classifies electricity, magnetism, and light as electromagnetic field phenomena, was proposed, realizing the second unification of physics since Newton.
The differential expression for the system of Maxwell's equations is:
It can be briefly summarized as:
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Gauss's law (electric field) : Describes the relationship between electric field and charge, showing that charge is the source of the electric field, which diverges from positive charge and converges toward negative charge.
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Gauss's law (magnetic field) : suggests that the magnetic field has no monopoles (i.e., there are noisolated poles), the magnetic field lines are closed, i.e., the flux of the magnetic field is zero through any closed surface, and the north and south poles always exist in pairs.
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Faraday's law: Describe how a changing magnetic field produces an electric field; a changing magnetic field produces a rotating electric field in the surrounding space; this is the phenomenon of electromagnetic induction.
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Ampere-Maxwell law (physics): Describe how an electric current and a changing electric field generate a magnetic field. A changing electric field induces a magnetic field, and the propagation of electromagnetic waves relies on this changing electric and magnetic fields generating each other.
Maxwell proposed in his 1864 paper "Dynamical Theory of Electromagnetic Fields" that electric and magnetic fields propagate in space at the speed of light in the form of waves, and suggested that light is an electromagnetic perturbation that causes many phenomena in the electric and magnetic fields of the same medium, while theoretically predicting the existence of electromagnetic waves.
(2) Hertz test
In 1886 Heinrich Rudolf Hertz experimentally confirmed the existence of electromagnetic waves and measured the speed of electromagnetic wave propagation to be the same as the speed of light, and further observed that electromagnetic waves have the properties of focusing, directness, reflection, refraction and polarization.
Hertz test to prove the existence of electromagnetic waves, using a device that is a spark gap transmitter.
(1) Introduction to Circuit Components：
B: A battery or power supply that provides the electrical energy needed for a circuit.
SW: A switch that is used to control the on/off of the circuit and start the experiment.
C: Capacitors for storing electrical energy
T: Transformer to generate high voltage across S.
L: coil on the transformer, forming an LC resonant circuit with capacitors
I: An interrupter (intermittent) that periodically opens and closes a circuit to rapidly charge and discharge a capacitor, producing a series of damped waves.
S: Spark gap, when the capacitor is fully charged and the voltage rises to a sufficient level, a spark discharge occurs at the spark gap, releasing the energy of the capacitor C.
M: The spark gap in the receiver, where sparks are also generated when EM waves arrive, proves the presence of EM waves.
(2) Principle of operation：
energy storage：
When the switch SW is closed, the power supply B charges the capacitor C1, during the charging process, the current is changing, the changing current passes through the coil L1, generating a changing magnetic field, the changing magnetic field generates an induced electromotive force (EMF) in the coil L2, charging the two capacitors C. The current is changing, the changing current passes through the coil L1, generating a changing magnetic field.
The C1 capacitor gradually stores energy until there is no current flow in the circuit, or S produces an ionizing spark.
spark
When the voltage of capacitor C is increased to a sufficiently high level, the energy stored in capacitor C is released at spark gap S because the high voltage ionizes the air and produces a spark discharge under the action of the electric field.
When the spark discharges, it produces a rapid change in current (current pulse) that excites the current in the coil L and generates an electromagnetic wave in the surrounding space.
The discharging spark produced here is based on a similar principle to that of the Tesla coil, where a spark is produced by ionizing the air with a high voltage discharge.
electromagnetic wave propagation：
Due to the rapid change in current, according to Maxwell's equations, a changing current produces a changing magnetic field, and a changing magnetic field induces an electric field in a coil, and this changing electromagnetic field propagates through space in the form of electromagnetic waves.
receive electromagnetic waves：
When the electromagnetic wave reaches the loop antenna M of the receiver, it excites an induced electromotive force in the wire loop, causing a spark to be generated between the two spheres of the wire loop as well, indicating that the electromagnetic wave was received by this wire loop.
A fundamental limitation of spark gap transmitters is that they produce transient pulses, called damped waves, which cannot produce the continuous waves used in modern radio transmissions.(e.g., radio, wireless phone signals, etc.).
(ii) Dipole antennas
Dipole Antenna (Dipole Antenna) is one of the simplest and most basic types of antennas, and also one of the most widely used antennas in modern wireless communication systems, it consists of two conductors of the same length.
- In the metal conductor of a dipole antenna, by feeding an alternating current, the current varies periodically with time in the form of a sine wave
- As the alternating current continues to flow in the antenna, the direction of the current repeatedly changes, creating ever-changing electric and magnetic fields.
- These two fields induce each other: a changing electric field produces a magnetic field, and a changing magnetic field produces an electric field, creating an electromagnetic wave.
Electromagnetic waves can be visualized as self-propagating transverse oscillating waves of electric and magnetic fields
- If the two wires are close together, the electric field is bound between the two wires and thus the radiation is very weak
- By spreading the two wires apart, the electric field spreads out in the surrounding space and the electromagnetic wave radiation is enhanced.
- In practice, a dipole antenna emits electromagnetic waves in all directions, but the intensity of the emission is not uniform in different directions.
Typically, the main radiation direction of the antenna is perpendicular to the antenna axis, showing a circular radiation pattern.
These two wires in a dipole antenna which generate electromagnetic waves are calledvibrator. Generally, the size of the oscillator works best at half a wavelength, so it is also often called thehalf-wave oscillator。
With an oscillator, it is possible to send a continuous electromagnetic wave.
(iii) Direction of the antenna
The directionality of an antenna is the ability of an antenna to radiate electromagnetic waves in a certain direction. For a receiving antenna, directionality indicates the ability of the antenna to receive electromagnetic waves coming from different directions.
Antennas have different radiation or reception capabilities for different directions in space, which is the directionality of the antenna.
According to the directionality of the antenna, the antenna can be divided into two categories: omnidirectional antenna and directional antenna.
(1) Omni-directional antenna
- An antenna that radiates and receives without a maximum direction in a horizontal plane is called an omnidirectional antenna.
- Omni-directional antennas are mostly used in the center station of point-to-multipoint communications because of their non-directionality.
- Commonly used WiFi antennas are omnidirectional antennas
Radiation direction of a half-wave symmetric oscillator antenna
Horizontal (H-side)
- The circular figure on the left represents a horizontal section looking down from the top of the antenna, i.e., the antenna's radiation pattern in the horizontal direction.
- This antenna is omnidirectional (radiating uniformly) in the horizontal direction, similar to a circular radiation pattern, with arrows indicating that the signal extends uniformly in all directions.
- This means that in the horizontal direction, the signal strengths are equal
Vertical (E-side)
- The figure on the right shows the direction of radiation in the vertical plane, i.e. from the side of the antenna. The vertical radiation pattern of the antenna resembles a figure 8 in the vertical plane.
- Indicates that the antenna is not omnidirectional in the vertical direction. Signal strength is stronger at certain angles and weaker at other angles (e.g., up and down directions).
- In the direction of the axis of the oscillator it radiates zero
(2) Directional antennas
- An antenna with one or more maximum directions in a horizontal plane is called a directional antenna
- Directional antennas are suitable for long-distance communication because they have a maximum radiation or reception direction, thus concentrating the energy.
- Higher immunity to interference due to directionality
- Commonly, there are Panel Antenna and Parabolic Antenna.
Using a parabolic reflecting surface, the power is reflected in a unilateral direction and the energy is concentrated into a small three-dimensional angle, reflecting and thus obtaining a very high gain.
(iv) Antenna polarization
Antenna Polarization (Antenna Polarization) refers to the antenna's ability to radiate or receive electromagnetic waves.electric fieldThe direction of the vector.
Since the electric field has a constant relationship with the magnetic field, it is generally taken as theelectric field vectordirection as the direction of polarization of the electromagnetic waves radiated by the antenna, and is the direction of the electric field vector in the direction of maximum radiation of the antenna as the direction of polarization of the antenna.
There are three main categories of antenna polarization: linear polarization, circular polarization and elliptical polarization:
(1) Linear Polarization
Line polarization is the vibration of the electric field vector of an electromagnetic wave along a straight line, which can be further classified according to the direction:
- Vertical Polarization: The electric field vector vibrates in the vertical direction. This type of polarization is commonly used in terrestrial communications, broadcasting, and some mobile communications applications because it is easier for signals to propagate around obstacles.
- Horizontal Polarization: The electric field vector vibrates in the horizontal direction. It has better penetration in certain specialized environments, such as long-distance radio communications.
Line Polarization Characteristics
- Line-polarized antennas are typically used for fixed-direction point-to-point communications because they have a single electric field direction
- The match between vertically and horizontally polarized antennas is very important, and mismatched polarization can lead to signal loss (called polarization loss)
(2) Circular Polarization
Circular polarization is the spiral rotation of the electric field vector over time, with the direction of the electric field constantly changing to form circular vibrational trajectories. There are two types of circular polarization:
right circular polarization(Right-Hand Circular Polarization, RHCP) : The electric field vector rotates in a clockwise direction.
left circular polarization(Left-Hand Circular Polarization, LHCP) : The electric field vector rotates in a counterclockwise direction.
- The advantage of circular polarization is its ability to adapt to complex reflections in multipath propagation environments, as the direction of the electric field is constantly changing and the receiver is able to maintain good signal reception over different reflection paths.
- Circularly polarized antennas are widely used in satellite communication, UAV control, GPS and other scenarios, and are particularly suitable for communication with mobile and rotating devices.
(3) Elliptical Polarization
Elliptical polarization is a type of polarization between linear and circular polarization. The electric field vector rotates in an elliptical trajectory and the polarization is not exactly linear or circular. This type of polarization is usually found in antenna design for some special applications
(4)Dual-Polarized Antenna (DPA)
A dual-polarized antenna transmits and receives signals through two separate radiating units, each with a different polarization direction.
These two polarization directions are orthogonal to each other, so two different sets of signals can be sent and received at the same frequency, greatly improving the efficiency of data transmission.
This orthogonal polarization enables dual-stream signal transmission without interfering with each other
The picture below shows the built-in antenna of one AP model
Key advantages of dual-polarized antennas
Improvement of spectral efficiency: Dual-polarized antennas can transmit two separate signal streams simultaneously at the same frequency, thereby doubling the data transmission rate. This is particularly important for improving spectrum utilization, especially in cellular communications and wireless local area WiFi networks.
Enhanced signal quality (MIMO technical support) : Dual-polarized antennas can support MIMO (Multiple Input Multiple Output) technology, which can better cope with signal reflection, attenuation, and multipath effects by using multiple antenna units and polarization directions, thus enhancing the coverage and stability of wireless signals.
Cross-Polarization Isolation Reduction : A dual-polarized antenna reduces mutual interference because its two polarization directions are orthogonal (90 degrees phase difference). This isolation reduces interference in adjacent frequency bands, especially in high-density radio environments.
Improve anti-interference capability: By using two orthogonally polarized signals at the same time, dual-polarized antennas are better able to cope with interference in the environment, especially in complex multipath propagation environments. Signal paths with different polarizations will have different interference behaviors, thus effectively separating signal and noise.
Enhanced Spatial Multiplexing : In MIMO systems, dual-polarized antennas enable spatial multiplexing by using two orthogonal polarization directions at the same time, thus further increasing the data transmission rate. This is extremely important in modern wireless communication systems.
(5) Polarization matching
Polarization matching between antennas is critical to the efficiency of signal transmission. If the polarizations of the transmitting and receiving antennas are not matched (e.g., one antenna is vertically polarized while the other is horizontally polarized), this can lead to significant signal loss, a phenomenon known as polarization loss. In extreme cases, the antennas with mismatched polarizations may fail to receive signals at all.
co-polarized communications: Transmitting antenna and receiving antenna have the same polarization, high communication efficiency and low signal loss.
cross-polarization loss: When the polarization of the transmitting and receiving antennas do not coincide, the received signal is significantly weakened and the loss increases s
(v) Antenna gain
Antennas are usually passive devices which do not amplify electromagnetic signals.
Antenna gain does not indicate the actual antennazoom inup the signal, but rather indicates that the antenna is able to radiate the incoming power more centrally in one direction. Higher gain means that the antenna radiates or receives more power in one direction, but correspondingly less power in other directions. Therefore, the higher the antenna gain, the more directional the antenna is.
Antenna gain is a measure of an antenna's ability to send and receive signals in a particular direction, and is one of the most important parameters in selecting an antenna and is related to the specific antenna model.
(1) Unit of antenna gain
There are two units of antenna gain dBi and dBd
(a) dBi (gain relative to isotropic antenna)
- dBi is the antenna gain relative toIsotropic Antenna The gain of the An isotropic antenna is an ideal antenna that radiates energy uniformly in all directions, so its gain is the same in all directions and is defined as 0 dBi
- When dBi is used as a unit of gain, it indicates how many more decibels of power an antenna radiates in a given direction compared to an isotropic antenna under the same conditions.
- dBi is the international standard unit of antenna gain, so it is widely used in antenna specifications, especially in devices such as Wi-Fi routers, radios, and satellite communications.
(b) dBd (gain with respect to dipole antenna)
- dBd is the antenna gain with respect to Half-Wave Dipole Antenna. The Half-Wave Dipole Antenna is a commonly used reference antenna that radiates energy in a more directional manner than isotropic antennas and has a greater ability to radiate in the horizontal direction.
- A half-wave dipole antenna has a gain of 2.15 dBi, which means that the dipole antenna radiates 2.15 dB more power horizontally than an isotropic antenna.Therefore, when the antenna gain is expressed in dBd, it represents the antenna's gain compared to a half-wave dipole antenna.
Since the gain of the half-wave dipole antenna is 2.15 dBi, the relationship between dBi and dBd is:dBi=dBd+2.15
In most WiFi devices, antenna gain is usually labeled in dBi, indicating its radiation performance relative to an isotropic antenna. Common home WiFi router antenna gains range from 2 dBi to 5 dBi.
(vi) Flap width
The antenna's beamwidth generally refers to the half-power beamwidth of the main flap (Half-Power Beamwidth, HPBW). It represents the main valve in two directions on the signal strength down to half the maximum strength (-3 dB) when the formation of the angular difference, usually expressed in angular degrees (°).
Lobe : Each raised portion of the antenna's radiation direction map is called a flap. The main flap is the strongest radiating flap, and the side and back flaps are relatively weak flaps.
Half-Power Point : The flap width is defined as the angular range at which the main flap power of the antenna reaches 50% of its maximum power, i.e., the point at which the power is attenuated by 3 dB from the direction of maximum radiation.
Beamwidth : The flap width represents the angle between the main flap from the left half-power point to the right half-power point. This angle is typically defined as -3 dB of flap width
wide-bandwidth valve : Omni-directional antennas typically have wider wavelength widths and radiate or receive less directionally, making them suitable for covering larger areas, such as omni-directional antennas for home WiFi routers.
narrowband : Directional antennas have narrow flap widths and are suitable for long-distance and high-precision communication scenarios, such as microwave communications, satellite communications and radio transmissions.
(1) Relationship between flap width and antenna gain
There is an inverse relationship between the antenna's wavelength width and gain. Typically, the narrower the antenna's wavelength width, the higher the antenna's gain. This is because a narrow-wavelength antenna concentrates more energy in a smaller angular range, thus increasing the signal strength in that direction
(2) Measurement of wavefront width
The antenna's flap width is usually obtained by measuring the antenna's Radiation Pattern. The Radiation Pattern is a three-dimensional image that shows the distribution of the radiated power of the antenna in different directions.
Find the half-power point of the main flap by measuring the power density in different directions and calculate the angular difference between the two half-power points, i.e., the flap width
wind up
Antennas are an important part of a WiFi device, and wave propagation and antennas are a separate subject with a lot of knowledge involved. I am not a communications professional and have only a superficial knowledge of antennas.
In order to write this WiFi series introduction, I actually learned a lot of things off the top of my head, and although I did my best, mistakes are inevitable. I'd be grateful if you, the readers, could let me know if there are any mistakes.
The next article will fill in the basics of WiFi networking.