Contents
1. Electromagnetic waves
2. Direct waves
3. Reflected waves
4. Diffraction waves
5. Scattered waves
6. Skin effect
7. Multipath effect
8. Shadow effect
9. Fresnel zone 10.
Slow fading and fast fading
1. Electromagnetic
waves Electromagnetic waves are a type of energy. Any object with a temperature higher than absolute zero will release electromagnetic waves. Electricity and magnetism can be said to be two sides of the same coin. Current will produce a magnetic field, and a changing magnetic field will produce current. The changing electric field and the changing magnetic field constitute an inseparable unified field.
In low-frequency electrical oscillations, the mutual changes between magnetism and electricity are relatively slow, and almost all of the energy returns to the original circuit without energy being radiated; in high-frequency electrical oscillations, magnetoelectricity changes very quickly, and it is impossible for all the energy to return The original oscillator circuit, then electrical energy and magnetic energy propagate out into space in the form of electromagnetic waves along with the periodic changes of the electric field and magnetic field. Energy can be transferred outward without a medium. This is a kind of radiation.
In the sport of table tennis, many laws are very similar to the laws of electromagnetic waves. Next, we will understand the classification of radio frequency medium waves from the game of table tennis.
2. Direct wave
analogy: If there is no obstruction when hitting the center of the ball directly, the ball will move along the Running in a straight line is like a direct wave.
Radio waves that travel from the transmitting antenna to the receiving point in a straight line are called direct waves. Free space radio wave propagation is the propagation of radio waves in vacuum, which is an ideal propagation condition. When radio waves propagate in free space, they can be considered as direct waves, and their energy will neither be absorbed by obstacles nor reflected or scattered.
3. Reflected wave
analogy: Let’s take table tennis as an example. If the ball hits the edge of the table, it will move according to the rules of the reflection angle and other incident angles, just like a reflected wave.
Application: When selecting stations for high-speed railway wireless coverage, attention should be paid to the incident angle of radio waves. The alternative station site should not be too far away, otherwise the incident angle will be too large and the refraction ability into the carriage will be reduced. Generally, a station site about 100 meters away from the railway will be selected.
Wireless signals reach the receiving point after being reflected from the ground or other obstacles, which is called a reflected wave. Reflections occur from the earth's surface, buildings and wall surfaces. Reflected waves only occur at the interface between two propagation media with different densities. The greater the density difference between the media at the interface, the greater the amount of wave reflection and the smaller the amount of refraction. The smaller the incident angle of the wave, the smaller the amount of reflection and the greater the amount of refraction.
4. Diffraction wave
analogy: Taking table tennis as an example again, if after hitting the ball, the cue ball is tangent to another ball, depending on the strength and direction, it can bypass the ball within the visual distance, just like diffraction;
When the wireless path between the receiver and transmitter is blocked by a sharp edge, the phenomenon of radio waves traveling around the obstacle is called diffraction. During diffraction, the path of a wave is changed or bent. The secondary waves generated by the blocking surface are spread in space, even on the back side of the blocking body. Diffraction loss is the loss caused by various obstacles to radio wave transmission.
5. Scattered wave
analogy: Let’s take playing pool as an example. Assume that many balls in a range are no more than one ball apart from each other. When the cue ball hits the middle of these balls, it will cause many balls to move in different directions, much like scattering.
Scattering occurs when there are objects smaller than the wavelength in the medium through which radio waves travel, and the number of obstructions per unit volume is very large; scattered waves are generated from rough surfaces, small objects or other irregular objects. In real communication systems, scattering is caused by leaves, street signs, lamp posts, etc.
2. Multipath effect
3. Shadow effect
4. Fresnel zone
5. Slow fading and fast fading
6. Skin Effect
Analogy: After heavy rain, the middle of the dirt road is filled with water, and everyone has to queue up along the roadside to pass. The effective passing area of the road is reduced due to water accumulation, which affects people's travel efficiency.
Since the inductive reactance inside the conductor hinders alternating current more than the surface, when alternating current passes through the conductor, the current density in each part is uneven, and the current density on the surface of the conductor is large (reducing the cross-sectional area and increasing loss). This phenomenon is called It is the skin effect. The higher the frequency of alternating current, the more significant the skin effect. When the frequency is high to a certain extent, it can be considered that the current flows completely from the surface of the conductor. Practical applications: hollow wires replace solid wires to save materials; use multiple strands to interact with each other in high-frequency circuits. Insulated thin wires are braided into bundles to weaken the skin effect.
2. Multipath effect
analogy: When I was playing with mud when I was a child, I poured water on the top of a small mound. The water flowed away from all directions. A lot of water seeped into the soil or flowed in different directions. It is lost, and part of the water flows to a low-lying place through different paths and at different times.
The multipath effect of radio waves means that the signal often has many transmission paths with different delays and losses from the transmitting end to the receiving end, which can be direct radiation, reflection or diffraction. The same signal in different paths will be superimposed at the receiving end. A phenomenon that increases or decreases the energy of a received signal.
3. Shadow Effect
Analogy: When the warm sunshine shines on the earth, trees and houses have shadows. This shadow is not complete darkness, but a light with much weakened intensity.
On the propagation path, when radio waves encounter obstacles such as uneven terrain, buildings of varying heights, tall trees, etc., a shadow area with weak radio signal field strength will be formed behind the obstruction. This phenomenon is It's called the shadow effect.
4. Fresnel zone
analogy: Sometimes, the most effective vision range of the human eye is also an ellipsoid. Although things outside the ellipsoid can be seen, they are no longer particularly clear. A well-trained shooter's effective vision range must be concentrated within the ellipsoid with a very small radius between him and the target.
The Fresnel zone is an ellipsoid, and the transmitting and receiving antennas are located at the two foci of the ellipsoid. The radius of this ellipsoid is the first Fresnel radius. In free space, the electromagnetic energy radiated from the transmitting point to the receiving point mainly propagates through the first Fresnel zone. As long as the first Fresnel zone is not blocked, propagation conditions similar to free space can be obtained.
In order to ensure normal communication of the system, the height of the transmitting and receiving antennas should be set up so that the obstacles between them should not exceed 20% of their Fresnel zones. Otherwise, multipath propagation of electromagnetic waves will have adverse effects, leading to a decline in communication quality, or even Interrupt communication.
5.
Analogy between slow decline and fast decline: During the decline of the stock market, although its time-sharing curve fluctuates violently, the 5-week line changes relatively slowly; in another case, the time-sharing instantaneous value of the stock price changes drastically, much like a fast decline .
During the propagation of radio waves, the median value of the signal strength curve changes slowly, which is called slow fading. Slow fading reflects the median value of the weighted average of instantaneous values, reflects the average change in reception level in the medium range of hundreds of wavelengths, and generally follows a logarithmic normal distribution.
The causes of slow fading:
1) path loss;
2) signal fading caused by shadow effect, etc.
Fast fading is the instantaneous rapid fluctuation and rapid change of the received signal field strength value. Fast fading is caused by the superposition of multipath propagation signals caused by various terrains, ground objects, and moving objects at the receiving point. Due to the different phases, frequencies, and amplitudes of the received multipath signals, the amplitude of the superimposed signal fluctuates violently. . When the mobile station is operating at high speed, the carrier frequency range of the received wireless signal changes with time, which can also cause drastic changes in the amplitude of the superimposed signal.
Generally, fast fading can be subdivided into:
1) Multipath effect causes spatial selective fading, that is, the fading characteristics are different in different locations and different transmission paths;
2) Changes in carrier frequency cause the carrier width range to exceed the range of the coherent bandwidth. The signal distortion caused is called frequency selective fading;
3) The Doppler effect or multipath effect can cause different signals to arrive at the receiving point with different time differences, exceeding the coherence time, and the signal distortion caused is called time selective fading.