Uncover the Secrets of the Antenna -2
The reason why the antenna can transmit information at high speed is because it can launch electromagnetic waves carrying information into the air, propagate at the speed of light, and finally reach the receiving antenna.
It's like using high-speed trains to transport passengers. If you compare information to passengers, then the means of transporting passengers: high-speed trains are electromagnetic waves, and antennas are equivalent to stations, responsible for managing and dispatching the transmission of electromagnetic waves.
So, what are electromagnetic waves?
Scientists have studied the two mysterious forces of electricity and magnetism for hundreds of years. In the end, Maxwell in the United Kingdom proposed that electric current can generate electric field around it, changing electric field generates magnetic field, and changing magnetic field generates electric field. In the end, this theory was confirmed by Hertz's experiment.
In such a periodic transformation of the electromagnetic field, electromagnetic waves radiate and propagate into space. For details, see the article: "Electromagnetic waves are invisible and intangible. This young man's whimsical ideas changed the world."
As shown in the figure above, the red line represents the electric field, and the blue line represents the magnetic field. The propagation direction of electromagnetic waves is perpendicular to the direction of the electric field and the magnetic field.
So, how does the antenna send these electromagnetic waves out? After reading the picture below, you will understand.
The above two wires that generate electromagnetic waves are called "oscillators". In general, the size of the vibrator works best at half the wavelength, so it is often called a "half wave vibrator."
With the vibrator, electromagnetic waves can be emitted continuously. As shown below.
The real vibrator looks like the picture below.
The half-wave oscillator continuously propagates electromagnetic waves to space, but the signal strength is not uniformly distributed in space, like a tire-like ring.
But in fact, the coverage of our base station needs to be farther in the horizontal direction. After all, the people who need to call are on the ground; the vertical direction is high in the sky, and there are no people who need to fly and brush the TikTok (route Covering is another topic, I will talk about it next time), therefore, in the emission of electromagnetic wave energy, it is necessary to strengthen the horizontal direction and weaken the vertical direction.
According to the principle of energy conservation, energy will neither increase nor decrease. If you want to increase the emission energy in the horizontal direction, you must weaken the energy in the vertical direction. Therefore, the only way to flatten the energy radiation pattern of the standard half-wave array is as shown in the figure below.
So how do you slap it? The answer is to increase the number of half-wave oscillators. The emission of multiple vibrators is concentrated in the center, and the energy at the edges is weakened, which achieves the purpose of flattening the radiation direction and concentrating the energy in the horizontal direction.
In general base station systems, the use of directional antennas is the most common. In general, a base station is divided into 3 sectors, covered by 3 antennas, each antenna covers a range of 120 degrees.
The above figure is a base station coverage plan of a patch area. We can clearly see that each base station is composed of three sectors, and each sector is represented by a different color, which requires three directional antennas.
So, how does the antenna realize the directional emission of electromagnetic waves?
This is certainly not difficult for smart designers. Isn't it enough to add a reflector to the vibrator to reflect the signal radiated to the other side?
In this way, adding a vibrator to allow electromagnetic waves to travel farther in the horizontal direction, and then adding a reflector to control the direction. After these two tossings, the prototype of the directional antenna was born, and the electromagnetic wave emission direction became as shown below.
The horizontal main lobe is far away from the launch area, but the upper and lower side lobes are generated in the vertical direction. At the same time, due to incomplete reflection, there is a tail behind, called the back lobe.
At this point, the most important indicator of the antenna: "gain" is explained.
As the name implies, gain means that the antenna can enhance the signal. It stands to reason that the antenna does not need a power source, but only emits the electromagnetic waves transmitted to it. How can there be "gain"?
In fact, whether there is "gain" depends on who and how it is compared.
As shown in the figure below, compared to an ideal point radiation source and half-wave oscillator, the antenna can concentrate energy in the direction of the main lobe and send electromagnetic waves farther, which is equivalent to increasing in the direction of the main lobe. In other words, the so-called gain is relative to a point radiation source or a half-wave oscillator in a certain direction.
So, how do you measure the coverage and gain of the antenna's main lobe?
This requires the introduction of a "beam width" concept. We call the beam width the range when the electromagnetic wave intensity on both sides of the center line on the main lobe attenuates to half.
Because the intensity is attenuated by half, which is 3dB, the beam width is also called "half power angle", or "3dB power angle".
Common antenna half-power angles are mostly 60°, and there are also narrower 33° antennas. The narrower the half power angle, the farther the signal propagates in the main lobe direction, and the higher the gain.
Next, we combine the horizontal pattern of the antenna with the vertical pattern to get the three-dimensional radiation pattern, which looks much more intuitive.
Obviously, the existence of the backlobe destroys the directivity of the directional antenna, and it is necessary to minimize it. The energy ratio between the front and rear lobes is called the "front-to-back ratio". The larger the value, the better. It is an important indicator of the antenna.
The precious power of the upper side lobe is radiated to the sky in vain, which is not a small waste, so when designing a directional antenna, the upper side lobe should be minimized as much as possible.
In addition, there are some holes between the main lobe and the lower side lobes, which are also called lower nulls, which lead to poor signals in places close to the antenna. When designing the antenna, these holes should be minimized, which is called "zero point filling".
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